Transmission for electrically driven tool

Abstract: Transmission between electric motor and tool shaft, for instance for hand tools such as an electric screwdriver and the like, which transmission is provided with an adjustable breaking coupling for discontinuing the drive torque on the tool shaft when a predetermined resistance moment on this tool shaft is exceeded, wherein the breaking coupling in the form of two mutually slidable parts is provided with a signal generator for controlling a member influencing the motor feed, which signal generator comes into operation as soon as the two parts slide relative to one another when the set torque is exceeded, so that a disengagement takes place between motor and tool shaft immediately after the desired resistance moment is exceeded, wherein the inertia of the rotating parts no longer has any effect on the tool shaft so that it stops immediately.

Description: The invention relates to a transmission between electric motor and tool shaft, as defined in the preamble of claim 1. Such a transmission is known for instance from FR-A-2396 626. US-A-44712 456 describes a transmission between electric motor and tool shaft, for instance for tools such as a screwdriver and the like, which transmission is provided with an adjustable breaking coupling for discontinuing the drive torque on the tool shaft when a predetermined resistance moment on this tool shaft is exceeded, wherein the breaking coupling in the form of two mutually slidable parts is provided with a signal generator for controlling a member influencing the motor feed, which signal generator comes into operation as soon as the two parts slide relative to one another when the set torque is exceeded, said breaking coupling is a claw coupling under axial bias which has one or more protrusions and parts axially slidable relative to each other. In other known electric tools, particularly electric hand tools, it occurs that a slip or claw coupling is placed between the electric motor and the tool shaft, whereby in the case of overload the tool shaft is no longer subjected to the full torque of the electric motor. The drawback to such a system is that when the motor is driven a torque is still exerted continuously or intermittently on the tool shaft. This can be disadvantageous in particular applications. In addition, such couplings are noisy and greatly subject to wear. There also exist protection circuits which cause the motor feed to be switched off and/or braked as soon as overload of the motor occurs. Such a switch-off system is difficult to embody well in the case of battery-powered DC-motors, wherein during switch-off during overload quite high amperages are present, with all the adverse consequences this entails. The mass inertia of the rotating parts moreover continues to act on the tool shaft. The invention has for its object to provide a transmission wherein a disengagement takes place between motor and tool shaft immediately after the desired resistance moment is exceeded, wherein the inertia of the rotating parts no longer has any effect on the tool shaft so that it stops immediately, and to keep the overall length of the implement including said transmission as short as possible. This object is achieved by providing the transmission defined in claim 1. Due to said planetary gear wheel drive provided with an outer sleeve along the internal toothing of which the planet wheels roll, said outer sleeve being the one part of the breaking coupling brings the advantage that, because of the standstill of the outer sleeve during normal operation, the claw coupling does not rotate either. As soon as the claw coupling disengages, the sleeve will rotate and cause the drive to stop via the planet wheels. The claw coupling is preferably provided with one or more pairs of protrusions distributed regularly along the periphery, so allowing a determined angular rotation between the parts without the claw coupling again being in active engagement. Thus achieved is that the inertia of the rotating parts on the sides of the electric motor no longer has any influence on the stopping of the motor shaft which can therefore be stopped immediately. Thereto the spring bias on the parts of the claw coupling acts on the claw coupling via a lever system whereby the whole active range of torques becomes accessible and a relatively short fitting method is obtained. Due to the shifting of the pressure point of the spring relative to the lever, a relatively large adjustment range of the spring bias on the claw coupling is possible while retaining a fixed spring setting. The invention will be further elucidated in the detailed description hereinbelow of an embodiment which is shown in the annexed drawings. In the drawings: fig. 1 shows a longitudinal section of a part of a hand tool provided with electric motor, transmission and tool shaft, fig. 2a and b show in each case a variant of the spring-lever systems in the section along the line II-II in fig. 1, fig. 3 shows a section along the line III-III in fig. 1, Designated in the figures with the numeral 1 is the transmission in its entirety which is received between an electric motor 2 and a tool shaft 3. These components may or may not be directly accommodated in a housing 4 which can be of random type and construction. Housing 4 is provided with a hand-grip 5 (not further shown), whereby the whole can be used as electric hand tool. The motor shaft 6 is connected to a gear wheel shaft 7 which co-acts with a planetary gear wheel 8 which rolls on an internal toothing of a sleeve 9 which is rotatably mounted in a cylindrical sub-housing part 10. The planetary gear wheel 8 is rotatably mounted on a first rotation shaft 11 which is fixed to a freely rotating first disc 12 provided with a second toothed shaft 13. This toothed shaft 13 co-acts with a second planetary gear wheel 14 which likewise rolls on the same internal toothing of the sleeve 9. The second rotation shaft 15 of this planetary gear wheel 14 is mounted in a second intermediate disc 12' which is connected for fixed rotation with tool shaft 3. The shank of tool shaft 3 is rotatably supported by a first roller bearing 16 in a bearing collar 17 of sleeve 9, while a second bearing 18 is received between the tool shaft 3 and a bearing casing 19 of the cylindrical sub-housing 10. The transmission is supported in axial sense by a pivot bearing 20 which has a supporting surface with an annular end flange 21 which is fixed on the open end of the cylindrical sub-housing 10. A protruding part 22 of the motor housing 2 is supported in this annular flange 21. In the partition wall 23 of sub-housing 10 oriented perpendicularly of the shaft and the bearing sleeve 19 is arranged a number of openings, in each of which is arranged a freely movable pin 24. The pins 24, whereof there are three in the embodiment shown, see fig. 2a or b, are fixedly attached to a ring 25 extending round the bearing 16. Arranged on the mutually facing surfaces of ring 25 and the head end surface of outer sleeve 9 are protrusions 26, the preferred position of which is further elucidated in fig. 3. The head end of each pin 24 remote from the ring 25 is provided with a pressure nose 27 which is in contact with an arcuate strip 28, see fig. 2a and b, the action of which is further explained hereinbelow. Each arcuate strip 28 is pressed with the one end against the nose 27 by means of a ball 29, three of which are likewise arranged in the embodiment shown in a suitable opening of the bottom wall part 30 of a gear rim 31. The other end of the arcuate strip supports on or below the intermediate wall 23 (fig. 2a and b respectively) and there forms a pivot point. The gear rim is held fast by a closing nut 32 which can be screwed onto a thread of the bearing sleeve 19. Received between the balls 29 and the inner surface of the closing nut 30 is a pressure spring 33 with suitable pivot bearing 34. Arranged in the closing flange 21 of the sub-housing 10 is a pressure pin 35, the right-hand end of which falls into a recess in the head end surface of inner sleeve 9 of the planetary drive, while the left-hand end is connected to a switch 36 forming part of the power supply circuit of motor 2. The supply circuit is for instance a voltage source 37 in the form of a battery which is connected to the motor clamps 39 via a control circuit 38. The latter can include any known suitable control for the speed of revolution and rotational direction of the motor 2 as well as the on/off switch. The switch 36 serves respectively to break and close the current supply circuit for the motor 2, the function of which will be elucidated hereinafter. The operation of the transmission as described above is as follows. In normal use, when the motor 2 is energized, the motor shaft 6 will drive the planetary gear wheel transmission, wherein the planet wheels 8 and 14 roll along the internal toothing of the sleeve 9. The speed of revolution of the output shaft 3 will herein be considerably less than the speed of revolution of the motor shaft 6 due to the two-stage planetary drive. if the resistance on the tool shaft 3 increases the rolling resistance of planet wheels 8 and 14 on the internal toothing of sleeve 9 will also increase. When a determined resistance is reached a determined torque will be applied to the internal toothing of sleeve 9 which can increase such that the forces on one another of the protrusions 26, which rest against each other in the normal operating position, can become so great that the protrusions slide over one another. This means that the sleeve 9 could begin to rotate relative to the ring 25. At the same time however, because of the rotating of the sleeve 9, the actuating pin 35 will be pushed out of the recess axially to the left and the switch 36 which is normally in the closed position will be opened. The feed to the motor 2 is thereby broken off and motor 2 will come to a stop. Stopping of the motor 2 nevertheless has the result that some turning of the rotor with the planet wheels will still occur due to the mass inertia thereof. This rotation will not however carry through onto the tool shaft 3 since the outer sleeve 9 turns together with the planet wheels. When a determined torque on the tool shaft 3 is exceeded it will thereby come to an immediate stop as soon as the protrusions 26 have passed each other, despite the phenomenon that the rotor of motor 2 is still turning with the planetary drive. The pressure force with which the ring 25 is pressed against the sleeve 9 is determined by the biasing spring 33. This latter rests against the closing nut 32 and against the pivot bearing 34, which force is passed onto the balls 29 which press against the arcuate strips 28. The end edge 40 of the strip 28 rests directly against the partition wall 23 of sub-housing 10, while the end portion 41 rests against the nose 27 of pin 24. The force of the spring 33 is decreased correspondingly subject to the position of the ball 29 in relation to the lever 28. The ball 29 can in any case be placed directly opposite the pin 24 by rotating the gear rim 31, whereby the biasing force is transferred directly onto the pin 24 without lever action. When rotation is to the left in fig. 2a and b the ball is carried into a further position relative to the pin 24 or opposite thereto, whereby the pressure force thereon is proportionally reduced or increased respectively. This means that the biasing force on the pin 24 can be simply adjusted by turning the gear rim 31 without the spring length of spring 33 changing appreciably. The biasing force on the pin 24 and therefore on the protrusions 26 of the claw coupling can hereby be adjusted over a wide range without changing the spring bias. It will be apparent from the above that the claw coupling proposed by the invention is formed on the one hand by the sleeve 9 with associated protrusion 26 and on the other by the ring 25 with associated co-acting protrusion 26. When the protrusions 26 are placed at the same pitch diameter the rotating of the two parts of the claw coupling can take place through a maximum of 120° before the protrusions of both parts will touch each other again. The free degree of movement of sleeve 9 is therefore then limited to 120°, which could be too little in some applications. In order to be able to enlarge the degree of rotation of sleeve 9 and therefore to enable stopping of a greater mass inertia after switching off motor 2, it is recommended to place the co-acting protrusions 26 at different pitch diameters, see R1, R2 and R3 in fig. 3, such that the protrusions can slide past each another until the protrusions touch again at the same pitch diameter, which here is almost 360°. It will also be apparent that within the scope of the invention The switch 36 can be at a position other than shown in fig. 1 to switch off the power supply to the motor 2. In addition the switch can also serve to reverse polarity in the motor 2, whereby a rapid braking of the rotor of the motor can likewise be obtained. Other configurations of protrusions are of course also possible within the scope of the invention as defined by the claims.

Claims: Transmission between electric motor (2) and tool shaft (3), for instance for tools such as a screwdriver and the like, which transmission is provided with an adjustable breaking coupling for discontinuing the drive torque on the tool shaft when a predetermined resistance moment on this tool shaft is exceeded, wherein the breaking coupling in the form of two mutually slidable parts is provided with a signal generator (35) for controlling a member (36) influencing the motor feed, which signal generator (35) comes into operation as soon as the two parts of the breaking coupling slide relative to one another when the set torque is exceeded, said breaking coupling being under axial bias and having parts (9, 25) axially slidable relative to each other, said transmission being provided with a planetary gear wheel drive (8) and the one part of said breaking coupling being formed by the outer sleeve (9) of said planetary gear wheel drive (8), wherein said parts of the breaking coupling are subjected to a force by a spring (33) acting on the other part of the breaking coupling, characterized in that said breaking coupling is a claw coupling which has one or more protrusions (26), said planetary gear wheel drive (8) has more than one stage, and said spring (33) acts on said other part of the breaking coupling via a lever system, wherein the pressure point of the spring on the or each lever (28) is displaceable. Transmission as claimed in claim 1, characterized in that one protrusion (26) or a group of protrusions (26) of the claw coupling is arranged at a different diameter R (fig. 3).

Inventors: GEERTS JACOBUS FRANCISCUS, HOUBEN JAN PETER, MOOLENAAR ANTONY JOHANNES, GEERTS, JACOBUS FRANCISCUS, HOUBEN, JAN PETER, MOOLENAAR, ANTONY JOHANNES
Applicants: EMERSON ELECTRIC CO, EMERSON ELECTRIC CO.
Application year: 1992
Main classification: B25B23
IPC codes: B25B21, B25F5, B25B23