GB2527906A

GB2527906A - Torque sensor

Info

Publication number: GB2527906A
Application number: GB1507502.1A
Authority: GB
Inventors: Thomas R Stafford; John Greenslade
Original assignee: Rotork Controls Ltd
Current assignee: Rotork Controls Ltd
Priority date: 2014-04-30
Filing date: 2015-04-30
Publication date: 2016-01-06
Anticipated expiration: 2035-04-30
Also published as: GB2527906B; GB201507502D0; GB201407623D0

Abstract

A torque sensor 100 comprises a shaft 125 rotatable in use to cause a rotation of an output (for example, a gear 110), wherein the shaft is axially moveable in two directions in response to thrust loads caused by the applied torque. A housing at least partially surrounds 160 the shaft. A spring pack assembly 140 is disposed on the shaft 125, the spring pack comprising: first 146 and second 147 bearings. Each bearing is disposed between the shaft 120 and the housing 160 and is axially movable relative to both the shaft and the housing. A resiliently axially compressible spring member 142 is disposed between the first and second bearing assemblies. The shaft 120 has first 122 and second 128 axial stops respectively positioned on the outside of the first 146 and second 147 bearings and arranged to limit the axial position of the bearings relative to the shaft. The housing 160 has first 162 and second 166 axial stops respectively positioned on the outside of the first 146 and second 147 bearings and arranged to limit the axial position of the bearings relative to the housing.

Description

Torque Sensor This invention relates to a torque sensor and in particularly, but not exclusively, to a torque sensor for sensing torque in a worm gear driven actuator.

The force applied to a motor-driven actuating mechanism, such as a valve actuator, is often controlled by a torque limiter, in which torque reaction in the actuating mechanism is utilised to interrupt the power to the driving motor once a pre-set value of torque is reached. A typical motor-driven actuating mechanism may have a worm and wheel. In use the worm shaft may be able to move (at least to a limited degree) in the axial direction. Such axial movement of the worm shaft occurs as a result of the thrust loads on the shaft which are dependent upon the torque being produced.

Torque sensors are known for use in such torque limiters in which springs (such as a stack of disc springs) are provided on the motor shaft to provide mechanical resistance to the movement of the shaft under load. An example of such an arrangement is, for example, shown in UK Patent GB 1 446 005. The basic operation of such a torque limiter is illustrated schematically in Figure 1, the motor shaft 20 is driven in use by a motor 30 and includes a worm 25 for driving a gear 10 associated with a drive shaft (which may for example be arranged for actuating a valve in use). A torque sensor arrangement 40 includes a pair of stacks of disc springs 42, 44 (one for each axial direction) with a bearing 46 disposed between. The bearing 46 is fixed relative to a housing and the shaft 20 is axially moveable relative to the bearing 46. In figure 1(a) the shaft and spring assemblies are shown in their neutral position (i.e. undisplaced). Depending upon the direction of the thrust load on the shaft 20 the shaft moves relative to the bearing 46 against one of the springs as shown in figure 1(b) where the shaft has been displaced to the right and has compressed spring 44. The displacement of the shaft against the spring provides a measure of the torque applied to the worm and actuator output (and as a mechanical measure is impendent of voltage fluctuations). The resulting displacement may be utilised to switch off (or pause) the electric motor 30 by means of a torque limit switch 50 and contactor (or alternatively to disconnect a fluid-powered motor through a suitable valve system).

Conventionally, the torque sensor is located on the end of the motor because that is where the worm shaft can be accessed and adapted. However, in some cases, such as valve actuators where a separable motor is employed, the torque sensor cannot be positioned there because there is no access to the motor shaft at the enclosed end of the motor housing. This requires the sensor to be moved inboard and so adjusting and installation may be more problematic.

At least some embodiments of the present invention are intended to provide an alternative torque sensor arrangement which may address at least some of the above issues.

According to one aspect of the invention, there is provided a torque sensor comprising: a shaft rotatable in use to cause a rotation of an output (for example, a gear), wherein the shaft is axially moveable in two directions response to thrust loads caused by the applied torque; a housing at least partially surrounding the shaft; and a spring pack assembly disposed on the shaft, the spring pack comprising: first and second bearings, each bearing being disposed between the shaft and the housing and being axially movable relative to both the shaft and the housing; a resiliently axially compressible spring member disposed between the first and second bearing assemblies; wherein the shaft has first and second axial stops respectively positioned on the outside of the first and second bearings and arranged to limit the axial position of the bearings relative to the shaft; and the housing has first and second axial stops respectively positioned on the outside of the first and second bearings and arranged to limit the axial position of the bearings relative to the housing.

The axial stops on the housing and shaft may be arranged such that movement of the shaft in a first axial direction displaces the first bearings relative to the housing whilst the second bearing remains fixed relative to the housing.

Movement of the shaft in a second axial direction (i.e. the opposite direction to said first direction) displaces the second bearing relative to the housing whilst the first bearing remains fixed relative to the housing. As such, the spring may be compressed by displacement of the shaft in both axial directions.

Upon movement of the shaft in a first axial direction movement of the first bearing may be axially limited relative to the shaft and movement of the second bearing is axially limited relative to the housing. Upon movement of the shaft in the other axial direction movement of the first bearing may be axially limited relative to the housing and movement of the second bearing is axially limited relative to the shaft.

The shaft may be a worm shaft. The torque sensor may be a bi-directional torque sensor. Thus, embodiments of the invention may for example provide a simplified arrangement and/or a reduced part count and/or simplified assembly by avoiding the need to provide a separate spring assembly for each direction of thrust.

The spring member may comprise a stack of disc springs.

The spring pack assembly may further comprise at least one shim positioned between the spring member and one of the first or second bearing.

The, or each, shim may be provided between only one of the bearings and the spring member. The provision of shims within the spring pack assembly may allow for simple initial adjustment of the torque sensor. Advantageously, the stop positions on the shaft and/or housing may be predefined (for example being machined into the shaft and/or housing) and any adjustments/alignment may be made by the provision of the at least one shim.

The spring pack assembly may further comprise a compression member arranged to limit the relative inward axial movement of the first and second bearings. Thus the compression member may be arranged to prevent over compression of the spring member. The compression member may comprise a tubular member disposed around the shaft. The compression member may for example surround at least a portion of the spring. The compression member may have a diameter which is less than the outer diameters of the first and second bearings. Thus, the thrust faces of the bearings engage the compression member at the maximum compression of the spring.

The bearings may be deep groove bearings. Advantageously, deep groove bearings are able to transfer thrust loads to the spring assembly.

The axial spacing between the first and second stops on the shaft and the first and second stops of the housing may be substantially similar. As such, when the spring pack assembly is in its neutral position (i.e. the shaft is not axially displaced) the outer surface of each bearing may be generally aligned with the stops of both the shaft and the housing.

The axial spacing between the first and second stops on the shaft may be greater than the axial spacing between the first and second stops of the housing.

The stops on the housing may preload the spring member in use. For example, the relative sizing of the spring pack assembly and the spacing between stops on the housing may be such that the spring member is compressed when assembled within the housing. The relative sizing of the spring pack assembly and the spacing between stops on the shaft may be such that the spring member is substantially uncompressed when assembled on the shaft without being fully positioned within the housing. The difference between the axial spacing may only be small but may be sufficient to allow the spring pack assembly to be positioned on the shaft without loading the spring member and for the stops on the housing to preload the spring member upon assembly of the spring pack assembly within the housing.

The housing may define a bore through which at least a portion of the shaft extends. The bore may be provided with a locally increased diameter to define a space which encloses the spring pack. At least one of the axial stops of the housing may be formed by a radially extending shoulder. The, or each, shoulder may be defined by the local increas in diameter. The housing may be formed of interconnected housing parts and, for example, the first and second axial stops may be formed on separate housing parts.

At least a portion of the shaft may have an increased diameter providing a shoulder which forms one of the axial stops. Additionally or alternatively, at least one retaining ring may be provided at a fixed location on the shaft to form one of the axial stops.

According to a further aspect of the invention there is provided a torque limiter comprising a torque sensor as herein described and a controller arranged to interrupt driving power to the shaft in response to predetermined torque limit being exceeded.

According to a further aspect of the invention there is provided a valve actuator comprising: a motor for rotating the drive shaft in use; an output driven by the drive shaft and arranged for actuating a valve in use; and a torque limiter as herein described.

The output may comprise a gear. The gear may be driven by a worm associated with the drive shaft.

The housing may at least partially house components of the valve actuator as well at least partially surrounding the shaft of the torque sensor.

According to a further aspect of the invention there is provided a valve assembly including a valve and a valve actuator as herein described.

Whilst the invention has been described above with reference to a number of embodiments and aspects it is to be understood that it includes any inventive combination of the features set out above or in the following

description or drawings.

Specific embodiments of the invention will now be described in detail, by way of example only, and with reference to the accompanying drawings in which: Figure 1 schematically illustrating the operation of an existing torque limiter; Figure 2 schematically illustrates a cross sectional view of a torque sensor arrangement in accordance with an embodiment of the invention; and Figure 3 schematically illustrates the operation of the embodiment of figure 2.

As used herein, references to the axial direction will be understood to mean the direction extending generally parallel to the axis of the drive shaft for which torque related displacement is being measured. Thrust will be understood to refer to loads which act substantially in an axial direction (and, accordingly, a thrust surface will, for example be understood to refer to a surface which transfers a thrust load). Inner and outer are used herein, in particular with reference to the components of or associated with the spring pack, and will be understood to refer to directions or surfaces which are respectively directed towards the spring member (i.e. inwardly with reference to the spring) or away from the spring member (i.e. outwardly with respect to the spring).

A torque sensor assembly 100 in accordance with an embodiment of the invention is shown in figure 2. It will be appreciated that the torque sensor 100 will typically be part of a torque limiting arrangement, for example in a valve actuator. The incorporation of torque sensing and limiting arrangements into such actuators would be known to the skilled person. One example of a known actuator incorporating a torque limiting arrangement (of the general type shown in figure 1) is the "A" Range actuator produced by Rotork Controls Limited.

The assembly is at least partially contained within a housing 160 which may typically be formed as part of, or integrally with, the housing of the valve actuator (it will be appreciated that such housing may be formed as a casting with environmental sealing). The actuator comprises a motor or other power source (not shown) which is arranged to rotationally drive a shaft 120. The shaft includes, carries or is associated with a worm 125 (and typically the shaft 120 may be integrally formed with the worm 125) for meshing with and driving a worm gear 110. As will be appreciated by those in the art, the worm gear 110 may typically be connected to an output of the actuator and will, for example, be used to actuate a valve during use.

The torque sensor assembly 110 in accordance with an embodiment includes the shaft 120, which is associated with the worm 125. In use shaft 120 is axially displaceable in two directions as a result of thrust loads caused by the applied torque. The torque sensor 110 further includes a spring pack assembly mounted on the shaft 120 at a location which is axially spaced away from the worm 125.

A spring pack mounting location 121 is defined on the shaft 120 between two axial stops 122 and 126. The position of the axial stops 122 and 126 is fixed on the shaft. The first axial stop 122 is formed by a radially extending surface of a shoulder which is formed by a local reduction in diameter of the shaft 120 (and it will be noted that the diameter of the shaft to the outside of the shoulder is increased by a tapered section although it will be appreciated that this is optional and may depend upon the overall actuator configuration). As such, the position of the first axial stop 122 is pre-defined (and permanently fixed) relative to the shaft. The second axial stop 126 is provided by a retaining ring 128 provided on the shaft 120 (such that the spring pack assembly 140 may be removably mounted onto the shaft mounting location 121). The axial mounting position of the retaining ring 128 is pre-defined (and permanently fixed) on the shaft 120 by a mounting formation 127. In the illustrated example the mounting formation 127 includes a local reduction in diameter to provide an outwardly facing shoulder against which the retaining ring 128 abuts and is retained (for example by a circlip which fits into an outward section of the formation 127).

The spring pack assembly 140 includes two bearings 146 and 147. A spring member 142 is sandwiched between the bearings 146 and 147. In the illustrated embodiment the spring member 142 comprises a stack of disc springs (stacks of disc springs are generally used in the art although it will be appreciated that other spring arrangements such as a helical spring could provide a similar function).

The first 146 and second 147 bearings each comprise a deep groove ball bearings assembly. The bearings allow relative rotation between the shaft 120 and the housing 160 but may also transfer thrust loads. Each bearing includes an inner thrust surface 146a and 147a which face the spring 142 and an outer thrust surface 146b and 147b which is outwardly facing relative to the spring pack assembly 140 (and face the stops 122 and 126 of the shaft). The bearings 146 and 147 are slidably mounted on mounting location 121 the shaft 120 such that they may be inwardly compressed against the spring 142 (as will be explained in detail below). The outermost position of the first 146 and second 147 bearings are respectively limited by the first axial stop 122 and second axial stop 126 provided on the shaft. In the initial position shown in figure 2 (which it will be understood is generally intended to represent the neutral or non-displaced position of the assembly), the outer thrust surfaces 146b and 147b of the bearing 146 and 147 generally adjacent to the respective stops 122 and 126 on the shaft. In order to assist in assembly, the axial spacing between the stops 122 and 126 on the shaft is slightly greater than the desired axial length of the spring pack assembly 140 (which, as will be explained below corresponds to the spacing between the stops 162 and 166 of the housing). Thus, in the assembled (and undisplaced) arrangement shown in figure 2 there is a slight clearance gap between the stops 122 and 126 of the shaft and the outer thrust surfaces of the bearings 146 and 147 (although it will be appreciated that any such clearance gaps are not shown to scale in in the schematic drawings for 1] clarity purposes) Between the stops 122 and 126 the mounting location 121 of the shaft 120 has a substantially constant and smooth external surface to enable sliding of the components of the spring pack assembly 140 relative to the shaft.

In order to simplify assembly of the spring pack assembly 140 at least one shim 143 may be provided between the spring 142 and one of the bearings 146 and 147 (although it will be appreciated that in some embodiments no shims may be needed -although this may, for example, be dependent upon the manufacturing tolerance of any given individual assembly). In the illustrated example two shims are disposed between the spring 142 and second bearing 147 (although alternatively a shim could be provided between the spring and both the first and second bearing). The shim(s) 143 enable adjustment to be made to the spring pack assembly 140, for example to allow for manufacturing tolerances. Adjustments allow the clearance between the spring pack assembly and the axial stops 122 and 126 on the shaft 120 to be set at an appropriate amount. The shim(s) 143 may, therefore, be used to reduce the play/lost-motion in the spring pack 140 and provide an arrangement with relatively low hysteresis. Advantageously, embodiments of the invention can be arranged such that the shimming of the spring pack assembly 140 is the only adjustment required in assembly and/or maintenance of the system (with the mounting position of the spring pack assembly fixed by the mounting location 121 defined between axial stops 122 and 126 provided on the shaft).

The spring pack assembly 140 further comprises a compression member which is provided between the first 146 and second 147 bearings. The purpose of the compression member is to prevent over-compression of the spring 142. As shown in the illustrated embodiment, the compression member may be a tubular member which partially surrounds a portion of the shaft and the spring 142. The compression member 145 is positioned between the spring 142 and the housing 162. The compression member 145 is axially moveable relative to the shaft 120, housing 160 and spring 140. However, the diameter of the compression member is less than the outer diameter of the bearings 146 and 147 such that its end faces are disposed between the inner thrust surfaces 146a and 147a of the bearings. Thus, it will be appreciated that in the event that the spring 142 is sufficiently compressed the compression member 145 will transfer thrust loads between the inner thrust surfaces 146a and 147a so as to prevent over compression of the spring 142.

The housing 160 defines a bore 163 which is configured to receive at least a portion of the shaft 120. A portion of the bore 164 is configured to receive the spring pack assembly 140 (and may typically substantially surround the spring pack assembly). The spring pack receiving portion 164 may conveniently be formed by a portion of the bore having an increased internal diameter. It will be appreciated that the housing 160 may be formed from more than one interconnected housing portions and as such at least part of the spring pack receiving portion 164 may be formed from a separate housing portion to allow positioning of the spring pack assembly 140 within the receiving portion 164 during assembly or maintenance. The radial sides of the spring pack receiving portion 164 within the bore each provide a shoulder which respectively define a first 162 and second 166 axial stop (it will of course be appreciated that such axial stops could be formed in other ways so as to define the spring pack receiving portion therebetween -for example, the stops could comprise radial projections formed in the walls of an otherwise substantially constant diameter bore). The first 162 and second 166 axial stops are axially spaced apart by a fixed distance (corresponding to the axial length of the increased diameter portion 164). Between the stops 162 and 166 the receiving portion 164 of the housing 160 has a substantially constant and smooth internal surface to enable sliding of the components of the spring pack assembly 140 relative to the housing. It will be noted that the axial distance between the stops 162 and 166 of the housing is substantially similar to the axial spacing between the stops 122 and 126 provided on the shaft. However, the axial spacing between the stops 162 and 164 of the housing is slightly less than the stops 122 and 126 of the shaft 120. Thus, in contrast to the stops 122 and 126 of the shaft 120, the stops 162 and 164 of the housing are intended to engage the thrust surfaces 146b and 147b of the bearings in the neutral (undisplaced) position of the spring pack 140 shown in Figure 2. Thus, the stops of the housing 162 and 166 act to preload the spring member 142 of the spring pack 140 when assembled.

Operation of the torque sensor of the embodiment will now be described with reference to figures 2 and 3. It should be appreciated that the drawings are purely schematic and for the purpose of understanding the operation of the invention and, accordingly, the illustrated displacements are not intended to be representative of the actual scale or amount of deflection. As discussed above, with the shaft 120 in the initial, undisplaced, position of figure 2 the spring 142 biases the bearings 146 and 147 outwardly. The spring pack assembly 140 is typically assembled on the shaft 120 prior to positioning within the housing 160 (or prior to fully assembling the housing 160 around the shaft and/or spring pack). The outer facing thrust surfaces 146b and 147b are adjacent to the respective first 122 and second 126 stops of the shaft. An appropriate shim 143 has been provided between the spring 142 and one of the bearings to set an appropriate clearance gap between the bearings 146 and 147 and the stops 122 and 126 and, in particular to account for manufacturing tolerances.

The shaft 120 and spring pack 140 are then assembled relative to the housing 160 (which may comprise attaching a removable portion of the housing to a first portion which is already positioned relative to the spring pack/shaft).

Due to the relative spacing of the first 162 and second 166 stops of the housing 160, the outerfacing thrust surfaces 146b and 147b abut the respective first 162 and second 166 stops of the housing 160. Thus, the spring member 142 of the spring pack assembly 140 is compressed and preloaded by the housing 160.

Since the spacing between the shaft stops 122 and 126 and the housing stops 142 and 146 are fixed and any manufacturing tolerances in the spring pack assembly 140 are accounted for by the shims 143 a desired amount of preload of the spring 142 can be easily achieved. As the shaft 120 is driven to turn the gear 110 (via worm 125) resulting torque loads will cause a thrust load on the shaft 120 in one of the two axial directions (as shown by arrows A and B in figures 3(a) and 3(b)) depending upon the direction of rotation. This thrust load will displace the shaft 120 relative to the housing 160 in the direction of the load.

In the arrangement shown in figure 3(a) the shaft 120 is displaced away from the gear 110 (towards the right in the figure) moving in the direction of the thrust load shown by arrow A. It will be noted that as a result of this displacement the first axial stop 122 is moved to the right by a distance x and as a result displaces the first bearing 146 inwardly by said distance. The first bearing 146 slides relative to the housing 160 and, therefore, lifts away from the first stop 162 of the housing. At least a portion of the thrust load is transmitted through the shoulder of shaft 122 via the thrust surfaces of the first bearing 146 to the spring 142. The second axial stop 126 of the shaft is also displaced with the shaft 120 (by the same distance x) but axial movement of the second bearing 147 is restrained and the bearing is held in position by the engagement between the outer bearing surface 147b and the second stop 166 of the housing. As such, spring 142 is compressed between the bearings 146 and 147 with the resultant displacement of the shaft against the resistance of the spring providing a measure of the torque applied to the worm 230 and gear 130. When the thrust load in the first direction is removed from the arrangement the shaft and bearing will return to the initial position of figure 2 with the load on the spring removed.

In the arrangement shown in figure 3(b) the shaft 120 is displaced towards the gear 110 (towards the left in the figure) moving in the direction of the thrust load shown by arrow B. It will be noted that as a result of this displacement the second axial stop 126 is moved to the right by a distance x' and as a result displaces the second bearing 147 inwardly by said distance. The second bearing 147 slides relative to the housing 160 and, therefore, lifts away from the second stop 166 of the housing. At least a portion of the thrust load is transmitted through the retaining ring 128 of the axial stop 126 via the thrust surfaces of the second bearing 166 to the spring 142. The first axial stop 122 of the shaft is also displaced with the shaft 120 (by the same distance x) but axial movement of the first bearing 146 is restrained and the bearing is held in position by engagement betweenof the outer bearing surface 14Gb and the first stop 162 of the housing. As such, spring 142 is compressed between the bearings 146 and 147 with the resultant displacement of the shaft against the resistance of the spring providing a measure of the torque applied to the worm 230 and gear 130. Thus it can be seen that embodiments provide a simple spring pack arrangement 140 which can act in a bi-directional manner.

Although the invention has been described above with reference to a preferred embodiment, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. For example, it will be appreciated that the torque limiter can be used with various types of valve actuators, as well as other types of motor-driven actuating mechanisms.

Claims

CLAIMS1. A torque sensor comprising: a drive shaft rotatable in use to cause a rotation of an output, wherein the shaft is axially moveable in two directions response to thrust loads caused by the applied torque; a housing at least partially surrounding the shaft; and a spring pack assembly disposed on the shaft, the spring pack comprising: first and second bearings, each bearing being disposed between the shaft and the housing and being axially movable relative to both the shaft and the housing; a resiliently axially compressible spring member disposed between the first and second bearing assemblies; wherein the shaft has first and second axial stops respectively positioned on the outside of the first and second bearings and arranged to limit the axial position of the bearings relative to the shaft; and the housing has first and second axial stops respectively positioned on the outside of the first and second bearings and arranged to limit the axial position of the bearings relative to the housing.
2. A torque sensor as claimed in claim 1, wherein upon movement of the shaft in a first axial direction movement of the first bearing is axially limited relative to the shaft and movement of the second bearing is axially limited relative to the housing; and upon movement of the shaft in the other axial direction movement of the first bearing is axially limited relative to the housing and movement of the second bearing is axially limited relative to the shaft.
3. A torque sensor as claimed in claim 1 or 2, wherein the spring member comprises a stack of disc springs.
4. A torque sensor as claimed in any preceding claim, wherein the spring pack assembly further comprises at least one shim positioned between the spring member and one of the first or second bearing.
5. A torque sensor as claimed in any preceding claim, wherein the spring pack assembly further comprises a compression member arranged to limit the relative inward axial movement of the first and second bearings to prevent over compression of the spring member.
6. A torque sensor as claimed in claim 5, wherein the compression member comprises a tubular member disposed around the shaft.
7. A torque sensor as claimed in any preceding claim, wherein the bearings each comprise deep groove bearings.
8. A torque sensor as claimed in any preceding claim, wherein the axial spacing between the first and second stops on the shaft is greater than the axial spacing betweenthe first and second stops of the housing.
9. A torque sensor as claimed in any preceding claim, wherein the housing defines a bore through which at least a portion of the shaft extends and wherein the bore has a locally increased diameter to define a space which encloses the spring pack.
10. A torque sensor as claimed in any preceding claim, wherein the housing axial stops are formed by a radially extending shoulder.
11. A torque sensor as claimed in any preceding claim, wherein at least a portion of the shaft has an increased diameter providing a shoulder which forms one of the axial stops.
12. A torque sensor as claimed in any preceding claim, wherein at least one retaining ring is provided at a fixed location on the shaft to form one of the axial stops.
13. A torque limiter comprising a torque sensor according to any preceding claim and controller arranged to interrupt driving power to the shaft in response to predetermined torque limit being exceeded.
14. A valve actuator comprising: a motor for rotating the drive shaft in use; an output driven by said drive shaft and arranged for actuating a valve in use; and a torque limiter according to claim 13.
15. A valve actuator as claimed in claim 14, wherein said output comprises a gear, driven by a worm associated with the drive shaft.
16. A valve actuator according to claim 14 or 15, wherein the housing at least partially houses components of the valve actuator as well at least partially surrounding the shaft of the torque sensing assembly.
17. A valve assembly including a valve and a valve actuator according to any of claims l4to 16.
18. A torque sensor substantially as described herein and/or with reference to the accompanying drawings.