Portable Impact Wrench

Abstract

A portable impact wrench wherein a crank assembly reciprocates a cylinder which propels an impeller against the rear end of a tool holder in response to each of its forward strokes. The tool holder is rotatable stepwise in a single direction by a group of three sleeves the first of which is rotatable on but moves lengthwise with the cylinder and is normally held against rotation by a claw clutch one element of which is fixed to the housing. An intermediate sleeve has inclined teeth or threads meshing with complementary threads or teeth of the first sleeve so that the intermediate sleeve rotates back and forth in response to rearward and forward movements of the first sleeve. A third sleeve is rotatable in the housing and is connected with the intermediate sleeve by a one-way clutch. The tool holder is movable axially in but shares all angular movements of the third sleeve. Another element of the claw clutch is biased against the fixed element by a helical spring and has axially parallel teeth mating with complementary teeth of the first sleeve so that the latter can move axially but is normally held against rotation. The bias of the spring may be changed, terminated and/or assisted by a projection which can be manipulated by hand so that the user can apply his muscle power in order to increase the resistance of the other clutch element to disengagement from the fixed clutch element when the tool holder resists rotation with the third sleeve. When the claw clutch is disengaged and the cylinder continues to move forwardly and rearwardly, the intermediate sleeve rotates the first sleeve.

Description

BACKGROUND OF THE INVENTION

The present invention relates to portable power tools in general, and more particularly to improvements in portable impact wrenches, especially electrically powered impact wrenches wherein the tool holder is movable axially and is also rotatable during penetration into a workpiece or during the application or removal of a nut, screw, bolt or the like.

It is already known to provide a portable impact wrench with a drive wherein an electric motor operates a crank assembly which reciprocates an output member. The latter transmits motion to an axially movable impeller which strikes against the tool holder during or in response to each forward stroke of the output member. The means for rotating the tool holder includes a first sleeve which rotatably surrounds the output member, a one-way clutch which transmits torque from the first sleeve to the tool holder while the first sleeve rotates in a given direction, and a device which rotates the first sleeve in response to reciprocation of the output member. This device has a second sleeve which is provided with inclined teeth or threads mating with complementary teeth or threads on the first sleeve. The second sleeve is rigid with or forms part of the output member and tends to rotate the output member whenever the first sleeve rotates whereby the parts of the crank assembly must stand very high torsional stresses resulting in pronounced wear and relatively short useful life of the drive. In other words, the drive must take up and stand all such stresses which develop when the tool in the tool holder is rotated against the resistance of a material in which the tool rotates or against resistance of a nut, bolt or screw which must be driven home or removed. The torsional stresses to which the drive is subjected increase very substantially when the tool jams in or on a workpiece while the output member continues to move back and forth and to cause the impeller to strike against the tool holder. Furthermore, the just described impact wrenches are likely to injure the operator, especially a weak person who is incapable of holding the housing with a requisite force while the tool offers a very high resistance to rotation with its holder.

It was already proposed to connect the second sleeve with the reciprocable output member of the drive by means of a safety clutch which becomes disengaged in response to a preselected maximum resistance which the tool holder offers to rotation during forward movement of the output member. When the safety clutch is disengaged, the impact wrench operates simply as a hammer wherein the tool moves axially in but does not rotate relative to the housing as long as the safety clutch remains disengaged. As a rule, the safety clutch is designed or adjusted to become disengaged in response to a relatively low torque to thus insure that the impact wrench can be properly manipulated by a relatively weak person, e.g., an adolescent or a tinker using the power tool once an a while in his basement workshop or garage. The lower the torque which is needed to disengage the safety clutch, the more frequent is the disengagement of this clutch so that its parts undergo excessive wear. Moreover, the drive is still called upon to take up the torsional stresses because the safety clutch is provided between the reciprocable output member and the second sleeve.

SUMMARY OF THE INVENTION

An object of the invention is to provide a portable impact wrench, especially an impact wrench which can be used as a drill, screwdriver and/or for analogous purposes, and whose drive comprises an electric motor serving to transmit motion to a reciprocable output member of the drive by way of a crank assembly, wherein the torque which develops when the tool and its holder rotate or resist rotation need not be taken up by the drive so that the latter is subjected to less pronounced wear than in heretofore known power tools.

Another object of the invention is to provide a portable impact wrench whose operation is safer than that of conventional impact wrenches and which is more versatile than the aforediscussed conventional wrenches.

A further object of the invention is to provide in a portable impact wrench novel and improved means for rotating the tool in stepwise fashion and novel and improved means for preventing damage to the impact wrench when its tool jams or is held against angular movement for other reasons.

An additional object of the invention is to provide a portable impact wrench with novel and improved means for selecting the resistance which the tool holder must offer to rotation in order to deactivate the torque transmitting means for the tool holder.

Still another object of the invention is to provide a portable impact wrench with a device which enables the operator to determine, at his will and with his own muscle power, the exact magnitude of resistance which the tool holder must offer to rotation in order to terminate its angular movement while the output member of the drive continues to actuate the impeller which propels the tool forwardly.

A further object of the invention is to provide a portable impact wrench wherein the torque transmitting connection between the drive and the tool holder can be established, adjusted and/or terminated at the will of the operator and while the impact wrench is in actual use.

The invention is embodied in a portable impact wrench, drill, screwdriver or the like which comprises a housing, drive means provided in the housing and including a driving member or output member which is movable forwardly and rearwardly (preferably in response to rotation of the rotor of an electric motor which moves the output member by way of a crank assembly), a tool holder which is rotatably and reciprocably mounted in the housing in front of the output member, impeller means provided in the housing and being operative to strike against an anvil at the rear end of the tool holder in response to forward movements of the output member, and torque transmitting means for normally rotating the tool holder in response to forward movements of the output member.

The torque transmitting means comprises an input element which rotatably surrounds and is movable with the output member forwardly and rearwardly, blocking means provided in the housing for normally holding the input element against rotation (the blocking means preferably comprises a claw clutch a first element of which is fixedly mounted in the housing and a second element of which is provided with axially parallel teeth in mesh with complementary teeth of the input element so that the latter can move axially of the second clutch element, and biasing means including one or more springs and/or a manually held part for holding the second clutch element in engagement with the first clutch element), an intermediate element which is rotatably mounted in the housing and has an annulus of inclined teeth or threads mating with complementary inclined teeth or threads of the input element so that the intermediate element rotates in opposite directions in response to forward and rearward movements of the output member as long as the blocking means holds the input element against rotation, an output element which is rotatably mounted in the housing in such a way that the tool holder can move axially of but cannot rotate relative to the output element, and one-way clutch means provided between the intermediate element and the output element. The blocking means allows the intermediate element to rotate the input element relative to the output member of the drive means when the tool holder offers such resistance to rotation that the blocking means permits the input element to rotate on the output member.

Since the blocking means normally establishes a connection between the housing and the input element, the drive means is not required to take up any torsional stresses irrespective of whether the tool holder rotates or is held against rotation, for example, due to jamming of the tool in or on a workpiece.

The impact wrench preferably further comprises a device for changing the bias of the aforementioned spring or springs; such device may constitute or include the aforementioned manually held part so that the operator can apply his muscle power in order to assist the spring or springs in preventing disengagement of the safety clutch until the resistance which the tool holder offers to rotation reaches a value which might render further operation of the impact wrench dangerous.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved impact wrench itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a fragmentary partly elevational and partly longitudinal vertical sectional view of a portable impact wrench which embodies the invention; and

FIG. 2 is a sectional view as seen in the direction of arrows from the line II--II of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The impact wrench of FIG. 1 comprises a housing or support 1 including a handle 1a and a cylindrical barrel 1b. The rear portion of the housing 1 contains a motor M (e.g., an electric motor) forming part of a drive which further includes a reciprocable tubular output member or cylinder 6 movable forwardly and rearwardly in the barrel 1b. The rotor R of the motor M reciprocates the cylinder 6 by way of a crank assembly including a rotary crank arm 5 having a crank pin 5a and a connecting rod 10 which is mounted on the crank pin 5a at one end and the other end of which is coupled to the rear end of the cylinder 6 by means of a vertical shaft 9. The shaft 9 is mounted in a rear end wall 11 of the cylinder 6. The reference character 8 denotes a cylindrical bore provided in the barrel 1b for the cylinder 6. The latter comprises a larger-diameter rear portion 7, a smaller-diameter front portion 15, an a rearwardly and outwardly flaring intermediate portion 28 between the portions 7 and 15. The cylinder 6 surrounds an impeller 12 having at its rear end a piston 13 which is reciprocable in the portion 7. The front portion or shank 14 of the impeller 12 is reciprocable in the smaller-diameter portion 15 of the cylinder 6. The shank 14 can strike against an anvil 60 at the rear end of a turnable and reciprocable tool holder 19 having a socket 62 for reception of a suitable tool 20 (e.g., a boring tool, a screw driver or the like). The piston 13 can move into and out of register with apertures 16 which are provided in the larger-diameter portion 7 of the cylinder 6. The chamber 107 in the portion 7 is then sealed from the atmosphere and its volume decreases while the cylinder 6 moves forwardly under the action of the connecting rod 10. The cushion of air which is entrapped in the chamber 107 urges the impeller 12 forwardly so that the shank 14 bears against the anvil 60 and moves the tool holder 19 and the tool 20 forwardly with the result that the tip of the tool penetrates into or moves with a workpiece. For example, the tool may penetrate into a wall which is to be provided with a hole.

The torque-transmitting means for rotating the tool holder 19 while the latter moves forwardly (i.e., away from the connecting rod 10) comprises a sleeve-like output element 22 which allows the tool holder to move axially but prevents the tool holder from rotating relative thereto. The output element 22 is rotatable in the barrel 1b and is coupled to an intermediate sleeve-like element 24 by a one-way clutch 52. The element 24 is rotatable but cannot move axially in the housing 1. The tool holder 19 moves forwardly when its anvil 60 is struck by the shank 14 of the impeller 12. The rearward movement of the tool holder 19 takes place in response to rebounding of the tool 20 on the adjacent material of the workpiece.

A sleeve-like input element 26 of the torque transmitting means surrounds the smaller-diameter portion 15 of the cylinder 6 with a relatively small clearance. The front end portion of the input element 26 is coupled to the portion 15 by a ring 27 and the rear portion of the input element 26 forms a collar which abuts against the intermediate portion 28 of the cylinder 6. In this way, the input element 26 invariably shares all forward and rearward movements of the cylinder 6. The just mentioned flange at the rear end of the input element 26 has an annulus of axially parallel external teeth 26a whose root diameter is at least equal to but preferably at least slightly greater than the outer diameter of the portion 7. The front end portion of the input element 26 has an annulus of inclined external teeth or threads 26b which mate with similarly inclined internal threads or teeth 24b of the intermediate element 24 which is coupled to the output element 22 by means of the aforementioned one-way clutch 52.

The axially parallel teeth 26a at the rear end of the input element 26 mate with internal teeth 29a of a sleeve-like clutch element 29 which surrounds the portion 7 of the cylinder 6 and whose front end portion is provided with one or more claws 30 engaging complementary claws at the rear end of a ring-shaped clutch element 31 fixedly mounted in the housing 1. The claws 30 are normally held in engagement with the claws of the clutch element 31 by a strong helical spring 34 which surrounds the portion 7 and reacts against a split ring 35 in the barrel 1b. The foremost convolution of the spring 34 bears against an annular retainer 33 which abuts against the rear end face of the clutch element 29. The reference character 38 denotes a distancing ring which is inserted in front of the split ring 35 to increase the bias of the spring 34. The distancing ring 38 can be removed, replaced with a distancing ring of greater or smaller axial length, or used together with one or more additional distancing rings 38 of identical or different axial length. The distancing ring or rings can be installed between the split ring 35 and the rearmost convolution of the spring 34 and/or between the foremost convolution of this spring and the retainer 33. The latter resembles a cylindrical sleeve which surrounds the rear end portion of the clutch element 29. The front end portion of the clutch element 29 has a collar 36 which is immediately adjacent to the claws 30 and defines with the front end portion of the retainer 33 an annular compartment or space 37 for the eccentric projection or pin 39 of a stud 40 which can be rotated to thereby change the bias of the spring 34 and hence the force which is needed to disengage the claws 30 of the clutch element 29 from the complementary claws of the fixedly mounted clutch element 31. The stud 40 extends radially of and into the housing 1 and is surrounded by a cylindrical sleeve 41 the upper end portion of which meshes with the barrel 1b. The stud 40 can rotate within but cannot move axially of the sleeve 41 which can be said to constitute a component part of the housing 1. In order to change the angular position of the stud 40 and to thereby change the bias of the spring 34 by way of the projection 39, the user of the impact tool must manipulate a handle 42 which surrounds the sleeve 41 and is movable angularly with the stud 40. The handle 42 can be pulled downwardly, as viewed in FIG. 1 (i.e., away from the barrel 1b) by stressing a helical spring 44 which bears against a washer 45 surrounding a threaded stem 40a of the stud and abutting against a nut 46. When the user exerts a pull in the direction indicated by arrow A. The spring 44 is compressed and the handle 42 moves relative to the sleeve 41 and stud 40. At the same time, a locking bolt 48 of the handle 42 is moved to a level below a fixed stop 49 of the barrel 1b so that the handle 42 is free to turn with the stud 40 and to thereby change the angular position of the projection 39 in the annular compartment 37. When the handle 42 is rotated in a direction to move the projection 39 toward and into engagement with the collar 36, the sleeve-like clutch element 29 is held by hand (muscle power) against movement away from the tool holder 19 and its claws 30 remain in engagement with the claws of the fixed clutch element 31, i.e., the operator's hand assists the bias of the spring 34. This insures that each and every forward stroke of the cylinder 6 normally results in an axial as well as in an angular displacement of the tool 20. In other words, the user can hold the handle 42 by hand in such angular position that the clutch element 29 is held against disengagement from the fixed clutch element 31 by the spring 34 as well as by that force which the user applies to hold the projection 39 in engagement with the collar 36.

If the user decides to rotate the handle 42 in a different direction, the projection 39 moves toward the retainer 33 and shifts the latter against the opposition of the spring 34 so that the claws 30 of the clutch element 29 can become disengaged from the claws of the fixed clutch element 31. In such angular position of the handle 42, each forward stroke of the cylinder 6 results in a mere axial movement of the holder 19 and tool 20. The barrel 1b is preferably provided with a second stop (not shown) which may be similar to the stop 49 and against which the locking bolt 48 abuts when the pin 39 of the stud 40 stresses the spring 34 and allows the clutch element 29 to become disengaged from the clutch element 31. It is preferred to locate the second stop for the locking bolt 48 in such position that the bolt 48 bears against the second stop under the action of the stressed spring 34 and cannot automatically return into engagement with the stop 49. The intermediate element 24 can rotate the tool holder 19 through the one-way clutch 52 and output element 22 only while the claws 30 of the clutch element 29 engage the claws of the clutch element 31. The apparatus is then operated as a simple impact hammer without any rotation of the tool 20.

When the clutch element 29 engages the clutch element 31, each forward and rearward movement of the cylinder 6 results in a rotary movement of the intermediate element 24 because the inclined teeth 24b mate with the inclined teeth 26b of the input element 26 which latter moves back and forth with the cylinder 6 and is held against rotation by the teeth 26a, 29b as long as the claws 30 engage the claws of the fixed clutch element 31. The extent of angular movement of the intermediate element 24 is determined by the inclination of the teeth 26b, 24b. The aforementioned one-way clutch 52 causes the output element 22 (and hence the tool holder 19 and tool 20) to rotate while the intermediate element 24 rotates in a first direction (in response to a forward movement of the cylinder 6). The clutch 52 is ineffective when the intermediate element 24 rotates in the opposite direction (in response to a rearward movement of the cylinder 6) so that the material into which the tool 20 is caused to penetrate then prevents the holder 19 and input element 22 from rotating with the element 24.

The construction of the one-way clutch 52 is shown in FIG. 2. This clutch is installed in a flange 53 at the rear end of the output element 22. The flange 53 is provided with tangential bores 54 for clutch pins 55 which are biased outwardly by helical springs 56 so that they extend into suitably configurated internal notches 57 of the intermediate element 24. Each notch 57 is bounded by a surface 57a against which the respective pin 55 bears while the intermediate element 24 rotates in a first direction (counterclockwise, as viewed in FIG. 2) whereby the intermediate element 24 rotates the output element 22 and tool holder 19. Each notch 57 is further bounded by a suitably inclined surface 57b along which the respective pin 55 slides while the intermediate element 24 rotates in a second direction (clockwise, as viewed in FIG. 2). The springs 56 insure that the pins 55 invariably bear against the adjacent internal surface of the intermediate element 24. When the intermediate element 24 rotates clockwise, as viewed in FIG. 2, each pin 55 can move from a preceding notch 57 into the next-following or further notch. As mentioned above, the housing 1 allows the intermediate element 24 to rotate back and forth but holds this element against appreciable axial movement.

The hollow front portion or socket 62 of the tool holder 19 which receives the shank of the tool 20 is reciprocable and rotatable in an axial bore 61 of the barrel 1b. The rear portion of the tool holder 19 is movable axially in but cannot rotate relative to the output element 22. An external annular groove 63 of the tool holder 19 immediately behind the socket 62 receives a portion of an elastic cushion 64 which is installed in the barrel 1b. The axial length of the groove 63 determines the extent of axial displacement of the tool holder 19.

An important advantage of the improved impact wrench is that, when the tool 20 jams in the material of the workpiece, the torque is transmitted to the housing 1 by way of the input element 26, clutch element 29, claws 30, the corresponding claws of the clutch element 31 and clutch element 31. Thus, the cylinder 6 need not take up any torsional stresses and this also holds true for all other parts of the drive means for the impeller 12. Such construction protects the crank assembly between the electric motor M and the cylinder 6 against torsional stresses. Consequently, the wear upon the parts of the crank assembly is minimal and its useful life is much longer than in heretofore known impact wrenches.

When the torque exceeds the value which is selected by the bias of the spring 34 and/or projection 39, the claws 30 of the clutch element 29 become disengaged from the claws of the fixed clutch element 31 with the result that the tool 20 ceases to turn as long as the claws 30 remain disengaged from the complementary claws. The minimum torque which is needed to disengage the clutch 29, 31 is determined by the bias of the spring 34. As mentioned before, the bias of the spring 34 can be assisted by the operator if the handle 42 is manipulated in the aforedescribed manner so that the projection 39 bears against the collar 36 and thus participates in retention of claws 30 in engagement with the claws of the fixed clutch element 31. Also, the user can decide to intentionally disengage the clutch element 29 from the fixed clutch element 31 by moving the locking bolt 48 against the aforementioned second stop on the barrel 1b so that the clutch element 29 has sufficient freedom of axial movement to disengage its claws 30 from the claws of the clutch element 31. The device then merely acts as a hammer by causing the impeller 12 to strike against the anvil 60 in response to each forward stroke of the cylinder 6. The clutch 29, 31 constitutes a safety device or blocking means which insures that the operator cannot be injured when the resistance which the tool 20 offers to rotation exceeds a permissible value. The parts 39-48 also constitute a safety device because it enables the operator to select that torque which can be handled by the operator without risking an injury. The minimum resistance which the tool 20 must offer to disengage the clutch 29, 31 is preferably low so that the impact wrench can be properly and safely manipulated by a relatively weak person. On the other hand, the projection 39 enables the operator to select a relatively high torque, i.e., the tool 20 must offer a very high resistance to rotation before the clutch element 29 becomes disengaged from the clutch element 31. The aforementioned positioning of the second stop for the locking bolt 48 prevents an accidental or unintentional change in the bias of the spring 34 so that the minimum torque which is needed to disengage the clutch element 29 from the clutch element 31 remains unchanged until the operator decides to move the projection 39 away from the collar 36. In other words, the operator must decide to move the handle 42 axially and to change the angular position of the stud 40; otherwise the magnitude of selected torque remains unchanged. If desired, the impact wrench can be provided with means for adjusting the position of one or both stops so that the user can select an optimum torque for a particular operation.

The claws 30 and the complementary claws of the clutch element 31 are preferably conical. This is desirable because such claws can transmit substantial torque while taking up a small amount of space.

The provision of a single projection 39 which can change the bias of the spring 34, disengage the spring 34 from the clutch element 29 or serve to transmit muscle power for retention of the clutch element 29 in engagement with the clutch element 31 contributes to simplicity, compactness, convenience of manipulation and lower cost of the impact wrench. The spring 44 cooperates with the locking bolt 48 to insure that the angular position of the handle 42 cannot be changed accidentally, i.e., the operator knows that he must exercise great care whenever the handle 42 is moved against the opposition of the spring 44 because the projection 39 is then likely to change its angular position under the bias of the spring 34 or in response to disengagement of the clutch element 29 from the clutch element 31.

It is also within the purview of the invention to omit the input element 26 and to provide the teeth 26a, 26b directly on the output member 6. The clutch element 29 then meshes with axially parallel teeth on the output member 6 and the inclined teeth or threads of the output member 6 then mesh with the teeth or threads 24b of the intermediate element 24. Such modified torque transmitting means operates satisfactorily if the threads or teeth on the parts 29, 6, 24 are machined with a high degree of precision; otherwise, the crank assembly of the drive means would be subjected to substantial torsional stresses.

The structure which is shown in the drawing is preferred at this time because the crank assembly is not subjected to any torsional stresses, even if the input element 26 has substantial freedom of radial movement with respect to the output member 6 and even if the teeth 26a, 29a and/or 24b, 26b are not machined with a high degree of precision. Moreover, a damaged or worn element 26 and/or 24 can be replaced by a fresh one with minimal losses in time and at a cost which is a small fraction of the cost of the output member 6.

All such parts which are subject to extensive wear are readily accessible to allow for replacement with minimal losses in time. Moreover, the adjustment of the assembled impact wrench is either unnecessary or minimal so that the initial cost of the impact wrench is reasonable in spite of its versatility and ability to transmit substantial torque.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

Claims

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In a portable power tool, particularly in an impact wrench or drill, a combination comprising a housing; drive means provided in said housing and including an output member movable forwardly and rearwardly; a tool holder rotatably and reciprocably mounted in said housing in front of said output member; impeller means provided in said housing and operative to strike against said tool holder in response to forward movements of said output member; and means for normally rotating said tool holder in response to forward movements of said output member, including an input element rotatably surrounding and movable with said output member, blocking means provided in said housing for normally holding said input element against rotation, an intermediate element rotatably mounted in said housing, said elements having tubular portions telescoped into each other and provided with mating inclined teeth or threads so that the forward and rearward movements of said input element result in rotation of said intermediate element in opposite directions, an output element rotatably mounted in said housing, said tool holder being movable axially relative to and being rotatable with said output element, and one-way clutch means provided between said intermediate element and said output element to rotate said tool holder by way of said output element in response to forward movements of said input element.

2. A combination as defined in claim 1, wherein said drive means further comprises an electric motor having a rotor and a crank assembly connected between said rotor and said output member for converting the rotary movements of said rotor into reciprocatory movements of said output member.

3. A combination as defined in claim 1, wherein said blocking means comprises a clutch having a first clutch element rigid with said housing, a second clutch element rotatably and axially movably mounted in said housing, said clutch elements having complementary male and female portions, and means for biasing said second clutch element against said first clutch element, said second clutch element having first axially parallel teeth and said input element having second axially parallel teeth mating with said first teeth so that said input element is rotatable with said first clutch element relative to said output member only when said second clutch element is disengaged from said first clutch element.

4. A combination as defined in claim 3, wherein said means for biasing comprises resilient means.

5. A combination as defined in claim 3, wherein said means for biasing comprises a manually operable biasing device.

6. A combination as defined in claim 5, wherein said means for biasing further comprises prestressed resilient means and said biasing device comprises means for changing the stressing of said resilient means and for disengaging said resilient means from said second clutch element.

7. A combination as defined in claim 3, wherein said male portions of said clutch elements comprise conical claws.

8. A combination as defined in claim 7, wherein each of said clutch elements is an annulus and said annuli having adjacent end faces provided with said claws.

9. A combination as defined in claim 8, wherein said means for biasing comprises a prestressed spring provided in said housing and arranged to normally bear against said second clutch element so as to urge the claws thereof into engagement with the claws of said first clutch element, and a biasing device having a projection movable relative to said housing to thereby change the stressing of said spring or to disengage said spring from said second clutch element.

10. A combination as defined in claim 9, wherein said biasing means further comprises a handle which is accessible from without said housing and is operable by hand to thereby move said projection relative to said housing.

11. A combination as defined in claim 9, wherein said projection is movable by hand to at least one position in which the force furnished by the hand maintains the claws of said second clutch element in engagement with the claws of said first clutch element.

12. A combination as defined in claim 11, wherein said biasing device further comprises a handle rotatably mounted in said housing and being operable by hand to thereby move said projection relative to said housing and to maintain said projection in said one position.

13. A combination as defined in claim 12, wherein said biasing device further comprises at least one stop provided in said housing and cooperating with locking means provided on said handle to maintain said handle in a selected angular position, said handle being movable radially of said clutch elements to thereby disengage said locking means from said stop and further comprising resilient means yieldably opposing the movement of said handle radially of said clutch elements in a direction to disengage said locking means from said stop.