Abstract:
A hardness tester generally comprises a tester assembly is supported on a frame assembly. The tester assembly has a tester housing that supports a motor drivingly connected to a load cell and an indentor. The frame assembly includes a vertically oriented plate having a general C-shape defined by upper and lower arms connected by a main body. The plate is fabricated from a unitary sheet of metal to provide rigidity in the vertical direction. The tester assembly is selectively attachable to the frame assembly, and may be remotely employed as a stand-alone unit. A unique ball screw assembly is also provided for applying the testing load.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to equipment for testing material properties, and more specifically relates to hardness testers used to determine the hardness of a material. 
     BACKGROUND OF THE INVENTION 
     Penetration hardness testers are well-known in the art. These hardness testers generally include an indentor tip that is driven to apply a load to the test specimen. Some hardness testers known as deadweight testers utilize gravity acting on weights to create the force which drives the indentor into the test specimen. Other hardness testers utilize an electric motor to drive the indentor. In either case, the force or load on the indentor relative to the depth of penetration of the indentor into the surface of the specimen, or relative to the diameter of the indentation, will give a hardness number measured on a common scale such as Rockwell or Brinell. 
     Inaccuracies in testing arise due to many sources, such as relative movement between mechanical components. Recent improvements to such hardness testers have come by the use of various electronic equipment, such as load cells to measure the applied force and displacement sensors to measure the depth of penetration. Closed-loop systems utilizing a central processor or computer, especially in conjunction with load cells and displacement transducers, have provided improved reliability to hardness testers. Despite these advances, there remains a need to provide a hardness tester with improved accuracy. 
     Further, known hardness testers utilize a vertically adjustable anvil to properly position the test specimen at a discrete position for testing. However, not all specimens may easily be positioned on the anvil for testing. Hence, there exists a need to provide a hardness tester that is adaptable for different specimens, including for a variety of situations. 
     The invention provides a hardness tester which not only obtains improved accuracy, but also is adaptable to a variety of situations and specimens. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein when taken in conjunction with the accompanying drawings. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of he present invention provides a hardness tester for determining the hardness of a specimen. The hardness tester generally comprises a tester assembly is supported on a frame assembly. The tester assembly has a tester housing that supports a motor drivingly connected to a load cell and an indentor. The frame assembly includes a vertically oriented plate having a general C-shape defined by upper and lower arms connected by a main body. The plate is fabricated from a unitary sheet of metal to provide rigidity in the vertical direction. 
     According to more detailed features of the embodiment, by virtue of the C-shaped plate, the upper and lower arms deflect less than 0.0015 inches apart in response to a separating force of 330 lbs. The lower arm supports an anvil for supporting the specimen. It is also preferable to provide a second vertically oriented plate having a general C-shape, the second plate being laterally spaced from the first plate to define side walls of the frame. Here, the free ends of the upper arms are connected by a mounting plate having a mounting surface engaging the tester housing. 
     In another embodiment of the present invention, a hardness tester is provided that generally comprises a frame assembly and a tester assembly. The frame assembly includes an upper arm and a lower arm, and the tester assembly includes a tester housing supporting a motor drivingly connected to a load cell and an indentor. The tester assembly is selectively attachable to the frame assembly at a frame mounting surface defined by the upper arm of the frame assembly. 
     According to more detailed features of this embodiment, the tester assembly includes a tester mounting surface for engaging the frame mounting surface. Preferably, the tester mounting surface and frame mounting surface are keyed together. It is also preferable that the tester mounting surface and the frame mounting surface extend at least 4 inches in the vertical direction. The tester mounting surface and frame mounting surface each include a plurality of corresponding mounting holes for selectively attaching the tester and frame assemblies, typically by threaded fasteners. 
     In yet another embodiment of the present invention, a hardness tester is provided that generally comprises a tester housing supporting a motor operatively connected to a ball screw assembly. The ball screw assembly is connected to the tester housing and includes a ball screw, a ball spline, and a ball spline bearing. The ball screw is operatively connected to the motor. The ball spline has a hollow center for receiving the ball screw, and includes a ball nut securely attached thereto. A downward end of the ball spline is directly connected to a load cell and an indentor. The ball spline bearing is attached to the housing and receives the ball spline to prevent rotation of the ball spline. The ball nut threadingly engages the ball screw for transforming the rotational movement of the ball screw into linear movement of the ball spline and indentor. 
     According to more detailed aspects of this embodiment, the ball spline includes a plurality of circumferentially spaced grooves extending along the length of the ball spline, the spine bearing engaging each groove to prevent rotation of the ball spline and guide linear movement. Preferably, a displacement sensor is provided that has a read head and a tape scale; the tape scale being attached to the outer periphery of the ball spline and the read head being attached to the tester housing and positioned to read the tape scale. A CPU is operatively connected to the motor, displacement sensor, and load cell to control the movement of the ball spline and indentor to perform hardness tests. Preferably, first and second limit switches are connected to the tester housing, the first and second limit switches being vertically spaced to define an operating range for the ball spline and indentor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially cut-away side view of an embodiment of the hardness tester constructed in accordance with the teachings of the present invention; 
     FIG. 2 is a partially cut-away front view of the hardness tester of FIG. 1; 
     FIG. 3 is a partially cut-away top view of the hardness tester of FIG. 1; 
     FIG. 4 is a cross-sectional view, taken about the line  4 — 4  in FIG. 2; 
     FIG. 5 is a cross-sectional view, taken about the line  5 — 5  in FIG. 2; 
     FIG. 6 is a cross-sectional view, taken about the line  6 — 6  in FIG. 2; 
     FIG. 7 is a cross-sectional view, taken about the line  7 — 7  in FIG. 2; and 
     FIG. 7 a  is an enlarged view, partially cut-away, taken about the circle  7   a  in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the figures, the hardness tester  20  generally comprises a frame assembly  30  connected to a tester assembly  60 . The frame assembly  30  is best seen with reference to FIGS. 1-3, and generally comprises a very rigid housing including two fabricated sidewalls  32  of a general C-shape. The sidewalls  32  are connected to a bottom wall  34 , while six triangular supports  36  provide lateral support to the sidewalls  32 . Four holes in the bottom wall  34  can be used to securely mount and level the frame assembly  30 . The inner edge of the C-shaped sidewalls  32  are supported by a vertical plate  38  and two horizontal plates  39  extending between the sidewalls. 
     Adjacent the end of the lower arm of the C-shape frame  30  are two support plates  40  upon which a block  42  rests. The block  42  supports an anvil  44 , which in turn supports the test specimen. At the end of the upper arm of the C-shape frame  30  is a vertically oriented mounting plate  46 . The mounting plate  46  includes four mounting holes  47  for bolting the tester assembly  60  to the frame assembly  30 . The mounting plate  46  also includes a large hole  48  for passing wires and the like from the tester assembly  60 , since the frame  30  can be used to house various electronic components of the tester  20  such as the central processing unit (CPU), the power supply, the signal conditioner, and the capacitor to name a few. Finally, the mounting plate  46  includes two laterally spaced key members  49  that project forwardly to assist in mounting the tester assembly  60 , as will be described in more detail herein. 
     The frame assembly  30  is preferably welded together as shown, and a housing or cover (not shown) encloses the frame members. Importantly, the sidewalls  32  are fabricated from a single sheet of material such as sheet steel of at least  ⅜″ thickness. Alternately, the frame could be cast iron. By fabricating the frame in this manner, the cost and difficulty of casting the frame is eliminated, while a stronger frame is provided. More specifically, the fabricated C-shape reduces the deflection between the upper arm of the C-shape and the lower arm of the C-shape during testing.    
     It will be recognized that the tester assembly  60  applies a downward force on the specimen supported on the anvil  44 , and hence lower arm of the frame. This downward force thus attempts to separate the upper and lower arms of the frame, and they in fact do separate. This separation can be incorrectly attributed to penetration of the specimen, and computer calibration or control cannot completely eliminate this error. As an example, the major load on a Rockwell test is typically 60 kg, 100 kg or 150 kg (330 lbs.), while one Rockwell Hardness Number is equal to about 0.00008 inches of penetration. While the frames of some hardness testers have been measured to separate or deflect 0.005 inches or more at 150 kg, the frame  30  of the present invention deflects less than 0.0015 inches at 150 kg. 
     Turning now to the tester assembly  60 , the assembly generally includes an electric servomotor  62  driving a ball screw assembly  80 , which in turn is attached to an indentor  110  and load cell  120  for applying a measured load to the specimen. More specifically, the tester assembly  60  generally includes a tester housing  65  comprising a top plate  66 , a bottom plate  67 , two opposing side plates  68 , and a mounting plate  69 . The mounting plate  69 , and hence the tester housing  65  and assembly  60  are securely attached to the upper arm of the C-shaped frame via the mounting plate  46 . A cover (not shown) encloses the tester housing  65  to keep the components free from dirt and dust or other impediments. 
     The tester mounting plate  69  includes keyways  69 ′ sized and positioned to correspond with key members  49  of the frame mounting plate  46 . By keying the tester mounting plate  69  to the frame mounting plate  46 , the tester assembly  60  may be easily and quickly positioned on the frame assembly  30  for attachment via mounting holes  47  that mate with mounting holes on the tester mounting plate  69 . Bolts are used to rigidly attach the mounting plates  49 ,  69 , which interface over a long vertical surface, ensuring a rigid mount and transfer of force from the tester housing  65  to the frame assembly  30 . In the illustrated embodiment, the mounting plates  49 ,  69  mate over a surface that is about 11″ tall and about 5″ wide, although other sized mating surfaces may be utilized. 
     It will also be recognized that by allowing the tester assembly  60  to be selectively attachable to the frame assembly  30 , the tester assembly may be separately employed without the frame. Some particular applications or specimens cannot be brought to the hardness tester, or may not fit within a particular frame. For example, the hardness of large metal gears is ideally tested, but their large size and shape prevent testing on existing hardness testers. With the present invention, the tester assembly  60  may be directly applied to such gears, typically by way of a mounting plate structured similarly to that of frame mounting plate  46 , which then can be attached by any convenient manner to the gear. For example, the similar mounting plate may be provided with addition mounting holes to mount the plate to the gear, or another structure fixed to the gear. Hence other fixtures may be developed for any particular mounting situation. It will also be recognized that the tester assembly  60  may simply be attached to a different frame assembly of alternate sizes or shapes. Many remote applications are possible with the selectively attachable tester assembly as will be readily recognized by those skilled in the art. 
     The upper end of the tester housing  65  supports the servomotor  62  in a position above the tester housing. The servomotor includes an encoder, the motor and encoder combination being purchased as a single unit, such as the Kollmorgan Servomotor with encoder. The encoder monitors the rotation of a driveshaft within the servomotor itself and can be used for various control applications as will be described in more detail herein. The output of the servomotor is operatively connected to a planetary gear reducer  64 , the output shaft  65  (FIG. 4) of which is operatively connected to the ball screw  82  of the ball screw assembly  80  by way of a coupler  70 . 
     The ball screw assembly  80  includes a driving screw  82 , the upper end of which is held within the housing by a double row angle contact bearing  84  suitably connected to the housing by support plates  72  (see also FIG.  5 ). The driving screw  82  operatively engages a ball nut  86  to translate the rotational movement of the screw into linear movement of the ball nut  86 , and ultimately the indentor  110 . The ball nut  86  is attached to the upper end of a ball spline  88 , and preferably the ball nut  86  and ball spline  88  are threadingly engaged. 
     A plate  87  is attached to the upper surface of the ball spline  88 , preferably between the ball nut  86  and spline  88 , and has a trigger arm  89  extending laterally therefrom (FIGS. 7 and 7 a ). As best seen in FIGS. 2 and 6, the tester assembly  60  includes an upper limit switch  91  and a lower limit switch  92 , each having a limit arm  90  that is contacted by the trigger arm  89  as it passes by one of the switches  91 ,  92 . The limit switches  91 ,  92  are attached to the tester housing  65  at upper and lower ends to define the extents of the operating range (travel) for the tester, the triggering of which causes the controller to stop the motor  62  and hence vertical movement of the ball spline  88  and indentor  110 . 
     Returning to the ball screw assembly  80 , the ball nut  84  is attached to the ball spline  88  and operatively engaged (via threads) to ball screw  82 . The lower end of the ball spline  88  is held against rotation by a ball spline bearing  94 . More specifically, the ball spline includes three outwardly opening and vertically extending grooves which receive spring loaded ball bearings of the ball spline bearing  94 . Accordingly, as the screw  82  is rotated, the ball spline  88  moves upwardly or downwardly via the threaded connection with the nut  84 . 
     With reference to FIGS. 2,  7  and  7   a , a position or displacement sensor  100 , which includes a tape scale  102  and read head  104 , is employed in the tester assembly  60 . The ball spline  88  is generally circular, but includes a flat strip ground on its outer surface, the flat strip extending vertically along its length. The tape scale  102  (FIGS. 2 and 7 a ) attached to the strip, preferably by an adhesive. The read head  104  is suitably mounted to the tester housing  65 , preferably by an attachment plate  106  attached to a block  108  welded to the mounting plate  69  of the tester assembly  60  (FIG.  7 ). The read head  104  is positioned to read the tape scale  102  and produce output signals corresponding to the position of the tape scale  102 , and hence the ball spline  88 . In one embodiment, the displacement sensor may comprise a Renishaw Read Head #RGH22Z30F00A, Mark Actuator #RGM22S, and Micron Pitch Tape Scale #RGS-S-20. 
     The ball spline  88  extends through the lower end of the spline bearing  94  and the tester housing  65 , and is directly connected to a load cell  120 . An accordion-type cover  109  may be attached to the load cell  120  and a bottom of the tester assembly  60  and housing  65  to keep the tester components clean and free of debris. The load cell  120  is in turn connected to an adapter  115 , which is designed to receive various types of indenters  110 . 
     Operation of the hardness tester  20  occurs, in part, through a closed-loop feedback system which will be described with reference to a Rockwell Hardness test, although it will be recognized that other hardness or penetration tests may be employed by the present invention. The system generally includes the CPU and user interface, the servomotor and encoder, the ball screw assembly, the load cell, and the displacement sensor. During operation, the specimen is placed on the anvil and the test is initiated via the user interface. The indentor begins at a default position. The CPU sends a signal to the servomotor to rotate the ball screw, driving the ball spline, load cell, and indentor downwardly until the indentor reaches the material. This portion of the test does not rely on the displacement sensor or on the servo motor encoder, and thus is “open-loop”, although the feedback from the load cell is monitored to determine when the indentor first touches the specimen. 
     At this point, the control switches to closed loop operation. The position of the indentor is monitored via the displacement sensor, while the readings from the load cell are used to place a first minor load on the material. The CPU will recognize from the load cell output when the minor load is reached and will hold that minor load for a fixed period of time, noting the position of the indentor. Next, the CPU will cause the servomotor to drive the ball spline, load cell, and indentor to induce a larger, major load. The CPU will recognize when this load has been reached by the output received from the load cell and will hold the major load for a fixed amount of time, again noting the position of the indentor via the displacement sensor. Then, the CPU will instruct the servomotor to raise the ball spline, load cell, and indentor until the minor load is again applied to the material being tested. The CPU will again record the position of the indentor when the minor load is applied for the second time, and then the displacement sensor will end its monitoring duties. 
     The CPU will then instruct the servomotor to raise the ball spline, load cell, and indentor to the default position. This movement is controlled by the CPU via feedback from the encoder in the servomotor. Generally, the encoder operates on a much coarser scale than the displacement sensor, and is not capable of conducting the hardness tests at the required precision and accuracy. However, the use of the encoder during non-load movement allows the ball spline, load cell, and indentor to be returned to its position very quickly. Further, the encoder is rotary based and monitors the rotation of the shaft of the motor which is translated into a linear displacement, but does not directly monitor the linear movement. Alternately, the motor is driven in an open-loop system to move the ball spline and indentor upwardly, relying on the upper limit switch to be triggered and stop the ball spline and indentor in the default position. The CPU, based on the particular minor and major loads applied and the positions sensed at each of these three load conditions, can compute the hardness of the specimen according to a standard hardness scale such as Rockwell or Brinell. 
     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.