Patent Description:
<CIT> discloses an impact rotary tool. The impact rotary tool includes: a drive shaft configured to be rotated by a motor; a hammer configured to be fit to an outer perimeter of the drive shaft in rotatable, movable forward, and movable backward manner; a hammer projection provided to the hammer; and an anvil including an anvil projection engageable with the hammer projection. In the impact rotary tool, rotation of the hammer exerts an impact on the anvil in a rotation direction, and via a socket or the like mounted on the anvil, strong torque is instantaneously given to a screw, thereby tightening the screw. <CIT> describes an impact rotary tool including a driver, a spindle rotated by the driver, an anvil disposed in front of the spindle in a rotation axis direction, a main hammer structured to apply a rotation force to the anvil, and a sub-hammer structured to apply, to the main hammer having applied the rotation force to the anvil, a rotation force in the same direction. Document <CIT> (D2) describes methods and systems for a rotary impact device having an annular exterior surface for use with an impact wrench for providing torque to a fastener. The rotary impact device includes an input member having an input recess for receiving the anvil of the impact wrench, an output member having an output recess for receiving the fastener, and an inertia member. The inertia member is stationary and positioned on the exterior surface of the rotary impact device for increasing the torque applied to the fastener.

For an impact tool as the impact rotary tool described in <CIT>, an improvement in energy efficiency may be required.

The claimed invention provides an impact tool defined by the features set forth in the appended independent claim. Particular embodiments are set forth in the dependent claims.

An impact tool of the present embodiment will be described with reference to the drawings. Figures described in the following embodiment are schematic views, and therefore, the ratio of sizes and the ratio of thicknesses of components in the drawings do not necessarily reflect actual dimensional ratios.

An impact tool <NUM> (see <FIG>) of the present embodiment includes a motor <NUM>, a hammer <NUM>, and an anvil <NUM>. The hammer <NUM> is rotated around a rotation axis Ax1 by motive power provided from the motor <NUM>. The anvil <NUM> is rotated around the rotation axis Ax1 by receiving striking force (rotation striking force) from the hammer <NUM> in a circumferential direction of the rotation axis Ax1.

In the impact tool <NUM> of the present embodiment, moment of inertia of the hammer <NUM> around the rotation axis Ax1 (i.e., with the rotation axis Ax1 being the center) is <NUM> or more times moment of inertia of the anvil <NUM> around the rotation axis Ax1 (i.e., with the rotation axis Ax1 being the center).

In the impact tool <NUM> of the present embodiment, energy efficiency can be improved.

The structure of the impact tool <NUM> according to the present embodiment will be described with reference to <FIG>.

In the following description, a direction in which the hammer <NUM> and the anvil <NUM> are aligned with each other is defined as a forward/backward direction, the side of the anvil <NUM> viewed from the hammer <NUM> is defined as a "front", and the side of the hammer <NUM> viewed from the anvil <NUM> is defined as a "back". That is, the anvil <NUM> is located forward of the hammer <NUM>. Moreover, the direction of the rotation axis Ax1 of the hammer <NUM> and the anvil <NUM> is along the forward/backward direction. Moreover, in the following description, a direction in which a body <NUM> and a grip <NUM> described later are aligned with each other is defined as an up/down direction, the side of the body <NUM> when viewed from the grip <NUM> is defined as upward, and the side of the grip <NUM> when viewed from the body <NUM> is defined as downward. Note that the directions are merely defined for the sake of explanation of the positional relationship between components of the impact tool <NUM> but do not intend to limit directions in which the impact tool <NUM> is used.

The impact tool <NUM> of the present embodiment is a portable electric tool. The impact tool <NUM> is, for example, an impact driver or an impact wrench.

The impact tool <NUM> operates with power supplied from a battery pack B1. The battery pack B1 is a power supply that supplies a current for driving the motor <NUM>. The battery pack B1 is not a component of the impact tool <NUM>. Note that the impact tool <NUM> may include the battery pack B1 as one of components thereof. The battery pack B1 includes: an assembled battery including a plurality of secondary batteries (e.g., lithium ion batteries) connected in series; and a case in which the assembled battery is housed.

As shown in <FIG> and <FIG>, the impact tool <NUM> includes a housing <NUM>, the motor <NUM>, a transmission mechanism <NUM>, an operating part <NUM>, and a controller <NUM>.

The housing <NUM> houses the motor <NUM>, part of the transmission mechanism <NUM>, and the controller <NUM>.

The housing <NUM> includes the body <NUM>, the grip <NUM>, and a fitting part <NUM>.

The body <NUM> has a tubular shape with its tip end (front end) having an opening (through hole <NUM>) and with its rear end having a bottom.

The grip <NUM> protrudes downward from a side surface of the body <NUM>.

The battery pack B1 is detachably attached to the fitting part <NUM>. In the present embodiment, the fitting part <NUM> is provided at a tip end part (lower end part) of the grip <NUM>.

The operating part <NUM> is provided at an upper end part of the grip <NUM> to protrude forward. The operating part <NUM> includes, in this embodiment, a trigger controller configured to receive, from a user, an operation for controlling rotation of the motor <NUM>. An operation (backward pulling operation) of pulling the operating part <NUM> enables the motor <NUM> to be switched on from off. Moreover, the rotational speed of the motor <NUM> is adjustable by pulled amount of the operating part <NUM> indicating how deep the operating part <NUM> is pulled. As the pulled amount increases, the rotational speed of the motor <NUM> increases.

The motor <NUM> is, for example, a brushless motor. The motor <NUM> includes: a rotor <NUM> including a rotary shaft <NUM> and a permanent magnet; and a stator <NUM> including a coil. The motor <NUM> is disposed at a relatively rear part in an interior space of the body <NUM>. The motor <NUM> converts electric power supplied from the battery pack B1 (see <FIG>) into rotary drive force of the rotary shaft <NUM>.

As shown in <FIG>, a drive circuit block <NUM> is disposed behind the motor <NUM> in the body <NUM>. The drive circuit block <NUM> includes a substrate <NUM> and a plurality of electronic components mounted on the substrate <NUM>. The plurality of electronic components include a plurality of power elements that constitute an inverter circuit. Each power element is, for example, a Field Effect Transistor (FET) element.

The controller <NUM> controls operation of the motor <NUM>. The controller <NUM> rotates or stops the motor <NUM> and controls the rotational speed of the motor <NUM> in accordance with the pulled amount of the operating part <NUM>. The controller <NUM> switches on and off the plurality of FET elements of the drive circuit block <NUM>, thereby controlling electric power supplied to the motor <NUM> via the plurality of FET elements (inverter circuit).

The transmission mechanism <NUM> is located forward of the motor <NUM> in the internal space of the body <NUM>. The transmission mechanism <NUM> includes an impact mechanism <NUM> and a planet gear mechanism <NUM>.

The impact mechanism <NUM> includes a drive shaft (spindle) <NUM>, the hammer <NUM>, a return spring <NUM>, the anvil <NUM>, and two steel balls <NUM> (rolling elements).

The planet gear mechanism <NUM> is a deceleration device. The torque of the rotary shaft <NUM> of the motor <NUM> is transmitted via the planet gear mechanism <NUM> to the drive shaft <NUM>. Thus, the drive shaft <NUM> is rotated around the rotation axis Ax1 by motive power provided from the motor <NUM>. The torque of the drive shaft <NUM> is transmitted to the hammer <NUM>. Thus, the hammer <NUM> rotates around the rotation axis Ax1. The torque of the hammer <NUM> is transmitted to the anvil <NUM>. Thus, the anvil <NUM> rotates around the rotation axis Ax1.

As shown in <FIG> and <FIG>, the hammer <NUM> includes a hammer body <NUM>, two hammer projections <NUM>, and a skirt part <NUM>.

The hammer body <NUM> has a columnar shape. The two hammer projections <NUM> protrude from a surface (front surface), facing the anvil <NUM>, of the hammer body <NUM>. The hammer projections <NUM> are pillars in the shape of a fan in front view. The skirt part <NUM> has a cylindrical shape and protrudes backward from a peripheral part of a rear surface of the hammer body <NUM>. As shown in <FIG>, a rear end of the skirt part <NUM> is located backward of the other portions of the hammer <NUM>. Thus, the hammer <NUM> has a peripheral edge part thicker than a center part of the hammer <NUM> in the forward/backward direction (a direction along the axis line of the rotation axis Ax1). The hammer <NUM> includes the skirt part <NUM> and thus has greater moment of inertia around the rotation axis Ax1 than a hammer of an impact tool of a comparative example including no skirt part <NUM>.

The hammer body <NUM> has a through hole <NUM> at the center thereof, and the drive shaft <NUM> is inserted in the through hole <NUM>. The hammer body <NUM> has, in its inner peripheral surface defining the through hole <NUM>, two groove parts <NUM> extending in V shape in the forward/backward direction. The drive shaft <NUM> has, in its outer peripheral surface, two groove parts <NUM> extending in V shape in the forward/backward direction. The two groove parts <NUM> are continuous.

The two steel balls <NUM> are disposed between the two groove parts <NUM> and the two groove parts <NUM>. The two groove parts <NUM>, the two groove parts <NUM>, and the two steel balls <NUM> constitute a cam mechanism.

While the two steel balls <NUM> move in the groove parts <NUM> and <NUM>, the hammer <NUM> is movable, with respect to the drive shaft <NUM>, in the direction (forward/backward direction) of the rotation axis Ax1 and is rotatable with respect to the drive shaft <NUM>. As the hammer <NUM> moves toward an anvil shaft <NUM> or away from the anvil shaft <NUM> along the direction of axis of the drive shaft <NUM>, the hammer <NUM> rotates with respect to the drive shaft <NUM>. That is, the hammer <NUM> is coupled to an outer peripheral surface of the drive shaft <NUM> to be movable in the forward/backward direction (direction along the axis line of the rotation axis Ax1) and rotates around the rotation axis Ax1 along with the rotation of the drive shaft <NUM>.

As shown in <FIG> and <FIG>, the anvil <NUM> includes an anvil body <NUM>, the anvil shaft <NUM>, a fitting part <NUM>, and two anvil projections <NUM>.

The anvil body <NUM> has an annular shape. The anvil body <NUM> is located forward of the hammer body <NUM>. The anvil body <NUM> faces the hammer body <NUM> in the forward/backward direction such that the axis line of the anvil body <NUM> coincides with the axis line of the hammer body <NUM>.

Each of the two anvil projections <NUM> is in the shape of a rectangular parallelepiped. The two anvil projections <NUM> are connected to the anvil body <NUM>. The two anvil projections <NUM> protrude from the anvil body <NUM> in a radial direction of the anvil body <NUM>.

The anvil shaft <NUM> has a columnar shape. The anvil shaft <NUM> also protrudes from the anvil body <NUM> in the axis direction of the anvil body <NUM>. The anvil shaft <NUM> protrudes forward from the anvil body <NUM>. As shown in <FIG>, the anvil shaft <NUM> is inserted in the through hole <NUM> formed in the housing <NUM>. The anvil shaft <NUM> has a tip end exposed outside the housing <NUM>.

The tip end (front end) of the anvil shaft <NUM> has the fitting part <NUM> provided integrally with the anvil shaft <NUM>. The fitting part <NUM> has a quadrangular prism shape. The fitting part <NUM> rotates along with rotation of the anvil <NUM>. To the fitting part <NUM>, a tip tool <NUM> is to be detachably attached. For example, a tip end surface (front end surface) of the fitting part <NUM> has a recess having a hexagonal prism shape, and in the recess, a rear end having a hexagonal prism shape of the tip tool <NUM> is fit.

In the present embodiment, the tip tool <NUM> is coupled to the fitting part <NUM> via a chuck <NUM> (see <FIG>). The anvil <NUM> receives torque from the motor <NUM> to rotate around the rotation axis Ax1 together with the chuck <NUM> and the tip tool <NUM>.

Neither the chuck <NUM> nor the tip tool <NUM> is a component of the impact tool <NUM>. Note that the impact tool <NUM> may include at least one of the chuck <NUM> or the tip tool <NUM> as its component.

The tip tool <NUM> is fit to a fastening member (e.g., a screw or a bolt) which is a work target. A fastening member can be tightened or loosen by the rotating the tip tool <NUM> fitted to the fastening member.

The tip tool <NUM> is, for example, a driver bit. A tip end of the driver bit is fit in a cross hole or groove formed in the head of a screw as the fastening member, and thereby, the driver bit is fit to the screw. The tip tool <NUM> is not limited to the driver bit but may be, for example, a socket. The socket has, in its tip end (front end), a recess having a hexagonal prism shape, and in the recess, the head of a bolt as the fastening member is fit, and thereby, the socket is fit in the bolt.

In the impact tool <NUM> of the present embodiment, the tip tool <NUM> for tightening or loosening a fastening member smaller than or equal to M8 or smaller than or equal to <NUM>/<NUM> inches is attachable to the fitting part <NUM>. That is, the impact tool <NUM> of the present embodiment is a tool for tightening a small fastening member. In such an impact tool <NUM>, an increase in the size of the housing <NUM> (the size of the body <NUM>) does not tend to be desired from the viewpoint of usability, portability, and the like.

As shown in <FIG>, the return spring <NUM> is sandwiched between the hammer <NUM> and the planet gear mechanism <NUM>. The return spring <NUM> of the present embodiment is a cone coil spring.

The impact mechanism <NUM> further includes a plurality of spherical bodies (steel balls <NUM>) (only two of which are shown in <FIG>) and a ring <NUM> between the hammer <NUM> and the return spring <NUM>. Thus, the hammer <NUM> is rotatable with respect to the return spring <NUM>. The hammer <NUM> receives forward force from the return spring <NUM> via the spherical bodies and the ring <NUM>.

When the impact mechanism <NUM> does not perform impact operation, the hammer <NUM> and the anvil <NUM> together rotate around the rotation axis Ax1 with the two hammer projections <NUM> being in contact with the two anvil projections <NUM>. Thus, at this time, the hammer <NUM>, the anvil <NUM>, and the tip tool <NUM> rotate together.

The impact mechanism <NUM> performs the impact operation when a torque condition is satisfied, where the torque condition relates to the magnitude of a load torque, and the load torque refers to the torque applied to the anvil shaft <NUM>. The impact operation is operation of applying striking force from the hammer <NUM> to the anvil <NUM>. In the present embodiment, the torque condition is that the load torque is greater than or equal to a prescribed value. That is, as the load torque increases, a force component, contained in force generated between the hammer <NUM> and the anvil <NUM>, which moves the hammer <NUM> backward increases. When the load torque comes to have a prescribed value or greater, the hammer <NUM> moves backward while compressing the return spring <NUM>. Then, this backward movement of the hammer <NUM> rotates the hammer <NUM> while the two hammer projections <NUM> of the hammer <NUM> climb over the two anvil projections <NUM> of the anvil <NUM>. The hammer <NUM> then moves forward by return force applied from the return spring <NUM>. When the drive shaft <NUM> makes a substantially half turn, the two hammer projections <NUM> of the hammer <NUM> collide with the two anvil projections <NUM> of the anvil <NUM>, and the hammer <NUM> applies rotation striking force (impact force) to the anvil <NUM>. Each time the drive shaft <NUM> in the impact mechanism <NUM> makes a substantially half turn, the hammer projections <NUM> collide with the anvil projections <NUM>, and thereby, the hammer <NUM> applies the rotation striking force to the anvil <NUM>. That is, each time the drive shaft <NUM> makes a substantially half turn, the hammer <NUM> applies pulsed rotation striking force to the anvil <NUM>.

As described above, the hammer <NUM> in the impact mechanism <NUM> repeatedly applies the rotation striking force to the anvil <NUM> in the impact operation. In the impact tool <NUM>, a fastening member such as a screw can be more strongly tightened with torque resulting from the rotation striking force than in an electric tool which does not perform the impact operation.

The inventors of the present application found that in such an impact tool <NUM>, clearances (backlash) may be formed between the anvil <NUM> and the tip tool <NUM> and between the tip tool <NUM> and the fastening member and these clearances may reduce the energy efficiency of the impact tool <NUM>. Here, the energy efficiency is, for example, defined as a ratio of energy used to tighten the fastening member to the rotation energy of the hammer <NUM> in one impact which is the operation that the hammer <NUM> applies pulsed rotation striking force to the anvil <NUM> once.

The inventors of the present application first of all used an impact tool of a comparative example and measured tightening force (tightening torque) which the tip tool <NUM> applied to the fastening member to tighten the fastening member by the impact tool of the comparative example. The impact tool of the comparative example has a structure similar to the impact tool <NUM> of the embodiment. However, the hammer <NUM> of the impact tool of the comparative example includes no skirt part <NUM>, and the moment of inertia of the hammer <NUM> around the rotation axis Ax1 is five times the moment of inertia of the anvil <NUM> around the rotation axis Ax1.

<FIG> shows a waveform indicating a change with time of the torsion amount of the anvil shaft <NUM>, in one impact, measured for the impact tool of the comparative example. The torsion amount of the anvil shaft <NUM> is measurable by a distortion sensor disposed to, for example, the anvil shaft <NUM>.

In the impact tool, when the impact mechanism <NUM> performs the impact operation, collision of the hammer <NUM> with the anvil <NUM> results in that the anvil shaft <NUM> of the anvil <NUM> gives to the tip tool <NUM> impact force around the rotation axis Ax1. Then, the anvil shaft <NUM> receives reaction force in the circumferential direction of the rotation axis Ax1 from the tip tool <NUM> and is twisted around the rotation axis Ax1 by the reaction force. The anvil shaft <NUM> thus twisted returns to its initial state, thereby giving force around the rotation axis Ax1 to the tip tool <NUM>. Thus, it can be said that the torsion amount of the anvil shaft <NUM> (the ordinate in <FIG>) represents the magnitude of force applied from the anvil <NUM> to the tip tool <NUM>, and consequently, represents the magnitude of the tightening force which the tip tool <NUM> applies to the fastening member to tighten the fastening member.

As can be seen from <FIG>, a measured waveform indicating a change with time of the torsion amount of the anvil shaft <NUM> includes a plurality of mountains M1 to M3 respectively having peaks (local maximum values) P1 to P3. This shows that due to the clearances (backlash), the hammer <NUM> and the anvil <NUM> collide with each other for a plurality of number of times during one impact.

That is, in the impact tool, the operation as described below is performed in one impact.

First of all, the hammer projections <NUM> climb over the anvil projections <NUM>, the hammer <NUM> rotates around the rotation axis Ax1 in one direction (hereinafter, this direction is referred to as a "first direction"), and the hammer projections <NUM> collide with the anvil projections <NUM> on an opposite side (first collision).

According to the impact of the first collision, the anvil <NUM> rotates in the first direction at a speed higher than the speed of the hammer <NUM> to close up the clearance between the anvil <NUM> and the tip tool <NUM>, thereby colliding with the tip tool <NUM> (time point t1). In the anvil <NUM>, reaction force of the collision with the tip tool <NUM> causes torsion of the anvil shaft <NUM> around the rotation axis Ax1, thereby increasing the torsion amount.

According to the impact of the collision with the anvil <NUM>, the tip tool <NUM> rotates in the first direction at a speed higher than that of the anvil <NUM> to close up the clearance between the tip tool <NUM> and the fastening member and collides with the fastening member. The impact of the collision tightens the fastening member in the first direction.

When the tip tool <NUM> collides with the fastening member, the tip tool <NUM> receives, from the fastening member, reaction force in a direction opposite to the first direction (hereinafter this direction is referred to as a "second direction"), is thus decelerated, and then collides with the anvil <NUM> rotating in the first direction (time point t2). The anvil <NUM> collides with the tip tool <NUM>, thereby receiving force in the second direction from the tip tool <NUM>, which reduces the torsion amount of the anvil shaft <NUM>. Moreover, the rotational speed in the first direction of the anvil <NUM> is reduced by the collision with the tip tool <NUM>. Then, the anvil projections <NUM> of the anvil <NUM> collide again (second collision) with the hammer projections <NUM> of the hammer <NUM> rotating in the first direction.

According to the impact of the second collision, the anvil <NUM> is accelerated again in the first direction and collides with the tip tool <NUM> (time point t3). In the anvil <NUM>, reaction force of the collision with the tip tool <NUM> causes torsion of the anvil shaft <NUM> around the rotation axis Ax1, thereby increasing the torsion amount.

The tip tool <NUM> is accelerated in the first direction by the collision with the anvil <NUM> and collides with the fastening member. The impact of the collision tightens the fastening member in the first direction.

Hereafter, the collision is repeated between the components, thereby tightening the fastening member.

As can be seen from <FIG>, in the impact tool, the peak P2 at the time of the second collision is higher than the peak P1 at the time of the first collision in one impact. The reason for this is that in the first collision, the energy of the anvil <NUM> is consumed, for example, for closing up the clearances, and therefore, the peak P2 at the time of the second collision is higher than the peak P1 at the time of the first collision.

Here, in the impact tool, in one impact, force that mainly contributes to tightening of the fastening member is force applied to the fastening member at and after a time point at which the force applied to the fastening member becomes maximum, that is, at and after a time point at which the torsion amount of the anvil shaft <NUM> is maximum. For example, in the example shown in <FIG>, a great tightening force is applied to the fastening member at the time point t4, which is greater than tightening force applied to the fastening member prior to the time point t4. Thus, the tightening force applied to the fastening member before the time point t4 is "overwritten" with the tightening force at the time point t4 and thus makes a small contribution to tightening of the fastening member.

In sum, in the impact tool, due to existence of the clearances, energy given from the hammer <NUM> to the anvil <NUM> by the first collision in one impact is consumed without contributing to tightening of the fastening member.

Based on the knowledge, the inventors of the present application found that in the impact tool <NUM>, increasing the ratio of the moment of inertia of the hammer <NUM> around the rotation axis Ax1 to the moment of inertia of the anvil <NUM> around the rotation axis Ax1 (hereinafter also referred to as an "inertia ratio") improves the energy efficiency.

<FIG> shows a graph of a simulation result of a calculation of the energy efficiency [%] of the impact tool <NUM> when the inertia ratio is varied.

In the example shown in <FIG>, a simulation is performed provided that the following operation is performed in one impact. That is, the hammer <NUM> at first collides with the anvil <NUM>, and as a result of the collision, the anvil <NUM> rotates in the first direction at a speed higher than that of the hammer <NUM>, and the anvil <NUM> thus collides with the tip tool <NUM>, and as a result of this collision, the tip tool <NUM> rotates in the first direction at a speed higher than that of the anvil <NUM>, and the tip tool <NUM> thus collides with the fastening member. The collision of the tip tool <NUM> with the fastening member rotates the tip tool <NUM> in the second direction, and the tip tool <NUM> thus collides with the anvil <NUM>, and as a result of the collision, the anvil <NUM> rotates in the second direction and collides with the hammer <NUM>. As a result of the collision, the anvil <NUM> rotates in the first direction at a speed higher than (substantially the same as) the speed of the hammer <NUM>, and the anvil <NUM> thus collides with the tip tool <NUM>, and as a result of this collision, the tip tool <NUM> rotates in the first direction at a speed higher than (substantially the same as) the speed of the anvil <NUM>, and the tip tool <NUM> thus collide with the fastening member. As a result, the hammer <NUM>, the anvil <NUM>, and the tip tool <NUM> rotate together and tighten the fastening member. In this simulation, the "energy efficiency" is calculated as the ratio of: energy of the hammer <NUM> at the time of the second collision of the hammer <NUM> with the anvil <NUM>, that is, energy consumed by the second and following collisions; to rotation energy which the hammer <NUM> initially has (rotation energy which the hammer <NUM> has at the time of the first collision with the anvil <NUM>). Note that in this simulation, the rotation energy which the hammer <NUM> initially has is constant (same as each other). Note that the relationship between the inertia ratio and the energy efficiency shown in <FIG> is also experimentally confirmed by the inventors of the present application.

From <FIG>, it can be seen that in the case where the inertia ratio is <NUM>, (i.e., in the case of the impact tool of the comparative example shown in <FIG>), the energy efficiency is about <NUM>%. That is, it can be seen that in the impact tool of the comparative example, about <NUM>% of the rotation energy which the hammer <NUM> initially has is consumed without contributing to tightening of the fastening member.

Moreover, it can be seen from <FIG> that as the inertia ratio increases, the energy efficiency is improved, and in the case where the inertia ratio is <NUM>, the energy efficiency is higher than or equal to <NUM>%. In sum, it can be seen that an inertia ratio of <NUM> or greater enables a half or more of the rotation energy which the hammer <NUM> initially has to be used to tighten the fastening member, thereby improving the energy efficiency.

Moreover, it can be seen from <FIG> that in the case where the inertia ratio is <NUM>, the energy efficiency is higher than or equal to <NUM>%, and in the case where the inertia ratio is <NUM>, the energy efficiency is higher than or equal to <NUM>%, which indicate that the energy efficiency is further improved.

In sum, increasing the inertia ratio enables the ratio of energy consumed by the first collision to be reduced, and the energy efficiency to be improved.

<FIG> and <FIG> show, for the impact tool <NUM> in the cases of the inertia ratio being <NUM> and <NUM> respectively, waveforms of a result of measurement of the change with time of the torsion amount of the anvil shaft <NUM> in one impact. <FIG> shows a measured waveform of the impact tool <NUM> in the case of the inertia ratio being <NUM>. <FIG> shows a measured waveform of the impact tool <NUM> in the case of the inertia ratio being <NUM>. Note that in <FIG>, <FIG>, and <FIG>, the scale on the ordinate and the scale on the abscissa are the same. Moreover, in <FIG>, <FIG>, and <FIG>, a measurement condition is that the rotation energy which the hammer <NUM> initially has is constant (same as each other).

It can be seen from <FIG>, <FIG>, and <FIG> that as the inertia ratio increases, the ratio of the peak P2 at the time of the second collision to the peak P1 at the time of the first collision increases. Moreover, it can be seen from <FIG>, <FIG>, and <FIG> that as the inertia ratio increases, the maximum value (magnitude of the peak P2) of the torsion amount of the anvil shaft <NUM> increases, which indicates that the fastening member is tightened with increased tightening force. Moreover, it can be seen from <FIG>, <FIG>, and <FIG> that as the inertia ratio increases, a time period during which the anvil shaft <NUM> is twisted increases, which indicates that tightening force is applied to the fastening member for a long period of time. In sum, it can be seen that as the inertia ratio increases, "impulse" applied from the anvil shaft <NUM> to the tip tool <NUM> increases, and the fastening member is thus efficiently tightened.

<FIG> show measurement results of the increment of the tightening torque when one impact is made. <FIG> shows a measurement result of the increment of the tightening torque when the tightening torque of a bolt as the fastening member is <NUM> N·m (i.e., when the bolt is already tightened with a tightening torque of <NUM> N·m). <FIG> shows a measurement result of the increment of the tightening torque when the tightening torque of a bolt as the fastening member is <NUM> N·m. <FIG> shows a measurement result of the increment of the tightening torque when the tightening torque of a bolt as the fastening member is <NUM> N·m. In each of <FIG>, a graph denoted by A1 shows a measurement result using the impact tool <NUM> having the inertia ratio of <NUM>, whereas a graph denoted by B1 shows a measurement result of an impact tool of a comparative example having the inertia ratio of <NUM>. In <FIG>, a measurement condition is that the rotation energy which the hammer <NUM> initially has is constant.

For example, when the tightening torque of a bolt is <NUM> N·m (see <FIG>), the increment of the tightening torque is <NUM> N·m in the impact tool of the comparative example having the inertia ratio of <NUM>, whereas the increment of the tightening torque is <NUM> N·m in the impact tool <NUM> having the inertia ratio of <NUM>. Thus, increasing the inertia ratio can increase the increment of the tightening torque of a bolt in one impact. In sum, increasing the inertia ratio improves the energy efficiency which is the ratio of energy used to tighten the fastening member to the rotation energy of the hammer <NUM>.

Thus, in the impact tool <NUM> of the present embodiment, the inertia ratio (ratio of moment of inertia of the hammer <NUM> around the rotation axis Ax1 to the moment of inertia of the anvil <NUM> around the rotation axis Ax1) is <NUM> or greater, and therefore, the energy efficiency can be improved. Moreover, an improvement in the energy efficiency can reduce the ratio of energy consumed as a sound or heat to the rotation energy which the hammer <NUM> initially has. Thus, the impact tool <NUM> of the present embodiment can reduce noise generated from the impact tool <NUM> and can reduce heat generated from the impact tool <NUM>.

Moreover, in the impact tool <NUM> of the present embodiment, the hammer <NUM> is provided with the skirt part <NUM>, thereby increasing the moment of inertia of the hammer <NUM> around the rotation axis Ax1. Thus, the moment of inertia of the hammer <NUM> around the rotation axis Ax1 can be increased while the size of the hammer <NUM> in its radial direction is suppressed, and consequently, the size of the housing <NUM> is suppressed.

The embodiment described above is merely an example of various embodiments of the present disclosure. Various modifications may be made to the embodiment depending on design and the like as long as the object of the present disclosure is achieved. Variations of the embodiment described above will be enumerated below. The variations described below are applicable accordingly in combination.

In the present variation, a hammer <NUM> has an edge part <NUM> as shown in <FIG> and <FIG>. The edge part <NUM> is provided at an outer periphery of a hammer body <NUM>. The edge part <NUM> is cylindrical and protrudes as far as front surfaces of hammer projections <NUM> in the forward/backward direction. Since the hammer <NUM> has the edge part <NUM>, the moment of inertia of the hammer <NUM> around the rotation axis Ax1 can be increased while the size of the hammer <NUM> in its radial direction is suppressed.

Note that the edge part <NUM> may protrude forward farther than the hammer projections <NUM>.

Moreover, in the present variation, as shown in <FIG>, the hammer <NUM> includes no skirt part <NUM>, and the thickness of the hammer <NUM> is smaller in its peripheral part than in its center part in the forward/backward direction. However, this should not be construed as limiting, but the hammer <NUM> may further include the skirt part <NUM>. The moment of inertia of the hammer <NUM> around the rotation axis Ax1 can be further increased while the size of the hammer <NUM> in its radial direction is suppressed.

In the present variation, a hammer <NUM> includes a main hammer <NUM> and a sub-hammer <NUM> as shown in <FIG>. The main hammer <NUM> is coupled to an outer peripheral surface of a drive shaft <NUM> (see <FIG>) to be movable in the forward/backward direction (direction along the axis line of the rotation axis Ax1). The main hammer <NUM> rotates around the rotation axis Ax1 along with the rotation of the drive shaft <NUM>. The sub-hammer <NUM> is restricted from moving in the forward/backward direction. The sub-hammer <NUM> is coupled to the main hammer <NUM> to rotate together with the main hammer <NUM> as the main hammer <NUM> rotates around the rotation axis Ax1.

More specifically, the main hammer <NUM> includes a main hammer body <NUM>, hammer projections <NUM>, and groove parts in a similar manner to the hammer <NUM> of the embodiment.

The sub-hammer <NUM> has a hollow cylindrical shape and is configured to house the main hammer <NUM> therein. The sub-hammer <NUM> has an inner peripheral surface having four pin grooves <NUM> which extend in the direction of the rotation axis Ax1 and which are formed at an interval of <NUM> degrees. Into the four pin groove <NUM>, four pins <NUM> which are columnar are to be fit. In this state, a stop member <NUM> which is annular is fit in a groom formed in a front edge of the sub-hammer <NUM>, so that the pins <NUM> do not fall off the pin grooves <NUM>.

On the other hand, the main hammer body <NUM> has an outer peripheral surface having four pin grooves <NUM> which extend in the axis direction of the rotation axis Ax1 and which are formed at an interval of <NUM> degrees. The main hammer <NUM> is disposed in the sub-hammer <NUM> such that the four pins <NUM> are fit in the four pin grooves <NUM>. Thus, the main hammer <NUM> is held by the sub-hammer <NUM> such that the main hammer <NUM> and the sub-hammer <NUM> are rotatable together around the rotation axis Ax1 and the main hammer <NUM> is movable with respect to the sub-hammer <NUM> in the forward/backward direction.

When the main hammer <NUM> performs the impact operation, the main hammer <NUM> rotates around the rotation axis Ax1 while moving in the forward/backward direction in a manner similar to the hammer <NUM> of the embodiment, and the main hammer <NUM> collides with the anvil <NUM> to apply rotation striking force to the anvil <NUM>. On the other hand, the sub-hammer <NUM> is restricted by the housing <NUM> from moving in the forward/backward direction, and thus, the sub-hammer <NUM> rotates around the rotation axis Ax1 together with the main hammer <NUM>, but the sub-hammer <NUM> does not move in the forward/backward direction.

In the present variation, the sum of the moments of inertia of the main hammer <NUM>, the sub-hammer <NUM>, the four pins <NUM>, and the stop member <NUM> around the rotation axis Ax1 is <NUM> or more times the moment of inertia of the anvil <NUM> around the rotation axis Ax1.

In the present variation, since the hammer <NUM> includes the sub-hammer <NUM>, the moment of inertia of the hammer <NUM> around the rotation axis Ax1 can be increased while the size of the hammer <NUM> in its radial direction is suppressed.

Note that the moment of inertia of only the main hammer <NUM> around the rotation axis Ax1 may be <NUM> or more times the moment of inertia of the anvil <NUM> around the rotation axis Ax1.

In a variation, the hammer <NUM> does not have to include the skirt part <NUM>. Alternatively, the density (mass per unit volume) of a peripheral edge part of the hammer <NUM> may be higher than the density of the center part of the hammer <NUM> such that the inertia ratio is <NUM> or greater. For example, the peripheral edge part and the center part of the hammer <NUM> may be made of materials having different densities and may be integrated with each other by welding or the like.

As can be seen from the embodiment and the variations described above, the present specification discloses the following aspects.

An impact tool (<NUM>) of a first aspect includes a motor (<NUM>), a hammer (<NUM>), and an anvil (<NUM>). The hammer (<NUM>) is configured to be rotated around a rotation axis (Ax1) by motive power provided from the motor (<NUM>). The anvil (<NUM>) is configured to be rotated around the rotation axis (Ax1) by receiving striking force from the hammer (<NUM>) in a circumferential direction of the rotation axis (Ax1). In the impact tool (<NUM>), moment of inertia of the hammer (<NUM>) around the rotation axis (Ax1) is <NUM> or more times moment of inertia of the anvil (<NUM>) around the rotation axis (Ax1).

With this aspect, energy efficiency is improved.

An impact tool (<NUM>) of a second aspect referring to the first aspect further includes a fitting part (<NUM>). The fitting part (<NUM>) is configured to rotate along with rotation of the anvil (<NUM>). A tip tool (<NUM>) is attachable to the fitting part (<NUM>). The tip tool (<NUM>) is configured to tighten or loosen a fastening member smaller than or equal to M8 or smaller than or equal to <NUM>/<NUM> inches.

With this aspect, the energy efficiency of the impact tool (<NUM>) is improved while the impact tool (<NUM>) is small in size.

In an impact tool (<NUM>) of a third aspect referring to the first or second aspect, the hammer (<NUM>) is coupled to an outer peripheral surface of a drive shaft (<NUM>) to be movable in a direction along an axis line of the rotation axis (Ax1). The drive shaft (<NUM>) is configured to be rotated around the rotation axis (Ax1) by the motive power provided from the motor (<NUM>). The hammer (<NUM>) is configured to be rotated around the rotation axis (Ax1) along with rotation of the drive shaft (<NUM>).

In an impact tool (<NUM>) of a fourth aspect referring to the first or second aspect, the hammer (<NUM>) includes a main hammer (<NUM>) and a sub-hammer (<NUM>). The main hammer (<NUM>) is coupled to an outer peripheral surface of a drive shaft (<NUM>) to be movable in a direction along an axis line of the rotation axis (Ax1). The drive shaft (<NUM>) is configured to be rotated around the rotation axis (Ax1) by the motive power provided from the motor (<NUM>). The main hammer (<NUM>) is configured to be rotated around the rotation axis (Ax1) along with rotation of the drive shaft (<NUM>). The sub-hammer (<NUM>) is restricted from moving in the direction along the axis line of the rotation axis (Ax1). The sub-hammer (<NUM>) is coupled to the main hammer (<NUM>) to rotate together with the main hammer (<NUM>) as the main hammer (<NUM>) rotates around the rotation axis (Ax1).

In an impact tool (<NUM>) of a fifth aspect referring to any one of the first to fourth aspects, the hammer (<NUM>) has a peripheral edge part thicker than a center part of the hammer (<NUM>) in a direction along an axis line of the rotation axis (Ax1).

With this aspect, the moment of inertia of the hammer (<NUM>) is increased while the size of the hammer (<NUM>) in its radial direction is suppressed.

In an impact tool (<NUM>) of a sixth aspect referring to any one of the first to fifth aspects, the hammer (<NUM>) includes a hammer body (<NUM>) and a hammer projection (<NUM>) protruding from a front surface of the hammer body (<NUM>). The front surface faces the anvil (<NUM>). The anvil (<NUM>) includes an anvil body (<NUM>) located forward of the hammer body (<NUM>) and an anvil projection (<NUM>) connected to the anvil body (<NUM>) and configured to collide with the hammer projection (<NUM>) in a rotation direction of the hammer (<NUM>). The hammer (<NUM>) further includes an edge part (<NUM>) at an outer periphery of the hammer body (<NUM>), the edge part (<NUM>) protruding forward as far as a front surface of the hammer projection (<NUM>) or protruding forward farther than the hammer projection (<NUM>).

An impact tool (<NUM>) of a seventh aspect includes a motor (<NUM>), a hammer (<NUM>), and an anvil (<NUM>). The hammer (<NUM>) is configured to be rotated around a rotation axis (Ax1) by motive power provided from the motor (<NUM>). The anvil (<NUM>) is configured to be rotated around the rotation axis (Ax1) by receiving striking force from the hammer (<NUM>) in a circumferential direction of the rotation axis (Ax1). The anvil (<NUM>) includes an anvil shaft (<NUM>) configured to transmit the striking force to a tip tool (<NUM>). The impact tool (<NUM>) is configured such that a waveform, which represents a measured torsion amount of the anvil shaft (<NUM>) during one impact for impact operation of giving the striking force to the anvil (<NUM>) from the hammer (<NUM>), has a plurality of peaks (P1, P2, P3), and such that in the one impact, rotation energy of the hammer (<NUM>) transmitted to the anvil (<NUM>) after a time point (t4) at which a highest peak (P2) of the plurality of peaks (P1, P2, P3) is measured is greater than rotation energy of the hammer (<NUM>) transmitted to the anvil (<NUM>) before the time point (t4).

Claim 1:
An impact tool (<NUM>) comprising:
a motor (<NUM>);
a hammer (<NUM>) configured to be rotated around a rotation axis (Ax1) by motive power provided from the motor (<NUM>); and
an anvil (<NUM>) configured to be rotated around the rotation axis (Ax1) by receiving striking force from the hammer (<NUM>) in a circumferential direction of the rotation axis (Ax1),
characterized in that the moment of inertia of the hammer (<NUM>) around the rotation axis (Ax1) being <NUM> or more times moment of inertia of the anvil (<NUM>) around the rotation axis (Ax1).