GB2379490A

GB2379490A - Actuator having a worm gear mechanism

Info

Publication number: GB2379490A
Application number: GB0121882A
Authority: GB
Inventors: John Herbert Harvey
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 2001-09-10
Filing date: 2001-09-10
Publication date: 2003-03-12
Also published as: GB0121882D0

Abstract

An actuator including a worm gear mechanism 11, bearing means 15 and 16 that permits limited axial movement of shaft 14 relative to the housing 13 in response to axial loading of the shaft. The actuator being characterised by co-operable first and second breaking components (32, 15b; 33, 16b) wherein the first of the components 32, 33 rotates with the shaft 14 and a second of the components 15b is fixed with respect to the housing 13. One of the breaking components having an internal frusto-conical surface 32b and the other braking component having a corresponding external frusto-conical surface 32a, the frusto-conical surfaces being inter-engageable by axial movement of the shaft 14 from an axial rest position.

Description

ACTUATORS This invention relates to actuators of the kind incorporating a worm gear reduction mechanism between an input and an output of the actuator.

One particular form of actuator of the above kind is used for positioning a control surface of an aircraft and is driven by a rotary prime mover, for example an electric or hydraulic motor coupled to the input shaft. The worm gear of the mechanism is carried on the input shaft and meshes with a worm wheel in known manner such that rotation of the shaft about its longitudinal axis at a first rotational speed results in rotation of the worm wheel about its axis, lying at right angles to the worm shaft axis, at a much reduced speed.

The worm wheel is coupled to the axially stationary, but rotatable nut of a ball-screw mechanism which converts the rotational movement of the worm wheel into axial displacement of a push rod coupled to the control surface of the aircraft.

It is recognised that in such an arrangement should the control surface be held against movement, for example by an obstruction, or should the push rod be obstructed in some way then by virtue of the mechanical advantage achieved by the worm gear mechanism substantial loading can be applied to the push rod and the control surface with a high resultant risk of damage to the push rod or control surface. It is an object of the present invention to provide a braking mechanism limiting the torque which can be applied through the worm gear mechanism, and thus limiting the axial loading which can be applied to the push rod.

A related mechanism is disclosed in our European patent EP 0486223 B in which as loading on the actuator mechanism increases a ball bearing assembly is distorted such that fixed and moveable parts associated with the bearing assembly are pressed into facial contact in a plane transverse to the rotational axis of the mechanism and friction between the parts brakes the transmission of torque.

The mechanism disclosed in European patent EP 0486223 B applies braking action to the rotation of the worm wheel of the worm gear mechanism and is able to generate only a limited braking action by virtue of the area of the contact between said fixed and movable parts and the loading which can be applied thereto. Increase of the braking force can be achieved by increasing the contact area but this in turn necessitates increasing the diameter of the brake components which is undesirable in most applications. It is an object

t of the present invention to provide an actuator with significantly improved braking performance without a corresponding increase in component diameter.

In accordance with the present invention there is provided an actuator including a worm gear mechanism comprising a rotatable shaft carrying the worm gear of the worm gear mechanism for rotation about a first axis and a worm wheel meshing with said worm gear and rotatable thereby about a second axis to operate the actuator, bearing means mounting said shaft in a relatively stationary housing, said bearing means permitting limited axial movement of said shaft relative to said housing in response to an axial loading applied to the shaft, and, the actuator further including co-operable first and second braking components a first of which rotates with said shaft,

and a second of which is fixed with respect to said housing, one of said braking components having an internal frusto-conical surface, and the other of said braking components having a corresponding external frusto-conical surface, said frusto-conical surfaces being inter-engageable by axial movement of said shaft from an axial rest position.

Preferably said axial movement of said shaft from an axial rest position takes place against a resilient bias.

Desirably said resilient bias is provided by said bearing means at least part of which distorts resiliently during loading of said shaft axially from its rest position.

Conveniently the resilient bias is provided by a self-centring action of said bearing means.

Preferably the cone angle of said frusto-conical surfaces is substantially equal, and lies in the range 200 to 400.

Conveniently said bearing means includes a ball bearing assembly having inner and outer races separated by rolling balls, and one of said internal and external frusto-conical surfaces is provided on the inner or outer race of said ball bearing assembly.

Conveniently in said actuator said worm wheel drives the axially stationary nut of a ball-screw mechanism.

One example of the invention is illustrated in the accompanying drawings wherein: Figure 1 is a transverse cross-sectional view of an actuator, Figure 2 is an enlargement of part of Figure 1, and Figure 3 is a cross-sectional view of the actuator in a plane at right angles to the plane of the cross-section of Figure 1.

Referring to the drawings, the actuator which is illustrated is an actuator for an aircraft control surface and includes a worm gear reduction mechanism 11 driving a recirculating ball-screw mechanism 12. The ball-screw mechanism 12 converts rotational movement of the output of the gear reduction mechanism into rectilinear movement of a push rod.

The actuator includes a housing 13 which is fixed in use. A rotary input shaft 14 extends through part of the housing 13 and is journaled therein for rotation in bearings 15,16 to be described in more detail hereinafter. A midregion of the shaft 14 is formed integrally with a worm gear 17 which lies within the housing and protrudes radially from the shaft. Joumaled in the housing 13 for rotation about an axis transverse to and spaced from the axis of the shaft 14, is a worm wheel 18 meshing with the worm gear 17. It will be recognised that when the shaft 14 is rotated relative to the housing 13 the worm wheel 18 is driven by the worm gear 17, and rotates at a rotational speed much reduced from that of the shaft 14, but with significantly increased mechanical advantage.

The worm wheel 18 is hollow and encircles the nut 19 of a generally conventional recirculating ball-screw mechanism 12. The nut 19 is keyed to the worm wheel 18 to rotate therewith, the nut 19 being journaled for rotation within the housing 13, but being fixed against axial movement relative to the housing 13. The threaded shaft 21 of the mechanism 12 extends through the nut 19 and projects at its ends from the housing 12. The shaft 21 constitutes the push rod of the actuator and is coupled at one end in use to the control surface of the aircraft. It will be well understood that rotation of the worm wheel 18 rotates the nut 19 relative to the threaded shaft 21 and since, in use, the shaft 21 is held against rotation relative to the housing 13 then rotation of the nut 19 relative to the shaft 21 drives the shaft 21 axially relative to the housing. The gear reduction and mechanical advantage increase provided by the worm gear reduction mechanism 11 ensures fine control over the axial movement of the shaft 21 as a relatively large rotation of the input shaft 14 is needed to effect a relatively small axial movement of the shaft 21. The nature and operation of the recirculating ballscrew mechanism of the actuator will be well understood by those familiar with actuators, and is not of particular significance to the present invention.

It will be recognised that in use if the shaft 21 is held against axial movement, for example by jamming of the associated control surface, then considerable load can be applied to the shaft 21 as the input shaft 14 continues to rotate. The mechanical advantage of the actuator, that is to say the combined mechanical advantage of the worm gear reduction mechanism and the recirculating ball-screw mechanism is such that sufficient load may be applied to the shaft 21 to seriously damage the shaft 21 and/or the control

surface. Furthermore, balls and corresponding threads in the recirculating ball-screw mechanism may well be damaged. In order to minimise this risk a braking arrangement is incorporated into the actuator in association with the shaft 14.

With particular reference to Figures 1 and 2 it can be seen that the shaft 14 is supported on opposite sides of the worm 17 in the housing 13 by means of the bearing assemblies 15,16. Each of the bearing assemblies 15, 16 includes an inner race indicated by the suffix a secured to the shaft 14, an outer race indicated by the suffix b secured to the housing 13, and a plurality of bearing balls indicated by the suffix c interposed between the races in the usual manner.

Interposed between the worm gear 17 and each of the bearing inner races dz 16a are respective spacing collars 22,23 which position the races 15a, 16a relative to the worm gear 17. The outer race 16b of the bearing assembly 16 abuts, at its axially outermost end, a thrust washer 24 which in turn abuts an internal shoulder of the housing 13. The face of the thrust washer 24 remote from the outer race 16b defines one wall of an annular chamber in the housing encircling the shaft 14, the opposite wall being defined by a shoulder 26 of the housing 13. An O-ring seal 25 is received between the shoulder 26 and the thrust washer 24 and engages the outer surface of the shaft 14 to seal the interface of the shaft 14 and the housing 13 at the right hand end of the shaft as shown in Figure 1.

A similar thrust washer 27 engages the axially outer end of the outer race 15b of the bearing assembly 15. The thrust washer 27 is received within the

housing 13, and its opposite face abuts an axial end of an annular closure member 28 rigidly secured within the housing 13. The closure member 28 has an internal shoulder 29 defining, with the thrust washer 27, a chamber within which an annular sealing ring 31 similar to the sealing ring 25, is received to seal the interface of the shaft 14 and the housing 13 at the right hand end of the shaft as illustrated in Figure 1.

In the usual manner the bearing balls of the bearing races are located by respective cages parts of which are visible in Figure 1.

The inner races 15a, 16a of the bearing assemblies 15,16 are formed integrally, at their axially innermost ends, with respective radially outwardly extending flanges 32,33. The radially outermost edge of each flange 32,33 is chamfered so as to provide thereon a frusto-conical surface 32a, 33a respectively. The axially innermost end of each outer race 15b, 16b terminates adjacent, but spaced from the respective collar 22,23 and is chamfered to provide a frusto-conical surface 32b, 33b parallel to the corresponding surface 32a, 33a of the respective flange 32,33.

During normal operation of the actuator the self-centring action of the bearing assemblies 15,16 centres the shaft 14 in the housing relative to the wheel 18 so that the central region of the worm gear 17 meshes with the periphery of the worm wheel 18. Furthermore, in the centred position of the shaft 14 each frusto-conical surface 32a is spaced from the mating frustoconical surface 32b.

The operation of the actuator is as follows. Let it be assumed that the shaft 14 is rotating in a direction to cause counter-clockwise rotation of the worm wheel 18 as viewed in Figure 1. The recirculating ball-screw mechanism 12 is sufficiently friction-free that there is substantially no impediment to rotation of the shaft 14 and the worm wheel 18. Thus there is substantially no axial loading on the shaft 14 during its rotation. However, let us now assume that the shaft 21 of the actuator is impeded, so that further axial movement thereof is prevented. Continued rotation of the shaft 14 attempts to continue the rotation of the worm wheel 18 in a counter-clockwise direction and as this is not possible the shaft 14 is driven to the left from the position illustrated in Figure 1. Such movement is of course opposed by the bearing assembly 15 which in effect is interposed between the collar 22 and the closure member 28. Thus the race 15a is moved to the left with respect to the race 15b from the positions illustrated in Figure 2 and the gap between

the frusto-conical surface 32a and the frusto-conical surface 32b is taken up.

At this point it is useful to recognise that the relative movement of the race 15a axially relative to the race 15b can be accommodated either by the normal operating clearance of the bearing, or by resilient distortion of the race 15a, the balls 15c and the race 15b under the loading imposed by continued rotation of the shaft 14. It will be understood that the frustoconical surface 32a of the flange 32 is rotating with the shaft 14 while the frusto-conical surface 32b of the race 15b is fixed. Thus immediately the surface 32a contacts the surface 32b a braking effect is generated and the more that the surface 32a is loaded axially against the surface 32b the greater is the braking effect. Thus the more axial loading that is applied to the shaft 14 then the greater is the braking effect resisting further rotation of the shaft

14 and thus protecting the actuator, and perhaps also the aircraft control surface, against damage.

In the event that the shaft 21 is impeded from axial movement during rotation of the shaft 14 in the opposite direction then it will be recognised that the bearing 16 performs the braking action, but that the operation is essentially as described above with respect to the bearing assembly 15.

Where the operation of the braking mechanism relies upon resilient distortion of the bearing assemblies it is to be recognised that the distortion will be sufficiently small for there to be no permanent damage to the bearing assemblies, and that when the over-load situation is removed, the bearings will restore to their normal operating configuration. Furthermore, it will be understood that in some applications it may be possible to dispense with the spacer collars 22,23 and to have the inner races abutting the axial ends of the worm gear 17 directly. Still further, although it is convenient to provide the frusto-conical braking surface on the races of the bearing assemblies this is not essential, and one or both frusto-conical surfaces could be provided on integral parts of the shaft 14 and housing 13 respectively, or on other components carried by the shaft and the housing respectively.

The braking effect which can be achieved by two mating frusto-conical surfaces is significantly greater than the braking effect which could be obtained from two planar surfaces of equivalent area. Furthermore, the degree of braking available is to some extent dependent upon the cone angle of the frusto-conical surfaces. Naturally the larger the cone angle becomes then the lower is the braking force for a given contact area. However, as the

cone angle is reduced there is an increasing risk that the inner frusto-conical surface will become wedged within the outer frusto-conical surface, and that the surfaces will not disengage when the over-load condition is removed.

Consequently the brake would not reset and the actuator would not subsequently be usable. Clearly this is undesirable, and although precise cone angles have not been determined for some applications, since this will be determined by the maximum diameters which can be permitted within the application, and the maximum torque that the brake is expected to experience

in use, it is anticipated that a cone angle of at least 20 (giving a face contact angle of around 10 ) will be necessary to prevent seizure of the components and that cone angles in excess of 400 are unlikely to give the required braking forces without significantly increasing component size.

Claims

CLAIMS 1. An actuator including a worm gear mechanism (11) comprising a rotatable shaft (14) carrying a worm gear (17) of the worm gear mechanism for rotation about a first axis and a worm wheel meshing (18) with said worm gear (17) and rotatable thereby about a second axis to operate the actuator, bearing means (15,16) mounting said shaft (14) in a relatively stationary housing (13), said bearing means permitting limited axial movement of said shaft relative to said housing in response to an axial loading applied to the shaft, and, the actuator further including co-operable first and second braking components (32, 15b ; 33, 16b) and being characterised in that a first of said components (32;

33) rotates with said shaft (14), and a second (15b) of said components is fixed with respect to said housing (13), one of said braking components having an internal frusto-conical surface (32b), and the other of said braking components having a corresponding external frusto-conical surface (32a), said frusto-conical surfaces being inter-engageable by axial movement of said shaft (14) from an axial rest position.
2. An actuator as claimed in Claim 1 characterised in that said axial movement of said shaft (14) from an axial rest position takes place against a resilient bias.
3. An actuator as claimed in Claim 2 characterised in that said resilient bias is provided by said bearing means (15,16) at least part of which distorts resiliently during loading of said shaft axially from its rest position.

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4. An actuator as claimed in Claim 2 characterised in that said resilient bias is a self-centring action of said bearing means.
5. An actuator as claimed in any one of Claim s 1 to 4 characterised in that the cone angle of said frusto-conical surfaces is substantially equal, and lies in the range 200 to 400.
6. An actuator as claimed in any one of the preceding claims characterised in that said bearing means includes a ball bearing assembly having inner and outer races separated by rolling balls, and one of said internal and external frusto-conical surfaces is provided on the inner or outer race of said ball bearing assembly.
7. An actuator as claimed in any one of the preceding claims characterised in that said worm wheel (18) drives the axially stationary nut (19) of a ball-screw mechanism (12).