Patent Publication Number: US-6213222-B1

Title: Cam drive mechanism

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to power tools and, more particularly, to an impacting drive mechanism for a power tool. 
     A hammer drill is one type of power tool including an impacting drive mechanism or hammer mechanism. Typically, the hammer mechanism includes first and second cam members having mating ratchet surfaces and a spring to bias the cam members and ratchet surfaces out of engagement. An externally applied biasing force is necessary to overcome the spring bias to cause the ratchet surfaces into engagement. Normally, the first cam member is connected to a rotating spindle and is rotated relative to a second cam member rotatably-fixed to the hammer drill housing to provide a ratcheting action. The relative rotation causes the cam member surfaces to slide and cause the second cam member to separate and move axially relative to the first cam member as the external force is overcome. After the apexes of the ratchet surfaces pass one another, the continually applied external biasing force causes the ratchet surfaces to re-engage, providing an impact. 
     A rotary hammer is another type of power tool including a hammer mechanism. This hammer mechanism typically includes a free floating impacting mass pneumatically driven by a reciprocating piston. 
     SUMMARY OF THE INVENTION 
     One problem with the above-described hammer drill is that, typically, the ratchet surfaces have a low angle of rise and, because a high external biasing force is required for effective impacting, a high rotational frictional force is developed, making the hammering operation inefficient. 
     Another problem with the above-described hammer drill is that the cam members generally have a large number of ratchet surfaces (10-20). This reduces the impact energy per blow (due to a large number of impacts for a given amount of input energy). 
     Yet another problem with the above-described hammer drill is that, because the impact-receiving ratchet surfaces are radially spaced from the axis of the spindle and the tool element, the impact energy is transmitted at a radial distance from the axis of the spindle and from the axis of the tool element, resulting in inefficient energy transmission to the tool element. Also, because the impact-receiving ratchet surfaces are angled relative to the axis, a transverse impact force causes an unnecessary moment on the cam members and a further reduction in energy transmission to the tool element. 
     A further problem with the above-described hammer drill is that, to operate effectively and generate impacts, the hammer mechanism requires a substantial axial force be applied by the operator to accelerate the mechanism forward so that contact is maintained between the ratchet surfaces. The operator becomes a part of the hammer mechanism and, as a result, influences the magnitude of the impact energies developed and the frequency of the impacts. For example, if the operator applies an insufficient axial force, some of the ratchet surfaces can be skipped over as the cam members separate and rotate, decreasing the number of impacts per rotation. Also, the operators application of axial force determines the magnitude of the impact energy that can be converted from a given magnitude of input energy. Further, since the axial force applied by the operator is part of the mechanical system, a constant application of a significant axial force and effort is required. 
     Another problem with the above-described hammer drill is that, to allow for rotation of the spindle without hammering action, the hammer mechanism includes a mechanism, generally requiring numerous additional components, to prevent the spindle from moving axially and/or to prevent the ratchets from contacting while the spindle rotates. These additional components increase the cost and complexity of the hammer mechanism. 
     Yet another problem with the above-described hammer drill is that, typically, the rotational speed and torque of the spindle for hammering and drilling in masonry materials is inappropriate for large accessories used for other materials. As a result, a secondary gear set, for speed and torque selection by the operator, is necessary as an option in the hammer drill. Misuse of this option can reduce the performance of the accessory and reduce the life of the hammer mechanism. 
     A further problem with the above-described hammer drill is that, because one of the cam members is rotatably fixed, the number of impacts per spindle rotation and the resulting impact pattern on the workpiece, with a given tool element, is determined solely by the number of ratchet teeth. The combination of impact pattern, frequency and energy cannot be optimized for cutting of the material of the workpiece. 
     One problem with the above-described rotary hammer is that the rotary hammer is more expensive to manufacture and maintain. The hammering mechanism of the rotary hammer has more critical components and is more complex and therefore is more susceptible to mechanical failure. The hammering mechanism of the rotary hammer requires the high precision and prevention of contamination typical of these systems. 
     Another problem with the above-described rotary hammer is that part of the hammer mechanism, such as a slider crank, wobble plate or other secondary hammer drive mechanism, contributes to the overall mechanism being relatively large and cumbersome. 
     Yet another problem with the above-described rotary hammer is the impact force is dependent on the speed of the motor. Specifically, when the motor speed is reduced, the speed of the piston and the force applied to the impacting mass are reduced. As a result, at lower motor speeds, the impact force of the hammering mechanism is reduced. Such low speed operations may occur when the operator reduces the motor speed to conduct detailed hammering or to operate on a fragile workpiece. Lower speed operations may also result when operating in a cordless mode on battery power (as compared to operations in a corded mode). 
     The present invention provides a drive mechanism for a power tool that alleviates the problems with the above-described hammer drill and rotary hammer. The present invention provides a drive mechanism including a drive mechanism housing connectable to the housing of the power tool, a first cam member, a second cam member and a gear assembly for drivingly connecting the first cam member and the second cam member to the drive shaft for counter-rotation. The first cam member and the second cam member each have a plurality of cam surfaces, the cam surfaces being oriented at a steep angle with respect to the axis of the tool element, each of the cam surfaces being complementary and engageable. The second cam member includes an impacting surface for engaging the tool element to provide an impact. 
     As the cam members counter-rotate, the cam surfaces engage so that the second cam member is axially moved in a direction relative to the first cam member. As the cam members continue to counter-rotate, the cam surfaces disengage so that the second cam member is axially moved in an opposite direction relative to the first cam member to provide an impact on the tool element. 
     Preferably, each cam member includes at least one cam surface, and, with the minimum or maximum number of cam surfaces being determined by the response of the spring and mass system for a given input that results in impact energy transfer to the tool element before the cam surfaces re-engage. The cam surfaces are preferably oriented at between 30° and 60° with respect to the axis of the tool element. 
     Also, the cam members are counter-rotated relative to one another at a rate of counter-rotation. The gear assembly may include a first gear drivingly connected to the first cam member and a second gear drivingly connected to the second cam member. In addition, the rate of counter-rotation of the cam members is selectable to change the impact pattern of the cutting tooth of the tool element in the workpiece. 
     Preferably, the drive mechanism is formed as a modular assembly and is connected to the housing of the power tool and to the motor. 
     The drive mechanism preferably further comprises a spring for biasing the cam members into engagement, and a spring housing supporting the spring and the second cam member, the spring being between the spring housing and the second cam member. The spring housing is preferably rotatably supported by said housing and connected between the gear assembly and the second cam member. The drive mechanism may further comprise a striker member supported force transmitting relation to the tool element and having an impact-receiving surface engageable by the impacting surface of the second cam member. Preferably, before the cam surfaces re-engage, the impacting surface impacts the impact receiving surface to provide an impact to the tool element. 
     The drive mechanism may further comprise a preventing mechanism to prevent the drive mechanism from imparting axial motion on the tool element, said preventing mechanism being operable to one of selectively disconnect one of the cam members from the drive shaft. 
     Also, the present invention provides a power tool including a housing, a motor supported by the housing and connectable to a power source, the motor including a rotatably driven drive shaft, a support member supported by the housing, the support member being adapted to support a tool element so that the tool element is movable relative to the housing, the tool element having an axis and being driven by the power tool to work on a workpiece, and a drive mechanism connectable to the drive shaft and operable to impart an axial motion on the tool element. 
     In addition, the present invention provides a method of optimizing a power tool. The method includes selecting a first gear ratio between the first cam member and the drive shaft, selecting a second gear ratio between the second cam member and the drive shaft, and changing one of the first gear ratio and the second gear ratio to optimize the impact pattern of the cutting tooth of the tool element on the workpiece. 
     One advantage of the present invention is that, because of the steeper angle of rise of the cam surfaces on the cam members, the hammer mechanism provides a higher mechanical efficiency due to more efficient cam angles. 
     Another advantage of the present invention is that due to the fewer number of cam surfaces, compared to the number of ratchet surfaces in a typical hammer drill, a given amount of rotational energy can be converted to a higher energy per impact (due to fewer impacts for a given period of time). 
     Yet another advantage of the present invention is that, because the impacting projection of the impacting cam extends along the axis of the spindle and along the axis of the tool member, the longitudinal impacts are provided along the axis of the hammer mechanism and the tool element, decreasing the impact energy lost from off axis and transverse forces. 
     A further advantage of the present invention is that a lower axial force is required to generate higher impact energies because the energy developed is stored in a spring. This results in less operator exertion. In addition, the operator&#39;s link to the hammer mechanism is softened by the spring and through various cushioning interfaces throughout the hammer mechanism. Also, the axial force that must be supplied by the operator to achieve optimum performance is minimized. 
     Another advantage of the present invention is that the hammer mechanism is more compact than other conventional hammer mechanisms, such as those employing a slider crank or a wobble plate or requiring a secondary system to drive the hammer mechanism. The drive system of the hammer mechanism of the present invention, in power tools including a rotary drive system, is coupled to the spindle through the rotary drive system. Also, the hammer mechanism can be employed with power tools providing only axial hammering impacting motion or providing both axial hammering motion with spindle rotation or providing only spindle rotation. In addition, the hammer mechanism is provided in a modular assembly which is connectable with a motor housing and motor of a power tool to replace another hammering mechanism. 
     Yet another advantage of the present invention is that the means for selecting the operating mode, such as hammering with spindle rotation or spindle rotation only, is easily accomplished, and the hammering mechanism does not require numerous additional components for mode selection. As a result, the power tool and the hammering mechanism of the present invention are simpler and less expensive to manufacture and maintain. 
     A further advantage of the present invention is that if rotation of the spindle is necessary without hammering motion, the speed and torque of the spindle is appropriate for applications requiring larger accessories in materials other than concrete or masonry. 
     Another advantage of the present invention is that, if hammering and spindle rotation is necessary, the parallel drive path allows for optimization of an indexing ratio, controlling the degree of angular rotation of the spindle between impacts. Because the indexing ratio can be optimized, the impact pattern of the tool element on the workpiece can be controlled and optimized for the tool element and the material of the workpiece. 
     Yet another advantage of the present invention is that, because the spindle is axially fixed, the spindle can accommodate a chucking device for grasping smooth shank tool elements, other accessory capturing devices, and other accessories that are common in the industry without the requirement of a special adapter. 
     A further advantage of the present invention is that the hammer mechanism is less complex and more durable than the hammer mechanism of the rotary hammer. 
     Another advantage of the present invention is that the impact force of the present hammer mechanism is substantially independent of the speed of the motor. The impact force is related to the biasing force of the spring and the mass of the impacting cam. As a result, at any speed, the impact force of the present hammer mechanism is substantially constant. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a power tool including a hammer mechanism embodying the invention. 
     FIGS. 2A-D are perspective views of the hammer mechanism shown in FIG.  1  and illustrating the operation of the hammer mechanism. 
     FIG. 3 is an exploded perspective view of a portion of the hammer mechanism shown in FIG.  2 A. 
     FIG. 4 is a perspective view of the hammer mechanism shown in FIG.  2 A and illustrating the hammer mechanism in a mode without hammering action. 
     FIG. 5 is a perspective view of a first alternative construction of the hammer mechanism shown in FIG. 2A with portions cut away. 
     FIG. 6 is a perspective view of a second alternative construction of the hammer mechanism shown in FIG. 2A with portions cut away. 
     FIG. 7 is a perspective view of a third alternative construction of the hammer mechanism shown in FIG. 2A with portions cut away. 
     FIGS. 8A-B illustrate exemplary impact patterns on a workpiece created by a tool element driven by the hammer mechanism. 
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A power tool  10  including a cam drive hammer mechanism  14  embodying the invention is illustrated in FIG.  1 . As explained in more detail below, the hammer mechanism  14  is operable to drive a tool element  18  for reciprocating, impacting or hammering movement along an axis  22 . It should be understood that the power tool  10  can be any type of power tool in which the tool element  18  is driven for axial movement. Such power tools include chippers, nailers, hammer drills, rotary hammers, chipping hammers and, in general, impacting devices. It should be understood that the power tool  10  can also include a mechanism to drive the tool element  18  for rotary motion about the axis  22 . In the illustrated construction, the power tool  10  is operable to, in one mode, drive the tool element  18  for both a rotary or drilling motion and a reciprocating or hammering motion. In the illustrated construction, the tool element  18  includes at least one carbide or cutting tooth  24 , and preferably, at least two cutting teeth  24   a  and  24   b.    
     The power tool  10  includes a motor housing  26  having a handle portion  30 . A reversible electric motor  34  (schematically illustrated) is supported by the motor housing  26 . An on/off switch  38  is supported on the handle  30  and is operable to connect the motor  34  to a power source (not shown). The motor  34  is operable to rotatably drive a drive shaft  42  (partially shown in FIG.  1 ). 
     The power tool  10  also includes (see FIG. 1) a forward housing  46  supporting the hammer mechanism  14 . An auxiliary side handle  50  is supported on the forward housing  46 . In the illustrated construction, the auxiliary handle  50  is of a band clamp type and is releasably secured about the forward housing  46 . 
     In the illustrated construction, the forward housing  46  surrounds the hammer mechanism  14  to provide a modular hammer mechanism assembly  52 . The modular hammer mechanism assembly  52  is connected to the motor housing  26  and the motor  34  to form the power tool  10 . It should be understood that, in other constructions (not shown), the power tool  10  may be formed as a single unit including a non-modular hammer mechanism (similar to hammer mechanism  14 ) and a forward housing (similar to forward housing  52 ). 
     The hammer mechanism  14  includes (see FIG. 2A) a gear assembly  54 . A pinion shaft  58  is drivingly connected to the drive shaft  42 . The pinion shaft  58  drives an intermediate gear  66  fixed to an intermediate shaft (not shown). An intermediate pinion  70  is also fixed to the intermediate shaft and is driven with the intermediate gear  66  at the same rotational speed and in the same direction. 
     The gear assembly  54  also includes a spindle gear  74  fixed to a rotatable spindle  78 . Spindle gear  74  is driven by intermediate pinion  70 . The spindle  78  is supported by bearings  60  and  61  so that the spindle  78  is rotatable but axially immovable. The spindle  78  is generally hollow and, within its forward portion, defines a plurality of axially-extending splines  80 , the purpose for which is explained in more detail below. 
     The gear assembly  54  also includes an idler gear  82  fixed to an idler shaft  86 . Idler gear  82  is also driven by intermediate pinion  70 . An idler pinion  90  is also fixed to the idler shaft  86  so that the idler gear  82 , the idler shaft  86  and the idler pinion  90  rotate in the same direction and at the same speed. 
     The gear assembly  54  also includes a housing gear  94  fixed to a rotatable spring housing  98 . The housing gear  94  is driven by the idler pinion  90 . In this manner, the spring housing  98  and the spindle  78  rotate in opposite directions, i.e., counter-rotate. The spring housing  98  defines a plurality of axial slots  100 , the purpose for which is explained in more detail below. 
     The hammer mechanism  14  also includes (see FIGS. 2A and 3) a drive cam  102  supported by the spindle  78 . In the illustrated construction, the drive cam  102  is axially fixed within the spindle  78  and, as explained in more detail below, is rotatable, in some instances, with the spindle  78 . In the illustrated construction, a central opening  104  is defined by the drive cam  102 . The purpose for the opening  104  is explained in more detail below. 
     The drive cam  102  includes at least one and, preferably, a plurality of cam driving surfaces  106 . In the illustrated construction, the drive cam  102  has two cam driving surfaces  106 . The cam driving surfaces  106  are helical in shape and have a relatively steep angle, i.e., greater than 30° and less than 65°, with respect to the axis  22 . Preferably, the cam driving surfaces  106  are angled at least 35° with respect to the axis  22 . The drive cam  102  also includes a plurality of ratchet members  110  facing opposite the cam driving surfaces  106 . The purpose for the ratchet members  110  is explained in more detail below. 
     The hammer mechanism  14  also includes an impacting cam  114 . The impacting cam  114  is supported by the spring housing  98  so that the impacting cam  114  is rotatable with the spring housing  98 . The impacting cam  114  is also axially movable relative to the spring housing  98 . The impacting cam  114  includes a plurality of lateral projections  118  which extend into respective axial slots  100  formed in the spring housing  98 . The lateral projections  118  and the axial slots  100  cooperate so that the impacting cam  114  is rotatably fixed to the spring housing  98 . 
     The impacting cam  114  also includes cam surfaces  122  which are complementary to, mate with and conform to the cam driving surfaces  106  on the drive cam  102 . The cam surfaces  122  are also helical in shape and also have a relatively steep angle, i.e., greater than 30° and less than 65°, with respect to the axis  22 . Preferably, the cam surfaces  122  are angled at least 35° with respect to the axis  22 , the same angle as the cam driving surfaces  106 . The cam surfaces  106  and  122  are configured to slide against one another when the drive cam  102  is rotated in the direction of arrow A (in FIG. 2A) while the impacting cam  114  is counter-rotated in the direction opposite to arrow A. 
     It should be understood that, in the illustrated construction, both the drive cam  102  and the impacting cam  114  are rotated and, preferably, are counter-rotated relative to one another. However, in some constructions (not shown), only one of the drive cam  102  and the impacting cam  114  may be rotated. Also, in some other constructions (not shown), the drive cam  102  and the impacting cam  114  may be rotated in the same direction but at different rates of rotation. 
     The impacting cam  114  also includes (see FIGS. 2B,  2 D and  3 ) a forwardly extending impacting projection  126  having an impacting surface  130 . The impacting cam  114  is supported so that the impacting projection extends into the opening  104  in the drive cam  102 . Preferably, the impacting surface  130  is substantially perpendicular to and centered on the axis  22 . 
     The hammer mechanism  14  also includes (see FIG. 2A) a spring  134  positioned between the spring housing  98  and the impacting cam  114 . The spring  134  biases the impacting cam  114  forwardly into engagement with the drive cam  102 . The spring  134  is axially restrained and has a small amount of preloading. 
     The hammer mechanism  14  also includes (see FIGS. 2A and 3) a striker  138 . The striker  138  is rotatably coupled to the spindle  78 . In the illustrated construction, the striker  138  includes a plurality of axially-extending splines  142  which are engageable with the splines  80  formed on the spindle  78  so that the striker  138  rotates with the spindle  78  but is axially movable relative to the spindle  78 . 
     A plurality of ratchet members  146  are formed on the rear surface of the striker  138 . The ratchet members  146  are engageable with ratchet members  110  of the drive cam  102 . In the construction shown in FIG. 3, the ratchet members  146  and  110  are configured so that, when the striker  138  is driven in the direction of arrow A (in FIG.  2 A), the ratchet members  146  and  110  are drivingly engaged and the drive cam  102  rotates with the striker  138  and with the spindle  78 . When the striker  138  is rotated in the direction opposite to arrow A (in FIG.  2 A), the ratchet members  146  and  110  do not drivingly engage but slide over one another so that the drive cam  102  does not rotate with the striker  138  and the spindle  78 . In the illustrated construction, the striker  138  defines a circumferential groove  148 , the purpose of which is explained in more detail below. 
     The striker  138  has (see FIGS. 2B,  2 D and  3 ) a rearwardly-extending impacting projection  150  having an impact-receiving surface  152 . The impact-receiving surface  152  is complementary to and engageable with the impacting surface  130  on the impacting projection  126 . Preferably, the impact-receiving surface  152  is also substantially perpendicular to and centered on the axis  22 . In the illustrated construction, the impact projection  150  extends into the opening  104  formed in the drive cam  102 . 
     The impacting projections  126  and  150  have a sufficient length so that, during an impact, the impacting projections  126  and  150  impact before the cam surfaces  106  and  122  re-engage. This ensures that no energy loss occurs due to transverse forces. Also, because the impacting projections  126  and  150  are centered on the axis  22 , impact energy is transmitted efficiently. Also, impacting cam  114  and spring  114  have a spring and mass relationship to cause impacting cam  114  to achieve the acceleration and impact velocity necessary to ensure that impact occurs before cam surfaces  106  and  122  re-engage as drive cam  102  and impacting cam  114  counter-rotate. 
     The hammer mechanism  14  also includes (see FIGS. 2A and 4) a mechanism  154  for disengaging the hammering mode. The mechanism  154  includes a plurality of balls  158  engageable with the groove  148  formed in the striker  138 . The balls  158  are supported in radial openings  162  formed in the spindle  78 . The mechanism  154  also includes a rotatable locking collar  166  having a locking cam surface  170  formed on its inner surface and defining positions  170   a  and  170   b . An axially-movable cam rider  174  is positionable in the positions  170   a  and  170   b . Portions of the cam rider  174  extends through openings  176  formed in the forward housing  46  to engage an axially-movable locking ring  178 . A spring  180  biases the mechanism  154  to a position in which the cam rider  174  is in position  170   a.    
     In the position shown in FIG. 2A, the hammer mechanism  14  is in the hammer mode. The cam rider  174  is in position  170   a , and the locking ring  178  is positioned to allow the balls  158  to extend through the openings  162 . The balls  158  do not engage the groove  148  formed in the striker  138 , and the striker  138  is free to engage the drive cam  102  so hammering is provided. The geometry of groove  148  facilitates balls  158  to move out of groove  148  and into openings  162 . 
     To disengage the hammer mode, the tool element  18  is lifted from the workpiece W. As shown in FIG. 4, the spring  134  forces the impacting cam  114  and the striker  138  forwardly so that the groove  148  is aligned with the balls  158  and the openings  162 . The locking collar  166  is rotated so that the cam rider  174  moves to position  170   b . In this position, the locking ring  178  covers the openings  162  and forces and restrains the balls  158  into the groove  148 . The striker  138  cannot engage the drive cam  102 , and the drive cam  102  does not counter-rotate relative to the impacting cam  114 . Hammering action is thus prevented. 
     To re-engage the hammer mode (see FIG.  2 A), the locking collar  166  is rotated so that the balls  158  can move out of the groove  148 . 
     The power tool  10  also includes (see FIG. 2A) a support member or chucking device  182  for supporting the tool element  18 . The chucking device  182  is supported by the spindle  78  for rotation with the spindle  78 . The chucking device  182  may be any type of chucking device capable of securely holding the tool element  18  during operations including drilling only, hammering only, or both drilling and hammering. In the illustrated construction, the chucking device  182  permits limited axial movement of the tool element  18  relative to the chucking device  182 . 
     In operation, the motor  34  rotatably drives the drive shaft  42  in a forward mode. The drive shaft  42  drives the gear assembly  54  so that the spindle  78  rotates in the direction of arrow A and so that the spring housing  98  and the impacting cam  114  counter-rotate. The striker  138 , the chucking device  182  and the tool element  18  rotate with the spindle  78 . In the mode shown in FIG. 4, the drive cam  102  is disengaged from the striker  138  and does not rotate with the spindle  78 . Instead, the drive cam  102  rotates with the impacting cam  114 . 
     The operator selects the hammering mode by rotating the locking collar  166  to allow the balls  158  to move out of the groove  148 . The striker  138  is now free to move axially. When the operator engages the tool element  18  against the workpiece W, the tool element  18  is pushed rearwardly against the striker  138  (as shown in FIG.  2 A). The striker  138  is forced rearwardly so that the ratchet members  110  and  146  engage. As a result, the drive cam  102  now rotates with the striker  138  and the spindle  78 . Continued counter-rotation of the spring housing  98  and the impacting cam  114  causes the cam surfaces  106  and  122  to slide against one another. The impacting cam  114  is forced rearwardly (from the position shown in FIG. 2A to the position shown in FIG. 2C) against the biasing force of the spring  134 . 
     As the drive cam  102  and the impacting cam  114  continue to counter-rotate, the cam surfaces  106  and  122  eventually move past their respective apexes and disengage (see FIG.  2 C). As a result, the impacting cam  114  is released, and the spring  134  forces the impacting cam  114  forwardly. As shown in FIG. 2D, the impacting surface  130  slams into the impact-receiving surface  152  on the striker  138 , and the striker  138  transmits the impact to the tool element  18 . After the impact, the cam surfaces  106  and  122  re-engage (as shown in FIG.  2 A). The drive cam  102  and the impacting cam  114  continue to counter-rotate to cause the next impact. 
     If the motor  34  is reversed to drive the drive shaft  42  in an opposite or reverse direction, the spindle  78  and the striker  138  are driven in the direction opposite to arrow A, and the spring housing  98  and the impacting cam  114  driven in the direction of arrow A. Because of the configuration of the ratchet members  110  and  146 , the drive cam  102  does not rotate with the spindle  78  and the striker  138 , and the normal impacts are not generated by the hammer mechanism  14 . Also, in this mode, the hammer mechanism  14  is usually placed in the non-hammering mode by the preventing mechanism  154  (i.e., in the mode shown in FIG.  4 ). 
     When the operator disengages the tool element  18  from the workpiece W, the striker  138  moves forwardly under the biasing force of the spring  134 . The striker  138  and the drive cam  102  do not engage so the hammer mechanism  14  does not provide hammering. The hammer mechanism  14  may be prevented from moving to the hammer mode (ie., by moving the hammer mechanism  14  to the position shown in FIG.  4 ). To prevent the hammer mechanism  14  from being moved to the hammer mode, the locking collar  166  is rotated so that the balls  158  engage in the groove  148 . The locking ring  178  prevents the balls from moving out of the groove  148 . The striker  138  is thus prevented from moving rearwardly to engage the drive cam  102 . 
     During hammering operations, the tool element  18  is rotated through a given degree of angular rotation between impacts. This continuing rotation, in combination with the number of cutting teeth  24  formed on the tool element  18 , results in the creation of an impact pattern in the workpiece W. 
     The resulting impact pattern is a finction of the number of cutting teeth  24  on the tool element  18  and the rate of counter-rotation between impacts of the drive cam  102  relative to the impacting cam  114 . With a tool element  18  having a selected number of cutting teeth  24 , the resulting impact pattern can be selected to provide an optimal impact pattern for the material of the workpiece W by changing the rate of counter-rotation of the drive cam  102  and the impacting cam  114 . The rate of counter-rotation can be adjusted by changing the gear ratio between the drive cam  102  and the drive shaft  42  and/or the gear ratio between the impacting cam  114  and the drive shaft  42 . 
     FIG. 5 illustrates a first alternative construction for a hammer mechanism  14 ′ embodying the invention. Common elements are identified by the same reference numbers “′”. 
     In this construction, the need for the ratchet members  110  and  146 , formed on the drive cam  102  and the striker  138 , respectively, is eliminated. Instead, straight-sided driving members  186  and  190  are formed on the drive cam  102 ′ and the striker  138 ′, respectively. Also, the idler gear  82 ′ is fixed to a roller clutch  194 . The roller clutch  194  only transmits torque in the direction of arrow B (in FIG. 5) and overruns in the other direction. When the motor  34 ′ (not shown) is reversed, the spindle  78 ′ rotates in the direction opposite to arrow A′. The striker  138 ′ and the drive cam  102 ′ rotate with the spindle  78 ′. In this direction, the roller clutch  194  slips so that the spring housing  98 ′ and the impacting cam  114 ′ are not driven. Instead, the impacting cam  114 ′ is driven in the same direction by the drive cam  102 ′, and impacts are not generated by the hammer mechanism  14 ′. 
     FIG. 6 illustrates a second alternative construction for a hammer mechanism  14 ″ embodying the invention. Common elements are identified by the same reference numbers ‘″’. 
     In this construction, the drive cam  102 ″ and the striker  138 ″ (not shown but similar to drive cam  102 ′ and striker  138 ′ shown in FIG. 5) include straight-sided driving members (not shown but similar to driving members  186  and  190  shown in FIG.  5 ). As shown in FIG. 6, the idler gear  82 ″ is freely rotatable but axially fixed on the idler shaft  86 ″. A shifter  198  is fixed to the roller clutch  194 ″ so that the shifter  198  transmits torque in the direction of arrow B″ and overruns in the other direction. The idler gear  82 ″ and the shifter  198  include inter-engaging driving projections  202  and  206 , respectively. The shifter  198  is movable on the idler shaft  86 ″ so that the projections  202  and  206  are engageable. 
     When the projections  202  and  206  are engaged, the idler gear  82 ″ transmits torque to the idler shaft  86 ″ only in the direction of arrow B″. When the spindle  78 ″, the striker  138 ″ and the drive cam  102 ″ are driven in the direction of arrow A″, the impacting cam  114 ″ (not shown but similar to impacting cam  114 ′) is counter-rotated, and hammering action is provided. When the spindle  78 ″ is rotated in the opposite direction, the impacting cam  114 ″ is not counter-rotated, and no hammering action is provided. 
     When the projections  202  and  206  are disengaged, the idler gear  82 ″ freely rotates on the idler shaft  86 ″. When the spindle  78 ″ is rotated in either direction, the impacting cam  114 ″ is not counter-rotated, and no hammering action is provided. 
     FIG. 7 illustrates a third alternative construction for a hammer mechanism  14 ′″. Common elements are identified by the same reference numbers “′″”. 
     In this construction, the striker  138 ′″ includes a forward projection  210  having axially-extending splines  214 . A chucking device  182 ′″ includes mating axial splines  218  and is mounted directly on the forward projection  210  of the striker  138 ′″ so that the chucking device  182 ′″ is axially fixed to the striker  138 ′″. The splines  214  and  218  ensure that rotary motion is transmitted from the striker  138 ′″ to the chucking device  182 ′″ and to the tool element  18 ′″. 
     Various features of the invention are set forth in the following claims.