BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiment of the present invention will now be described, by way of example only with reference to the accompanying drawings, in which:
FIG. 1 is a perspective view of a worm gear drive according to a preferred embodiment of the present invention;
FIG. 2 is a view similar to FIG. 1 with some parts removed to reveal the gear train;
FIG. 3 is an exploded view of the worm wheel and gear box housing being parts of the drive of FIG. 1;
FIG. 4 is an exploded view of the worm wheel of FIG. 3 from a different angle;
FIG. 5 is an enlarged view of a thrust bearing of the drive of FIG. 1;
FIG. 6 is a schematic used to explain the connection between the motor shaft and the thrust bearing; and
FIG. 7 is a schematic view of the motor shaft and two thrust bearings according to a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The apparatus of FIG. 1 is a worm gear drive 10 according to the preferred embodiment of the present invention. The drive comprises a motor 12 and a gearbox 14. The motor is bolted directly to the gearbox. This drive is used for moving a window in a passenger vehicle and is of the type commonly referred to as a window lift motor.
The motor 12 is a PMDC motor having a deep drawn steel housing 16 having a closed lower end 17 and an open upper end 18. The open end has a flange 19 for mounting an electrical connector 20 and the gearbox 14. A bearing retainer or boss 21 is formed in the closed end 17 for mounting a thrust bearing and a shaft bearing.
The gearbox 14 has a gearbox housing 22 accommodating the gear train. The gearbox housing 22 includes a lid 23 for closing the gearbox. The gearbox housing has three mounting holes 25 for mounting the drive to a support bracket in the door of a vehicle. The lid 23 is shown fixed to the gearbox housing 22.
Extending through the lid 23 is the output 26 of the drive. Output 26 comprises a drive plate and an integrally formed drive cog 27 which is mounted for rotation on an axle 28 which also extends through the lid 23. Just visible on a side of the gearbox housing opposite the motor is what appears to be a small slotted button. This is thrust bearing 29. Slot 30 in the outer end of the button facilitates screwing of the thrust bearing 29 into a receiving recess in the gearbox housing. By being screwed, axial position of the thrust bearing 29 can be easily adjusted to set the end play for the motor.
Inside the gearbox, as shown in FIG. 2, is the worm drive gear train. Also visible is an end cap 31 which closes off the open end 18 of the motor housing 16. It also supports a bearing (not shown) for the motor shaft 32 and brushes and optionally, noise suppression components for the motor. The end cap 31 is shown with a cylindrical tubular extension 33 surrounding the shaft 32. This is for keying the motor 12 to the gearbox 14. Shaft 32 supports a worm which is in mesh with a worm wheel. The worm is a helical cog pressed onto the shaft though it is known to cut or form the worm directly on the shaft. The worm wheel 35 is of a dish shape with a central boss 36 which is rotatably mated on the axle 28. Three ribs 39 extend from the boss to a peripheral rim 37. Teeth 38 which mesh with the threads of the worm 34 are formed on the radially outer surface of the rim. Optionally, the teeth are of the half arc form for greater strength. A seal 40 of elastomeric material is provided between the end cap and the gearbox housing to seal the commutator against ingress of water.
Thrust bearing 29 is shown adjacent to or butting against the distal end of the shaft 32. Although not shown, a sleeve bearing, optionally of the self-aligning, oil impregnated, sintered bushing type, may be held by the gearbox housing and rotationally supporting the distal end of the shaft. Alternatively, the distal end of the shaft may be rotationally supported by a part of the gearbox housing. Such rotating bearing support is provided, if desired, between the thrust bearing 29 and the worm 34.
The gearbox is shown exploded in FIG. 3 illustrating the connection between the worm wheel 35 and the drive plate 41. A damper 42 of, preferably, elastomeric material is used to transmit the driving torque between the worm wheel 35 and the drive plate 41. The drive plate has a lower surface from which extends three spigots 43. The spigots extend into holes 44 in the damper 42. The damper 42 has three slots 45 which receive the ribs 39 of the worm wheel 35. Damper 42 also has a number of small projections 46 on its upper surface to space the damper slightly from the drive plate 41 to allow room for resilient deformation of the damper. The damper absorbs sudden shocks applied to the output by external forces. The axle 28 is supported by the gearbox housing 22 which may be directly molded thereto. Optionally, bearings, not shown, rotatably support the drive plate and worm wheel on the axle. A central through passage 48 in the drive plate accommodates the axle 28. One or more seals 47 seal the drive plate to the gearbox lid and to the axle to prevent ingress of water.
FIG. 4 illustrates the worm wheel/drive plate/damper connection from a different angle showing more clearly the spigots 43. Also visible in this view is an enlargement 49 of the central through passage 48 in the drive plate 41. The enlargement 49 serves as a bearing housing 50 supporting a bearing of the drive plate 41 that rides on axle 28.
A spring clip 52 locates in a circumferential groove 53 in the axle 28 to prevent axial separation of the drive plate 41 from the axle 28. Spring clip 52 may be an E-clip, C-clip, circlip, etc.
FIG. 5 illustrates the thrust face 54 of the first thrust bearing 29. The thrust bearing 29 as briefly described before, has a cylindrical body having a screw thread 57 formed on the cylindrical surface for screw positioning in the thrust bearing recess of the gearbox housing. The thrust face 54 is substantially planar with a recess 56 forming a hole which, in use, is approximately co-axial with the motor shaft 32. The thrust face 54 optionally is stepped from the outer surface of the thrust bearing.
FIG. 6 illustrates the contact between the rounded first end of the shaft 32 and the first thrust bearing 29. The thrust bearing is shown in cross-section. The body of the thrust bearing is cylindrical with the screw thread 57 formed on the outer cylindrical surface. A slot 30 is formed in one end of the thrust bearing for receiving a blade of a screw driver for positioning of the thrust bearing in the gearbox housing.
The opposite end is the thrust face 54. At the centre of the thrust face is the recess 56 formed as a round blind hole. The hole is co-axial with the shaft and the rounded end of the shaft contacts the thrust face along the edge of the recess, thus forming a circular line contact, spaced from the axis of the shaft. Recesses of other shapes are possible, including irregular recesses, but round holes are preferred due to simplicity in making.
As the frictional force is related to the radius of the contact position, the greater the circular line contact is spaced from the axis of the shaft, the greater the force. Thus, the frictional force generated can be increased by moving the contact further away from the axis.
The friction can also be modified by selecting an appropriate material for the thrust bearing.
FIG. 7 illustrates a bi-directional self-locking worm drive. First thrust bearing 29 is as described above. A second thrust bearing 60 adjacent the second end of the shaft 32 which is also rounded, has a thrust face which is also formed with a recess arranged such that the axis of shaft 32 passes through this recess. Optionally, the recesses of both thrust bearings are round holes co-axial with the shaft and arranged to increase the friction between the shaft and the thrust bearings. As the second thrust bearing is located within the bearing recess of the motor housing, it is not adjustable axially and does not require a screw threaded outer surface.
In use, when driving the worm wheel, either clockwise or anti-clockwise (up or down), the reaction between the worm and the worm wheel causes the shaft to press against a respective one of the thrust bearings. Should the worm wheel try to drive the motor, the worm wheel, through the worm, will cause the shaft to press against the other one of the thrust bearings, at which point, because of the increased friction between the thrust bearing and the shaft and because of the gear ratio between the worm and the worm wheel, the shaft will lock against the thrust bearing and the motor will not rotate, hence locking the worm drive.
Hence, by providing a high friction thrust bearing at one or both ends of the motor shaft or driving shaft, the worm drive can be made self-locking in a simple manner without degrading the gear interface between the worm and the worm wheel.
While the invention has been described with respect to one embodiment of one specific application, the invention has far greater application and many variations will be readily apparent to those skilled in the art. Accordingly, the invention should only be limited to that defined by the following claims.
Patent Application
20080087123
