Tool driving or impacting – Processes
Reexamination Certificate
2000-01-06
2001-04-10
Smith, Scott A. (Department: 3721)
Tool driving or impacting
Processes
C173S104000, C173S109000, C173S205000
Reexamination Certificate
active
06213222
ABSTRACT:
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
Michael & Best & Friedrich LLP
Milwaukee Electric Tool Corporation
Smith Scott A.
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