This invention relates to improvements in the use of self-locking transmissions that transmit movement between two gear members without relative movement between the gear teeth. There are many types of transmissions for moving an element. In general, transmissions include a driven element which is driven by a driving element. Some source of movement is applied to the driving element, and the driving element engages and drives the driven element.

One type of transmission is known as a self-locking or irreversible transmission. In a self-locking transmission, two gear members have inter-engaging teeth. The lead angle of the gear teeth, and the coefficient of friction of the material of the gears are selected such that the gear transmission is "self-locking". One example of such gear is shown in U.S. Patent No. 3,859,700. In a self-locking transmission, the gears will not result in motion from static condition or the gears will not slide relative to each other unless one of the gears is rotated.

One self-locking transmission would include a driving and driven gear set that may be a screw and nut gear set, a worm and worm gear set or a cam-driven indexer. One of the two gears is selected as a driving element. The driving element is moved in a direction other than rotation about its axis of rotation, and drives the driven gear in a power stroke.

As one example, in a worm and worm gear combination proposed in the prior art, the worm is selected as the driving element. It is driven about the axis of rotation of the worm gear. Rotation is transmitted to the worm gear by the inter engaged worm and worm gear teeth.

In proposed screw and nut transmissions, either the screw or nut is selected as the driving element. The driving element is moved axially along the axis of rotation of the screw and nut, thus causing both elements to move linearly.

The prior art systems do propose transmissions that would have valuable characteristics. In particular, these transmissions would be able to transmit very high torque levels with relatively inexpensive components. It could be said that the above-described gear sets move without relative movement between the gear teeth.

Thus, efficiency is high. The proposed systems have not been fully developed, however, and the systems as proposed in the prior art would not be capable of functioning on a practical basis.

One major problem with the prior art systems is the inevitable presence of a gap or clearance between the teeth on the two gears. All gear combinations have a gap between the gear teeth that is taken up during the initial movement of the driving element in the prior art systems. Thus, with the prior art system, if the driving element is a worm in a worm gear set, the worm would initially rotate about the axis of the worm gear until its teeth took up the gap, and engaged the teeth of the worm gear. One cannot predict how much movement is required before the gap is eliminated. Until contact is made, the driven worm gear is not driven by the driving worm. There is thus some inevitable lost motion, and it has not been possible to provide extreme accuracy of movement with such systems. Moreover, upon contact of the gears at the high torque levels used in the driving mode, there is a good deal of noise and potential damage to the gear. Finally, it cannot be insured that the gear teeth will be brought into engagement at an optimum location.

Another practical deficiency in the prior art is the failure to unload the driving and driven gears after the power stroke. As an example, once the worm has been rotated about the axis of the worm gear to drive the worm gear, the two will remain in contact unless they are returned to having a gap. It would be desirable to return the two to having a gap between them after the power stroke, or to

"unload" the driving element. The prior art systems have not provided a method for achieving such unloading.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENT

In a method according to the present invention, prior to beginning the driving power stroke in a self-locking transmission of the type wherein there is no relative movement between the gear members during the power stroke, an initial step of taking up the gap between the gear teeth is performed. In preferred embodiments, this initial step is performed by a lower torque motor than that used for the power stroke. The power stroke is then performed by the relatively high torque motor. It may be that a single motor is utilized for both movements, with the power being delivered to the motor controlled to achieve the relative low and high torque movement. No complex control is necessary for deciding when the gap is taken up, as the low torque motor would be unable to drive the worm gear once the gap has been taken up. One could simply drive the low torque motor until there is no further movement and then identify that the gap is gone and that there is now contact.

In other aspects of this invention, it is sometimes preferred that the movement to take up the gap be along a distinct direction than the power stroke. As an example, in a worm and worm gear set, the gap may be taken up by rotation

of the worm aborts its axis until the teeth engage. The worm may then be driven into the power stroke by a distinct motor about the axis of the worm gear.

Similarly, in a screw and nut transmission, it may be that the contact is taken up by rotation of one of the screw and nut. The driving element is then driven axially along the axis of rotation of the screw and nut.

With the inventive method of taking up the contact prior to the driving movement, one is able to accurately predict the amount of movement of the driven member during the power stroke. The entire power stroke is delivered to the driven element. Moreover, the noise that resulted in the prior art is eliminated. In another feature of this invention, once the power stroke is complete, the driving element is unloaded relative to the driven element. This allows the driving element to be returned to its original location without having its gear teeth in engagement with the gear teeth of the driven element. In one example, a stop is placed on the driven element that prevents the driven element from rotation after completion of movement in the power stroke. In one example, an electromagnetic clutch may be applied to prevent further movement of the driven element once the power stroke is complete.

The invention is described in relation to both a worm and a worm gear set, and also a screw and nut combination. Either the worm or worm gear can be the driven or the driving element, and the screw and nut may also be either the driven or driving element. The term "controlling element" is applied to the element which is moved to take up contact. In the worm and worm gear set, the worm typically is the controlling element. Either the nut or screw may be the controlling element.

These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A shows a first step in movement of a first embodiment.

Figure IB shows a detail of Figure 1A.

Figure 2 shows a subsequent step in the movement of a first embodiment.

Figure 3A shows a return step after the movement of Figure 2.

Figure 3B shows a detail of a feature of this invention. Figure 4 shows a final step in the movement of this invention.

Figure 5A shows a second type transmission in a first step of movement.

Figure 5B shows a detail of Figure 5A.

Figure 6 shows a subsequent step.

Figure 7 shows yet another subsequent step. Figure 8 shows a final step in the transmission with the second embodiment.

Figure 9 shows a third embodiment.

Figure 10 shows the subsequent movement of the third embodiment.

Figure 1 1 shows a fourth embodiment in its first step of movement.

Figure 12 shows a subsequent step. Figure 13 shows an additional step.

Figure 14 shows a final step in the fourth embodiment.

DET ILED DESCRIPTION OF A PREFERRED EMBODIMENT

A first embodiment transmission according to the present invention is illustrated in Figures 1-4. Worm 1 is mounted in a worm housing 1A shown schematically. A motor selectively rotates housing 1A. The worm 1 engages teeth on a worm gear 2. In this embodiment, the worm 1 is the driving element while the worm gear 2 is the driven element. The initial step shown in Figure 1 A is to take up a gap, such as gap IG shown in Figure IB, prior to beginning a power stroke. Contact may be taken up by rotating housing 1A about the axis of worm gear 2 a small amount. Alternatively, worm gear 1 may be rotated about its axis of rotation until its teeth come into engagement with the teeth on worm gear 2. It should be emphasized that the gap IG is rather small, and no large amount of movement is necessary to take up the gap. Even so, by having taken up this gap as an initial step, the transmission of motion will be greatly improved as described above. If the housing 1A is rotated about the axis of worm gear 2 to take up the gap, it is preferred that a relatively low torque motor be utilized to rotate the housing. In one embodiment, the power to motor M is controlled to provide low torque operation while taking up the gap, and to provide high torque in the power stroke. Alternatively, a relatively low torque motor may be placed on worm 1 as shown at IB to rotate the worm to take up clearance.

As shown in Figure 2, worm housing 1A and worm 1 has now been driven about the axis of worm gear 2 in the power stroke. The teeth of worm 1 rotate worm gear 2 a discrete amount. Since the gap has been taken up, the amount of movement of a worm gear 2 is strictly controlled. This transmission is thus able to

provide very close control over the amount of rotation of worm gear 2. The motor

M for driving the worm housing 1A in this power stroke is a very high torque motor. The transmission allows the transmission of very high torque loads between elements 1 and 2. If a single motor is utilized for taking up the gap and rotating the housing

1A as shown in Figure 1A, then driving the housing 1A in the power stroke to turn drive the worm gear 2, that motor is preferably supplied with a variable torque.

The torque is initially low to take up the gap, and then a higher torque is utilized to provide the movement shown in Figure 2. A simple motor control is able to achieve such a result.

As shown in Figure 3A, in some transmissions, it may be desirable to return housing 1A, and worm 1 to an initial position after the power movement shown in Figure 2. To this end, housing 1 A is rotated back relative to the axis of worm gear 2. At the same time, the worm 1 is rotated in a reverse fashion such that its teeth roll along the teeth of worm gear 2. Thus, no transmission is provided between the two elements during this return movement. As shown in Figure 4, the worm 1 and worm housing 1A have now returned to the position shown in Figure 1A. The worm gear 2 remains at the position shown by the arrow it was driven to in the Figure 2 movement. Figure 3B shows one further feature of this invention. Once the movement shown in Figure 2 is complete, a control for the system preferably actuates a stop, such as controlling brake 2C having an electromagnetic control. Brake 2C is preferably actuated at the completion of the power stroke shown in Figure 2 and locks the shaft 2S of the worm gear 2. Without a stop, it is not inevitable that some

inertia will cause the worm gear 1 to rotate after the stopping of movement of the housing 1A in the driving stroke. Under an negative external torque worm gear 1 may rotate backwards. Since the controlling brake 2C locks shaft 2S immediately upon the ending of the power stroke, the worm gear 2 will not move due to such negative external torque. Thus, the gear 2 will return to a position where there is a gap. This is "unloading" the driving element as described above. Due to this unloading, when the driving element is returned to the Figure 4 position as shown in Figure 3A, there is smooth movement and little torque required.

With the above described invention, there is smooth, reliable transmission and very accurately controllable movement of worm gear 2.

In an alternative embodiment shown in Figures 5-8, the driving and controlling element is nut 4 of a nut and screw combination. The screw 3 has gear teeth that engage gear teeth within the nut 4 as is known is this type transmission. In the contact step shown in Figure 5A, either the nut or the screw may be rotated about its axis to take up the gap 3G (see Figure 5B), or clearance. The nut is shown as the driving element, and is preferably driven along its axis through housing 4A as shown in Figure 5A to engage the screw 3 and drive the screw 3 as shown in this figure. A drive for driving the gears and housing as described may be of any known type. This provides a discrete controllable amount of axial movement to the screw 3. An element which one wishes to move axially is fixed to the screw 3 and shown schematically at 3A.

As an alternative to rotating one of the screw and nut to take up the gap, nut 4 may move axially to take up the gap. As before, if a single motor is utilized to move the nut, and then drive the nut and screw in the power stroke, it is preferred

that a low torque is supplied to the motor during the contact step. As with the prior embodiment, it is preferred that a stop also be provided on the shaft of the driven screw 3.

As shown in Figure 7, the nut 3 is now being returned to its initial position. The nut 3 has been unloaded as described with the first embodiment. As an example, a stop is used on the screw for unloading the two gears. The nut 3 is now rotated about its axis and moved axially back to is original position. As shown in

Figure 8, the nut has now returned, the screw has been driven through a small step of axial movement, and the system is prepared to perform the next movement step. Figures 9 and 10 show a third embodiment, wherein the worm gear 6 is the driving member and the worm 5 is the driven member. Once again, worm 5 is placed in a housing 5 A. As shown, a motor 5B may drive the worm 5 within the housing 5A. As with the other methods, the initial step is to take up the contact between the worm 5 and the worm gear 6. This may be accomplished by rotating the worm 5 through the motor 5B, or alternatively, rotating the worm gear 6 at a relatively low torque level as with the earlier embodiments. The power stroke is then initiated by driving the worm gear 6 at a relatively high torque load. The housing 5 A is thus moved to a distinct rotational position as shown in Figure 10. The other method steps apply with this method, and as shown schematically, the worm gear 6 could be rolled along the teeth of the worm 5, with the worm 5 also being rotated to allow the worm gear 5 to return to its original position. In the embodiment also, it may be preferable to apply a stop on the housing 5A once the power stroke is completed.

As shown in Figures 11-14, the screw 7 is the driving element and the nut

8 is the driven element. A housing 8A moves with the nut. In this embodiment, contact is first taken up as in the earlier embodiment. It may be that the screw 7 is moved at a relatively low torque, or that one of the screws 7 or nut 8 is rotated relative to the other to take up the gap. Once the gap is eliminated, the screw 7 is driven along its axis to in turn drive the nut 8 to the position shown in Figure 12.

A member 8A is connected to the nut, and is the item to be moved. Once the elements are in the Figure 12 position, the screw may be returned to its initial position. One way of allowing the screw and nut to rotate relative to each other during this return movement would be to rotate the nut as shown schematically in Figure 13. Now the teeth of the nut roll relative to the teeth of the screw 7 as the screw 7 is withdrawn. Since the screw 7 is preferably unloaded after the end of the power stroke, this is a relatively low torque, low noise return stroke. Eventually, the screw and nut are returned to the position shown in Figure 14, and the nut has been driven a small controlled axial amount.

The method of this invention could be utilized in a number of different ways. Either the worm or worm gear may be a drive or driving or driven element. Similarly, either the screw or nut can be the driving or driven element. Preferably, the worm is the controlling element which takes up the gap. In the screw and nut type embodiment, either the screw or nut can be the controlling element. The gap may be taken up by moving the controlling element in the direction of the power stroke at a relatively low torque until the gap is eliminated. Alternatively, the controlling element may be rotated about an axis other than the driving direction to take up the gap. Finally, in some embodiments the controlling element may utilize

a combination of movements to take up the gap. As an example, it may be that the nut is driven axially and rotated relative to the screw to take up the gap. Again, it is preferred that the contact movement be performed with a relatively low torque motor. The power stroke preferably occurs without relative movement between the gear teeth of the drive and driven element. In this way, the movement efficiency is maximized.

The drives and housings are shown schematically. Any known drive and connections may be utilized. In one preferred application of the method of transmission, at least a pair of the gear combinations disclosed in this method can be utilized in tandem to provide continuous movement to an output shaft. In a second preferred application the driven member is an output for a one-way clutch. Preferred embodiments of this invention have been disclosed, however, a worker of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.