Apparatus for measuring coefficient of restitution and hardness tester

An apparatus for measuring for measuring coefficient of restitution which is capable of reducing a mass effect and performing tests in free directions, is disclosed. The apparatus for measuring coefficient of restitution includes a holder for holding a spherical indenter, an ejection mechanism configured to eject the indenter held by the holder from the holder to a specimen, a speed measuring unit configured to measure an impact speed that is a speed of the indenter before the indenter impacts against the specimen, and a rebound speed that is a speed of the indenter after the indenter is rebounded from the specimen; and an arithmetic unit configured to calculate a coefficient of restitution that is a ratio of the rebound speed to the impact speed.

CROSS REFERENCE TO RELATED APPLICATIONS

This document claims priorities to Japanese Patent Application No. 2015-217868 filed Nov. 5, 2015 and Japanese Patent Application No. 2016-043386 filed Mar. 7, 2016, the entire contents of which are hereby incorporated by reference.

BACKGROUND

Rebound hardness tests have been used to measure the hardness of specimen, particularly metal material. In the rebound hardness test, generally, an impactor which is constructed by an indenter made of a hard material such as diamond or the like and an indenter support member to which the indenter is secured, is impacted against the surface of the specimen, and a rebound height or a rebound speed of the impactor that has been rebounded from the specimen is measured to thereby measure the hardness of the specimen. Such rebound hardness tests include the Shore hardness test and the Leeb hardness test.

The Shore hardness test, which is specified as a rebound hardness test in JIS (Japanese Industrial Standards), is a hardness testing method in which a hammer serving as the impactor freely drops from a predetermined height (drop height) onto a specimen, and the rebound height representing a maximum point reached by the hammer that has been rebounded from the specimen is measured (see JIS B 7727:2000 “Shore hardness test—Verification of testing machines”). In the Shore hardness test, the hammer is the impactor and includes an indenter and an indenter support member to which the indenter is secured. The Shore hardness is obtained by multiplying a ratio of the rebound height to the drop height of the hammer by a predetermined proportionality constant.

The Leeb hardness test is a hardness testing method in which an impact body serving as the impactor is ejected toward a specimen by a spring, and an impact speed of the impact body before impacting against the specimen and a rebound speed of the impact body when impacting against the specimen and rebounded therefrom (i.e., the speed of the impact body after impacting against the specimen) are measured (see U.S. Pat. No. 4,034,603). In the Leeb hardness test, the impact body is an impactor and includes an indenter and an indenter support member to which the indenter is secured. In the Leeb hardness test, a ratio of the rebound speed of the impact body to the impact speed of the impact body serving as the impactor is measured as a coefficient of restitution. The Leeb hardness is obtained by multiplying the coefficient of restitution by a predetermined proportionality constant.

The rebound hardness tests, which are typified by the Shore hardness test and the Leeb hardness test, are advantageous in that testing of the hardness can be simply and quickly performed, compared with indentation hardness tests such as the Rockwell hardness test and the Vickers hardness test. Furthermore, testers for use in the rebound hardness tests are advantageous in that they have a simple structure and excellent portability, compared with testers for use in the indentation hardness tests.

However, a mass of the hammer for use in the Shore hardness test, and a mass of the impact body for use in the Leeb hardness test are comparatively large. For example, the mass of the hammer of a D-type Shore hardness tester is 36.2 g and the mass of the impact body of the Leeb hardness tester is 5.45 g. When the hardness of a small and light specimen is measured using such hammer or impact body, only a hardness value lower than the true hardness value of this specimen may be obtained.

The reason of this is that the kinetic energy of the impactor (i.e., the hammer or the impact body including the indenter and the indenter support member) which impacts against the small and light specimen is consumed by not only plastic deformation and elastic deformation of the specimen, but also vibrations or the like, of the specimen, resulting in the measurement of a rebound height or a rebound speed which is smaller than the rebound height or the rebound speed to be measured normally. This phenomenon, i.e., the phenomenon that a smaller rebound height or rebound speed than the rebound height or rebound speed to be measured normally, is measured as a result of the consumption of the kinetic energy of the impactor by vibration or the like, of the specimen, will be referred to as “mass effect” in the present specification.

If the mass effect occurs when the hardness of the specimen is measured, a correct hardness of the specimen cannot be obtained. Therefore, when the hardness of a specimen having a mass of 4 kg or less is to be measured by the Shore hardness test, it is necessary to perform the test while the specimen is firmly secured to a dedicated steel anvil having a sufficiently large mass. In the Leeb hardness test, a dedicated anvil is not prepared. Thus, when the hardness of the small and light specimen is to be measured by the Leeb hardness test, it is necessary for the user to prepare an appropriate support having a sufficiently large mass, and firmly secure the specimen to the support using a dedicated paste. Specifically, there have been restrictions on the size and mass of the specimen to be tested when its hardness is to be correctly measured by conventional rebound hardness testers.

In the Shore hardness test, since it is necessary to measure the rebound height after the hammer has dropped freely from a predetermined drop height to impact with the specimen, the testing direction of the Shore hardness test is limited to a vertical direction. On the other hand, in the Leeb hardness test, the impact body is ejected toward the specimen by the spring, and the coefficient of restitution which represents the ratio of the rebound speed of the impact body to the impact speed thereof before the impact body impacts against the specimen is measured. Accordingly, the Leeb hardness test is capable of measuring, in a free direction, the coefficient of restitution and the hardness based on the coefficient of restitution.

According to the hardness testing method, such as the Leeb hardness test, in which an impactor is ejected by the spring to measure the coefficient of restitution of the specimen and this coefficient of restitution is used as an index for evaluating the hardness of the specimen, the hardness test can be performed while the tester is oriented in a free direction. However, when the specimen to measure the hardness is small and light, the mass effect occurs even in the Leeb hardness test. Therefore, there have been demands for an apparatus for measuring coefficient of restitution and a hardness tester which are capable of reducing the mass effect and performing tests in free directions.

SUMMARY OF THE INVENTION

According to embodiments, there are provided an apparatus for measuring coefficient of restitution and a hardness tester which are capable of reducing the mass effect and performing tests in free directions.

Embodiments, which will be described below, relate to an apparatus for measuring coefficient of restitution that is used as an index for evaluating hardness of a specimen. The below-described embodiments further relate to a hardness tester for measuring hardness of a specimen.

In an embodiment, there is provided an apparatus for measuring coefficient of restitution, including: a holder for holding a spherical indenter; an ejection mechanism configured to eject the indenter held by the holder from the holder to a specimen; a speed measuring unit configured to measure an impact speed that is a speed of the indenter before the indenter impacts against the specimen, and a rebound speed that is a speed of the indenter after the indenter is rebounded from the specimen; and an arithmetic unit configured to calculate a coefficient of restitution that is a ratio of the rebound speed to the impact speed.

In an embodiment, the holder has a tubular shape, a front end of the holder is constituted of a plurality of divided portions by forming slits extending parallel to an axis of the holder, and the holder holds a circumferential surface of the indenter at the divided portions.

In an embodiment, the ejection mechanism includes an inner cylinder with a through hole formed therein, an outer cylinder having an inner circumferential surface slidably supported by an outer circumferential surface of the inner cylinder, an indenter pushing member movable in the through hole, and a biasing spring which is disposed between the outer cylinder and the indenter pushing member, and is compressed by movement of the outer cylinder to apply a biasing force to the indenter pushing member, and the outer cylinder has an ejection lever which can engage with a groove formed on an outer surface of the indenter pushing member.

In an embodiment, the indenter pushing member is a striker which collides with the indenter held by the holder.

In an embodiment, the indenter pushing member is a piston rod which applies an air pressure to the indenter held by the holder.

In an embodiment, the speed measuring unit includes a speed measuring body having an indenter channel which is connected to the through hole, and a first passage sensor and a second passage sensor which are arrayed along the indenter channel.

In an embodiment, the first passage sensor and the second passage sensor are optical sensors.

In an embodiment, the speed measuring unit further includes a third passage sensor, the first passage sensor, the second passage sensor, and the third passage sensor are arrayed along the indenter channel, and the arithmetic unit calculates an acceleration of the indenter from a speed of the indenter passing between the first passage sensor and the second passage sensor, and a speed of the indenter passing between the second passage sensor and the third passage sensor, and further calculates an impact speed at the instant at which the indenter impacts against the specimen, and a rebound speed at the instant at which the indenter is rebounded from the specimen.

In an embodiment, the first passage sensor, the second passage sensor, and the third passage sensor are optical sensors.

In an embodiment, the speed measuring unit includes a speed measuring body having an indenter channel which is connected to the through hole, and a first passage sensor and a second passage sensor which are arrayed along the indenter channel, the first passage sensor is an optical sensor having a first light emitter and a first light receiver, the second passage sensor is an optical sensor having a second light emitter and a second light receiver, the first light emitter emits light through a first optical fiber into the indenter channel, and the first light receiver receives the light emitted into the indenter channel through a second optical fiber, the second light emitter emits light through a third optical fiber into the indenter channel, and the second light receiver receives the light emitted into the indenter channel through a fourth optical fiber.

In an embodiment, the speed measuring unit includes a speed measuring body having an indenter channel which is connected to the through hole, and a first passage sensor which is disposed in the indenter channel, the arithmetic unit detects a detection starting point of time of the indenter at the first passage sensor and a detection ending point of time of the indenter at the first passage sensor, and the arithmetic unit calculates the impact speed and the rebound speed by dividing a diameter of the indenter by a time between the detection starting point of time and the detection ending point of time.

In an embodiment, the speed measuring unit includes a speed measuring body having an indenter channel which is connected to the through hole, and the speed measuring body has a shutter mechanism which opens an opening of the indenter channel when the speed measuring unit contacts the specimen, and closes the opening of the indenter channel when the speed measuring unit is separated from the specimen.

In an embodiment, the shutter mechanism includes a door disposed at the opening of the indenter channel, an opening/closing rod whose front end projects from the speed measuring body, and a link mechanism for converting movement of the opening/closing rod into opening/closing movement of the door.

In an embodiment, the speed measuring unit includes a speed measuring body having an indenter channel which is connected to the through hole, a lid is fixed to a front end of the speed measuring body, the lid having a lid through hole which is connected to the indenter channel, and the lid through hole has a diameter which is greater than 0.2 times a diameter of the indenter, and is smaller than the diameter of the indenter.

In an embodiment, a wall surface of the lid through hole is forming in a curved surface, and a radius of curvature of the wall surface is greater than a radius of curvature of the indenter.

In an embodiment, a vent hole extending from a side surface of the speed measuring body to the indenter channel, is formed in the speed measuring body.

In an embodiment, the indenter channel has a diameter which is 1.4 times the diameter d of the indenter or greater.

In an embodiment, the speed measuring unit includes a speed measuring body having an indenter channel which is connected to the through hole, a coupling mechanism for coupling the holder to the outer cylinder is further provided, and when the outer cylinder moves toward the speed measuring unit, the holder moves in the forward direction in the indenter channel to hold the indenter which exists in the indenter channel.

In an embodiment, the indenter is made of ceramics.

In an embodiment, the indenter is a bearing ball made of alumina.

In an embodiment, a diameter of the indenter is in a range from 0.5 mm to 5 mm.

In an embodiment, there is provided a hardness tester, comprising: a holder for holding a spherical indenter; an ejection mechanism configured to eject the indenter held by the holder from the holder to a specimen; a speed measuring unit configured to measure an impact speed that is a speed of the indenter before the indenter impacts against the specimen, and a rebound speed that is a speed of the indenter after the indenter is rebounded from the specimen; and an arithmetic unit configured to decide hardness of the specimen based on a ratio of the rebound speed to the impact speed.

According to the above-described embodiments, an impactor (object) that impacts against the specimen for measuring the coefficient of restitution is only the spherical indenter. More specifically, the impactor that impacts against the specimen does not include an indenter support to which the indenter would be fixed, unlike the hammer for use in the Shore hardness test and the impact body for use in the Leeb hardness test, for example. As a result, since the mass of the impactor (object) that impacts against with the specimen is greatly reduced, the mass effect occurring when the coefficient of restitution is to be measured is greatly reduced, thereby enabling the coefficient of restitution of the specimen to be correctly measured. Further, the spherical indenter is held in the holder, and ejected from the holder toward the specimen by the ejection mechanism. Therefore, since there is no limitation on the direction in which the indenter is ejected, the test can be performed in a free direction.

Further, according to the above-described embodiments, an impactor (object) that impacts against the specimen for measuring the hardness is only the spherical indenter. As a result, since the mass of the impactor (object) that impacts against with the specimen is greatly reduced, the mass effect occurring when the hardness is to be measured is greatly reduced, thereby enabling the hardness of the specimen to be correctly measured.

DESCRIPTION OF EMBODIMENTS

Embodiments will be described below with reference to the accompanying drawings.

FIG. 1is a schematic cross-sectional view showing an apparatus1for measuring coefficient of restitution according to an embodiment. For convenience in description, in the present specification, the direction indicated by arrow A shown inFIG. 1is defined as a forward direction, and the direction indicated by arrow B is identified as a backward direction.

The apparatus1for measuring coefficient of restitution shown inFIG. 1has a holder3for holding a spherical indenter2, an ejection mechanism5for ejecting the indenter2held by the holder3from this holder3toward a specimen8, a speed measuring unit6for measuring an impact speed which represents the speed of the indenter2before the indenter2impacts against the specimen8and a rebound speed of the indenter2after the indenter2impacts against the specimen8and rebounds therefrom, and an arithmetic unit7for calculating a coefficient of restitution which represents the ratio of the rebound speed to the impact speed. The arithmetic unit7is disposed in a display unit10having a display10afor displaying the coefficient of restitution calculated by the arithmetic unit7.

The holder3shown inFIG. 1has a hollow cylindrical shape. As shown inFIGS. 2A and 2B, the front end of the holder3is constituted of a plurality of divided portions3aby forming slits3bextending parallel to the axis of the holder3. In the illustrated example, the front end of the holder3is constituted of four divided portions4bby four slits3b. The front end of the holder3may be constituted of three or less divided portions3aor five or more divided portions3a. The diameter of the inner circumferential surface of the holder3is slightly smaller than the diameter of the spherical indenter2, so that when the front end of the holder3is pressed against the indenter2, the divided portions3aare slightly spread in an outer circumferential direction of the holder3and an outer circumferential surface of the indenter2is held by the plurality of divided portions3a.

The shape of holder3is not limited to the hollow cylindrical shape, but may be of a tubular shape. For example, the holder3may have a polygonal tubular shape such as a square tubular shape, a pentagonal tubular shape, or the like. Even though the holder3has a polygonal tubular shape, the slits extending parallel to the axis of the holder3are formed in the front end of the holder3and the front end of the holder3is constituted of a plurality of divided portions. When the front end of the holder3having a polygonal tubular shape is pressed against the indenter2, the divided portions are slightly spread outwardly of the holder3, and the inner circumferential surface of the indenter2is held by the divided portions.

As shown inFIG. 1, the ejection mechanism5according to the present embodiment has an inner cylinder13with a through hole13aformed therein, an outer cylinder12having an inner circumferential surface12aslidably supported by an outer circumferential surface13bof the inner cylinder13, a striker15movable in the through hole13a, and a biasing spring16which is disposed between the outer cylinder12and the striker15, and is compressed by movement of the outer cylinder12to thereby apply a biasing force to the striker15. Furthermore, the ejection mechanism5shown inFIG. 1has a stopper20for restricting movement of the striker15in the through hole13a. Although not shown, the stopper20may be omitted. For example, by fixing a front end of the biasing spring16to the striker15, forward movement of the striker15in the through hole13acan be restricted. A wall surface of the through hole13ais the inner circumferential surface of the inner cylinder13. As will be described later, the striker15constitutes an indenter pushing member that collides with the indenter2, which is held by the holder3, with use of the spring force from the biasing spring16.

The striker15according to the present embodiment has a rod shape. More specifically, this striker15includes a cylindrical striker body15aand a cylindrical body support15bhaving a greater diameter than the diameter of the striker body15a. The striker body15ahas its back end embedded in the front end of the body support15b, so that the striker body15ais fixed to the body support15b. The central axis of the striker body15ais in alignment with the central axis of the body support15b. The striker body15amay be integrally formed with the body support15b. For example, a cylindrical member may be ground to form the strike body15aon the striker15.

A front end of the biasing spring16is inserted into a guide hole15cwhich is formed in the body support15b. The guide hole15cextends from the back end toward front end of the body support15b, and the central axis of the guide hole15cis in alignment with the central axis of the body support15b. To the back end of the outer cylinder12, a plug18that closes an opening of the outer cylinder12is fixed, and a back end of the biasing spring16is supported by the plug18. The plug18according to the present embodiment has a cylindrical shape, and a screw thread is formed on an outer circumferential surface of the plug18. An opening formed on the back end of the outer cylinder12is constructed as a threaded hole into which the screw thread formed on the outer circumferential surface of the plug18is screwed. The screw thread formed on the plug18engages with the threaded hole, thereby securing the plug18to the outer cylinder12. By rotating the plug18, the plug18can be moved forwards or backwards relative to the outer cylinder12. As a result, the biasing force applied to the striker15from the biasing spring16can be easily changed, because a length of the biasing spring16can be easily changed.

With these configurations, the biasing spring16is disposed between the outer cylinder12and the striker15. The biasing spring16is compressed by movement of the outer cylinder12, thereby allowing the biasing force to be applied to the striker15. The compressing operation of the biasing spring16will be described later. As shown inFIG. 1, when the biasing spring16is compressed, the biasing spring16applies the biasing force for moving the striker15in the forward direction of the ejection mechanism5, to the striker15. This position of the striker15is an ejection position of the striker15.

An annular groove15dextending along a circumferential direction of the striker15is formed on the outer surface of the body support15b. The groove15dmay be formed on a part of the outer surface of the body support15b. An ejection lever15is fixed to the outer surface of the outer cylinder11, the ejection lever15having a hook14athat can engage with the groove15dwhen the striker15is in the ejected position shown inFIG. 1. The ejection lever14has a through hole, and a rotational shaft17which is secured to a bracket (not shown) extending radially outwardly from the outer surface of the outer cylinder11, is inserted into this through hole. Therefore, the ejection lever14is mounted to the outer cylinder12so as to be able to pivot around the rotational shaft17. As shown inFIG. 1, the outer cylinder12has a through hole12bwhich penetrates through a side wall of the outer cylinder12, and the inner cylinder13has a first elongated hole13ewhich penetrates through a side wall of the inner cylinder13and extends along a longitudinal direction of the inner cylinder13. The hook14aof the ejection lever14passes through the through hole12bof the outer cylinder12and the first elongated hole13eof the inner cylinder13to engage with the groove15dof the striker15.

FIG. 3is a schematic cross-sectional view showing a state in which the striker15is pushed out in the forward direction of the ejection mechanism5with use of the spring force of the biasing spring16when the hook14aof the ejection lever14disengages from the groove15dof the striker15. As shown inFIG. 3, the striker15that has been pushed out in the forward direction of the ejection mechanism5, collides with the stopper20, which has a hollow cylindrical shape and is fixed to the inner circumferential surface of the inner cylinder13, and as a result, the forward movement of the striker15is restricted. More specifically, the front end of the body support15bof the striker15collides with the back end of the stopper20, and as a result, the forward movement of the striker15is restricted. During the striker15moves from the ejection position shown inFIG. 1to a collision position shown inFIG. 3where it collides with the stopper20, the front end of the striker body15acollides with indenter2held by the front end of the holder3, thereby ejecting only the indenter2from the holder3toward the specimen8. As shown inFIG. 3, when the striker15has been ejected in the forward direction of the ejection mechanism5, the hook14aof the ejection lever14passes through the through hole12bof the outer cylinder12and the first elongated hole13eof the inner cylinder13, and is in contact with the outer surface of the striker15.

As described above, the length of the biasing spring16can be adjusted by moving the plug18supporting the back end of the biasing spring16forwards or backwards relative to the outer cylinder12. Therefore, the impact speed of the indenter2ejected by collision with the striker15can be adjusted, because the biasing force applied to the striker15from the biasing spring16can be adjusted. When the coefficients of restitution of different specimens are to be compared, it is preferred that the material of the indenter2and the impact speed of the indenter2are constant. The apparatus1for measuring coefficient of restitution according to the present embodiment can easily adjust the impact speed of the indenter2.

Although the striker15according to the above described embodiment has the rod-shape, the shape of the striker15is not limited to the rod shape. The shape of the striker15is arbitrary if it can collides with the indenter2held by the holder3to thereby eject the indenter2from the holder3toward the specimen8.

The speed measuring unit6for measuring the impact speed which represents the speed of the indenter2before the indenter2ejected by the striker15impacts against the specimen8, and the rebound speed of the indenter2after the indenter2impacts against the specimen8and rebounds therefrom, is fixed to the front end of the ejection mechanism5, or more specifically, to the front end of the inner cylinder13. The speed measuring unit6according to the present embodiment has a speed measuring body23which is mounted on the front end of the inner cylinder13and has an indenter channel23athrough which the indenter2passes, and a first passage sensor24and a second passage sensor25which are arrayed along the indenter channel23a. The indenter channel23ais connected to the through hole13aof the inner cylinder13. The first passage sensor24and the second passage sensor25are arrayed along the indenter channel23a, and the first passage sensor24is positioned forwardly of the second passage sensor25in the speed measuring body23. The “passage sensor” collectively refers to sensors capable of detecting the passage of an object. The passage sensor includes an optical sensor or a magnetic sensor, for example.

The first passage sensor24according to the present embodiment is an optical sensor having a first light emitter24afor emitting light into the indenter channel23aand a first light receiver24bfor receiving the light emitted from the first light emitter24a. The second passage sensor25according to the present embodiment is an optical sensor having a second light emitter25afor emitting light into the indenter channel23aand a second light receiver25bfor receiving the light emitted from the second light emitter25a. The embodiment in which the first passage sensor24and the second passage sensor25are the optical sensors will be described below.

In the present embodiment, light-emitting diodes (LEDs) are used as the first light emitter24aand the second light emitter25a, and photodiodes are used as the first light receiver24band the second light receiver25b. Light from the first light emitter24ais emitted into the indenter channel23athrough a light projection slit (not shown) disposed in a wall surface of the indenter channel23a, and the first light receiver24breceives light that has passed through a light receiving slit (not shown) disposed in a wall surface of the indenter channel23a. Similarly, Light from the second light emitter25ais emitted into the indenter channel23athrough a light emission slit (not shown) disposed in a wall surface of the indenter channel23a, and the second light receiver25breceives light that has passed through a light receiving slit (not shown) disposed in a wall surface of the indenter channel23a. In the present embodiment, the first light emitter24aand the first light receiver24bof the first optical sensor24are disposed at a position which is spaced from the front end of the speed measuring body23by 10 mm, and the second light emitter25aand the second light receiver25bof the second optical sensor25are disposed at a position which is spaced from the front end of the speed measuring body23by 20 mm.

The first light receiver24bdetects that the light emitted from the first light emitter24ais blocked by the passage of the indenter2, whereby the first optical sensor24detects that the indenter2has passed the first optical sensor24. Similarly, the second light receiver25bdetects that the light emitted from the second light emitter25ais blocked by the passage of the indenter2, whereby the second optical sensor25detects that the indenter2has passed the second optical sensor25. The arithmetic unit7measures a passage time taken after the indenter2has passed the first optical sensor24until the indenter2passes the second optical sensor25. The distance between the first optical sensor24and the second optical sensor25(10 mm in the present embodiment) is determined in advance. Therefore, the arithmetic unit7can calculate the impact speed of the indenter2from the passage time taken after the indenter2has passed the second optical sensor25until the indenter2passes the first optical sensor24, and the distance between the first optical sensor24and the second optical sensor25. Similarly, the arithmetic unit7can calculate the rebound speed of the indenter2from the passage time taken after the indenter2has passed the first optical sensor24until the indenter2passes the second optical sensor25, and the distance between the first optical sensor24and the second optical sensor25.

FIG. 4is a schematic diagram showing an arrangement of an electric circuit of the first optical sensor24. The arrangement of the electric circuit of the first optical sensor24will be described below with reference toFIG. 4. An arrangement of an electric circuit of the second optical sensor25is identical to the arrangement of the electric circuit of the first optical sensor24, and its repetitive explanation will be omitted.

As shown inFIG. 4, a reverse bias voltage Vr is applied to the first light receiver24bwhich is a photodiode. The first light receiver24breceives the light from the first light emitter24awhich is an LED, thereby generating an electric current. The electric current generated from the first light receiver24bis input to a negative input terminal30bof a first operational amplifier30. A voltage VA is input to a positive input terminal30aof the first operational amplifier30. From an output terminal30cof the first operational amplifier30, a voltage Vopa, which is converted from the electric current generated from the first light receiver24bby a conversion resistor35using the voltage VA as a reference, is output (current-to-voltage conversion).

The voltage Vopa output from the first operational amplifier30is input to a negative input terminal31bof a second operational amplifier31through a delay circuit32. A predetermined base voltage Vdet is input to a positive input terminal31aof the second operational amplifier31. The second operational amplifier31outputs, from a output terminal31cof the second operational amplifier31, a voltage Vopb which is a difference between the voltage Vopa input to the negative input terminal31band the base voltage Vdet input to the positive input terminal31a. The voltage Vopb output from the output terminal31cof the second operational amplifier31is converted by a conversion resistor36into an electric current, this electric current being applied to the first light emitter24a.

As shown inFIG. 4, the voltage Vopa output from the output terminal of the first operational amplifier30is input to a positive input terminal38aof a comparator38. A voltage Vcmp, which is a preset threshold value, is input to a negative input terminal38bof the comparator38. The comparator38compares the voltage Vopa input to the positive input terminal38aand the voltage Vcmp input to the negative input terminal38bwith each other. When the voltage Vopa is larger than the voltage Vcmp, the comparator38output a signal VS from its output terminal38c. The signal VS is input to the arithmetic unit7.

According to the circuit arrangement shown inFIG. 4, when the light from the first light emitter24ais not blocked due to the passage of the indenter2, the amount of light emitted from the first light emitter24acan automatically be adjusted so that the voltage Vopa output from the first operational amplifier30converges on the preset base voltage Vdet at all times. Specifically, the electric circuit shown inFIG. 4includes a feedback circuit for automatically changing the amount of light emitted from the first light emitter24awhich is the photodiode, depending on the output electric current from the first light receiver24bso that the output electric current from the first light receiver24bwhich is the photodiode becomes constant.

When the light from the first light emitter24ais not blocked due to the passage of the indenter2, it is preferred that the first light receiver24b, which is the photodiode and receives the light emitted from the first light emitter24awhich is the LED, outputs a constant electric current at all times. However, the amount of light emitted from an LED differs depending on the environment (e.g., temperature and humidity) in which the LED is placed. Similarly, the electric current output from a photodiode differs depending on the environment (e.g., temperature and humidity) in which the photodiode is placed. Furthermore, since there are individual differences between LEDs, the amounts of light emitted from the LEDs are different from each other when the same electric current flows through the different LEDs that are of the same structure. Similarly, since there are individual differences between photodiodes, the currents output from the photodiodes are different from each other when the different photodiodes, which are of the same structure, receive the same amount of light.

Therefore, in a case where the electric circuit shown inFIG. 4is not used, the voltage Vopa output from the operational amplifier30varies depending on the environment in which the apparatus1for measuring coefficient of restitution is placed, and thus there is the possibility that a correct coefficient of restitution cannot be measured. In order to measure the correct coefficient of restitution, it is necessary to adjust, in every measurement, the amount of light emitted from the first light emitter24aand/or the output electric current from the first light receiver24b. Similarly, in a case where the electric circuit shown inFIG. 4is not used, the voltages Vopa output from the operational amplifiers30of different apparatus1for measuring coefficient of restitution are different in the respective apparatus1for measuring coefficient of restitution, and thus there is the possibility that a correct coefficient of restitution cannot be measured. In order to measure the correct coefficient of restitution, it is necessary to adjust, in each apparatus1for measuring coefficient of restitution, the amount of light emitted from the first light emitter24aand/or the output electric current from the first light receiver24b.

In a case where the electric circuit shown inFIG. 4is used, the amount of light emitted from the first light emitter24acan automatically be adjusted so that the voltage Vopa output from the first operational amplifier30converges on the preset base voltage Vdet at all times. Accordingly, it is not necessary to adjust, in every measurement, the amount of light emitted from the first light emitter24aand/or the output electric current from the first light receiver24b. Similarly, it is not necessary to adjust, in each apparatus1for measuring coefficient of restitution, the amount of light emitted from the first light emitter24aand/or the output electric current from the first light receiver24b. Inasmuch as the arrangement of the electric circuit of the second optical sensor25is identical to the arrangement of the electric circuit of the first optical sensor24, the identical advantages are available for the optical sensor25.

FIG. 5is a schematic diagram illustrating a method of measuring a passage time T1taken after the indenter2has passed the first optical sensor24until the indenter2passes the second optical sensor25. In other words,FIG. 5is a schematic diagram illustrating a method of measuring the rebound speed of the indenter2which impacts against specimen8and rebounds therefrom.FIG. 5shows in its upper area a graph representing the output voltage Vopa from the first operational amplifier30and the signal VS output from the comparator38, which vary over time, of the first optical sensor24.FIG. 5shows in its lower area a graph representing the output voltage Vopa from the first operational amplifier30and the signal VS output from the comparator38, which vary over time, of the second optical sensor25.

The indenter2which impacts against the specimen8and rebounds therefrom moves to the position where the first optical sensor24is disposed. When the indenter2blocks the light emitted from the first light emitter24aof the first optical sensor24, and thereby the amount of light received by the first light receiver24bis reduced, the output voltage Vopa from the first operational amplifier30increases. As the indenter2passes the first optical sensor24, the blocked amount of light emitted from the first light receiver24bgradually decreases and thereafter gradually increases. Therefore, because the amount of light received by the first light receiver24bgradually decreases and thereafter gradually increases, the output voltage Vopa from the first operational amplifier30is plotted as a wave SA having a convex waveform shown in the upper area ofFIG. 5. As described above, the output voltage Vopa from the first operational amplifier30is input to the comparator38(seeFIG. 4) and compared with the voltage Vcmp serving as the threshold value. When the output voltage Vopa from the first operational amplifier30is larger than the voltage Vcmp, the comparator38outputs the signal VS. When the output voltage Vopa from the first operational amplifier30is smaller than the voltage Vcmp, the comparator38stops outputting the signal VS. As a result, a rectangular wave RA converted from the wave SA having the convex waveform by the comparator38is input to the arithmetic unit7.

The arithmetic unit7detects a point of time Ta when the signal VS is output from the comparator38and a point of time Tb when the outputting of the signal VS is stopped. Specifically, the point of time Ta is a detection starting point of time when the arithmetic unit7starts detecting of the passage of indenter2at the first optical sensor24, and the point of time Tb is a detection ending point of time when the arithmetic unit7ends detecting of the passage of indenter2at the first optical sensor24. The arithmetic unit7calculates a time Td by halving a time Tc taken after the point of time Ta has been detected until the point of time Tb is detected. The time Tc corresponds to a time during which the arithmetic unit7is detecting the passage of indenter2at the first optical sensor24.

The indenter2further moves on to the position where the second optical sensor25is disposed. When the indenter2blocks the light emitted from the second light emitter25aof the second optical sensor25, reducing the amount of light detected by the second light receiver25b, the output voltage Vopa from the first operational amplifier30increases. As the indenter2passes the second optical sensor25, the blocked amount of light emitted from the second light receiver25bgradually decreases and thereafter gradually increases. Therefore, because the amount of light detected by the second light receiver25bgradually decreases and thereafter gradually increases, the output voltage Vopa from the first operational amplifier30is plotted as a wave SB having a convex waveform shown in the lower area ofFIG. 5. As described above, the output voltage Vopa from the first operational amplifier30is input to the comparator38(seeFIG. 4) and compared with the voltage Vcmp as the threshold value. When the output voltage Vopa from the first operational amplifier30is larger than the voltage Vcmp, the comparator38outputs the signal VS. When the output voltage Vopa from the first operational amplifier30is smaller than the voltage Vcmp, the comparator38stops outputting the signal VS. As a result, a rectangular wave RB converted from the wave SB having the convex waveform by the comparator38is input to the arithmetic unit7.

The arithmetic unit7detects a point of time Te when the signal VS is output from the comparator38and a point of time Tf when the outputting of the signal VS is stopped. Specifically, the point of time Te is a detection starting point of time when the arithmetic unit7starts detecting of the passage of indenter2at the second optical sensor25, and the point of time Tf is a detection ending point of time when the arithmetic unit7ends detecting of the passage of indenter2at the second optical sensor25. Further, the arithmetic unit7calculates a time Th by halving a time Tg taken after the point of time Te has been detected until the point of time Tf is detected. The time Tg corresponds to a time during which the arithmetic unit7is detecting the passage of indenter2at the second optical sensor25.

The arithmetic unit7determines the time between the point of time Td and the point of time Th, as a passage time T1taken after the indenter2has passed the first optical sensor24until the indenter2passes the second optical sensor25. In addition, the arithmetic unit7calculates the rebound speed of the indenter2by dividing the distance between the first optical sensor24and the second optical sensor25by the passage time T1.

In the present embodiment, the time between the point of time Td and the point of time Th is used as the passage time T1taken after the indenter2has passed the first optical sensor24until the indenter2passes the second optical sensor25. In a case where the time between the point of time Td and the point of time Th is used as the passage time T1, it is possible to make a measurement error of the passage time T1smaller compared with a case where the time between the point of time Ta and the point of time Te is used as the passage time, so that a more correct rebound speed can be calculated.

The impact speed of the indenter2is calculated by the arithmetic unit7, using a similar method. Specifically, the arithmetic unit7calculates the impact speed by determining the passage time taken after the indenter2has passed the second optical sensor25until the indenter2passes the first optical sensor24. The arithmetic unit7also calculates the coefficient of restitution which represents the ratio of the rebound speed to the impact speed.

Although not shown, the second optical sensor25may be omitted, and thus the arithmetic unit7may calculate the impact speed and the rebound speed of the indenter2, using only the first optical sensor24. For example, the arithmetic unit7may calculate the rebound speed of the indenter2by dividing the diameter of the indenter2by the time Tc between the point of time Ta (i.e., the detection starting point of time of the indenter2at the first optical sensor24) and the point of time Tb (i.e., the detection ending point of time of the indenter2at the first optical sensor24) inFIG. 5. Similarly, the arithmetic unit7can calculate the impact speed of the indenter2, using only the first optical sensor24.

In the case where the impact speed and the rebound speed of the indenter2are measured by using only the first optical sensor24, the size of the speed measuring body23can be reduced. Furthermore, since the first optical sensor24can be disposed near the surface of the specimen8, it is possible to measure a correct coefficient of restitution.

As shown inFIG. 1, the holder3is inserted from the front end toward back end of the inner cylinder13. The outer cylinder12has an outer cylinder flange12cprojecting radially outwardly of the outer cylinder12, and the inner cylinder13has an inner cylinder flange13cprojecting radially outwardly of the inner cylinder13. A holder flange member40is fixed to the outer circumferential surface of the holder3. The holder flange member40includes a cylindrical static part40awhich is fixed to the outer circumferential surface of the holder3at its back end, and a projecting flange40bwhich projects from the static part40aradially outwardly of the holder3. The inner cylinder13has a second elongated hole13dpassing through a side wall of the inner cylinder13and extending along a longitudinal direction of the inner cylinder13. The projecting flange40bof the holder flange member40extends through the second elongated hole13dand projects outwardly of the outer circumferential surface of the inner cylinder13.

The apparatus1for measuring coefficient of restitution has a first return spring43disposed between the inner cylinder flange13cand the outer cylinder flange12c, and the first return spring43biases the inner cylinder flange13cin a direction away from the outer cylinder flange12c. Further, the apparatus1for measuring coefficient of restitution has a second return spring44disposed between the projecting flange40band the outer cylinder flange12c, and the second return spring44biases the projecting flange40bin a direction away from the outer cylinder flange12c. A first guide rod46is fixed to the outer cylinder flange12c, the first guide rod46extending through a first through hole40cformed in the projecting flange40bto pass through the projecting flange40b. The first guide rod46extends through the inside of the second return spring44. An anchor48is fixed to the first guide rod46for restricting the projecting flange40bto move in a direction away from the outer cylinder flange12cby a biasing force of the second return spring44. A second guide rod47is fixed to the inner cylinder flange13c, the second guide rod47extending through a second through hole40dformed in the projecting flange40bto pass through the projecting flange40b.

The outer cylinder flange12cof the outer cylinder12, the first guide rod46, the second return spring44, the holder flange member40, and the anchor48constitutes a coupling mechanism39for coupling the holder3to the outer cylinder12.

FIG. 6is a schematic view showing a state in which the indenter2is held in the holder3again after the coefficient of restitution has been measured. As shown inFIG. 6, in order to hold the indenter2in the holder3again, an operator pushes the outer cylinder12toward the inner cylinder13against the biasing force of the first return spring43until the hook14aof the ejection lever14engages with the groove15dformed on the outer surface of the striker15. At this time, the holder3that is coupled to the outer cylinder12by the coupling mechanism39moves forwards in the indenter channel23aof the speed measuring unit6. Since the projecting flange40bof the holder flange member40can move along the second elongated hole13dformed in the side wall of the inner cylinder12, the holder flange member40can also move in the forward direction of the apparatus1for measuring coefficient of restitution. Since the second return spring44applies its biasing force to the flange holder member40fixed to the holder3, the holder3moves in the forward direction of the apparatus1for coefficient of restitution while the distance between the holder3and the outer cylinder12is maintained. The holder3moving in the forward direction of the apparatus1for coefficient of restitution moves forwards in the indenter channel23aformed in the speed measuring body23of the speed measuring unit6to reach the front end of the indenter channel23a, and as a result, the indenter2is held in the front end of the holder3.

When the outer cylinder12is pushed in the inner cylinder13, the striker15is unable to move, because movement of the striker15in the forward direction of the apparatus1for measuring coefficient of restitution is restricted by the stopper20. On the other hand, the biasing spring16is compressed. The hook14aof the ejection lever14fixed to the outer cylinder12moves along the first elongated hole13eformed in the side wall of the inner cylinder12, and engages with the groove13dformed on the outer surface of the striker15. When the pushing force for pushing the outer cylinder12toward the inner cylinder13is removed, the outer cylinder12moves in the backward direction of the apparatus1for measuring coefficient of restitution by the biasing force of the first return spring43. At this time, the striker15, with which the hook14aengages, also moves in the backward direction, and thus the striker15waits in the ejection position shown inFIG. 1.

When the outer cylinder12is pushed in the inner cylinder13, the holder3is guided by the first guide rod46. Similarly, when the outer cylinder12is pushed in the inner cylinder13, the holder3is guided by the second guide rod47. Therefore, the holder3is prevented from rotating with respect to the outer cylinder12and the inner cylinder13.

With these configurations, the indenter2can be held in the holder3again by a simple operation in which the outer cylinder12is pushed in the inner cylinder13. Accordingly, a burden on the operator can be reduced when the coefficients of restitution are to be measured successively.

According to the apparatus1for measuring coefficient of restitution, of this embodiment, an impactor (object) that impacts against the specimen8for measuring the coefficient of restitution is only the spherical indenter2. Unlike the hammer for use in the Shore hardness test and the impact body for use in the Leeb hardness test, the impactor that impacts against the specimen8does not include an indenter support to which the indenter2would be fixed. As a result, since the mass of the impactor (object) that impacts against with the specimen8is greatly reduced, the mass effect occurring when the coefficient of restitution is to be measured is greatly reduced, thereby enabling the coefficient of restitution of the specimen to be correctly measured. The spherical indenter2is held in the holder3, and ejected from the holder3toward the specimen8by the ejection mechanism5. Therefore, since there is no limitation on the direction in which the indenter2is ejected, the test can be performed in a free direction. Furthermore, according to the apparatus1for measuring coefficient of restitution of this embodiment, the impact speed can be easily adjusted by moving the plug18forwards or backwards with respect to the outer cylinder12.

The apparatus1for measuring coefficient of restitution according to the present embodiment, which is capable of greatly reducing the mass effect and performing the tests in a free direction, can measure coefficients of restitution of various specimens8. For example, not only coefficients of restitution of metal materials, but also coefficients of restitution of foods such as chocolate and dried bonito or coefficients of restitution of non-metal materials such as ceramics, marble, and glass, can be measured. Further, the impactor that impacts against the specimen8is only the spherical indenter2, and thus has a very small volume. Therefore, the heat capacity of the impactor is small. The indenter2contacts the specimen8instantaneously. As a result, the coefficient of restitution of a specimen8which is of high or low temperature can be correctly measured, because the amount of change in a surface temperature of the specimen due to the test is greatly reduced.

The apparatus1for measuring coefficient of restitution according to the present embodiment can use indenters2having various diameters by appropriately selecting a size of the holder3and a diameter of the striker body15adepending on the diameter of the indenter2. Therefore, the apparatus1for measuring coefficient of restitution according to the present embodiment can use an indenter2having a very small diameter, so that the mass effect can further be reduced.

From the viewpoint of reducing the mass effect that occur when the test is performed, it is preferable to use an indenter2having as small a diameter as possible. In contrast, when the coefficient of restitution of a specimen8which has small irregularities on its surface is measured, the direction in which the indenter2is rebounded from the specimen8may greatly deviate from (be inclined with respect to) the direction in which the indenter2is ejected from the ejection mechanism5. The reason of this is that the diameter of the indenter2is too small with respect to the size of the irregularities formed on the surface of the specimen8. Therefore, in order to stabilize the direction in which the indenter2is rebound, while reducing the occurrence of the mass effect, it is preferable that the diameter of the indenter2is equal to or less than 5 mm.

As the diameter of the indenter2decreases, the burden on the operator for measuring the coefficient of restitution increases. For example, when the indenter2having a very small diameter is used, the operator is likely to lose the indenter2. Therefore, from the viewpoint of working efficiency, it is preferable that the diameter of the indenter2is equal to or more than 0.5 mm. Furthermore, in order to reliably confirm a fracture or defect of the indenter2prior to the measurement of the coefficient of restitution, it is more preferable that the diameter of the indenter2is equal to or more than 2 mm. More specifically, the diameter of the indenter2is preferably in the range from 0.5 mm to 5 mm, and more preferably in the range from 2 mm to 5 mm.

The speed measuring unit6of the apparatus1for measuring coefficient of restitution according to this embodiment measures the impact speed and the rebound speed of the indenter2using the optical sensors24,25. Therefore, there is no limitation on the material of the indenter2, and the indenter2may be made of a metal material such as cemented carbide or the like, or a non-metal material such as ceramics, diamond, or the like. Preferably, the indenter2is a bearing ball made of alumina which is ceramics. Since the bearing ball made of alumina has high sphericity, the apparatus1for measuring coefficient of restitution can measure correct coefficients of restitution with high repeatability. The bearing ball made of alumina is inexpensive and easily commercially available.

The speed measuring body23of the speed measuring unit6may have a shutter mechanism50which opens the opening of the indenter channel23ain the speed measuring body23when the speed measuring unit6contacts the specimen8, and which closes the opening of the indenter channel23ain the speed measuring body23when the speed measuring unit6is separated from the specimen8.FIG. 7is a schematic cross-sectional view showing an example of the speed measuring body23that is provided with the shutter mechanism50. As shown inFIG. 7, the shutter mechanism50of this embodiment includes a door51disposed at the opening of the indenter channel23a, an opening/closing rod54whose front end projects from the speed measuring body23, and a link mechanism55for converting movement of the opening/closing rod54into opening/closing movement of the door51. The speed measuring body23of this embodiment is constructed by a first member23bin which the indenter channel23ais formed and the door51is disposed, and a second member23cin which the opening/closing rod54is disposed.

FIG. 8Ais a view as viewed along arrows D inFIG. 7,FIG. 8Bis a cross-sectional view taken along line F-F ofFIG. 8A, andFIG. 8Cis a cross-sectional view taken along line G-G ofFIG. 8B. As shown inFIG. 8A, the door51of this embodiment has a first door51aand a second door51bwhich open and close the opening of the indenter channel23ain the speed measuring body23by the link mechanism55. The link mechanism55will be described later. When the first door51aand the second door51babut against each other, the opening of the indenter channel23ais closed, and when the first door51aand the second door51bare separated from each other, the opening of the indenter channel23ais opened.FIG. 8Aillustrates a state in which the first door51aand the second door51babut against each other, and thus the opening of the indenter channel23ais closed.

As shown inFIGS. 8A and 8B, a first opening/closing guide52having a first arc-shaped surface52ais fixed to a side surface of the first door51a, and a second opening/closing guide53having a second arc-shaped surface53ais fixed to a side surface of the second door51b. The first arc-shaped surface52aof the first opening/closing guide52and the second arc-shaped surface53aof the second opening/closing guide53face each other. The first opening/closing guide52is pivotably secured to the first member23bby a first opening/closing shaft56. In other words, the first opening/closing guide52can pivot about the first opening/closing shaft56, and as a result, the first door51afixed to the first opening/closing guide52can pivot about the first opening/closing shaft56. The second opening/closing guide53is pivotably secured to the first member23bby a second opening/closing shaft57. In other words, the second opening/closing guide53can pivot about the second opening/closing shaft57, and as a result, the second door51bfixed to the second opening/closing guide53can pivot about the second opening/closing shaft57.

As shown inFIGS. 8B and 8C, in a central region of the first door51a, a first receiving portion51c(seeFIG. 8B) is formed, in which a first sloping surface51dwith a circular-arc-shape is formed. In a central region of the second door51balso, which is illustrated by the imaginary lines (dot-and-dash lines) inFIG. 8C, a second receiving portion51fis formed, in which a second sloping surface51ewith a circular-arc shape is formed. When the first door51aand the second door51babut against each other, the indenter2can be housed in an internal space formed by the first receiving portion51cand the second receiving portion51f.

FIG. 9Ais a view as viewed along arrows E inFIG. 7, andFIG. 9Bis a cross-sectional view taken along line H-H ofFIG. 9A. As shown inFIG. 9A, one end of an opening/closing spring58is fixed to the back end of the opening/closing rod54, and the other end of the opening/closing spring58is fixed to the second member23c. In this state, the front end of the opening/closing rod54projects forwards from the front end of the second member23c. As shown inFIGS. 9A and 9B, a first protrusion54aand a second protrusion54b, which are cylindrical in shape, are formed on a side surface of the opening/closing rod54. The first protrusion54ais positioned forwardly of the second protrusion54bon the opening/closing rod54. The first protrusion54aand the second protrusion54bproject from the side surface of the opening/closing rod54toward the first member23b.

The link mechanism55of the shutter mechanism50is constructed by the first opening/closing guide52, the second opening/closing guide53, the first protrusion54aand the second protrusion54bformed on the opening/closing rod54, and the opening/closing spring58.FIG. 10is a schematic view showing a state in which the first door51aand the second door51bare pivoted in directions away from each other by the link mechanism55, and thus the opening of the indenter channel23ais opened.FIG. 11is a schematic view showing a state in which the first door51aand the second door51bare brought into abutment against each other by the link mechanism55, and thus the opening of the indenter channel23ais closed.

As shown inFIG. 10, when the speed measuring body23of the speed measuring unit6is brought into contact with the surface of the specimen8, the opening/closing rod54is pushed in against the biasing force of the opening/closing spring58, until a front end of the opening/closing rod54is housed into the speed measuring body23. At this time, the first protrusion54aformed on the opening/closing rod54contacts the first arc-shaped surface52aof the first opening/closing guide52and the second arc-shaped surface53aof the second opening/closing guide53, thereby rotating the first opening/closing guide52and the second opening/closing guide53respectively, about the first opening/closing shaft56and the second opening/closing shaft57in directions to separate the first door51aand the second door51baway from each other. As a result, the opening of the indenter channel23aof the speed measuring body23is opened, allowing the indenter2to impact against the surface of the specimen8.

As shown inFIG. 11, when the speed measuring body23of the speed measuring unit6is spaced from the surface of the specimen8, the opening/closing rod54moves forwards until the front end of the opening/closing rod54projects from the front end of the speed measuring body23with the spring force of the opening/closing spring58. At this time, the second protrusion54bformed on the opening/closing rod54contacts the first arc-shaped surface52aof the first opening/closing guide52and the second arc-shaped surface53aof the second opening/closing guide53, thereby rotating the first opening/closing guide52and the second opening/closing guide53respectively, about the first opening/closing shaft56and the second opening/closing shaft57in directions to bring the first door51aand the second door51binto abutment against each other. As a result, the opening of the indenter channel23aof the speed measuring body23is closed, housing the indenter2in the internal space formed by the first receiving portion51cand the second receiving portion51f(seeFIG. 8C). At this time, the indenter2is guided by the first sloping surface51dof the first receiving portion51cand the second sloping surface51eof the second receiving portion51f, preventing the indenter2from being pinched by the first door51aand the second door51b.

According to the shutter mechanism50of this embodiment, the opening of the indenter channel23ais opened, only when the speed measuring body23contacts the specimen8. On the other hand, when the speed measuring body23is spaced from the specimen8, the opening of the indenter channel23ais closed. Therefore, a loss of the indenter2ejected from the ejection mechanism5is effectively prevented, so that the burden on the operator can be reduced.

FIG. 12is a schematic view showing the speed measuring unit6according to another embodiment. The speed measuring unit6shown inFIG. 12has a third passage sensor26in addition to the first passage sensor24and the second passage sensor25. As described above, the “passage sensor” collectively refers to sensors capable of detecting the passage of an object, and includes an optical sensor or a magnetic sensor, for example. The embodiment in which the first passage sensor24, the second passage sensor25, and the third passage sensor26are optical sensors, will be described below.

The first optical sensor24, the second optical sensor25, and the third optical sensor26are arrayed along the indenter channel23a. The first optical sensor24is positioned forwardly of the second optical sensor25and the third optical sensor26in the speed measuring body23, and the second optical sensor25is positioned forwardly of the third optical sensor26in the speed measuring body23.

The third optical sensor26is identical in structure to the first optical sensor24and the second optical sensor25. Specifically, the third optical sensor26has a third light emitter26afor emitting light into the indenter channel23aand a third light receiver26bfor receiving the light emitted from the third light emitter26a. The third light emitter26ais an LED, and the third light receiver26bis a photodiode. The arrangement of an electric circuit of the third optical sensor26is identical to the arrangement of the electric circuit of the first optical sensor24described with reference toFIG. 4. Specifically, the amount of light emitted from the third light emitter26acan automatically be adjusted so that the voltage Vopa output from the first operational amplifier30in the electric circuit of the third optical sensor26converges on the preset base voltage Vdet at all times.

The apparatus1for measuring coefficient of restitution1according to the embodiments described herein are able to perform tests in free directions. For example, the indenter2can be ejected upwardly or downwardly. If the indenter2is ejected in different directions, gravitational forces in different directions act on the indenter2. Therefore, depending on the direction in which the ejected indenter2is ejected, small errors may be produced in the measured coefficients of restitution. In particular, the gravitational force tends to have a greater effect when the impact speed and the rebound speed of the indenter2are low. Accordingly, it is preferable to calculate the coefficient of restitution using, as the impact speed, the speed of the indenter2at the instant at which the indenter2impacts against the surface of the specimen8, and using, as the rebound speed, the speed of the indenter2at the instant at which the indenter2is rebounded from the surface of the specimen8. A method of determining, through calculations, the impact speed of the indenter2at the instant at which the indenter2impacts against the specimen8, and the rebound speed of the indenter2at the instant at which the indenter2is rebound from the specimen8will be described below with reference toFIG. 13.

FIG. 13is an explanatory diagram illustrating a method of determining, through calculations, the impact speed of the indenter2at the instant at which the indenter2impacts against the surface of the specimen8and the rebound speed of the indenter2at the instant at which the indenter2is rebounded from the specimen8. InFIG. 13, a point Ps represents the surface of the specimen8, a point P1represents the position of the first optical sensor24, a point P2represents the position of the second optical sensor25, and a point P3represents the position of the third optical sensor26. A method of calculating the impact speed of the indenter2at the instant at which the indenter2reaches the point Ps after passing the point P3, the point P2, and the point P1in this order, will be described below.

The speed of the indenter2passing the point P3(i.e., the third optical sensor26) is represented by v3, the speed of the indenter2passing the point P2(i.e., the second optical sensor25) is represented by v2, the speed of the indenter2passing the point P1(i.e., the first optical sensor24) is represented by v1, and the speed of the indenter2at the instant at which the indenter2reaches the point Ps (i.e., the surface of the specimen8) is represented by vs. Furthermore, the average speed between the point P3and the point P2is represented by v23, the average speed between the point P2and the point P1is represented by v12, and the average speed between the point P3and the point P1is represented by v13. A distance from the point P3to the point P2is represented by L23, a distance from the point P2to the point P1is represented by L12, a distance from the point P3to the point P1is represented by L13, and a distance from the point P3to the point Ps is represented by Ls. The controller7measures, according to the method described with reference toFIG. 5, a time T23taken after the indenter2has passed the point P3until it passes the point P2, a time T12taken after the indenter2has passed the point P2until it passes the point P1, and a time T13taken after the indenter2has passed the point P3until it passes the point P1.

The average speed v23between the point P3and the point P2can be calculated from the time T23measured by the controller7and the known L23according to the following equation (1).
v23=L23/T23  (1)

Similarly, the average speed v12can be calculated according to the following equation (2), and the average speed v13can be calculated according to the following equation (3).
v12=L12/T12  (2)
v13=L13/T13  (3)

Assuming that the indenter2is making uniformly accelerated motion at an acceleration α, the speed v23, the speed v2, and the speed v3satisfy the relationship according to the following equation (4).
v23=(v2+v3)/2  (4)

Similarly, the speed v12, the speed v1, and the speed v2satisfy the relationship according to the following equation (5), and the speed v13, the speed v1, and the speed v3satisfy the relationship according to the following equation (6).
v12=(v1+v2)/2  (5)
v13=(v1+v3)/2  (6)

From the equation (4), the equation (5), and the equation (6), the following equation (7), equation (8), and equation (9) are obtained.
v1=−v23+v12+v13  (7)
v2=v23+v12−v13  (8)
v3=v23−v12+v13  (9)

The acceleration α of the indenter2can be determined according to the following equation (10).
α=(v1−v3)/T13  (10)

Since the arithmetic unit7has obtained the speed v23, the speed v12, and the speed v13according to the above-described equations (1), (2), (3), the arithmetic unit7can calculate the speed v1, the speed v2, and the speed v3according to the equation (7), the equation (8), and the equation (9). As a result, the arithmetic unit7can calculate the acceleration α of the indenter2according to the equation (10).

If it is assumed that the indenter2is making uniformly accelerated motion at the acceleration α, the impact speed vs at the instant at which the indenter2impacts against the point Ps (i.e., the surface of the specimen8) can be determined from the following equation (11) or equation (12).
vs=(v32+2Ls·α)1/2(11)
vs=v3·(1+((v1/v3)2−1)(Ls/L13)))1/2(12)

As described above, since the arithmetic unit7has obtained the speed v3, the speed v2, the speed v1, and the acceleration α through calculations, and the distance Ls and the distance L13are known, the arithmetic unit7can calculate the impact speed vs at the instant at which the indenter2impacts against the surface of the specimen8.

Next, a method of determining, through calculations, the rebound speed at the instant at which the indenter2is rebounded from the surface of the specimen8will be described below with reference toFIG. 13. InFIG. 13, the point Ps represents the surface of the specimen8, the point P1represents the position of the first optical sensor24, the point P2represents the position of the second optical sensor25, and the point P3represents the position of the third optical sensor26. The indenter2which has been rebounded from the surface of the specimen8passes the point P1, the point P2, and the point P3in this order.

The speed of the indenter2passing the point P1(i.e., the first optical sensor24) after being rebounded from the point Ps is represented by v1′, the speed of the indenter2passing the point P2(i.e., the second optical sensor25) after being rebounded from the point Ps is represented by v2′, and the speed of the indenter2passing the point P3(i.e., the third optical sensor26) after being rebounded from the point Ps is represented by v3′. Furthermore, the average speed between the point P1and the point P2is represented by v12′, the average speed between the point P2and the point P3is represented by v23′, and the average speed between the point P1and the point P3is represented by v13′. The distance from the point P1to the point P2is represented by L12, the distance from the point P2to the point P3is represented by L23, the distance from the point P1to the point P3is represented by L13, and the distance from the point Ps to the point P3is represented by Ls. The controller7measures, according to the method described with reference toFIG. 5, a time T12′ taken after the indenter2rebounded from the point Ps has passed the point P1until it passes the point P2, a time T23′ taken after the indenter2rebounded from the point Ps has passed the point P2until it passes the point P3, and a time T13′ taken after the indenter2rebounded from the point Ps has passed the point P1until it passes the point P3.

The average speed v12′ between the point P1and the point P2can be calculated from the time T12′ measured by the controller7and the known L12according to the following equation (13).
v12′=L12/T12′  (13)

Similarly, the average speed v23′ can be calculated according to the following equation (14), and the average speed v13′ can be calculated according to the following equation (15).
v23′=L23/T23′  (14)
v13′=L13/T13′  (15)

Assuming that the indenter2is making uniformly accelerated motion at an acceleration α′, the speed v12′, the speed v1′, and the speed v2′ satisfy the relationship according to the following equation (16).
v12′=(v1′+v2′)/2  (16)

Similarly, the speed v23′, the speed v2′, and the speed v3′ satisfy the relationship according to the following equation (17), and the speed v13′, the speed v1′, and the speed v3′ satisfy the relationship according to the following equation (18).
v23′=(v2′+v3′)/2  (17)
v13′=(v1′+v3′)/2  (18)

From the equation (16), the equation (17), and the equation (18), the following equation (19), equation (20), and equation (21) are obtained.
v1′=−v23′+v12′+v13′  (19)
v2′=v23′+v12′−v13′  (20)
v3′=v23′−v12′+v13′  (21)

The acceleration α′ of the indenter2can be determined according to the following equation (22).
α=(v3′−v1′)/T13′  (22)

Since the arithmetic unit7has obtained the speed v12′, the speed v23′, and the speed v13′ from the above-described equations (13), (14), and (15), the arithmetic unit7can calculate the speed v1′, the speed v2′, and the speed v3′ according to the equation (19), the equation (20), and the equation (21). As a result, the arithmetic unit7can calculate the acceleration α′ of the indenter2rebounded from the point Ps according to the equation (22).

If it is assumed that the indenter2is making uniformly accelerated motion at the acceleration α′, the rebound speed vs' of the indenter2at the instance at which it has rebounded from the point Ps (i.e., the surface of the specimen8) can be determined from the following equation (23) or equation (24).
vs′=(v1′2+2Ls·α)1/2(23)
vs′=v1′·(1+((v3′/v1′)2−1)·((L13−Ls)/L13))1/2(24)

As described above, since the arithmetic unit7has obtained the speed v1′, the speed v2′, the speed v3′, and the acceleration α′ through calculations, and the distance Ls and the distance L13are known, the arithmetic unit7can calculate the rebound speed vs' at the instance at which the indenter2is rebounded from the surface of the specimen8.

In this manner, the arithmetic unit7is capable of determined, through calculations, the impact speed vs of the indenter2at the instant at which the indenter2impacts against the specimen8, and the rebound speed vs' of the indenter2at the instant at which the indenter2is rebounded from the surface of the specimen8. When the arithmetic unit7calculates the coefficient of restitution based on the impact speed vs at the instant at which the indenter2impacts against the specimen8and the rebound speed vs' at the instant at which the indenter2is rebounded from the surface of the specimen8, the arithmetic unit7can obtain the coefficient of restitution in which the effect of the gravitational force acting on the indenter2is removed.

FIG. 14is a schematic view showing the speed measuring unit6according to still another embodiment. The first light emitter24aand the first light receiver24bof the first optical sensor24, and the second light emitter25aand the second light receiver25bof the second optical sensor25of the speed measuring unit6shown inFIG. 14are disposed in the arithmetic unit7. In other words, the first optical sensor24and the second optical sensor25are disposed in the arithmetic unit7. The speed measuring body23has four through holes23d,23e,23f, and23gformed therein, which extend from side faces of the speed measuring body23to wall surfaces of the indenter channel23a. Although only the through holes23d,23fare illustrated inFIG. 14, the through hole23eis formed in a position facing the through hole23d, and the through hole23gis formed in a position facing the through hole23f.

A first optical fiber60extending from the first light emitter24ais fitted in the through hole23d, so that the light emitted by the first light emitter24apasses through the first optical fiber60and is emitted into the indenter channel23a. A second optical fiber61extending from the first light receiver24bis fitted in the through hole23e, so that the light emitted by the first light emitter24apasses through the second optical fiber61and is received by the first light receiver24b. In other words, the first light emitter24aof the first optical sensor24emits the light through the first optical fiber60into the indenter channel23a, and the first light receiver24bof the first optical sensor24receives the light emitted into the indenter channel23athrough the second optical fiber61. Similarly, a third optical fiber62extending from the second light emitter25ais fitted in the through hole23f, so that the light emitted by the second light emitter25apasses through the third optical fiber62and is emitted into the indenter channel23a. A fourth optical fiber63extending from the second light receiver25bis fitted in the through hole23g, so that the light emitted by the second light emitter25apasses through the fourth optical fiber63and is received by the second light receiver25b. In other words, the second light emitter25aof the second optical sensor25emits the light through the third optical fiber62into the indenter channel23a, and the second light receiver25bof the second optical sensor25receives the light emitted into the indenter channel23athrough the fourth optical fiber63.

According to the constructions shown inFIG. 14, the size of the speed measuring body23can be reduced. Therefore, the speed measuring body23can be inserted into a small gap. For example, as shown inFIG. 14, it is possible to measure the coefficient of restitution of a bottomland65bformed between two adjacent teeth65a,65bof a gear65.

FIG. 15is a schematic cross-sectional view showing the apparatus1for measuring coefficient of restitution according to another embodiment. In the apparatus1for measuring coefficient of restitution shown inFIG. 15, instead of the striker15shown inFIG. 1, a piston rod19is used as an indenter pushing member. The piston rod19applies an air pressure to the indenter2held in the holder3, ejecting the indenter2from the holder3toward the material8by use of this air pressure. Structures of the apparatus1for measuring coefficient of restitution according to the present embodiment other than the piston rod19are identical to those of the apparatus1for measuring coefficient of restitution described above, and their repetitive descriptions are omitted.

The piston rod19includes a cylindrical rod body19a, a cylindrical rod support19bhaving a diameter greater than the diameter of the rod body19a, and a disk-shaped piston head19efixed to the front end of the rod body19a. The rod body19ahas its back end embedded in the front end of the rod support19b, so that the rod body19ais fixed to the rod support19b. The central axis of the rod body19ais in alignment with the central axis of the body support19b. The piston head19ehas its outer circumferential surface slidable against the inner circumferential surface of the inner cylinder13. The central axis of the piston head19eis in alignment with the central axis of the rod body19a.

The front end of the biasing spring16is inserted into a guide hole19cformed in the rod support19b. The guide hole19cextends from the back end toward front end of the rod support19b, and the central axis of the guide hole19cis in alignment with the central axis of the rod support19b. To the back end of the outer cylinder12, a plug18that closes the opening of the outer cylinder12is fixed, and the back end of the biasing spring16is supported by the plug18. The plug18according to the present embodiment has a cylindrical shape, and the screw thread is formed on the outer circumferential surface of the plug18. The opening formed on the back end of the outer cylinder12is constructed as the threaded hole into which the screw thread formed on the outer circumferential surface of the plug18is screwed. The screw thread formed on the plug18engages with the threaded hole, thereby securing the plug18to the outer cylinder12. By rotating the plug18, the plug18can be moved forwards or backwards relative to the outer cylinder12. As a result, the biasing force applied to the piston rod19from the biasing spring16can be easily changed, because a length of the biasing spring16can be easily changed.

With these configurations, the biasing spring16is disposed between the outer cylinder12and the piston rod19. As described above with reference toFIG. 6, the biasing spring16is compressed by movement of the outer cylinder12, thereby allowing the biasing force to be applied to the piston rod19. As shown inFIG. 15, when the biasing spring16is compressed, the biasing spring16applies the biasing force for moving the piston rod19in the forward direction of the ejection mechanism5, to the piston rod15. This position of the piston rod19is an ejection position of the piston rod19.

An annular groove19dextending along a circumferential direction of the rod support19bis formed on the outer surface of the rod support19b. The groove19dmay be formed on a part of the outer surface of the rod support19b. The ejection lever15is fixed to the outer surface of the outer cylinder12, the ejection lever15having the hook14athat can engage with the groove19dwhen the piston rod19is in the ejected position shown inFIG. 15. The ejection lever14has the through hole, and the rotational shaft17which is secured to a bracket (not shown) extending radially outwardly from the outer surface of the outer cylinder12, is inserted into this through hole. Therefore, the ejection lever14is mounted to the outer cylinder12so as to be able to pivot around the rotational shaft17. As shown inFIG. 15, the outer cylinder12has the through hole12bwhich penetrates through the side wall of the outer cylinder12, and the inner cylinder13has the first elongated hole13ewhich penetrates through the side wall of the inner cylinder13and extends along a longitudinal direction of the inner cylinder13. The hook14aof the ejection lever14passes through the through hole12bof the outer cylinder12and the first elongated hole13eof the inner cylinder13to engage with the groove19dof the piston rod19.

FIG. 16is a schematic cross-sectional view showing a state in which the piston rod19is pushed out in the forward direction of the ejection mechanism5with use of the spring force of the biasing spring16when the hook14aof the ejection lever14disengages from the groove19dof the piston rod19. As shown inFIG. 16, the piston rod19that has been pushed out in the forward direction of the ejection mechanism5, collides with the stopper20, which has a hollow cylindrical shape and is fixed to the inner circumferential surface of the inner cylinder13, and as a result, the forward movement of the piston rod19is restricted. During the piston rod15moves from the ejection position shown inFIG. 15to a collision position shown inFIG. 16where it collides with the stopper20, the piston head19ecompresses air that is present in a space extending from the piston head19eto the indenter2. The pressure of the compressed air acts on the indenter2held in the front end of the holder3, and the indenter2is ejected toward the specimen8by this pressure of the compressed air. Since movement of the piston head19eis restricted by the stopper20, the piston head19edoes not contact the indenter2. In this manner, the indenter2may be ejected toward the specimen8by the air pressure acting on the indenter2.

As described above, the length of the biasing spring16can be adjusted by moving the plug18supporting the back end of the biasing spring16forwards or backwards relative to the outer cylinder12. Therefore, since the biasing force applied from the biasing spring16to the piston rod19can be adjusted, the impact speed of the indenter2ejected by the air which is compressed by the piston rod19can be adjusted. When coefficients of restitution of different specimens are to be compared, it is preferred that the material of the indenter2and the impact speed of the indenter2are constant. The apparatus1for measuring coefficient of restitution according to the present embodiment can easily adjust the impact speed of the indenter2.

An experiment was carried out to confirm that the mass effect had no effect on a coefficient of restitution measured using the apparatus1for measuring coefficient of restitution shown inFIG. 1. Specimens used in the experiment were standard blocks, having a cylindrical shape, for the Shore hardness test, and three standard blocks whose nominal hardnesses were Shore hardness90, Shore hardness60, and Shore hardness30respectively, were used. Coefficients of restitution were measured in a case where these standard blocks were secured to a steel anvil having a mass of 5.7 kg, and coefficients of restitution were measured in a case where these standard blocks were secured to a wood anvil having a mass of 0.12 kg.FIG. 17is a plan view of the standard block used in the experiment, and coefficients of restitution were measured at five measurement points Pa, Pb, Pc, Pd, and Pe shown inFIG. 17. The measurement point Pc is positioned at the center of the standard block, and the measurement points Pa, Pe are positioned at outer circumferential areas of the standard block. The measurement point Pb is positioned between the measurement point Pa and the measurement point Pc, and the measurement point Pd is positioned between the measurement point Pc and the measurement point Pe. As comparative examples, the D-type Shore hardness test and the Leeb hardness test were carried out on the same measurement points Pa, Pb, Pc, Pd, and Pe.

The indenter2used in the experiment is a bearing ball made of alumina and having a diameter of 2 mm. The mass of the indenter2was 0.055 g. The indenter2was ejected vertically downwardly. The length of the biasing spring16was adjusted so that the impact speed of the indenter2measured using the first optical sensor24and the second optical sensor25would be 10 m/s. The length of the biasing spring16can be adjusted by moving the plug18forwards or backwards relative to the outer cylinder12.

FIG. 18Ais a graph indicating coefficients of restitution in the case where the standard blocks fixed to the steel anvil were tested by the apparatus1for measuring coefficient of restitution, andFIG. 18Bis a graph indicating coefficients of restitution in the case where the standard blocks fixed to the wood anvil were tested by the apparatus1for measuring coefficient of restitution. As shown inFIGS. 18A and 18B, it was confirmed that there were no differences in the coefficients of restitution measured by the apparatus1for measuring coefficient of restitution between the case where the standard blocks were fixed to the steel anvil and the case where the standard blocks were fixed to the wood anvil, and that the coefficients of restitution were not affected by the mass effect. It was also confirmed that the same coefficients of restitution were measured at all of the five measurement points Pa, Pb, Pc, Pd, and Pe.

FIG. 19Ais a graph indicating Shore hardnesses in the case where the standard blocks fixed to the steel anvil were tested by the D-type Shore hardness tester, andFIG. 19Bis a graph indicating Shore hardnesses in the case where the standard blocks fixed to the wood anvil were tested by the D-type Shore hardness tester. In the D-type Shore hardness test, a hammer including an indenter made of diamond and an indenter support member to which the indenter is secured at its front end, drops from a predetermined drop height onto the standard block, and a rebound height reached by the hammer when it has been rebounded, is measured. The Shore hardness is obtained by multiplying the ratio of the rebound height to the drop height by a predetermined proportionality constant. The mass of the hammer used in the D-type Shore hardness test was 36.2 g (including the mass of the indenter) and the drop height was 19 mm.

As shown inFIG. 19A, in the case where the standard blocks were fixed to the steel anvil, the same shore hardnesses as the nominal Shore hardnesses of the standard blocks were measured. In contrast, as shown inFIG. 19B, in the case where the standard blocks were fixed to the wood anvil, the measured Shore hardnesses were clearly smaller than the nominal Shore hardnesses, and thus it was confirmed that the mass effect due to the hammer had an effect on the measured Shore hardnesses. Furthermore, a phenomenon that the measured Shore hardnesses were gradually smaller from the central measurement point toward the measurement points in the peripheral areas was confirmed.

FIG. 20Ais a graph indicating Leeb hardnesses in the case where the standard blocks fixed to the steel anvil were tested by the Leeb hardness tester, andFIG. 20Bis a graph indicating Leeb hardnesses in the case where the standard blocks fixed to the wood anvil were tested by the Leeb hardness tester. The Leeb hardness test is a hardness testing method in which an impact body including an indenter and an indenter support member to which the indenter is secured at its front end, is ejected toward a specimen by a spring, and an impact speed of the impact body before impacting against the specimen and a rebound speed of the impact body when impacting against the specimen and rebounded therefrom are measured. In the Leeb hardness test, the rebound speed of the impact body with respect to the impact speed of the impact body before impacting against the specimen is measured as a coefficient of restitution. The Leeb hardness is obtained by multiplying the coefficient of restitution by a predetermined proportionality constant. The mass of the impact body used in the Leeb hardness test was 5.45 g (including the mass of the indenter), and the impact speed before the impact body impact against the specimen was 2.1 m/s.

As shown inFIGS. 20A and 20B, the Leeb hardnesses of the standard blocks fixed to the wood anvil were smaller than the Leeb hardnesses of the standard blocks fixed to the steel anvil. Therefore, it was confirmed that even in the Leeb hardness test, the mass effect due to the impact body had an effect on the measured hardnesses. Furthermore, a phenomenon that the measured Leeb hardnesses were gradually smaller from the central measurement point toward the measurement points in the peripheral areas was confirmed.

Instead of the shutter mechanism50described with reference toFIGS. 7 through 11, a lid may be fixed to the front end of the speed measuring body23of the speed measuring unit6, and the lid may have a lid through hole which is connected to the indenter channel23aformed in the speed measuring body23.FIG. 21Ais a cross-sectional view showing an example of the lid70fixed to the front end of the speed measuring body23, andFIG. 21Bis a view as viewed along arrow I inFIG. 21A. Structures of the present embodiment other than the lid70are identical to those of the embodiments described above, and their repetitive descriptions are omitted.

As shown inFIG. 21A, the lid70fixed to the front end of the speed measuring body23has a lid through hole71which is connected to the indenter channel23aof the speed measuring body23. The lid through hole71has a hollow cylindrical shape extending from a front face70ato a rear face70bof the lid70. In other words, the diameter D of the lid through hole71is constant from the front face70ato the rear face70b. A central axis of the lid through hole71is in alignment with the central axis of the indenter channel23a. Further, the lid through hole71has a diameter D smaller than the diameter d of the indenter2(i.e., D<d). Therefore, the lid70prevents the indenter2from being expelled out of the apparatus1for measuring coefficient of restitution, because the indenter2cannot pass fully through the lid through hole71.

When the indenter2is ejected from the ejection mechanism5in a state in which the front face70aof the lid70is being placed in contact with the surface of the specimen8, the impact point where the indenter2impacts against the surface of the specimen8may deviate slightly from the point of intersection of the surface of the specimen8with the central axis of the lid through hole71.FIG. 22is a schematic view showing an example of a distribution of impact points S where the indenter2impacts against the surface of the specimen8. As shown inFIG. 22, the impact points S where the indenter2ejected from the ejection mechanism5impacts against the surface of the specimen8, lie within a circle having a radius Rs around the point X of intersection of the surface of the specimen8with the central axis of the lid through hole71. It has been experimentally confirmed that the radius Rs is smaller than 0.1 times the diameter d of the indenter2. Therefore, when the lid through hole71has a diameter D greater than at least 0.2 times the diameter d of the indenter2(i.e., 0.2 d<D), the indenter2ejected from the ejection mechanism5can impacts against the surface of the specimen8without colliding with the lid70.

The lid through hole71has a diameter D smaller than the diameter d of the indenter2so that the indenter2is not expelled out of the apparatus1for measuring coefficient of restitution. On the other hand, when the indenter2ejected from the ejection mechanism5collides with the lid70, the lid70may be deformed. Therefore, in order to reliably prevent the indenter2ejected from the ejection mechanism5from colliding with the lid70, the lid through hole71preferably has a diameter D that is as large as possible within the range smaller than the diameter d of the indenter2. The diameter D of the lid through hole71is preferably 0.5 times the diameter d of the indenter2or greater, and more preferably 0.7 times the diameter d of the indenter2or greater, or much more preferably be 0.9 times the diameter d of the indenter2or greater.

As shown inFIG. 21B, the lid70is fastened to the speed measuring body23by three screws (fasteners)73. The screws73engage with threaded holes (not shown) formed in the speed measuring body23, so that the lid70is fastened to the speed measuring body23. The screws73are screwed into the threaded holes formed in the speed measuring body23so as not to project from the front face70aof the lid70. With this structure, the front face70aof the lid70can directly be contacted with the surface of the specimen8. The number of screws73may be less than three or more than three. Instead of the screws73, hexagonal bolts may be used as fasteners for fastening the lid70to the speed measuring body23.

The lid through hole71allows a part of the indenter2to pass through the lid70, but the indenter2in its entirety cannot pass through the lid70. The lid70having such lid through hole71is simpler in structure than the shutter mechanism50, and thus can be manufactured less expensively than the shutter mechanism50. As a result, a manufacturing cost of the apparatus1for measuring coefficient of restitution can be reduced. Further, since the diameter D of the lid through hole71is smaller than the diameter d of the indenter2, the indenter2ejected from the ejection mechanism5is reliably prevented from being expelled out of the speed measuring body23. As a result, when the indenter2is ejected from the ejection mechanism5in a state in which the front face70aof the lid70is not placed in contact with the surface of the specimen8, the indenter2does not collide with the operator in the vicinity of the apparatus1for measuring coefficient of restitution. Accordingly, the safety of operator is increased. Furthermore, a loss of the indenter2is effectively prevented, so that the burden on the operator is reduced.

As shown inFIG. 21A, the speed measuring body23may have vent holes75extending from side surfaces of the speed measuring body23to the indenter channel23a. The vent holes75provided with the speed measuring body23prevent air in the indenter channel23afrom being compressed by the indenter2passing through the indenter channel23a. If air in the indenter channel23ais compressed by the indenter2ejected from the ejection mechanism5, the impact speed of the indenter2may be reduced, and the rebound speed of the indenter2may be increased. The vent holes75provided with the speed measuring body23enables the indenter channel23ato communicate with the exterior of the speed measuring body23. Preferably, the vent holes75are provided at positions close to the front end of the speed measuring body23. Accordingly, a more correct coefficient of restitution of the specimen8can be measured, because air in the indenter channel23ais not compressed by the indenter2passing through the indenter channel23a. In this embodiment, though two vent holes75are formed in the speed measuring body23, the number of vent holes75may be one, or three or more.

A cross-sectional shape of the vent hole75can be determined arbitrarily. For example, the vent holes75may have a circular cross-sectional shape or a rectangular cross-sectional shape. The size of the vent hole75is preferably smaller than the size of the indenter2. In this case, the indenter2is prevented from being expelled out of the speed measuring body1through the vent holes75.

FIG. 23is a cross-sectional view showing a modified example of the lid70fixed to the front end of the speed measuring body23. In the embodiment shown inFIG. 23, a wall surface71aof the lid through hole71in the lid70is formed in a curved surface. The wall surface71aof the lid through hole71has a radius rc of curvature which is constant from the front face70ato the rear face70bof the lid70and greater than the radius rd of curvature of the outer surface of the indenter2. Since the indenter2has a spherical shape, the radius rd of curvature of the outer surface of the indenter2is the same as a radius (=d/2) of the indenter2.

Because the wall surface71aof the lid through hole71is constructed by a curved surface having the constant radius rc of curvature, the diameter D of the lid through hole71is gradually reduced from the rear surface70btoward the front face70aof the lid70. Therefore, a minimum diameter Dminof the lid through hole71is represented by a diameter of the lid through hole71that is open at the front face70aof the lid70. The minimum diameter Dminof the lid through hole71is smaller than the diameter d of the indenter2. Therefore, since the indenter2cannot pass fully through the lid through hole71, the lid70prevents the indenter2from being expelled out of the apparatus1for measuring coefficient of restitution. Furthermore, the minimum diameter Dminof the lid through hole71is at least 0.2 times the diameter d of the indenter2. Therefore, the indenter2ejected from the ejection mechanism5can impact against the surface of the specimen8without colliding with the lid70. The wall surface71aformed in a curved surface effectively prevents the indenter2ejected from the ejection mechanism5from colliding with the lid70.

In order to reliably prevent the indenter2ejected from the ejection mechanism5from colliding with the lid70, the minimum diameter Dminof the lid through hole71is preferably 0.5 times the diameter d of the indenter2or greater, and more preferably 0.7 times the diameter d of the indenter2or greater, or much more preferably 0.9 times the diameter d of the indenter2or greater.

FIG. 24is a cross-sectional view showing another modified example of the lid70fixed to the front end of the speed measuring body23. In the embodiment shown inFIG. 24, the front end of the speed measuring body23has a hollow cylindrical portion23h, and the indenter channel23aextends to the front end of the hollow cylindrical portion23h. The lid70has a hollow cylindrical step70cinto which the hollow cylindrical portion23his fitted. Although the lid through hole71according to the present embodiment has a hollow cylindrical shape, the wall surface71aof the lid through hole71may be formed in a curved surface having a constant radius rc of curvature, as shown inFIG. 23. The central axis of the lid through hole71can easily be aligned with the central axis of the indenter channel23aby fitting the step70cof the lid70into the hollow cylindrical portion23hof the speed measuring body23.

In this embodiment, the vent holes75extend from side surfaces of the lid70to the indenter channel23a. The vent holes75pass through the hollow cylindrical portion23hof the speed measuring body23. As shown inFIG. 23, the vent holes75may extend from side surfaces of the lid70through the hollow cylindrical portion23hto the indenter channel23a.

Experiments were carried out to measure coefficients of restitution using the apparatus1for measuring coefficient of restitution in which the lid70shown inFIG. 21was fixed to the front end of the speed measuring body23. Specimens used in the experiment were standard blocks, having a cylindrical shape, for the Shore hardness test, and three standard blocks whose nominal hardnesses were Shore hardness90, Shore hardness60, and Shore hardness30respectively, were used. Using the same apparatus1for coefficient of restitution, coefficients of restitution of standard blocks, having a cylindrical shape, for the Rockwell hardness test were also measured. In the experiments, the ratio of the diameter da (seeFIG. 21) of the indenter channel23ato the diameter d of the indenter2was changed, and an effect that the relationship between the diameter da of the indenter channel23aand the diameter d of the indenter2has on the measured coefficients of restitution was also evaluated. Coefficients of restitution of the same standard blocks were measured using the apparatus1for measuring coefficient of restitution with the shutter mechanism50mounted in the speed measuring body23, and the obtained measured results were used as reference values.

The indenter2used in a first experiment was a bearing ball made of alumina, and the diameter d of the indenter2was 3 mm. The diameter D of the lid through hole71was 2.8 mm Therefore, the lid through hole71had a diameter D that was 0.93 times the diameter d of the indenter2. The diameter da of the indenter channel23awas 5 mm. Therefore, the indenter channel23ahad a diameter da that was 1.67 times the diameter d of the indenter2. The length of the biasing spring16was adjusted so that the impact speed of the indenter2would be 10 m/s.

The indenter2used in a second experiment was a bearing ball made of alumina, and the diameter d of the indenter2was 3 mm. The diameter D of the lid through hole71was 2.8 mm. Therefore, the lid through hole71had a diameter D that was 0.93 times the diameter d of the indenter2. The diameter da of the indenter channel23awas 4 mm. Therefore, the indenter channel23ahad a diameter da that was 1.33 times the diameter d of the indenter2. The length of the biasing spring16was adjusted so that the impact speed of the indenter2would be 10 m/s.

The indenter2used in a third experiment was a bearing ball made of alumina, and the diameter d of the indenter2was 5 mm. The diameter D of the lid through hole71was 4.7 mm Therefore, the lid through hole71had a diameter D that was 0.94 times the diameter d of the indenter2. The diameter da of the indenter channel23awas 7 mm. Therefore, the indenter channel23ahad a diameter da that was 1.4 times the diameter d of the indenter2. The length of the biasing spring16was adjusted so that the impact speed of the indenter2would be 10 m/s.

In order to obtain reference values for comparing coefficients of restitution obtained in the first experiment, the second experiment, and the third experiment, coefficients of restitution of the same standard blocks were measured using the apparatus1for measuring coefficient of restitution with the shutter mechanism50mounted in the speed measuring body23. The indenter2used in the experiments for obtaining the reference values was a bearing ball made of alumina, and the diameter d of the indenter2was 3 mm. The diameter da of the indenter channel23awas 5 mm. Therefore, the indenter channel23ahad a diameter da that was 1.67 times the diameter d of the indenter2. The length of the biasing spring16was adjusted so that the impact speed of the indenter2would be 10 m/s.

Coefficients of restitution obtained in the first experiment, the second experiment, and the third experiment, and coefficients of restitution measured as reference values are indicated in Table 1. The coefficients of restitution and the reference values indicated in Table 1 are average values of seven coefficients of restitution measured at different positions on the specimens.

As is clear from Table 1, the coefficients of restitution obtained in the first experiment, the second experiment, and the third experiment are essentially the same as the coefficients of restitution measured as reference values. Therefore, the apparatus1for measuring coefficient of restitution with the lid70fixed to the front end of the speed measuring body23can measure coefficients of restitution without being affected by the mass effect occurring in a conventional rebound hardness tester. Further, it was confirmed from the results of the first experiment and the results of the second experiment that even though the indenters2having different diameters d are used, the same coefficients of restitution can be obtained by fixing the impact speed of the indenter2to a constant.

The coefficients of restitution obtained in the second experiment are slightly smaller than the coefficients of restitution obtained in the first experiment and the coefficients of restitution obtained in the third experiment. On the other hand, the coefficients of restitution obtained in the first experiment and the coefficients of restitution obtained in the third experiment are the same as the reference values. The reason of this is considered to be the fact that the wall surface of the indenter channel23aand the outer surface of the indenter2are too close to each other. Therefore, in order to obtain more correct coefficients of restitution, it is preferred that the indenter channel23ahas a diameter da which is 1.4 times the diameter d of the indenter2or greater. Also in the apparatus1for measuring coefficient of restitution with the shutter mechanism50mounted in the speed measuring body23, it is preferred that the indenter channel23ahas a diameter da which is 1.4 times the diameter d of the indenter2or greater.

In this manner, according to the above-described apparatus1for measuring coefficient of restitution, the coefficient of restitution can be measured without being affected by the mass effect occurring in a conventional rebound hardness tester. Furthermore, according to the above-described apparatus1for measuring coefficient of restitution, there is no limitation on the direction in which the indenter2is ejected, because the indenter2is held in the holder3. As a result, tests can be performed in free directions without being affected by the mass effect.

The apparatus1for measuring coefficient of restitution according to the above-described embodiments can be used as a hardness tester. Specifically, the hardness tester includes the holder3for holding the spherical indenter2, the ejection mechanism5for ejecting the indenter2held by the holder3from this holder3toward the specimen8, a speed measuring unit6for measuring the impact speed which represents the speed of the indenter2before the indenter2impacts against the specimen8and the rebound speed which represents the speed of the indenter2after the indenter2has been rebounded from the specimen8. Further, the hardness tester includes the arithmetic unit7, and the arithmetic unit7determines a hardness of the specimen8based on the ratio (i.e., corresponding to the coefficient of restitution) of the rebound speed of the indenter2to the impact speed of the indenter2.

In the hardness tester, the indenter2held by the holder3is ejected from the holder3toward the specimen by the ejection mechanism5. The speed measuring unit6measures the impact speed which represents the speed of the indenter2before the indenter2impacts against the specimen8, and the rebound speed which represents the speed of the indenter2after the indenter2has been rebounded from the specimen8. Further, the arithmetic unit7determines the hardness of the specimen8based on the ratio of the rebound speed of the indenter2to the impact speed of the indenter2. For example, the arithmetic unit7determines the hardness of the specimen8by multiplying the ratio of the rebound speed of the indenter2to the impact speed of the indenter2by a predetermined proportionality constant (e.g., 100 or 1000).

If the hardness of the specimen8is measured by this hardness tester, the hardnesses of different specimens8can be compared with each other by adjusting the impact speed, which represents the speed of the indenter2before the indenter2impacts against the specimen8, so as to be constant, and using the indenter2made of the same material. As described above, the impact speed of the indenter2can easily be adjusted by moving the plug18forwards or backwards relative to the outer cylinder12.