Impact wrench

Abstract

An electric impact wrench wherein the output shaft of the electric motor which is installed in a synthetic plastic housing can rotate the tool holder through the medium of a two-speed transmission and the output shaft can cause a plunger to repeatedly strike against the tool holder in response to rotation of a crankshaft which can receive torque from the output shaft. The two-speed transmission has several spur gears and the output shaft can drive the crankshaft by way of two bevel gears. The axes of all of the gears are located in a common plane which coincides with the central longitudinal symmetry plane of the housing.

Description

BACKGROUND OF THE INVENTION

The present invention relates to portable power tools in general, and more particularly to improvements in portable power tools of the type known as impact wrenches wherein a tool holder is rotatable in and is also movable axially with respect to the housing in response to repeated impulses against its shank. Such power tools can be used for chipping of rock, concrete or the like, for drilling or holes in wood, rock or other material, for the application or removal of screws, bolts, nuts or the like, and/or for other purposes wherein the working end of the tool is to rotate with or relative to and/or penetrate into a workpiece. The prime mover of the power tool is preferably an electric motor which can rotate the tool holder at one or more speeds, preferably at several speeds.

It is already known to construct an electric impact wrench in such a way that the tool holder can be rotated at several speeds and that the tool holder can receive axial impulses which drive its working end into a workpiece while the tool holder rotates or while the torque transmitting connection between the motor and the tool holder is interrupted. A drawback of presently known impact wrenches of the just outlined character is that their parts occupy a substantial amount of space so that the power tool must be provided with a relatively large, bulky and heavy housing. This is undesirable because the manipulation of such power tools is tiresome and the likelihood of injury or accident is more pronounced than if the impact wrench comprises a relatively small, lightweight and handy housing. Another drawback of presently known impact wrenches is that they comprise an excessive number of discrete parts, such as differently configurated and/or dimensioned gears, especially if the tool holder is to be rotated at several speeds.

SUMMARY OF THE INVENTION

An object of the invention is to provide a portable power tool, especially an electric impact wrench, which is smaller, lighter and handier than but just as versatile as presently known power tools.

A further object of the invention is to provide an impact wrench wherein the tool holder can be rotated and/or propelled axially in a novel and improved way.

An additional object of the invention is to provide the impact wrench with novel and improved transmissions which serve to rotate and to propel the tool holder into a workpiece.

Still another object of the invention is to provide a novel and improved multi-speed transmission for use in the above outlined impact wrench.

A further object of the invention is to provide a novel arrangement of gears in an electrically powered impact wrench.

The invention is embodied in a portable power tool, particularly in an impact wrench, which comprises a housing preferably consisting of synthetic plastic material, a tool holder which is rotatably and axially movably installed in the housing, a prime mover (preferably an electric motor) which is mounted in the housing and has a rotary output member, means for rotating the tool holder, and means for transmitting axial impulses to the tool holder. The means for rotating the tool holder preferably comprises a multi-speed transmission which is actuatable to transmit torque from the output member to the tool holder, and the means for transmitting axial impulses comprises a second transmission which is actuatable to transmit to the tool holder axial impulses in response to rotation of the output member. At least one of the two transmissions comprises a plurality of gears (for example, the multi-speed transmission may comprise several spur gears and the second transmission may comprise several bevel gears) and, in accordance with a feature of the invention, the axes of all of the gears are located in a single plane. Such plane preferably coincides with the central longitudinal symmetry plane of the housing.

In accordance with a presently preferred embodiment of the invention, the multi-speed transmission has an intermediate shaft which is parallel to the tool holder and to the output member and is integral with several gears, for example, with three gears which serve to transmit torque from the output member to the tool holder, and with a gear which serves to transmit torque to a crankshaft of the second transmission.

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 longitudinal vertical sectional view of an electric impact wrench which embodies one form of the invention;

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

FIG. 3 is an enlarged fragmentary sectional view as seen in the direction of arrows from the line III--III of FIG. 1;

FIG. 4 is a fragmentary longitudinal vertical sectional view of a second impact wrench; and

FIG. 5 is a similar fragmentary longitudinal vertical sectional view of a third impact wrench.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, there is shown an electric impact wrench which comprises a combined motor and gear housing including a front section 1, a median section 2 and a rear section 3. The rear section 3 is made integral with or is separably connected with a spade handle 3a. The sections 1-3 preferably consist of a suitable synthetic plastic material. The prime mover of the impact wrench is an electric motor 4 which is mounted in the central region of the lower part of the median housing section 2. The motor 4 has a rotor 5 which drives an output shaft 6; the latter extends beyond both ends of the motor casing and its left-hand end portions carries a customary flywheel 8. The leftmost part of the shaft 6 rotates in an antifriction roller bearing 7 which is installed in the front section 1 of the housing. The right-hand portion of the output shaft 6 rotates in an antifriction ball bearing 9 which is installed in a partition 2a of the median housing section 2. The rightmost part of the output shaft 6 constitutes a small spur gear or pinion 10 which meshes with a larger spur gear 12 on an intermediate shaft 11 mounted in the housing in parallelism with and at a level above the output shaft 6. The spur gear 12 constitutes an integral part of the intermediate shaft 11 the front end portion of which is mounted in an antifriction roller bearing 13 installed in the partition 2a. The rear end portion of the intermediate shaft 11 rotates on an antifriction ball bearing 14. The latter is installed in an axial bore 15 of the intermediate shaft 11 and its inner race surrounds a centering member 18 and is secured thereto by a screw 19 or an analogous fastener. The centering member 18 has a smaller-diameter front end portion or extension 16 which extends into the inner race of the bearing 14. The rear portion of the centering member 18 is mounted on a fixed bracket or support 17 which is installed in the housing section 3 in front of the handle 3a.

In addition to the aforementioned spur gear 12, the intermediate shaft is formed with spur gears 20, 21 and a bevel gear 22. The diameter of the spur gear 12 exceeds the diameters of the gears 20, 21 and 22, and this spur gear is located behind the gears 20, 21 but in front of the bevel gear 22. The gear 20 is smaller than the gear 21 and these gears respectively mesh with spur gears 24, 25 which are freely rotatable on but can be coupled to a hollow driven shaft 23 which is parallel to the shafts 6, 11 and is mounted in the housing at a level above the intermediate shaft 11. The gear 24 is rotatable relative to the gear 25, and vice versa. The arrangement is such that the gear 24 drives the shaft 23 when it is caused to move in a direction to the right, as viewed in FIG. 1, and that the gear 25 drives the shaft 23 when it is caused to move in a direction to the left, as viewed in FIG. 1. The shifter means for moving the gears 24, 25 axially of the driven shaft 23 comprises a manually operable actuating member in the form of a rotary knob 30 shown in FIG. 2. This knob is adjacent to one side of the median housing section 2 so that it can be grasped by one hand while the other hand grips the handle 3a. The axial lengths of the gears 20, 24 and 21, 25 are selected in such a way that the gears 24, 25 respectively remain in mesh with the gears 20, 21 even if they are shifted from the neutral positions shown in FIG. 1. The gear 24 can drive the shaft 23 when the latter can rotate with respect to the gear 25, and vice versa.

The shaft 23 is provided with a set of four equidistant external torque receiving teeth or claws 26 (see also FIG. 2) which are disposed in the internal recesses or grooves 27 of the gears 24, 25 when these gears assume the neutral positions of FIG. 1. The gear 24 has internal torque-transmitting teeth or claws 28a separated from each other by axially parallel grooves 28 which receive the claws 26 when the knob 30 is rotated in a direction to move the gear 24 to the right, as viewed in FIG. 1. The intermediate shaft 11 then rotates the shaft 23 through the medium of gears 20, 24 at the lower of two speeds. The gear 25 has internal axially parallel teeth or claws 29a separated from each other by axially parallel grooves 29 which receive the claws 26 when the knob 30 is rotated to move the gears 24, 25 in a direction to the left, as viewed in FIG. 1, whereby the shaft 23 is driven at the higher of two speeds.

The shaft 23 can rotate in a front antifriction (ball) bearing 31 and a rear antifriction (roller) bearing 32, both installed in the median housing section 2. The bearing 32 is located in front of and is spaced apart from the spur gears 24, 25. The outer race of the front bearing 31 for the shaft 23 is received in a metallic sleeve 33 which is embedded into the plastic material of the median housing section 2. A similar metallic sleeve 34, which is also embedded in the plastic material of the housing section 2, receives the outer race of the rear bearing 32 for the shaft 23. That portion of the shaft 23 which is located behind the front bearing 31 is formed with one or more radial openings or bores 35 for the median portions of spherical torque transmitting elements 36 each of which has an inner portion normally extending into an axially parallel external groove 40 machined into the external surface of a sleeve 41 for the shank 42 of a tool holder 43. A third or outer portion of each element 36 invariably extends outwardly beyond the periphery of the driven shaft 23 and abuts against the front end face of a ring 37 which is movable axially of the shaft 23 and is biased against the elements 35 by a strong helical spring 38. The rearmost convolution of the spring 38 reacts against a ring-shaped retainer 39 which abuts against an external shoulder of the shaft 23 in front of the bearing 32. The innermost portion 37a of the front end face of the ring 37 is of conical shape and diverges forwardly, i.e., away from the rear bearing 32. This conical portion 37a abuts against the spherical elements 36. The parts 35-41 constitute a simple but reliable safety clutch which is normally engaged (as long as the inner portions of the elements 36 extend into the grooves 40 and of the sleeve 41) so that the shaft 23 can rotate the tool holder 43. The safety clutch is disengaged when the tool holder 43 causes the sleeve 41 to offer a predetermined resistance to rotation with the shaft 23; the inner portions of the spherical elements 36 are then expelled from the respective grooves 40 and travel outwardly along the conical portion 37a of the front end face of the ring 37 (against the opposition of the spring 38) so that the shaft 23 can rotate with respect to the sleeve 41 and tool holder 43.

The sleeve 41 is installed in the front portion of and extends forwardly beyond the driven shaft 23. The axial bore of the sleeve 41 receives the shank 42 of the tool holder 43. The shank 42 constitutes an anvil which can receive impacts from an impeller assembly including a pneumatic cylinder 65 and a plunger 66 which is reciprocable in the cylinder 65. An intermediate portion 45a of the shank 42 of the tool holder 43 is provided with external involute teeth 45 which mate with complementary internal teeth 44 of the sleeve 41. The length of the teeth 45, as considered in the axial direction of the tool holder 43, is preferably between 30 and 60 percent of the length of internal teeth 44.

The maximum diameter of the shank portion 45a exceeds the diameters of the remaining cylindrical portion of the shank 42, and such cylindrical portion of the shank 42 is a tight sliding fit in the sleeve 41. The diameter of the cylindrical portion of the shank 42 in front of and behind the portion 45a equals or approximates the root diameters of teeth 45. The shank 42 preferably consists of a high-quality metal and its external surface is preferably finished to a high degree of polish to insure that it can move axially and rotate in the sleeve 41 with a minimum of play. The top lands of internal teeth 44 in the sleeve 41 are preferably chamfered, as at 46 (see FIG. 3). The sleeve 41 is preferably produced by extrusion molding. Its teeth 44 are polished and cooperate with the teeth 45 to enable the shank 42 (and hence the entire tool holder 43) to move axially in response to impacts which are being transmitted by the plunger 66 while the tool holder rotates with the sleeve 41 (provided that the safety clutch including the parts 35-41 is engaged). The shank 42 cannot rotate relative to the sleeve 41, or vice versa.

The front section 1 of the housing contains two annular sealing elements 47, 48 which prevent entry of dust or other foreign matter into the driven shaft 23 and/or into the space between the races of the front bearing 31.

The bevel gear 22 of the intermediate shaft 11 meshes with a bevel gear 50. The latter is rotatable about the axis of a hollow crankshaft 49 forming part of a second safety clutch which is installed in the motion transmitting connection between the bevel gear 22 of the intermediate shaft 11 and the cylinder 65 of the impeller assembly for the shank 42 of the tool holder 43. The crankshaft 49 is normal to the intermediate shaft 11. A plane which is common to the axes of the shafts 6, 11 and 23 also includes the axis of the crankshaft 49, i.e., the axes of gears 10, 12, 20-22, 24-25 and 50 are located in a common plane which is preferably the central longitudinal vertical symmetry plane of the housing 1-3. The hollow crankshaft 49 is mounted in two antifriction roller bearings 51, 52 which are installed in the aforementioned bracket or support 17. The latter is an aluminum casting, i.e., the rigidity of its material greatly exceeds the rigidity of the plastic material of housing sections 1, 2 and 3. As shown in FIG. 2, the support 17 is provided with four bores 53 for reception of screws or analogous fasteners (not shown) by means of which the support is mounted in the rear housing section 3.

The lower portion of the crankshaft 49 is provided with one or more radially extending openings or bores 54 for spherical torque transmitting elements 55 which extend outwardly beyond the peripheral surface of the crankshaft and into axially parallel internal flutes or grooves 56 of the bevel gear 50. The innermost portions of the spherical torque transmitting elements 55 normally abut against the adjacent intermediate portion of an axially movable plug 58 which is installed in the crankshaft 49 and serves to normally hold the outer portions of the elements 55 in the adjacent internal grooves 56. When the plug 58 is moved upwardly, as viewed in FIG. 1, against the opposition of a helical spring 57 which reacts against an internal surface 49a of the crankshaft 49, the neck portion 58a of the plug 58 allows the elements 55 to move radially inwardly so that the bevel gear 50 is free to rotate with respect to the crankshaft. This interrupts the transmission of motion from the intermediate shaft 11 to the cylinder 65.

The spring 57 in the axial bore of the crankshaft 49 biases the lower end portion or follower 58b of the plug 58 against an eccentric pin 60 which is mounted on a rotary actuating member 59. The latter is accessible from without the housing of the impact wrench so that it can be rotated by hand between a first position which is shown in FIG. 1 and a second position (e.g., in response to rotation through an angle of substantially 180.degree.. In the illustrated first position of the actuating member 59, the eccentric pin 60 allow the spring 57 to expand and to maintain a conical portion 58c of the plug 58 in engagement with the spherical elements 55 so that the elements 55 couple the bevel gear 50 with the crankshaft 49. When the actuating member 59 is rotated through 180 degrees to its second position, the pin 60 lifts the follower 58b of the plug 58 so that a conical surface 61 of the follower 58b bears against the spherical elements 55 and positively prevents the outer portions of such spherical elements from leaving the respective flutes 56 of the bevel gear 50. The safety clutch between the crankshaft 49 and bevel gear 50 is then inoperative because the crankshaft 49 is rotated whenever the motor 4 rotates the intermediate shaft 11.

The actuating member 59 is further movable to one or two third positions (clockwise or counterclockwise through 90.degree. with respect to the first position shown in FIG. 1) whereby the spherical elements 55 are adjacent to the neck portion 58a of the plug 58 and are free to leave the flutes 56 so that the bevel gear 50 can rotate relative to the crankshaft 49. When the plug 58 dwells in the position of FIG. 1 and the crankshaft 49 offers a predetermined resistance to rotation with the bevel gear 50, the plug 58 moves upwardly and allows the elements 55 to leave the flutes 56 so that the bevel gear 50 can rotate relative to the crankshaft.

The upper end portion of the crankshaft 49 is rigid with a disk-shaped crank arm 62 having an eccentric crank pin 63 which is coupled to the rear end of a connecting rod 64. The front end of the connecting rod 64 is coupled to a pin 63a in the rear end wall of the cylinder 65. Thus, when the crankshaft 49 is rotated by the intermediate shaft 11 through the medium of bevel gears 22, 50 and spherical elements 55, the connecting rod 64 moves the cylinder 65 forwardly and rearwardly whereby the plunger 66 strikes against the rear end face of the shank 42 whenever the cylinder 65 performs a forward stroke. The space between the plunger 66 and the rear end wall of the cylinder 65 contains a cushion of gas (e.g., air) which is compressed when the cylinder 65 moves forwardly relative to the plunger 66 (which is slidable in the cylinder) whereby the compressed cushion causes the plunger to strike against the shank 42 so as to propel a tool (not shown) which is connected to the holder 43 into a workpiece, e.g., a concrete wall, rock, wooden panel or the like. The cylinder 65 is slidably guided by the internal surface of the gear portion of the driven shaft 23.

When the spur gears 24, 25 assume the axial positions of FIG. 1, the multi-speed transmission between the output shaft 6 and the driven shaft 23 is ineffective. Thus, when the motor 4 is started (for example, in response to depression of a customary trigger, not shown, in the handle 3a), the output shaft 6 drives the intermediate shaft 11 and the latter drives the crankshaft 49 so that the cylinder 65 of the impeller assembly moves back and forth and causes the plunger 66 to repeatedly strike against the shank 42 so that the tool holder 43 drives the tool into a workpiece, e.g., a concrete wall or the like. For example, the tool can be used for chipping or to make a hole in a concrete wall.

If the user decides to rotate the knob 30 in a direction to engage the claws 26 of the driven shaft 23 with the claws 28a or 29a of the spur gear 24 or 25, the motor 4 can rotate the shaft 23 through the medium of the gear 24 or 25 so that the shaft 23 rotates the sleeve 41 by way of spherical elements 36. If the nature of the workpiece into which the tool is caused to penetrate is such that the sleeve 41 offers a predetermined resistance to rotation with the shaft 23, the spherical elements 36 are expelled radially outwardly by the ribs which separate the grooves 40 of the sleeve 41 from each other. The elements 36 shift the ring 37 against the opposition of the spring 38 and allow the shaft 23 to rotate with respect to the sleeve 41 and tool holder 43. The spring 38 expands immediately and causes the spherical elements 36 to reenter the adjacent grooves 40 as soon as the tool is disengaged from the workpiece or as soon as the resistance which the sleeve 41 offers to rotation with the shaft 23 decreases sufficiently to allow the spring 38 to move the ring 37 forwardly.

If the operator wishes to use the wrench exclusively for drilling, i.e., to disengage the bevel gear 50 from the crankshaft 49 while the motor 4 is on and the intermediate shaft 11 drives the shaft 23 by way of the spur gear 24 or 25, the handle 3a is employed to push the housing including the sections 1-3 forwardly while the tip of the tool bears against the workpiece. This causes the inclined rear edge faces 45b of teeth 45 to bear against the sleeve 41 (immediately behind the teeth 44) so that the operator supplies a force which urges the rotating tool into the workpiece. The edge faces 45b then abut against the inclined surfaces at the rear ends of the spaces between the teeth 44 of the sleeve 41. Such engagement of the edge faces 45b with the just mentioned inclined surfaces prevents the tool holder 43 from moving axially of the shaft 23 as long as the force which tends to move the tool holder toward the gears 24, 25 does not exceed the force with which the operator pushes the housing 1-3 of the impact wrench toward the workpiece. The sleeve 41 is held against axial movement with respect to the drive shaft 23 by a distancing ring 41A which engages the inner race of the antifriction bearing 31 and a nut 23A at the front end of the shaft 23.

The chamfers 46 on the teeth 44 of the sleeve 41 insure that burrs, ridges or fins which can develop on the teeth 44, especially if the impact wrench is used to remove material from or to penetrate into very hard workpieces (such as concrete or rock), do not reach and damage the precision-finished external surface of the cylindrical portion of the shank 42 in front of and/or behind the teeth 45. Such surface cooperates with the internal surface of the sleeve 41 to properly guide the tool holder 43 during rotation and/or axial movement with respect to the housing.

As mentioned above, the axes of the shafts 6, 11, 23 and 49 are located in a common plane which is preferably the central longitudinal vertical symmetry plane of the power tool. Consequently, the axes of all gears (10, 12, 20, 21, 22, 24, 25, 50) are also located in a common plane. This contributes to compactness of the tool, especially as considered in a direction at right angles to such plane. Additional savings in space are achieved because the gears 12, 20, 21 and 22 constitute integral parts of the intermediate shaft 11. The mating teeth 44 and 45 of the sleeve 41 and shank 42 reduce the play of the tool holder 43 in the sleeve 41 and shaft 23 to a minimum. The extent to which the shank 43 is movable axially of the sleeve 41 is shown as being equal or nearly equal to the extent to which the plunger 66 is movable axially in the cylinder 65. This is desirable because it enables the tool holder 43 to receive and to reliably transmit to the tool substantial boring or drilling moments.

FIG. 4 shows a portion of a modified impact wrench wherein the sleeve 41 in the hollow driven shaft 23 forms part of a modified safety clutch. The innermost portions of spherical elements 36 in the radial openings 35 of the driven shaft 23 normally extend into the grooves 40 of the sleeve 41 and the elements 36 are then held in such positions by two cooperating rings 77, 78 which replace the ring 37 of FIG. 1. The front end face of the ring 78 has a conical portion 78a and the rear end face of the ring 77 has a conical portion 77a which defines the conical portion 78a a circumferentially complete recess for the outermost portions of spherical elements 36. When the resistance which the sleeve 41 offers to rotation with the driven shaft 23 reaches a predetermined value, the ribs between the grooves 40 of the sleeve 41 expel the spherical elements 36 radially outwardly and these elements move the rings 77, 78 away from each other to stress a helical clutch spring 81. The latter bears against the rear end face of the ring 78 and reacts against a retainer 80 which is affixed to the rear end of a tubular extension or skirt 79 forming part of or being rigidly connected to the ring 77 and surrounding the ring 78.

An advantage of the safety clutch of FIG. 4 is that the bias of the spring 81 can constitute only one-half of the bias of the spring 38 of FIG. 1 but the rings 77, 78 will offer the same resistance to radially outward movement of the spherical elements 36 as the ring 37 of FIG. 1. If the mass of the ring 77 and its extension or skirt 79 matches the mass of the ring 78, the noise which the tool produces when the safety clutch including the parts 35, 36, 41, 77-81 is disengaged is surprisingly small. Also, the conical surfaces 77a, 78a insure a smooth and gradual shifting of spherical elements 36 to and from the positions shown in FIG. 4.

FIG. 5 illustrates a third safety clutch which uses roller-shaped, rather than spherical, torque-transmitting elements 86. Each of these elements normally extends into a discrete groove 40 of the sleeve 41 and into and through an opening 23D of the driver shaft 23. The roller-shaped elements 86 have conical front and rear facets which normally abut against the conical portions of adjacent end faces on two rings 87, 88 respectively corresponding to the rings 77, 78 of FIG. 4. The ring 87 has a tubular extension or skirt 89 which surrounds the ring 88 and the spring 81. The latter reacts against a retainer 90 in the rear end portion of the skirt 89. An advantage of the roller-shaped torque transmitting elements 86 is that they cannot perform any other but radial movements with respect to the shank 42 and sleeve 41. This enhances the safety and reliability of the clutch.

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, a combination comprising a housing; a tool holder rotatably and axially movably installed in said housing; a prime mover mounted in said housing and having a rotary output member; means for rotating said tool holder, including a multi-speed transmission actuatable to transmit torque from said output member to said tool holder, said multi-speed transmission including an intermediate shaft parallel to said output member, a driven shaft parallel to said intermediate shaft and coaxial with said tool holder, a safety clutch normally coupling said tool holder with said driven shaft, a first gear rigid with said output member, a second gear rigid with said intermediate shaft and meshing with said first gear, third and fourth gears rigid with said intermediate shaft, and fifth and sixth gears coaxial with and movable axially of said driven shaft to and from operative positions in which said fifth and sixth gears respectively mesh with said first and fourth gears and thereby transmit torque to said driven shaft, said fifth gear being movable to said operative position thereof when said sixth gear is out of its operative position and vice versa; and means for moving said tool holder lengthwise, including a second transmission actuatable to transmit to said tool holder axial impulses in response to rotation of said output member.

2. A combination as defined in claim 1, wherein said second, third and fourth gears are integral with said intermediate shaft and said second transmission also comprises a plurality of gears including a first bevel gear integral with said intermediate shaft and a second bevel gear meshing with said first bevel gear.

3. A combination as defined in claim 1, wherein said first to sixth gears are spur gears and the diameter of said second gear exceeds the diameters of said first, third, fourth, fifth and sixth gears.

4. A combination as defined in claim 1, wherein said intermediate shaft is provided with an axial bore at one end thereof and further comprising an antifriction bearing received in said bore and a support mounted in said housing and having an extension coaxial with said intermediate shaft and supporting said bearing, said second transmission including a further shaft normally receiving torque from said intermediate shaft and rotatably mounted in said support.

5. A combination as defined in claim 4, wherein said gears further include a first bevel bear coaxial with and driven by said intermediate shaft and a second bevel gear coaxial with said further shaft and meshing with said first bevel gear, said further shaft having an eccentric crankpin and said second transmission further comprising a clutch interposed between said second bevel gear and said further shaft.

6. A combination as defined in claim 4, wherein said housing consists at least in part of a synthetic plastic material and said support consists of a metallic material, such as cast aluminum.

7. A combination as defined in claim 1, wherein said second transmission comprises a pneumatic impeller assembly having a cylinder reciprocable with respect to and being coaxial with said tool holder and a plunger reciprocable in said cylinder and arranged to repeatedly strike against said tool holder in response to reciprocation of said cylinder.

8. A combination as defined in claim 7, wherein said second transmission further comprises a crankshaft rotatably mounted in said housing and means for reciprocating said cylinder in response to rotation of said crankshaft, said gears including a first gear receiving torque from said output member and a second gear mating with said first gear and arranged to normally rotate said crankshaft in response to rotation of said first gear.

9. In a portable power tool, particularly an impact wrench, a combination comprising a housing; a tool holder rotatably and axially movably installed in said housing; a prime mover mounted in said housing and having a rotary output member; means for rotating said tool holder, including a multi-speed transmission actuatable to transmit torque from said output member to said tool holder and comprising an intermediate shaft parallel to said output member, a driven shaft parallel with said intermediate shaft and coaxial with said tool holder, said tool holder having a shank axially movably installed in said driven shaft, a gear train arranged to transmit motion from said output member to said intermediate shaft and from said intermediate shaft to said driven shaft, and a safety clutch interposed between said shank and said driven shaft, said safety clutch comprising a sleeve interposed between said driven shaft and said shank and having internal teeth mating with external teeth provided on said shank so that the shank is movable axially of said sleeve while its external teeth remain in mesh with said internal teeth, said safety clutch further comprising means for normally transmitting torque from said driven shaft to said sleeve; and means for moving said tool holder lengthwise, including a second transmission actuatable to transmit to said tool holder axial impulses in response to rotation of said output member.

10. A combination as defined in claim 8, wherein the length of said external teeth, as considered in the axial direction of said shank, is between 30 and 60 percent of the length of said internal teeth.

11. A combination as defined in claim 8, wherein said shank includes a first portion which is provided with said external teeth and a cylindrical second portion having a diameter equal to or closely approximating the root diameters of said internal teeth.

12. A combination as defined in claim 11, wherein said second portion of said shank has a polished peripheral surface.

13. A combination as defined in claim 8, wherein said internal teeth and said external teeth are involute teeth.

14. A combination as defined in claim 8, wherein said internal teeth have top lands provided with chamfers.

15. In a portable power tool, particularly in an impact wrench, a combination comprising a housing; a tool holder rotatably and axially movably installed in said housing; a prime mover mounted in said housing and having a rotary output member; means for rotating said tool holder, including a multi-speed transmission comprising an intermediate shaft driven by said output member, a driven shaft co-axial with said tool holder and rotatable by said intermediate shaft at a plurality of speeds, and a safety clutch between said tool holder and said driven shaft, said safety clutch comprising a sleeve axially movably receiving a portion of said tool holder and rotatably mounted in said driven shaft, at least one torque-transmitting element having a first portion received in a radial opening of said driven shaft and a second portion normally extending into an external axially parallel groove of said sleeve, and means for yieldably holding said torque-transmitting element against radial movement away from the axis of said driven shaft to thereby maintain said second portion of said element in said groove; and means for moving said tool holder lengthwise, including a second transmission actuatable to transmit to said tool holder axial impulses in response to rotation of said output member.

16. A combination as defined in claim 15, wherein said torque-transmitting element is a sphere.

17. A combination as defined in claim 15, wherein said torque-transmitting element is a roller.

18. A combination as defined in claim 15, wherein said torque-transmitting element further comprises a third portion which normally extends radially outwardly beyond said opening and said means for yieldably holding comprises at least one ring surrounding said driven shaft and means for biasing said ring against said third portion of said element.

19. A combination as defined in claim 18, wherein said means for yieldably holding comprises two rings flanking said third portion of said torque-transmitting element and having conical surface portions abutting against said third portion.

20. A combination as defined in claim 19, wherein said means for biasing comprises a single spring arranged to react against one of said rings and to bear against the other ring to thereby urge both said rings against said third portion of said torque-transmitting element.

21. A combination as defined in claim 20, wherein one of said rings has a tubular extension axially movably surrounding the other of said rings, said single spring reacting against said extension and bearing against said other ring.

22. A combination as defined in claim 19, wherein the mass of one of said rings equals or closely approximates the mass of the other of said rings.