Actuating drive with a wrap spring arrangement

An actuating drive for transmitting drive torque to a power takeoff, comprising a drive unit; a braking device comprising a cylindrical surface; and a wrap spring having a plurality of turns which are concentric to the cylindrical surface, a pair of ends, a first actuating area on one of the ends, and a second actuating area on one of the ends. When the drive unit exerts a drive torque in a first direction, the first actuating area is nonrotatably connected to the drive unit, the turns undergo a diameter change away from the cylindrical surface, and the second actuating area is nonrotatably connected to the power takeoff for transmitting drive torque. When the drive torque is interrupted, the turns undergo a diameter change toward the cylindrical surface and frictionally engage the cylindrical surface to block transmission of restoring torque from the power takeoff to the drive unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to an actuating drive which is in working connection with a power takeoff by way of a wrap spring arrangement with a plurality of turns between two ends which serves as a load torque interlock.

2. Description of the Related Art

A technical article which deals with wrap springs as mechanical servo elements was published in the technical journalAntriebstechnik, Vol. 34, No. 11, 1995, beginning on page 67. Special designs of wrap spring arrangements are discussed in Section 2.3 of this article. These special designs can be used to produce a load torque interlock.

Although the basic function and design of wrap spring arrangements are described in this article, there is no concrete description in which a special design acting as a load torque interlock is discussed by way of example.

SUMMARY OF THE INVENTION

The invention has the task of designing an actuating drive with a wrap spring arrangement serving as a load torque interlock in such a way that, with the least possible effort, a precise holding function for a power takeoff can be ensured even when the wrap spring arrangement is not absorbing any torque from the drive.

According to the invention, advantage is taken of a certain property of wrap spring arrangements, which makes it possible to obtain a specific reduction ratio between the two ends of the spring by selecting the appropriate the number of turns of the wrap spring. When the number of turns is relatively large, only a negligibly small residual torque will be present at the power takeoff end of the spring even if a strong drive torque is being introduced into the end of the spring in working connection with the drive. This residual torque, however, does not necessarily have to be absorbed at the takeoff end of the spring. It is also possible according to the invention for some or all of the torque of the wrap spring to be absorbed around the circumference of the turns. For this reason, the wrap spring is installed in the actuating drive in such a way that the turns of the wrap spring can enter into frictional contact with a radially adjacent braking device, so that the drive torque introduced at the drive end of the turns can be absorbed by the preferably stationary braking device. In this design, the takeoff end of the turns of the wrap spring can remain completely unsupported.

According to the invention, each end of the turns of the wrap spring has a first actuating area assigned to a selected direction of rotation and a second actuating area assigned to the opposite direction of rotation. Of these two actuating areas at one end of the turns, the first is in working connection with the drive so that the drive torque can be introduced, the drive consisting of, for example, an electric motor, whereas the second actuating area at the end of the turns can be brought into working connection with the power takeoff. The power takeoff can be formed by a clutch device preceded by a gearbox, so that the second actuating area of the end of the turns is accordingly in working connection with the input part of the gearbox.

When a drive torque is introduced to the first actuating area of the adjacent end of the turns, for example by the motor shaft of the electric motor, the end of the turns is carried along as well—under the assumption of a certain rotational position of the shaft—and thus, because the power takeoff is in working connection with the second actuating area of the end of the turns, the power takeoff is carried along also. As soon as the motor shaft of the drive introduces the drive torque into the first actuating area of the corresponding end of the turns, the diameter of the turns of the wrap spring is changed, this change in diameter occurring in the direction pointing radially away from the braking device. As a result, the frictional connection between the turns of the wrap spring and the braking device is at least weakened, so that the drive is required to overcome only a limited amount of frictional resistance during the further course of its actuating movement.

As soon as the actuating process is over, the drive is turned off by shutting off the supply of current to the electric motor, for example. As a result, the actuating area of the end of the wrap spring assigned to the drive moves the drive, e.g., the motor shaft of the electric motor, back in the direction opposite the previous direction of rotation by a certain minimum angular distance, which is accompanied by a slight reduction in the pretension of the spring. As a result, the diameter of the turns of the wrap spring changes slightly in the radial direction toward the braking device. At the same time, a torque, referred to in the following as the restoring torque, which also acts in the direction opposite that of the previous direction of rotation, is applied by the power takeoff to the end of the spring adjacent to the power takeoff and thus to the second actuating area. As a result, a change in diameter is again produced in the turns of the wrap spring in the radial direction toward the braking device, this restoring torque originating from the power takeoff being sufficiently strong to expand the turns of the wrap spring to such an extent that they enter into frictional connection with the braking device with a high applied force. The wrap spring is now in a position in which the wrap spring arrangement is self-locking; any further movement of the actuating drive under the action of the restoring torque can to this extent be effectively prevented. As a result, the special advantage is obtained that the drive is turned on only for the actuating process itself, and if the drive is designed as an electric motor, it must be supplied with power for only short periods of time. The drive itself does not need to be in operation over the course of long holding times at the power takeoff, such as when the clutch device is engaged.

So that the power takeoff can be returned to its original position to disengage the clutch device, for example, the drive is made to rotate in the opposite direction and thus to act now on the previously torque-free end of the wrap spring, namely, on its first actuating area assigned to the drive. Thus the diameter of the wrap spring is again caused to change slightly in the direction away from the braking device, thus coming at least partially away from the braking device, until the power takeoff has resumed its original position.

Because both ends of the wrap spring have first and second actuating areas, the actuating movement can be realized regardless of the direction of rotation initially selected for the drive.

So that the drive torque can be transmitted by the drive to the adjacent end of the wrap spring turns, the drive is provided with a first control element for the first actuating area of the end of the turns, and the power takeoff is provided with a second control element for the second actuating area of the end of the turns. The first control element can be provided, for example, on the motor shaft of the drive, or, in accordance with an advantageous elaboration, it can be provided on a sleeve attached nonrotatably to the motor shaft. Similarly, the second control element on the power takeoff can be provided, for example, on an input element of the gearbox of the clutch device, or, in accordance with an advantageous elaboration, on a sleeve attached nonrotatably to the input part.

When the wrap spring arrangement is designed with the two sleeves, the drive torque is transmitted via the sleeves to the power takeoff. According to the invention, each of these sleeves has an axial projection, which extends a predetermined angular distance around the circumference. The circumferential ends of the axial projection provide the control elements for the associated end of the wrap spring turns.

The two sleeves are preferably coaxial to each other, and their axial projections preferably engage in corresponding openings in the other sleeve. In the circumferential direction, sufficient free spaces remain between two axial projections to accommodate the ends of the wrap spring turns. When designed in this way, the two sleeves also provide a reciprocal locking function in the axial direction, so that they can be pushed toward each other only up to a predetermined extent. After they have been positioned in this way, the two sleeves are then, in an advantageous embodiment, enclosed by the turns of the wrap spring, which for their own part are located radially inside the braking device, the latter being preferably designed as an essentially ring-shaped shell, mounted nonrotatably on a housing.

So that the wrap spring can be effectively prevented from being inserted into the two sleeves in a laterally reversed manner, the sleeves have blocking elements for this purpose, so that production problems arising from human error will not occur.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1show a drive1with a wrap spring arrangement9and a power takeoff21, which are mounted in brackets3of a housing5. The drive1can be realized in the form of an electric motor with a motor shaft7, on which a first sleeve23is mounted nonrotatably in a manner not shown. This sleeve cooperates by way of a wrap spring27with a second sleeve25, which is mounted nonrotatably on a journal11of an input part13of a gearbox17. The gearbox17and a clutch device19together form the previously mentioned power takeoff21, whereas the sleeves23and25together with the wrap spring27constitute the wrap spring arrangement9. In a manner to be described in greater detail below, the wrap spring27cooperates with a braking device22, which radially surrounds the wrap spring arrangement9. The braking device is designed as an essentially ring-shaped shell79and is permanently connected to a bracket3of the housing5by fastening means81.

FIG. 2shows a first sleeve23by itself, andFIG. 3shows the second sleeve25by itself. The first sleeve23has a cylindrical base part29, from which a radial collar31extends radially outward. This radial collar31supports an axial projection33, which extends around a predetermined portion of the circumference of the base part and has control elements43,45along parts of its two circumferential ends, i.e., the ends35and37. These control elements cooperate with first actuating areas73,74(FIG. 4), formed by the ends69,71of the turns67of the wrap spring27, these ends following after bent sections89,91.

The second sleeve25, shown inFIG. 3, also has a cylindrical base part49, from which a radial collar51extends radially outward. The collar supports an axial projection53, which extends a certain distance around the circumference of the base part and has control elements63,65along parts of its circumferential ends55,57. These control elements can be brought into working connection with the second actuating areas75,76on the ends69,71of the turns67of the wrap spring27(FIG. 4).

AsFIG. 1and especially asFIG. 5shows, the two sleeves23and25are pushed together coaxially. Each of the axial projections33and53extends over a distance of less than 180° in the circumferential direction, so that neither of the two axial projections33,35interferes with the other as the two sleeves23,25approach each other in the axial direction. The open spaces83,85, which are shown inFIG. 6(free space83only) and inFIG. 8, remain circumferentially between the two axial projections33,53. As is especially clear inFIG. 8, the free spaces83and85not only serve to accept the ends69,71of the turns67of the wrap spring27, but also give the two sleeves23,25the possibility of moving relative to each other in the circumferential direction to a certain limited extent, so that a change in the diameter of the turns67of the wrap spring27can be achieved by producing a change in the relative rotational position of the two sleeves23,25. This point will be described in greater detail at a later point in the specification.

To return toFIG. 2, this shows a free axial end39on the axial projection33; after the sleeves23and25have been pushed together, this free end cooperates with an axial stop61on the radial collar51of the second sleeve25, shown inFIG. 3. The second sleeve25also has a free axial end59on its axial projection53; this free end can be brought into contact with the axial stop41of the radial collar31of the first sleeve23, shown inFIG. 2. InFIG. 6, the two sleeves23,25are completely pushed together, and thus the axial stops41,61are shown in their actively functioning positions.

It should be mentioned thatFIGS. 2 and 3also show blocking elements47, which, because of their geometric form, allow the ends69,71of the wrap spring27to be inserted in only one exactly defined position, whereas insertion in the reversed position, whether because of human error or production problems, is effectively prevented.

To return toFIG. 1, the wrap spring27with its turns67at a certain diameter is placed around the sleeves23,25; this diameter is such that the radially outer side of the turns67makes friction-locking contact with the radially inner surface of the braking device22. As a result, the wrap spring arrangement9is locked in position.

In a system in which the drive1is implemented in the form of an electric motor, the supply of current to the motor results in the rotation of the motor shaft7in a selected direction of rotation, as a result of which the first sleeve23is carried along in the same direction. This first direction of rotation is indicated inFIG. 8by the directional arrow d1. When the first sleeve23is rotating in this direction around the common center axis87, the control element43of the first sleeve23moves in direction of the rotation d1and thus produces a defined pretension in the wrap spring27, the turns67of which are still in friction-locking connection with the braking device22; this pretension leads to a change in the diameter of the turns67in the radial direction away from the braking device22, that is, radially inward, and thus at least weakens the friction-locking connection between the wrap spring27and the braking device. The further rotational movement of the first sleeve23in the direction of rotation d1can therefore proceed with very little resistance.

After a movement phase in which, to achieve this change of diameter, the first sleeve23was moved alone in the direction of rotation d1, the free space83between the control element43of the first sleeve23and the control element63of the second sleeve25is used up, so that now the sleeve23drives the sleeve25in the direction of rotation d1by way of the end69of the turns. The movement in this case is transmitted by the control element43of the first sleeve23to the first actuating area73of the end69of the turns and from its second actuating area75to the control element63of the second sleeve25. The actuating movement can comprise a plurality of rotations of the sleeves23,25in the direction of rotation d1around the center axis87, where the sleeve25transmits the rotational movement to the journal11of the input part13of the gearbox17, which journal is connected nonrotatably to the sleeve25. As soon as the clutch device19, actuated by the gearbox17, has arrived in a defined end position, e.g., its engaged position, the current to the drive unit1is shut off, whereupon its motor shaft7comes to a stop and no more drive torque is transmitted to the first sleeve23. The previously pretensioned wrap spring27is able at this point to relax slightly and brings about in this way a slight reverse rotational movement of the motor shaft7and of the first sleeve23. Simply as a result of this, there is a slight change in the diameter of the turns67of the wrap spring27; that is, the turns67expand slightly in the radial direction. A torque acting on the second sleeve25, however, referred to in the following as the “restoring torque”, which attempts to move the engaged clutch device19back into its starting position, acts much more strongly. This restoring torque produced by the clutch device19is transmitted via the gearbox17with the input part13, the journal11, and the second sleeve25via its control element63to the second actuating area75of the end69of the wrap spring27, as a result of which the second sleeve25is moved back by a limited angular distance opposite the direction of rotation d1. As this occurs, the turns67of the wrap spring27change their diameter in the direction toward the braking device22, that is, radially outward, so that the turns67press more strongly in the radial direction against the inside surface of the braking device22. As soon as a certain restoring distance in the direction opposite the direction of rotation d1has been achieved, the frictional connection between the turns67and the braking device22is so high that the restoring torque coming from the power takeoff21is no longer able force any further restoring movement. The wrap spring arrangement9has now achieved a self-locking state, which has the effect of maintaining the clutch device19in its set position.

To disengage the clutch device19, the drive unit1is supplied with current in such a way that its motor shaft7is driven in the direction of rotation opposite d1. Now the control element45of the first sleeve23comes to rest against the first actuating area74of the second end71of the turns and in the course of its further movement tensions the wrap spring27slightly, so that its turns67undergo a change of diameter in the direction away from the braking device22, that is, radially inward, with the result that the previously mentioned self-locking state is released. Under the action of the restoring torque of the power takeoff21, the second sleeve25can now execute a rotational movement opposite the direction of rotation d1. To ensure that the actuating drive returns as smoothly as possible to its starting position and thus to ensure that the clutch device19is restored as smoothly as possible to its starting position, the drive1is then actuated in such a way that, until this starting position is reached, a torque equilibrium is maintained between the drive torque provided by the drive1, the braking torque produced by the residual frictional connection between the turns67of the wrap spring27and the braking device22, and the restoring torque of the power takeoff21. The drive1is then shut off.

It is easy to see that, because of the symmetry of the wrap spring arrangement9, the clutch device19could also arrive in the engaged position when the direction opposite d1were to be chosen as the initially selected direction of rotation, in which case the return movement of the clutch device to its disengaged position would occur in the direction of rotation d1.