EP1637289A1

EP1637289A1 - Method of manufacturing a power tool

Info

Publication number: EP1637289A1
Application number: EP04021706A
Authority: EP
Inventors: Takuo c/o Makita Corporation Arakawa; Hiroki c/o Makita Corporation Ikuta
Original assignee: Makita Corp
Current assignee: Makita Corp
Priority date: 2004-09-13
Filing date: 2004-09-13
Publication date: 2006-03-22
Anticipated expiration: 2024-09-13
Also published as: DE602004003383T2; EP1637289B1; DE602004003383D1; DE602004003383T8

Abstract

It is an object of the invention to provide a technique for optimizing a vibration reducing mechanism of a power tool (121). According to the invention, a method of manufacturing a power tool is provided. Such power tool may include a tool bit (137), a driving motor (131), driving force transmitting mechanism (133) and a counter weight (139). The representative method may include a step of determining the mass and travel of the counter weight based on the magnitudes of vibration caused in the power tool under loaded driving conditions in which the tool bit receives a load by performing the processing operation and under unloaded driving conditions in which the tool bit does not receive a load by performing the processing operation. By such determination, vibration reducing mechanism of the power tool can be optimized and thus, rational manufacturing technique of a power tool can be provided.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a technique of manufacturing a power tool such as a hammer and a hammer drill to drive a tool bit at a predetermined cycle.

Description of the Related Art

Japanese laid-open utility model publication No. 51-6583 discloses an electric hammer with a vibration reducing device. This known electric hammer includes a motion converting mechanism that converts a rotating output of a driving motor in order to drive a tool bit that performs a hammering operation on a workpiece, and a striker that reciprocates via the motion converting mechanism. Further, in order to reduce vibration acting on the electric hammer, a counter weight is provided and driven in a direction opposite to the reciprocating direction of the striker.
The counter weight is thus designed to reduce vibration of the electric hammer by moving in a direction opposite to the moving direction of the striker. However, the properties of vibration exerted to the electric hammer under loaded driving conditions in which the tool bit receives a load by performing the operation are different from those under unloaded driving conditions in which the tool bit does not receive a load. Due to such difference, the counter weight designed to perform a vibration reducing function under loaded driving conditions, may not be able to perform an appropriate vibration reducing function under unloaded driving conditions. It is therefore desired to reduce vibration of an electric hammer as much as possible under each operating condition.

SUMMARY OF THE INVENTION

It is, accordingly, an object of the invention to provide a technique for optimizing a vibration reducing mechanism of a power tool.
According to the invention, method of manufacturing a power tool is provided. Such power tool may include a tool bit, a driving motor, driving force transmitting mechanism and a counter weight. The tool bit performs a predetermined processing operation to a workpiece by a reciprocating movement. The driving motor drives the tool bit. The driving force transmitting mechanism converts a rotating output of the driving motor to a reciprocating movement and transmits the reciprocating movement to the tool bit. The counter weight reduces a vibration caused in the power tool by reciprocating in a direction opposite to a component of linear motion of the driving force transmitting mechanism.
The representative method may include a step of determining the mass and travel of the counter weight based on the magnitudes of vibration caused in the power tool under loaded driving conditions in which the tool bit receives a load by performing the processing operation and under unloaded driving conditions in which the tool bit does not receive a load by performing the processing operation. By such determination, vibration reducing mechanism of the power tool can be optimized and thus, rational manufacturing technique of a power tool can be provided.
Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an essential part of an electric hammer to be manufactured in the representative embodiment of the invention.
FIG. 2 is a diagram showing an entire configuration of a manufacturing system of the electric hammer according to the representative embodiment.
FIG. 3 is a graph showing the correlation between vibration caused in the electric hammer and the mechanism of reducing vibration by the counter weight.
FIG. 4 is a flow chart showing the procedure of determining the optimum design of the counter weight in the embodiment.
FIG. 5 is a graph relating to a modification of the embodiment of the invention.
FIG. 6 is a graph showing a curve of change of mean vibration in the modification of the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The representative method according to the invention is configured to determine the mass and travel of the counter weight based on the magnitudes of vibration caused in the power tool under loaded driving conditions in which the tool bit receives a load by performing the operation and under unloaded driving conditions in which the tool bit does not receive a load by performing the operation. Thus, vibration reducing mechanism of the power tool can be optimized. The "loaded driving conditions" in this invention typically corresponds to the conditions in which the operation is performed by a tool bit being pressed against a workpiece. The "unloaded driving conditions" in this invention typically corresponds to the conditions in which the tool bit is not in contact with the workpiece and the power tool is idled. Further, the "magnitude of vibration" caused in the power tool may widely include indexes relating to vibration caused in the power tool, such as acceleration and stress that act on the power tool, as well as frequency of vibration. Further, in measuring the magnitude of vibration, a coordinate system, such as three X-, Y-, Z-axis, cylindrical or polar coordinates system, can be appropriately used.
The counter weight is typically driven to reciprocate by a crank mechanism. However, the "kinetic energy of the counter weight" in this invention is obtained based on the value of the mass of the counter weight being multiplied by the travel of the counter weight. Further, the "travel of the counter weight" in this case can be defined by the eccentricity between the center of rotation of the counter weight and the counter weight mounting point. The manner in which a difference between the magnitudes of vibration under loaded and unloaded driving conditions is "within a predetermined range" includes the manner in which the magnitudes of vibration are substantially the same therebetween, as well as the manner in which the difference is smaller than a predetermined amount
Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide and manufacture improved power tools and method for using such power tools and devices utilized therein. Representative examples of the present invention, which examples utilized many of these additional features and method steps in conjunction, will now be described in detail with reference to the drawings. This detailed description is merely intended to teach a person skilled in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed within the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe some representative examples of the invention, which detailed description will now be given with reference to the accompanying drawings.
An embodiment of the present invention will now be described with reference to the drawings. An electric hammer will be explained as a representative example of the "power tool" in the embodiment of the present invention. As shown in FIG. 1, an electric hammer 121 to be manufactured in this representative embodiment mainly comprises a body 123 that defines the contours of the electric hammer 101. The body 123 includes a motor housing 125, a gear housing 127 and a tool holder 129. A hammer bit 137 is disposed in the tip end region of the body 123. The hammer bit 137 is a feature that corresponds to the "tool bit" according to the invention.
The motor housing 125 houses a driving motor 131, and the gear housing 127 houses a motion converting mechanism 133 and a striking mechanism 135. The motion converting mechanism 133 is adapted to convert the rotating output of the driving motor 131 to linear motion. Specifically, the motion converting mechanism 113 mainly includes a crank arm 134 that converts the rotation of an output shaft 132 of the driving motor 131 to linear motion in the axial direction of the hammer bit 137. In the striking mechanism 135, when the crank arm 134 rotates, a driver 135b slidingly reciprocates within a cylinder 135a. As a result, a striker 138 is caused to reciprocate within the cylinder 135a at a speed higher than the sliding speed of the driver 135b by the action of an air spring which is caused within the cylinder 135a by sliding movement of the driver 135b. Thus, an impact force is applied to the hammer bit 137 so that the hammer bit 137 performs a hammering operation. The driving mechanism of the hammer bit 137 is well known and therefore will not be explained in greater detail.
Further, a counter weight 139 is connected to the motion convecting mechanism 133. The counter weight 139 reciprocates with a 180° phase shift with respect to the reciprocating movement of the striker 138. Specifically, a mounting portion 139a of the counter weight 139 is located a predetermined distance apart from the center of a driving shaft 140 of the counter weight 139. When the driving motor 131 is driven, the crank arm 134 rotates via the output shaft 132. Then, the counter weight 139 is caused to reciprocate in the axial direction of the hammer bit 137 via the driving shaft 140 that is mounted on the crank arm 134. At this time, when the crank arm 134 rotates, the driving shaft 140 revolves. In this embodiment, the radius of the revolving movement is identical to the horizontal distance between the center of a driving shaft 134a of the crank arm 134 and the mounting portion 139a of the counter weight 139, which is set to be a distance "r". In other words, when the crank arm 134 rotates around the driving shaft 134a, the counter weight 139 linearly moves the distance "2r" in the axial direction of the hammer bit 137.
In driving conditions in which a hammering operation is performed with the hammer bit 137 held in contact with the workpiece W as shown in FIG. 2 and by the user applying the pressing load toward the workpiece W to the electric hammer 121, or, in loaded driving conditions in which the hammer bit 137 receives a load from the workpiece W by performing a hammering operation, the counter weight 139 reciprocates in a direction opposite to the reciprocating direction of the striker 138 which reciprocates to cause the hammer bit 137 to perform the hammering operation.
As shown in FIG. 2, a computer-aided manufacturing system for the electric hammer 121 mainly includes a controller 103, and a ROM 105, a RAM 107, an acceleration detector 109, a correlation output section 111 and an optimum value output section 113 which are connected to the controller 103.
Electric hammer 121 under each different driving condition is connected to the acceleration detector 109 of the computer-aided manufacturing system 101. A first driving condition is defined as loaded driving conditions (designated by (A) in FIG. 2) in which the hammer bit 137 is in contact with the workpiece W and the user provides pushing force to press the electric hammer 121 toward the workpiece W, so that the hammer bit 137 receives a load, in the form of a reaction force Fw, from the workpiece W by performing a hammering operation. A second driving condition is defined as unloaded driving conditions (designated by (B) in FIG. 2) in which the hammer bit 137 is not in contact with the workpiece W and the hammer bit 137 does not receive a load in the hammering operation.
Although it is not particularly shown, a gauge for measuring acceleration on appropriate X, Y and Z axes is provided in the body of the electric hammer 121. The detected value of the acceleration is inputted into the controller 103 via the acceleration detector 109. The controller 103 combines the accelerations in the direction of each axis and executes a predetermined frequency correction, and then produces three-axis combined frequency-corrected acceleration data (in m/sec²).
The ROM 105 stores index data relating to the mass value (in gram) and the travel (in mm) of the counter weight 139. Further, the ROM 105 stores a program for determining an optimum value for the counter weight 139 based on the index data and the above-mentioned acceleration data.
The correlation between the acceleration caused in the electric hammer 121 under loaded and unloaded driving conditions and the kinetic energy of the counter weight 139 is outputted to the correlation output section 111 in the form of a graph, of which example is shown in FIG. 3. Further, based on such correlation, an optimum design which can ensure the vibration reducing function of the electric hammer 121 as much as possible is outputted to the optimum value output section 113.
Method of manufacturing the electric hammer 121 by utilizing the above-described computer-aided manufacturing system 101 will now be explained. Each index utilized in the embodiment is defined as follows:

"ms" is the mass of the striker 138;
"vs" is the speed of the striker 138;
"mc" is the mass of the counter weight 139;
"2r" is the distance between the driving shaft 140 of the counter weight 139 and the mounting portion 139a;
"w" is the angular speed of the crank arm 134;
"θ" is the phase difference between the mounting portion (eccentric pin) of the driver 135b and the mounting portion (eccentric pin) of the counter weight 139 on the crank arm 134;
"M" is the mass of the electric hammer 121;
"t" is the sampling time;
"ANL" is the three-axis combined value of acceleration of the vibration which is caused in the electric hammer 121 under unloaded driving conditions;
"AL" is the three-axis combined value of acceleration of the vibration which is caused in the electric hammer 121 under loaded driving conditions.

In the electric hammer 121, most of vibrations are caused in the axial direction of the hammer bit 137, other than vibration caused by rotation of the crank arm 134 in the horizontal plane. Therefore, in the electric hammer 121, greater importance is placed on its vibration reducing performance with respect to vibration caused by the striker 138 being driven by the crank arm 134 in the axial direction of the hammer bit 137 and applying a strong impact force on the hammer bit 137, compared with vibration caused by rotation of the crank arm 134. Taking this into consideration, in the following description, the above-mentioned three-axis combined values "ANL" and "AL" are substantially identified as acceleration of vibration in the axial direction of the hammer bit 137 in the electric hammer 121.
In the representative embodiment, acceleration of vibration caused by the reciprocating movement of the counter weight 139 is defined as; $(mc / M) \cdot r w^{2} \sin (wt + θ)$
Likewise, acceleration of vibration caused in the electric hammer 121 in the axial direction of the hammer bit 137 under unloaded driving conditions is defined as; $(mc / M) \cdot r w^{2} \sin (wt + θ) + ANL (t;)$

wherein ANL (t) represents acceleration in the axial direction of the hammer bit 137 caused by the reciprocating movement of the crank arm 134, the driving motor 131 or the connecting rod under unloaded driving conditions.
Further, acceleration of vibration which is caused by the reciprocating movement of the striker 135b is defined as; $(1 / M) \cdot d / dt (ms \cdot vs)$
Further, acceleration of vibration which is caused in the electric hammer 121 in the axial direction of the hammer bit 137 under loaded driving conditions: $(1 / M) \cdot d / dt (ms \cdot vs) + (mc / M) \cdot r w^{2} \sin (wt + θ) + AL (t);$

wherein AL (t) represents acceleration in the axial direction of the hammer bit 137 caused by the reciprocating movement of the crank ann 134, the driving motor 131 or the connecting rod under loaded driving conditions.
The electric hammer 121 under loaded driving conditions and the electric hammer 121 under unloaded driving conditions are connected to the acceleration detector 109 of the computer-aided manufacturing system 101 shown in FIG. 2. Two electric hammers 121 under the different driving conditions may be prepared and connected. Alternatively, one electric hammer 121 may be connected and switched between the driving conditions as necessary. In this embodiment, one electric hammer 121 is connected to the computer-aided manufacturing system 101 and measurements are made on the hammer by switching it between loaded and unloaded driving conditions.
In this state, the controller 103 executes the program for determining an optimum value for the counter weight 139, which program is stored in the ROM 105. FIG. 4 shows the procedure of executing the program. In step S1, a value of "mc × r", or a value of the mass "mc" of the counter weight 139 of the electric hammer 121 multiplied by the radius "r" (corresponding half the distance "2r" between the driving shaft 140 of the counter weight 139 and the mounting portion 139a) of the revolving movement of the driving shaft 140 of the counter weight 139, is obtained from the ROM 105. The value is used as an index relating to the kinetic energy of the counter weight 139. Specifically, as mentioned above, the acceleration of vibration which is caused in the electric hammer 121 by the reciprocating movement of the counter weight 139 can be defined as "(mc/M) · rw²sin (wt + θ)", and further, mass "M" of the electric hammer 121 is defined as steady-state value. Therefore, in this embodiment, the value of "mc × r" is used as an index relating to the kinetic energy of the counter weight 139. As shown in FIG. 3, in this embodiment, the value of "mc × r" is stored in the ROM 105 as a variable ranging from 0.0 to 4000.0.
Next, the value of acceleration of vibration in the electric hammer 121 under loaded driving conditions is obtained (step S2). The vibration acceleration value is obtained based on the measured value via the acceleration detector 10 shown in FIG. 2. Likewise, the value of acceleration of vibration in the electric hammer 121 under unloaded driving conditions is obtained (step S3). Specifically, the index (mass and eccentricity) relating to the kinetic energy of the counter weight 139 obtained in step S1 is adopted in the electric hammer 121, and accelerations caused by vibration of the electric hammer 121 under loaded and unloaded driving conditions are measured.
In this manner, accelerations caused by vibration of the electric hammer 121 under loaded and unloaded driving conditions are measured with respect to each of the varied values of "mc × r" relating to the kinetic energy of the counter weight 139. The acceleration values are stored in the RAM 107 one after another. This process is repeatedly continued until the value of "mc × r" reaches 4000.0 (step S4). As a result, the change of accelerations of the electric hammer 121 under loaded and unloaded driving conditions with respect to the value of "mc × r" which is appropriately changed from 0.0 to 4000.0 is outputted to the correlation output section 111 (see FIG. 2) in the form of a graph shown in FIG. 3.
As seen from FIG. 3, the plot (shown in the form of a set of plotted rhombic points in FIG. 3) of change of acceleration of the electric hammer 121 under unloaded driving conditions is shown uprising generally linearly when the value of "mc × r" is gradually increased.
On the other hand, the plot (shown in the form a set of plotted square points in FIG. 3) of change of acceleration in the electric hammer 121 under loaded driving conditions is shown in a slightly downward-curved parabola when the value of "mc × r" is gradually increased.
Based on this correlation of the vibration characteristics of the electric hammer 121 and the kinetic energy characteristics of the counter weight 139, a set value of acceleration is determined in step S5 of the program shown in FIG. 4. In this embodiment, the "set value of acceleration" is related to the acceleration value at the intersection of the plot FL of change of acceleration of the electric hammer 121 under loaded driving conditions and the plot FNL of change of acceleration of the electric hammer 121 under unloaded driving conditions, which plots are obtained by varying the value of "mc × r". This intersection is shown by S in FIG. 3.
As mentioned above, acceleration of the electric hammer 121 under unloaded driving conditions generally linearly increases, while acceleration of the electric hammer 121 under loaded driving conditions shifts in a slightly downward-curved parabola. Therefore, the region of the intersection of the two plots can be defined as a region in which the vibration acceleration of the electric hammer 121 under loaded and unloaded driving conditions can be minimized in a manner of obtaining a greatest common divisor. In this embodiment, the controller 103 (see FIG. 2) determines the set value of acceleration to be approximately 8.0 m/sec² and the value "mc × r" in the region of the intersection to be approximately 1000.0 g· mm.
Next, according to step S6 in FIG. 4. optimum mass and the optimum eccentricity of the counter weight 139 are determined. Specifically, on the premises that the set value of acceleration and the value "mc × r" at the set acceleration value have been determined as mentioned above, the optimum mass and the optimum eccentricity of the counter weight 139 which satisfy the value "mc × r" are determined.
Preferably, the optimum mass and the optimum eccentricity of the counter weight 139 are appropriately determined considering design constraints relating to the distance between the center of the driving shaft 140 of the counter weight 139 and the mounting portion 139a of the counter weight 139 in the electric hammer 121 (or the radius of revolution of the driving shaft 140 of the counter weight 139). In this embodiment, it is determined that the optimum mass is 115 g and the optimum eccentricity is 9 mm, based on the value "mc × r" which has been determined to be approximately 1000.0 g · mm.
By thus configuring the counter weight 139 in the electric hammer 121 based on the above-mentioned design values, vibrations under loaded and unloaded driving conditions can be reduced as much as possible.
According to this embodiment, in designing the electric hammer 121, the optimum values of the mass "mc" of the counter weight 139 and the "eccentricity" of the clank arm 134 with respect to the counter weight 139 are determined based on the magnitude of vibrations caused in the electric hammer 121 under loaded and unloaded driving conditions. Such system can facilitate optimized designing of the vibration reducing mechanism of the electric hammer 121.
In the above-mentioned embodiment, in step S5 shown in FIG. 4, the set value of acceleration is defined as an acceleration value in the region in which the vibration acceleration which acts on the electric hammer 121 under loaded driving conditions is about the same as that under unloaded driving conditions. However, in order to reduce the vibration on electric hammer 121 as much as possible, the set value of acceleration may be determined to be a value in the region in which a difference between the accelerations under loaded and unloaded driving conditions is within a predetermined range.
Alternatively, considering the fact that vibration in the electric hammer 121 is more highly required under loaded driving conditions than unloaded driving conditions, the set value of acceleration may be determined to be a value in the region which is slightly shifted to the side where acceleration under loaded driving conditions is lower than in the region in which acceleration is about the same as that under unloaded driving conditions. For example, in the case as shown in FIG. 3, acceleration under loaded driving conditions is further reduced in the region in which the value "mc × r" is slightly larger than that at the intersection S. Therefore, the set value of acceleration can be determined to be a value in this region.
Further, in this embodiment, the manufacturing method of this invention is applied to an electric hammer, but it may also be applied to other power tools, such as a reciprocating saw.
A modification of the above-described representative embodiment will now be explained with reference to FIG. 5 and 6. In this modification, mean vibration caused in the electric hammer 121 is determined based on the plot FL reflecting the change of acceleration of the electric hammer 121 under loaded driving conditions and the plot FNL reflecting the change of acceleration of the electric hammer 121 under unloaded driving conditions. Specifically, the sum of a value obtained by multiplying the acceleration of the electric hammer 121 under loaded driving conditions by a first constant A and a value obtained by multiplying the acceleration of the electric hammer 121 under unloaded driving conditions by a second constant B is defined as the mean vibration MV in the electric hammer 121. In other words, the mean vibration MV can be obtained by utilizing an equation of "MV = A · FL + B · FNL". Because the vibration reduction is particularly important under loaded driving conditions, the first constant A may preferably be larger than the second constant B. In this modification, as one example, the first constant A and the second constant B are taken as 0.8 and 0.2, respectively. In FIG. 5, both the plot FL under loaded driving conditions and the plot FNL under unloaded driving conditions are shown as approximate plots based on a number of sample points. Further, the mean vibration MV obtained by the above expression is shown by a broken line as a curve of change in relation to the kinetic energy characteristic of the counter weight 139.
Based on the curve of change of the mean vibration MV thus obtained, as shown in FIG. 6, minimum zone in which the mean vibration MV is substantially at the minimum on the curve is determined. In this modification, the range of about 1200 to 1600 g·mm of the set value "mc × r" of acceleration is specifically defined as the minimum zone. When the set acceleration value "mc ×r" is within the minimum zone of about 1200 to 1600 g·mm, the mean vibration MV of the electric hammer 121 is at the minimum of about 7.7 to 7.8 m/sec². In this modification, the optimum set value of acceleration is determined to be within the minimum zone of the value "mc ×r" of about 1200 to 1600 g - mm, and specifically to be 1400 g - mm. Based on such determination, the optimum mass and optimum eccentricity of the counter weight 139 in manufacturing the electric hammer are determined to be 140 g and 10 mm, respectively.
It is explicitly stated that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure as well as for the purpose of restricting the claimed invention independent of the composition of the features in the embodiments and/or the claims. It is explicitly stated that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure as well as for the purpose of restricting the claimed invention, in particular as limits of value ranges.

Description of Numerals

101: manufacturing system
103: controller
105: ROM
107: RAM
109: acceleration detector
111: correlation output section
113: optimum value output section
121: electric hammer
123: body
125: motor housing
127: gear housing
129: tool holder
131: driving motor
132: output shaft
133: motion converting mechanism
134: crank arm
134a: driving shaft
135: striking mechanism
135a: cylinder
135b: driver
137: hammer bit (tool bit)
138: striker
139: counter weight
W: workpiece

Claims

A method of manufacturing a power tool, the power tool including a tool bit that performs a predetermined processing operation to a workpiece by a reciprocating movement, a driving motor for driving the tool bit, a driving force transmitting mechanism that converts a rotating output of the driving motor to a reciprocating movement and transmits the reciprocating movement to the tool bit, and a counter weight that reduces a vibration caused in the power tool by reciprocating in a direction opposite to a component of linear motion of the driving force transmitting mechanism
characterized in that the method includes a step of detennining the mass and travel of the counter weight based on the magnitudes of vibration caused in the power tool under loaded driving conditions in which the tool bit receives a load by performing the processing operation and under unloaded driving conditions in which the tool bit does not receive a load by performing the processing operation, thereby optimizing a vibration reducing mechanism of the power tool.
The method as defined in claim 1 comprising steps of determining a region in which a difference between the magnitudes of vibration under loaded and unloaded driving conditions which vary with kinetic energy of the counter weight is within a predetermined range, and determining the mass and travel of the counter weight according to the value of the kinetic energy of the counter weight in said region.
The method as defined in claim 1 or 2, wherein the counter weight is driven by the rotating output of the driving motor being mechanically converted to a reciprocating movement.
The method as defined in any one of claims 1 to 3 comprising steps of determining mean vibration in the power tool based on the magnitude of vibration which is caused in the power tool under loaded driving conditions and the magnitude of vibration which is caused in the power tool under unloaded driving conditions, and determining the mass and travel of the counter weight based on the mean vibration.
The method as defined in claim 4, wherein the mean vibration is obtained by the sum of a value obtained by multiplying the magnitude of vibration in the electric hammer under loaded driving conditions by a first constant and a value obtained by multiplying the magnitude of vibration in the electric hammer under unloaded driving conditions by a second constant and wherein the first constant is provided to be larger than the second constant.
The method as defined in claim 5, wherein the mass and travel of the counter weight are determined such that the mean vibration is substantially at a minimum.
The method as defined in any one of claims 1 to 6, wherein the driving force transmitting mechanism includes an air spring mechanism and the tool bit performs impulsive and cyclic hammering movement by the action of the air spring.
A power tool comprising a vibration reducing mechanism that includes a counter weight having the mass and travel determined by the method as defined in any one of claims 1 to 7.