Patent Application
20060266537
This invention relates generally to power driven hand tools and more specifically to a rotary impact wrench having an intermittent drive clutch mechanism.
Rotary impact wrenches of the type to which the present invention is related have employed different mechanisms for applying an impact force to an output shaft for turning a fastener element, such as a nut. These impacts develop relatively instantaneously high torque in the output shaft for tightening (or loosening) the fastener elements. Most rotary impact mechanisms include an output shaft including an anvil periodically impacted by hammers. The hammers are typically mounted for motion with respect to the anvil. A clutch mechanism is provided to periodically move the hammers between a position in which the hammers will strike the anvil, and a position in which they are clear of the anvil. When clear of the anvil, the hammers gain speed, and hence momentum, for the next impact with the anvil.
There are presently several types of impact mechanisms. One type of rotary impact wrench, such as shown in U.S. Pat. No. 3,661,217, uses a “swinging weight” mechanism in which hammer dogs are mounted for pivoting about axes parallel to, but spaced from the central axis of the output shaft. A lobe on the output shaft forms the anvil to be struck by the hammer dogs. The hammer dogs, which also rotate around the output shaft, periodically strike the anvil to deliver an impact to the output shaft. In another type of impact mechanism, a spring biases each hammer toward a position in which the hammer is in engagement with the anvil. However, cam balls riding in raceways in a motor driven shaft periodically force the hammers out of engagement with the anvil.
A third type of rotary impact wrench, such as shown in U.S. Pat. No. 2,881,884 and to which the present invention is particularly related, employs a “ski-jump” mechanism in which the output shaft is mounted for free rotation about its longitudinal axis in a tubular cage rotated by a motor about its longitudinal axis. The output shaft has two anvils projecting radially outward in opposite directions. Hammers mounted for rotation with the cage are spring biased axially away from the anvils, but connected to a cam follower for axial motion. A cam ball rotating with the cage periodically engages the cam follower, throwing the hammers forward into registration with the anvils so that they strike the anvils to deliver an impact force for turning the output shaft with a relatively instantaneous high torque.
Some of the prior “ski-jump” clutch mechanisms, such as that disclosed in U.S. Pat. No. 5,199,505, have a cam follower with a surface along which the cam ball travels. The cam follower includes a generally triangular shaped finger projecting radially outward. The sides of the finger serve as ramps so that the cam ball can travel over the finger.
When being used to tighten fastener elements, the output shaft is initially loaded with only a small torque, such as caused by the inertia of the fastener element being turned and the frictional interengagement between the turning and stationary fastener elements. The initial load is insufficient to overcome the force of a spring pushing the hammer pins and cam follower rearwardly. Thus, the cam ball remains engaged with one side of the finger, pushing it around the central longitudinal axis so that the cam follower and output shaft rotate.
As the torque experienced by the output shaft increases, the resistance to rotation of the output shaft and cam follower increases and the axial component of the force exerted by the cam ball on the finger increases until the cam ball is able to drive the cam follower forward far enough to pass over the finger and down the opposite side. Thereafter, the cage and cam ball rotate at high speed until they catch up with the cam follower finger. The cam ball passes over the cam follower finger rapidly, causing the hammer pins to be thrown forward so that the hammer pins are brought into registration with the anvils of the output shaft. Because the cam ball quickly passes the finger, the hammer pins quickly move out of registration with the anvils. Therefore, the hammer pins deliver a quick, sharp impact to the anvils to tighten the fastener element an incremental amount, and then release to regain momentum for the next impact.
However, in the prior designs, the ramps of the fingers are insufficiently shaped to prevent the cam ball from being pushed over the finger and thereby forcing the hammer pins into contact with the anvils when the shaft is minimally loaded or not loaded. As a result, the hammers, anvils, and other components of the tool are unnecessarily subject to stresses that wear on tools components. Moreover, the efficiency of the tool is diminished when hammer prematurely engages the anvil because the rotational speed of the output shaft is reduced.
In one aspect, the present invention is directed generally to a rotary impact tool comprising a housing and an output shaft for tightening and loosening fastener elements. The output shaft includes an anvil projecting radially outwardly from the output shaft. A motor is mounted in the housing for driving the output shaft. At least one hammer is adapted to be driven by the motor. A clutch mechanism can move the hammer to strike the anvil for delivering an impact to the output shaft. The clutch mechanism is connected to the output shaft such that when the output shaft is not loaded the hammer is not positioned to strike the anvil and when the output shaft is loaded the hammer is moved for intermittently striking the anvil. The clutch mechanism includes a cam ball and a cam follower. The cam follower includes a raceway for allowing the cam ball to travel along a surface of the cam follower. The raceway includes a ramp extending to a crest, the ramp being formed to increase resistance to movement of the cam ball as the cam ball nears the crest for inhibiting the cam ball from passing over the crest when the shaft is not loaded while allowing the cam ball to travel over the crest when the shaft is loaded to thereby push the cam follower and hammer axially forward causing the hammer to strike the anvil.
In another aspect, the present invention is directed generally to a clutch mechanism for a rotary impact tool comprising a cam ball and a cam follower. The cam follower includes a raceway for allowing the cam ball to travel along a surface of the cam follower. The raceway includes a ramp extending to a crest, the ramp being formed to increase resistance to movement of the cam ball as the cam ball nears the crest for inhibiting the cam ball from passing over the crest when the tool is not loaded while allowing the cam ball to travel over the crest when the tool is loaded to thereby push the cam follower axially forward to cause a hammer to impact an output shaft.
Other objects and features of the invention will be in part apparent and in part pointed out hereinafter.
Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.
Referring now to the drawings, and in particular to
The output shaft 16 is supported generally coaxially with the cage 14 for rotation relative to the cage by an annular member 30 at the rearward end of the cage, and a bushing 32 fitted in the forward end of the cage. Two wedge-shaped anvils 34, which are formed as one piece with the output shaft 16, project outwardly in radially opposite directions from the output shaft. Each anvil 34 has two generally flat impact surfaces 34A which lie in generally radial planes including the central longitudinal axis of the cage 14. A pair of hammer pins 36 made of cold-forged steel are received in two axially extending guide channels 40 formed in an internal wall of the cage 14. The other two channels 42 seen in
A clutch mechanism indicated generally at 46 intermittently moves the hammer pins 36 axially in the guide channels 40 between a retracted position (
The guide channels 40 are shaped for a close sliding fit with the hammer pins 36 to prevent movement of the pins radially out of the channel or lateral within the channel, and thus substantially restrict the hammer pins to movement longitudinally of the cage 14. As shown in
The rearward end of the cage 14 has a recess 54 which is generally circular and communicates with the opening 26 in the cage. The recess 54 has radially outwardly flaring extensions which define arcuate outer walls of pockets 56. The inner walls of the pockets 56 are defined by the annular member 30 which is positioned coaxially with the central longitudinal axis of the cage 14. As best seen in
A tubular cam follower 68 located forwardly of the annular member 30 fits around the output shaft 16 and is connected by internal splines 70 to splines 72 on the output shaft for conjoint rotation with the output shaft. However, the spline connection leaves the cam follower 68 free to move axially relative the output shaft 16. The cam follower 68 includes a radially outwardly projecting flange 74 which is formed with a finger 76 projecting rearwardly into the recess 54 in the cage 14 where it would be free to rotate in the recess about the central longitudinal axis of the cage 14 but for the presence of the cam ball 58.
As shown in
A thrust ring 80 is adapted for axial movement with the cam follower 68. As shown in
As shown in
In operation, the input shaft 18 of the motor 19 rotates the cage 14. As shown in
When the rotary impact wrench 10 is being used to tighten two fastener elements (not shown), the output shaft 16 is initially loaded with only a small torque resisting its rotation, such as caused by the inertia of the fastener element being turned and the frictional interengagement between the turning and stationary fastener elements. Although the cam ball 58 moves the cam follower 68, thrust ring 80, and hammer pins 36, the axial component of the force exerted by the cam ball on the finger 76 is insufficient to cause the hammer pin 36 to strike the anvils 34. The farther the cam ball 58 is driven up the ramp 94, the greater the slope of the ramp. The axial component of force exerted by the cam ball 58 on the cam follower decreases with the increasing slope of the ramp 94. Thus, a larger resistive torque must be experienced by the output shaft 16 before the cam ball 58 moves over the crest 90 thereby actuating the cam follower 68 to cause the hammer pins 36 to strike the anvil 34. Stated another way, the cam ball 58 experiences increased resistance to movement as it gets closer to the crest 90. The cam ball 58 remains captured by the ramp 94, pushing the finger 76 around the central longitudinal axis such that the cam follower 68 and output shaft 16 rotate with the input shaft 18 of the motor 19 for small increase in resisting torque.
As the fastener element being turned by the output shaft 16 engages the surface (not shown) to which it is being tightened, the torque experienced by the output shaft increases markedly. As the resistance to rotation of the output shaft 16 and cam follower 68 increases, the axial component of the force exerted by the cam ball 58 on the finger 76 increases until the cam ball is able to move up the ramp 94 and drive the cam follower forward far enough to pass over the crest 90 of the finger and down the descent portion 98. The engagement of the cam ball 58 with the ramp 94 is illustrated in
Thereafter, the cage 14 and cam ball 58 rotate at high speed about the central longitudinal axis until they catch up with the cam follower finger 76. The cam ball 58 hits the ramp 94 with at a high momentum, causing the hammer pins 36 to be thrown forwardly with great force against the resisting force of the spring 86 so that the striking surfaces 36A of the hammer pins are brought into registration with the impact surfaces 34A of the anvils 34 of the output shaft 16. Further revolution causes the flat striking surfaces 36A of the hammer pins 36 to impact the flat impact surfaces 34A of the anvils. Because the impact areas engage one another face-to-face over a relatively large area, momentum from the hammer pins and the cage 14 is efficiently transferred to the anvils 34 and output shaft 16. Because the cam ball 58 moves quickly past the crest 90 of the ramp 94, the hammer pins 36 are pushed quickly rearwardly out of registration with the anvils 34. Therefore, the hammer pins 36 deliver a quick, sharp impact to the anvils 34 to tighten the fastener element an incremental amount, and then release to regain momentum for the next impact.
The momentum of the cage 14, which has a significantly greater weight and hence greater momentum than the hammer pins 36, is also efficiently transferred to the anvils 34 because the hammer pins have a close-fitting relationship with the side walls 48 of the channels 40. Thus, rather than moving laterally or radially as a result of the impact with the anvils 34, the hammer pins 36 are held rigid by their close fit with the side walls 48 of the guide channels so that they transfer substantially the full momentum of the cage 14 to the anvils and output shaft 16. The engagement of the hammer pins 36 with the anvils 34 is brief, and a relatively large amount of torque is delivered to the output shaft 16.
The rotary impact wrench 10 of the illustrated embodiment works well at higher air pressures (e.g., above 90 psi up to about 140 psi). At high pressure, the cage 14 rotates so rapidly that the hammer pins 36 impact the anvils 34 before substantial portions of the striking surfaces 36A of the hammer pins move into registration with the impact surfaces 34A of the anvils. Although the area over which the force of the impact is applied to the hammer pins 36 is reduced from the optimum, it is still applied over a flat area of the hammer pin. Moreover, because the hammer pin is closely held in the channel, much of the impact load on the hammer pins 36 is supported by the cage 14. The channels 40 prevent any lateral or radial movement of the hammer pins 36 relative the channels so that stress developed at the notch 82 engaging the rim 81 of the thrust ring 80 is reduced. The provision of a notch on only one side of the hammer pins reduces stress concentration at the notch. Thus, the hammer pins 36 will not merely skip under the anvils 34, which would cause inefficient transfer of momentum and tend to chip the hammer pins. Therefore, the hammer pins 36 have a long operational life even when high pressure is used.
The rotary impact wrench 10 of the present invention provides more reliable and more consistent output than its predecessors. When the rotary impact wrench 10 is loaded with only a small torque resisting its rotation, the cam ball 58 remains captured by the ramp 94 of the cam follower 68 because of the increasing slope of the ramp near the crest 90. As a result, the cam ball 58 consistently pushes the cam follower 67 around the central longitudinal thereby to rotate the output shaft 16. In this first mode of operation (i.e., no or little torque), the output shaft 16 operates with less power but greater speed. The output shaft 16 is maintained constant rotational speed, which is approximately the same speed as the rotational speed of the cage 14. Once the torque experienced by the output shaft increases above a threshold torque, the cam ball is able to drive the cam follower 68 forward far enough to pass over the crest 90 of the finger 76 and down the descent portion 98, which causes the hammer pins 36 to be brought into registration with the anvils 34 of the output shaft 16. Further revolution of the cam ball 58 causes additional impacts to the anvils 34 by the hammer pins 36. In this second mode of operation (i.e, high torque), the output shaft 16 delivers more power but at a lower speed. The rotational speed of the output shaft 16 is this mode is variable and less than that of the rotational speed of the cage 14. Accordingly, the rotary impact wrench 10 of the present invention efficiently tightens and loosens fastening elements.
When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Moreover, the use of “up” and “down” and variations thereof is made for convenience, but does not require any particular orientation of the components.
As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
