GB2200714A

GB2200714A - Worm gear transmissions

Info

Publication number: GB2200714A
Application number: GB08702643A
Authority: GB
Inventors: Timothy John Holman; Roy Morris
Original assignee: Horstman Defence Systems Ltd
Current assignee: Horstman Defence Systems Ltd
Priority date: 1985-02-07
Filing date: 1987-02-06
Publication date: 1988-08-10
Anticipated expiration: 2007-02-06
Also published as: GB8702643D0; GB2200714B

Abstract

A transmission mechanism, such as a winch, comprises meshing worm gears (14, 16) which provide an irreversible mechanism with speed reduction. The output worm gear (16) is rotatably supported on a spigot (38) which is fixed to a wall of the housing (2) of the mechanism. The housing (2) comprises two parts which meet at a parting plane (60), one part supporting the input worm gear (14) and the other supporting the output worm gear (16). The parting plane (60) is parallel to the axes of the worm gears (14, 16). <IMAGE>

Description

MECHANICAL TRANSMISSIONS This invention relates to mechanical transmissions.

British Patent Specification No. 1139171 discloses a mechanical transmission in the form of two meshing worms which rotate about axes which are almost, but not quite, parallel to each other. By appropriate design of the worms, and in particular the lead angle of their threads, it is possible to obtain a transmission in which drive can be transmitted from an input shaft to an output shaft without there being any possibility of the output shaft driving the input shaft. Thus, the transmission is irreversible and can be considered as analogous to a worm and wheel transmission. However, a transmission in accordance with British Patent Specification 1139171 has several advantages over a worm and wheel transmission. For example, it can be designed to have a higher efficiency and a lower transmission ratio.Devices in accordance with British Patent Specification 1139171 can conveniently be 11 referred to as "twin worm drives".

Although twin worm drives have been known for several years, they have not so far been used in commercially available equipment. One possible reason for this is that the slightly inclined axes of the meshing worms create difficulties in mounting the worms accurately in a housing. Also, substantial axially directed forces are generated in twin worm transmissions, which need to be taken into account.

According to the present invention there is provided a transmission mechanism comprising two meshing worm gears mounted in a housing for rotation about axes which are almpst parallel to each other, one of the worm gears being rotatably supported on a spigot which is fixed to a wall of the housing.

The worm gear supported in a spigot may be mounted in such a way that axial forces in one or both directions are transferred to the spigot.

The spigot may be mounted on the side of the housing from which the input shaft of the transmission projects. The spigot preferably supports the worm gear which serves as the output of the transmission.

In one embodiment, the housing comprises two housing parts which meet each other at a plane which is parallel to at least one of the axes of the. worm gears.

The worm gears are independently mounted in the respective housing parts, and the housing parts are then brought together to bring the worm gears into mesh.

In another embodiment, the housing parts meet each other at a mating plane which is perpendicular to the axis of one of the worm gears.

Another aspect of the present invention provides a transmission mechanism comprising an input member and an output member which are drivingly interconnected by a twin worm device and a gear wheel transmission connected in series between the input and output members.

With such a construction, the twin worm device provides irreversibility, preventing the output member from driving the input member, while the gear wheel transmission provides a mechanical advantage or velocity ratio between the input and output members.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a sectional view of a winch, taken on the line I-I in Figure 2; Figure 2 is a view taken in the direction of the arrow II in Figure 1; Figure 3 is a sectional view of an irreversible reduction gear mechanism, taken on the line III-III in Figure 4; and Figure 4 is a view taken in the direction of the arrow IV in Figure 3.

The winch shown in Figure 1 comprisesa housing 2 in which an input shaft 4 and an output "shaft 6 are rotatable. The input shaft 4 is provided with a handle 8 for rotating the input shaft 4. The output member 6 comprises a winch drum 10 and a cylindrical projection 12. The input shaft 4 and the output member 6 are drivingly interconnected by input and output worm gears 14 and 16. The input worm gear 14 is formed integrally on the input shaft 4, while the output worm gear 16 is fastened to the cylindrical portion 12 of the output member 6 by screws 18. The input shaft 4 is supported in a cylindrical projection of the housing 2 by means of two spaced rows of needle bearings 20. At the end of the shaft 4 away from the worm gear 14, there is a drive element 22 having a portion 24 of square cross section to receive the handle 8.

A first thrust bearing 26 acts between the drive element 22 and a shoulder 28 -provided at the outer end of a cylindrical projection 29 of the housing 2. A further thrust bearing 30 acts between an abutment 32 on the input shaft 4 and a further shoulder 34 at the inner end of the cylindrical projection 29.

The output member 6 is rotatably supported on a spigot 38 which is fixed to the wall of the housing 2.

The spigot 38 has an enlarged boss 40 which is received in a bore 42 in the housing wall. A thrust bearing" 44 acts between the output worm gear 16 and a radial face of the boss 40. A further thrust bearing 46 acts between a shoulder 48 formed within the output member 6 and a plate 50 which is fastened to the end of the spigot 38. The output member 6 and the output worm gear 16 are supported on the spigot 38 by needle bearings 52 and 54 respectively.

As can be appreciated from Figure 1, the'housing 2 is in two parts, namely a first part 56 which carries the input shaft 5 and a second part 58 which carries the output member 6. These two parts are fitted together at a mating plane 60, which is parallel to the axes of the input shaft 4 and the output member 6.

Thus, the depth of meshing of the worm gears 14 and 16 can be controlled by introducing shims between the housing parts 56 and 58 at the parting plain 60.

In the embodiment shown in Figures 1 and 2, the lead angle of the worm gear 14 is 50, while that of the worm gear 16 is 110. The threads of the worm gears are of opposite hand, and this relationship means that the axes of the input shaft 4 and the output member 6 must be inclined to each other by 60 if the worm gears 14 and 16 are to mesh properly. This mutual inclination is not visible in Figures 1 and 2, but would be visible in a view taken in the direction perpendicular to the parting plane 60.

In operation, turning of the handle 8 would rotate the worm gear 14. This in turn would rotate the worm gear 16, although the peripheral speeds of the two gear wheels 14 and 16 would not be the same. The transmission ratio between the worm gear 14 and the worm gear 16 is approximately 2:1 or 3:1.

Should the handle 8 be released while a load is suspended from a cable 62 wound on the drum 10, the load would not be able to drive the handle 8 through the meshing worm gears 16 and 14 because the friction angle at the cooperating tooth faces of the worm gears 14 and 16 is greater than the lead angle (50) of the worm on the worm gear 14. The construction shown in Figures 1 and 2 thus provides an irreversible mechanism with a speed ratio between the handle and the winch drum 10 which is considerably less than would be the case if the worm gears 14 and 16 were replaced by an irreversible worm and wheel transmission.

Figures 3 and 4 show a different mechanism, which is suitable for turning a large rotating assembly. A section of a ring gear 102 is shown in Figure 1, and this ring gear would be secured to the large rotating assembly to be turned. In this embodiment, the ring gear 102 can be turned by means of a handle 104.

Rotation of the handle 104 turns an input worm gear 106 which then turns an output worm gear 108. An output pinion 110 is rotationally connected to the output worm gear 108 by a clutch pack 112. This pinion 110 meshes with the ring gear 102.

The input and output worm gears 106 and 108 are accommodated in a housing 114. The input worm gear 106 is formed integrally with an input shaft 116 which is supported at one end by a bearing 118 in a bore formed in the housing 114. At the other end, the input shaft 116 is supported by a bearing 120 in a bearing cap 122 which is fastened to the housing 114. Thrust bearings 124 and 126 are provided to retain the input worm gear 106 in the axial direction.

The output worm gear 108 is supported at one end on a spigot 128 which is fastened to the housing 114.

The worm gear 108 is formed integrally with a shaft portion 130 which is supported in a support body 132 by bearings 134. Thrust loads on the output worm gear 108 are resisted by a thrust bearing 136 positioned between the end of the spigot 128 and an adjacent face of the worm gear 108, and by a thrust plug 138 which is fitted into the support body 132 and engages the end face of the shaft portion 130.

Between the worm gear 108 and the shaft portion 130, the integral body from which these elements are formed is provided with a cylindrical portion 140, a splined portion 142 and a screwthreaded portion 144. A sealing element 146 surrounds the outer surface of the cylindrical portion 140. This sealing element is fastened to the pinion 110 and is provided with a spiral groove formation 147. This formation meshes with a drive pinion (not shown) of a gauge 149 (Figure 2) for indicating the angular position of the ring gear 102. The splines on the splined portion 142 engage alternate discs of the clutch pack 112, the other discs of this clutch pack engaging internal splines formed within the pinion 110. The screwthreaded portion 144 receives an adjusting nut, which acts on the pinion 110 through a belleville washer, and so determines the pressure applied to the clutch pack 112.

As in the embodiment of Figures 1 and 2, the input worm 106 has a lead angle of 50 and the output worm 108 has a lead angle of 110. As is apparent from Figure 2, the axes of the input and output worm gears 106 and 108 are inclined to each other by an angle of 60. In normal operation, turning of the handle 104 will rotate the input worm gear 106, this motion being transmitted to the output worm gear 108 and the pinion 110, which then rotates the ring gear 102. In the embodiment of Figures 3 and 4, the transmission ratio between the input worm gear 106 and the output worm gear 108 is 2.67:1. The pinion 110 and the ring gear 102, however, introduce a further, much larger, reduction ratio. As with the embodiment of Figures 1 and 2, the worm gears 106 and 108 will lock to prevent the transmission of motion from the ring gear 102 to the handle 104.By providing the worm gears 106 and 108 in series with the gear wheels 110 and 102 between the handle 104 and the body to be driven, the transmission of Figures 3 and 4 provides an irreversible mechanism with a high reduction ratio, having a greater efficiency than could be achieved using a worm and worm wheel mechanism.

The large rotating body to which the ring gear 102 is secured may have a considerable" mass, and consequently, even when rotating relatively slowly, its angular momentum can be very large. Since the worm gears 106 and 108 will lock to prevent further rotation of the worm gear 108 as soon as an operator stops turning the handle 104, very high inertia loads would be applied to the mechanism if the large rotating body were to be stopped almost dead. Such high loads are avoided by means of the clutch pack 112 which enables the kinetic energy of the large rotating body to be absorbed over a period of time.

In both embodiments, the worm gears 14, 16, 106 and 108 are formed with multi-start threads (from 2 to 8 starts) and have an axial extent which is at least three times the axial pitch of the threads.

Consequently, the meshing worm gears engage each other at at least three axially spaced locations, so spreading the load between these locations.

Although various features of the disclosed embodiments are indicated in this description as being inventive, other features may also be inventive, whether taken alone of in combination. Furthermore, features of one of the embodiments may be incorporated in the other embodiment. For example, a reduction gear mechanism similar to that of Figures 3 and 4 may be accommodated in a two-part housing similar to that shown in Figures 1 and 2, wherein each of the worm gears may be supported in a respective one of the housing parts.

Claims

1. A transmission mechanism comprising two meshing worm gears mounted in a housing for rotation about axes which are almost parallel to each other, one of the worm gears being rotatably supported on a spigot which is fixed to a wall of the housing.

2. A transmission mechanism as claimed in claim 1, in which axial forces applied to the said one worm gear are transferred to the spigot.

3. A transmission mechanism as claimed in claim 2, in which the said one worm gear and the spigot have oppositely disposed faces extending transversely of the axis of the worm gear, a thrust bearing being disposed between the faces.

4. A transmission mechanism as claimed in any one of the preceding claims, in which the housing comprises two separable housing parts, each housing part supporting a respective one of the worm gears.

5. A transmission mechanism as claimed in claim 4, in which the parting plane between the two housing parts is parallel to the axis of at least one of the worm gears.

6. A transmission mechanism as claimed in any one of the preceding claims, in which a gear wheel transmission is connected in series with the worm gears.

7. A transmission mechanism as claimed in claim 6, in which one of the worm gears is rotationally coupled to the gear wheel transmission by a torque limiting clutch device.

8. A transmission mechanism substantially as described herein with reference to, and as shown in, Figures 1 and 2 or 3 and 4 of the accompanying drawings.