GB1573317A

GB1573317A - Variable resistance device

Info

Publication number: GB1573317A
Application number: GB1907876A
Authority: GB
Original assignee: BOURNE Ltd
Current assignee: BOURNE Ltd
Priority date: 1976-05-10
Filing date: 1976-05-10
Publication date: 1980-08-20
Also published as: DE2720828A1; FR2351477A1; FR2351477B1

Description

(54) A VARIABLE RESISTANCE DEVICE (71) We, BOURNS (TRIMPOT) LI MITED, a British Company, of Hillend Industrial Estate, Inverkeithing, Fife, Scotland, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a variable resistance device for generating a voltage proportional to the angular position of a shaft relative to a reference position.

According to one aspect of the invention, a variable resistance device for generating a voltage proportional to the angular position of a shaft relative to a reference position, comprises a casing defining a chamber, a rotatable member turnable relative to the casing about an axis, variable resistive means located within the chamber and comprising a resistive track arcuately disposed about said axis and an electrical contact urged in an axial direction against said track, one of said variable resistive means being located on said rotatable member and the other on said casing whereby turning of said rotatable member relative to said casing moves the contact along said track thereby varying the electrical resistance between the contact and an end of said track, coupling means for rigidly coupling said rotatable member to a turnable shaft, first terminal means electrically connected to said end of said track, second terminal means electrically connected to said contact, and axial limiting means on, or forming part of, each of said rotatable member and said casing, one of said axial limiting means consisting of a projection which is abuttable against the other of said axial limiting means for limiting relative movement, in the axial direction, between the rotatable member and the casing to prevent excessive pressure between said contact and said track, occasioned when connecting a turn able shaft to the coupling means, the axial limiting means being dimensioned and arranged so that they are only able to make substantially point contact with each other, said point contact being on said turning axis of the rotatable member.

Since the axial limiting means can only make point contact with each other there is virtually no friction between the axial limiting means on turning of the rotatable member. In addition abutment of the axial limiting means when connecting a turnable shaft to the coupling means prevents a non-elastic deformation of the contact against the conductive track, and/or prevents an excessive pressure between the contact and the track causing the latter to wear excessively.

Suitably the projection terminates in a substantially conical tip having a common axis with the turning axis of the rotatable member.

Preferably, the other of said axial limiting means comprises a recess having a substantially conical bottom the axis of which is common with the said turning axis of the rotatable member. Suitably the projection terminates in a substantially conical tip, the cone angle of the tip being less than that of the conical bottom of the recess. In this case the recess then serves to centre the rotatable member within the chamber when the axial limiting means abut against each other.

Conveniently the cone angle of the tip is from between 30 and 1200, preferably from between 65" to 90 , and the cone angle of the recess is at least 50 greater than that of the tip and suitably from between 35 and 1300, preferably from between 70" and 100 .

Suitably the axial dimension of the chamber and the axial dimension of the rotatable mamber, or the part thereof positioned within the chamber, serve to limit relative movement between the rotatable member and the casing in the axial direction tending to decrease the contact pressure between the contact and said track.

Suitably the rotatable member has an axially positioned shaft portion extending through an aperture in the casing. The shaft portion may be hollow, having a noncircular internal cross-section constituting said coupling means, for rigidly coupling a turnable shaft having an end of a similar, non-circular cross-section. The portion of the rotatable member positioned in said chamber is suitably cylindrical in shape.

First and second stop means may be provided in or on the rotatable member and the casing, respectively, for limiting the angular turning movement of the rotatable member. Typically, if the rotatable member is cylindrical, the first stop means is provided in or on an end face of the cylindrical rotatable member, and the second stop means is provided in or on a confronting surface of the casing. One of the stop means suitably comprises a peg offset from said axis, and the other stop means comprises an arcuate slot in which the peg is received.

According to another aspect of the invention a variable resistance device for generating a voltage proportional to the angular position of a shaft relative to a reference position, comprises a casing defining a chamber, a resistance element carried by an electrically insulating part of said casing and providing a conductive track arcuately disposed about an axis, a rotatable member at least partly positioned within the chamber and carrying an electrical contact resiliently urged in a first axial direction against said conductive track, the rotatable member being turnable relative to the casing about said axis for moving the contact along said track thereby varying the electrical resistance between the contact and an end of said resistance element, first terminal means electrically connected to said end of the resistance element, second terminal means electrically connected to said contact, coupling means for rigidly coupling said rotatable member to a turnable shaft, and axial limiting means on, or forming part of, said rotatable member and either forming part of the casing defining said chamber or being rigidly fixed with respect to the casing in said chamber, one of said axial limiting means consisting of a projection which is abuttable against the other of said axial limiting means for limiting relative movement, in said first axial direction, between the rotatable member and the casing to prevent excessive pressure between said contact and said track occasioned when connecting a turnable shaft to the coupling means, the axial limiting means being dimensioned and arranged so that they are only able to make substantially point contact with each other, said point contact being on said turning axis of the rotatable member, the axial dimension of the chamber and the axial dimension of the rotatable member, or the part thereof positioned in the chamber, serving to limit relative movement, in a second axial direction opposite to said first axial direction, between the casing and the rotatable member.

According to a yet further aspect of the invention a variable resistance device for generating a voltage proportional to the angular position of a shaft relative to a reference position, comprises a casing defining a chamber, a carrier at least partly positioned within the chamber and being turnable about an axis, the carrier supporting a conducting strip electrically connected to an arcuate resistance element arcuately disposed about said axis, a first spring contact, carried by an electrically insulating part of said casing, pressing in a first axial direction against said strip and being electrically connected to said first terminal means, a second spring contact, carried by an electrically insulating part of said casing, pressing in said first axial direction against said resistance element and being electrically connected to a second terminal means, the carrier being turnable relative to the casing about said axis for sliding the first spring contact along the strip and the second spring contact along said resistance element thereby varying the electrical resistance between the electrically connected first and second terminal means, coupling means for rigidly coupling said carrier to a turnable shaft, and axial limiting means on, or forming part of, said carrier and either forming part of the casing defining said chamber or being rigidly fixed with respect to the casing in said chamber, one of said axial limiting means consisting of a projection which is abuttable against the other of said axial limiting means for limiting relative movement, in said first axial direction, between the carrier and the casing to prevent excessive pressure between each of said first and second spring contacts and said strip and said resistance element, respectively, occasioned when connecting a turnable shaft to the coupling means, the axial limiting means being dimensioned and arranged so that they are only able to make substantially point contact with each other, said point contact being on said turning axis of the carrier, the axial dimension of the chamber and the axial dimension of the carrier, or the part thereof positioned in the chamber, serving to limit relative movement, in a second axial direction opposite to said first axial direction, between the casing and the carrier.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure I is an end view of one embodiment of a variable resistance device according to the invention, Figure 2 is a sectional view taken on the line II-II of Figure 1, Figure 3 is a sectional view taken on the line III-III of Figure 2, Figure 4 is a view of the spring metal member shown in Figure 2, Figure 5 is a view of the resistance element mounted in the casing of the variable resistance device shown in Figures 1 and 2, Figure 6 is a sectional view of another embodiment of a variable resistance device according to the invention, and Figure 7 is a view showing an end view of the carrier of the variable resistance device shown in Figure 4.

Figures 1 and 2 show a potentiometer or variable resistance device, generally designated by the reference numeral 1. The device 1 comprises a cylindrical casing consisting of a first part 2 defining a lid of the potentiometer and a second, cup-shaped part 3, the parts 2 and 3 together defining a cylindrical member 4 within which is positioned a rotatable member 5.

The parts 2 and 3 are made of electrically insulating material, e.g. "Deritron T.A.R." (Trade Mark), and are held together by three screw-threaded bolts 6 (only one of which can be seen in Figure 2). The outer end face 2a of the part 2 may be provided with recesses (see arcuate recesses 2b, 2c, 2d and 2e in Figure 3 and circular recess 2f in Figure 2) to minimise the material used for manufacturing the part 2.

The bolts 6 pass, respectively, through three angularly spaced holes 7 (only one of which can be seen in Figure 2) provided in the first part 2 and screw into three angularly spaced, blind holes 8 in the second part 3, which holes 8 are aligned with the holes 7.

The parts 2 and 3 have confronting surfaces 9 and 10, respectively. The surface 9 has an arcuate groove 9a of V-cross-section which extends through approximately 350 , opposite ends of the groove 9a opening into a recess 11 through which passes a sheathed, three-core cable 12. Three spaced-apart arcuate ridges 10a, 10b and 10c of V-crosssection and lying on the circumference of a common circle are received in the arcuate groove 9a when the parts 2 and 3 are bolted together. When the parts 2 and 3 are bolted together, the ridge 10a clamps the sheathed cable 12 against the bottom 11a of the recess 11.The second part 3 has three angularly spaced lugs 40, 41 and 42 having apertures 43, 44 and 45, respectively, therein for the reception of bolts (not shown) to enable the device to be mounted to a housing (not shown).

A ceramic substrate 15 is received in a recess 14 provided in the part 2. The substrate 15 (see Figure 5) carries an arcuate strip 16 of palladium electrically isolated, and angularly spaced, from an arcuate resistance element 17 (e.g. resistive film or coating of plastics material), and is adhered to the inwardly facing end face 18 of the first part 2. To ensure accurate location of the substrate 15 in the recess 14 during manufacture, the substrate 15 and the recess 14 have a similar non-circular shape, as can be seen in Figures 2 and 3. The three terminal wires of the cable 12 are crimped and soldered to metallic termination clips 13 (see Figure 5). One of the clips 13 is soldered or silver welded to the palladium strip 16. The other two clips 13 are soldered or welded to palladium strips 22 and 23 provided at opposite ends of the resistance element 17.

The rotatable member 5 comprises a cylindrical portion 24, positioned wholly within the chamber 4, a hollow shaft portion 25, having a substantially "D" shaped internal cross-section, extending from one end face 26 of the portion 24 through an aperture 27 provided centrally in the second casing part 3, and a projection 28, terminating in a substantially conical tip 29, extending from the other end face 30 of the portion 24. The end face 30 carries a tiltable, spring metal member 31 (see Figure 4), typically made of beryllium copper, having palladium contacts 32a, 32b pressing axially against the resistance element 17 with a contact force of approximately 15 grammes, and palladium contacts 33a, 33b pressing axially against the strip 16 with a contact force also of approximately 15 grammes.The member 31 has a pair of apertures 31a and 31b which receive projections (not shown) integrally formed with the portion 24 and extending from the end face 30. These projections serve to locate the member 31 on the wiping member 5. The end face 26 has extending therefrom and offset from the axis of the member 5, a pin 34 which co-operates in an arcuate slot 35 opening into the inwardly facing surface 40 of the second casing part 3. The cooperating pin 34 and slot 35 limit the angular turning movement, about the axis of the device 1, of the member 5 relative to the casing preventing a full 3600 rotation of the member 5. The locus of the contacts 32a, 32b as the member 5 turns describes a track on the resistive element 17.

The device 1 is intended to be used to convert the angular position of a shaft (e.g.

the shaft carrying the throttle-controlled butterfly valve in the intake manifold of a vehicle's carburettor) to a voltage proportional thereto. The end of such a shaft (not shown) is provided with a "D" shaped cross-section enabling the shaft to be inserted in a close fitting manner into the hollow shaft portion 25. A circlip 36 may be provided around the portion 25 to prevent its fracture during this insertion. The process of push-fitting the shaft onto the portion 25 causes the rotatable member 5 to be pushed axially against the casing so that the tip 29 abuts against a conical recess 37 opening into the end face 18 of the casing part 2.The cone angle of the recess 37 is a few degrees greater than that of the tip 29 (typically the cone angle of the tip is from between 30 and 1200, e.g. 80 , and of the recess 37 is from between 35 and 1300, e.g.

90 , although the cone angles may be outside these typical ranges) so that only the pointed end of the conical tip 29 abuts against the bottom of the recess 37 to provide substantially point contact therebetween. This abutment serves to space the end face 30 from the substrate 15 so that the spring metal member 31 is not nonelastically deformed during coupling of the shaft to the shaft portion 25. This is particularly important when the rotatable member 5 is coupled to the butterfly valve shaft of a vehicle carburettor which shaft is constantly being turned in opposite directions during operation of the vehicle.In order to ensure that the device functions correctly for upwards of 3,000,000 cycles (e.g. approximately two years of average motoring), it is essential that the contact force between the resistance element 17 and the contacts 32a, 32b and between the strip 16 and the contacts 33a, 33b is maintained substantially constant, e.g. at approximately 15 grammes.

During operation of the device, the rotatable member is completely supported by the shaft which is coupled to the portion 25.

Initially there is little friction between the tip 29 and the recess 37 since there is only point contact therebetween. Subsequently the tip 29 may wear away in use so that there is no contact between the tip 29 and the recess 37 and consequently no friction therebetween. It should be realised that the conical recess 37 also serves to locate the rotatable member 5 centrally within the chamber 4 during coupling of the shaft to the shaft portion 25.

Figures 6 and 7 show another embodiment of a variable resistance device according to the invention and generally designated by the reference numeral 50. The device 50 comprises an electrically insulating casing defining a cylindrical chamber 54 and consisting of a base 51, a cup-shaped intermediate portion 52 and a cover 53, and an electrically insulating carrier 55 disposed at least partly within the chamber 54.

The carrier 55 is identical to the rotatable member 5 of the device 1 with the exception that the carrier 55 supports an annular ceramic substrate 56 instead of the spring metal member 31 supported by the rotatable member 5. The substrate 56 (shown in detail in Figure 7) carries an electrically conducting portion 57 comprising an arcuate resistance element 58, a narrower, arcuate strip 59 of metal, e.g. a palladium deposit, radially spaced from, but occupying the same angular position as the resistance element 58, and a further arcuate strip 60 of metal, e.g. palladium deposit, positioned diametrically opposite, but at the same radial distance from, the resistance element 58. One end of the resistance element 58 is connected, via an arcuate metal strip 61, to the strip 60 and the other end of the resistance element 58 is connected, via a short metal strip 62, to the metal strip 59.

Three metallic spring members 63 (only one member 63 is shown in Figure 6) are fixedly supported between the portion 52 and the cover 53. Each spring member 63 serves as a terminal and carries a contact 64 at one of its ends which presses, in the axial direction, against the electrically conducting portion 57 with a substantially constant force, e.g. 15 grammes. The cup-shaped, intermediate portion 52 is provided with a conical recess 65 having a cone angle a few degrees greater than the cone angle of the conical tip 70 provided on the carrier 55, the conical tip and recess serving the same purpOse as the tip 29 and recess 37 of the device 1.

The base 51, intermediate portion 52 and cover 53 are shaped so that the base 51 and cover 53 can be clipped to the portion 52.

The joints between the parts of the casing are then heat-formed all round.

The device 50 functions in a similar manner to the device 1, turning movement of the carrier 55 causing the contacts 64 to wipe against respective ones of the resistance element 58 and the strips 59 and 60 to vary the electrical resistance between the contact 64 which presses against the element 58 and both the other two contacts 64.

In other embodiments of the invention it is possible for the tip, e.g. a conical tip, of the projection to abut against a flat face of the casing of the device. However such a construction is not preferred since, although the rotatable member (or carrier) is limited against axial movement it is not radially centred within the casing as is the case when the tip of the projection is seated within a conical recess.

Although the tip of the projection has been described as being conical it should be realised that this is not essential. In other embodiments of the invention the tip could be rounded, or pointed without being conical, so that it can only make substantially point contact with the other axial limiting means.

WHAT WE CLAIM IS: 1. A variable resistance device for

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. hollow shaft portion 25. A circlip 36 may be provided around the portion 25 to prevent its fracture during this insertion. The process of push-fitting the shaft onto the portion 25 causes the rotatable member 5 to be pushed axially against the casing so that the tip 29 abuts against a conical recess 37 opening into the end face 18 of the casing part 2. The cone angle of the recess 37 is a few degrees greater than that of the tip 29 (typically the cone angle of the tip is from between 30 and 1200, e.g. 80 , and of the recess 37 is from between 35 and 1300, e.g. 90 , although the cone angles may be outside these typical ranges) so that only the pointed end of the conical tip 29 abuts against the bottom of the recess 37 to provide substantially point contact therebetween. This abutment serves to space the end face 30 from the substrate 15 so that the spring metal member 31 is not nonelastically deformed during coupling of the shaft to the shaft portion 25. This is particularly important when the rotatable member 5 is coupled to the butterfly valve shaft of a vehicle carburettor which shaft is constantly being turned in opposite directions during operation of the vehicle.In order to ensure that the device functions correctly for upwards of 3,000,000 cycles (e.g. approximately two years of average motoring), it is essential that the contact force between the resistance element 17 and the contacts 32a, 32b and between the strip 16 and the contacts 33a, 33b is maintained substantially constant, e.g. at approximately 15 grammes. During operation of the device, the rotatable member is completely supported by the shaft which is coupled to the portion 25. Initially there is little friction between the tip 29 and the recess 37 since there is only point contact therebetween. Subsequently the tip 29 may wear away in use so that there is no contact between the tip 29 and the recess 37 and consequently no friction therebetween. It should be realised that the conical recess 37 also serves to locate the rotatable member 5 centrally within the chamber 4 during coupling of the shaft to the shaft portion 25. Figures 6 and 7 show another embodiment of a variable resistance device according to the invention and generally designated by the reference numeral 50. The device 50 comprises an electrically insulating casing defining a cylindrical chamber 54 and consisting of a base 51, a cup-shaped intermediate portion 52 and a cover 53, and an electrically insulating carrier 55 disposed at least partly within the chamber 54. The carrier 55 is identical to the rotatable member 5 of the device 1 with the exception that the carrier 55 supports an annular ceramic substrate 56 instead of the spring metal member 31 supported by the rotatable member 5. The substrate 56 (shown in detail in Figure 7) carries an electrically conducting portion 57 comprising an arcuate resistance element 58, a narrower, arcuate strip 59 of metal, e.g. a palladium deposit, radially spaced from, but occupying the same angular position as the resistance element 58, and a further arcuate strip 60 of metal, e.g. palladium deposit, positioned diametrically opposite, but at the same radial distance from, the resistance element 58. One end of the resistance element 58 is connected, via an arcuate metal strip 61, to the strip 60 and the other end of the resistance element 58 is connected, via a short metal strip 62, to the metal strip 59. Three metallic spring members 63 (only one member 63 is shown in Figure 6) are fixedly supported between the portion 52 and the cover 53. Each spring member 63 serves as a terminal and carries a contact 64 at one of its ends which presses, in the axial direction, against the electrically conducting portion 57 with a substantially constant force, e.g. 15 grammes. The cup-shaped, intermediate portion 52 is provided with a conical recess 65 having a cone angle a few degrees greater than the cone angle of the conical tip 70 provided on the carrier 55, the conical tip and recess serving the same purpOse as the tip 29 and recess 37 of the device 1. The base 51, intermediate portion 52 and cover 53 are shaped so that the base 51 and cover 53 can be clipped to the portion 52. The joints between the parts of the casing are then heat-formed all round. The device 50 functions in a similar manner to the device 1, turning movement of the carrier 55 causing the contacts 64 to wipe against respective ones of the resistance element 58 and the strips 59 and 60 to vary the electrical resistance between the contact 64 which presses against the element 58 and both the other two contacts 64. In other embodiments of the invention it is possible for the tip, e.g. a conical tip, of the projection to abut against a flat face of the casing of the device. However such a construction is not preferred since, although the rotatable member (or carrier) is limited against axial movement it is not radially centred within the casing as is the case when the tip of the projection is seated within a conical recess. Although the tip of the projection has been described as being conical it should be realised that this is not essential. In other embodiments of the invention the tip could be rounded, or pointed without being conical, so that it can only make substantially point contact with the other axial limiting means. WHAT WE CLAIM IS:

1. A variable resistance device for

generating a voltage proportional to the angular position of a shaft relative to a reference position, comprising a casing defining a chamber, a rotatable member relative to the casing about an axis, variable resistive means located within the chamber and comprising a resistive track arcuately disposed about said axis and an electrical contact urged in an axial direction against said track, one of said variable resistive means being located on said rotatable member and the other on said casing whereby turning of said rotatable member relative to said casing moves the contact along said track thereby varying the electrical resistance between the contact and an end of said track, coupling means for rigidly coupling said rotatable member to a turnable shaft, first terminal means electrically connected to said end of said track, second terminal means electrically connected to said contact, and axial limiting means on, or forming part of, each of said rotatable member and said casing, one of said axial limiting means consisting of a projection which is abuttable against the other of said axial limiting means for limiting relative movement, in the axial direction, between the rotatable member and the casing to prevent excessive pressure between said contact and said track occasioned when connecting a turnable shaft to the coupling means, the axial limiting means being dimensioned and arranged so that they are only able to make substantially point contact with each other, said point contact being on said turning axis of the rotatable member.

2. A variable resistance device according to claim 1, in which the projection terminates in a substantially conical tip having a common axis with the said turning axis of the rotatable member.

3. A variable resistance device according to claim 1 in which the said other axial limiting means comprises a recess having a substantially conical bottom the axis of which is common with the said turning axis of the rotatable member.

4. A variable resistance device according to claim 3, in which the projection terminates in a substantially conical tip, the cone angle of the bottom of the recess being greater than the cone angle of the tip of said projection.

5. A variable resistance device according to claim 4, in which the cone angle of the tip is from between 30 and 1200 and of the recess is from between 35 and 1300.

6. A variable resistance device according to claim 5, in which the cone angle of the tip is from between 65" and 90" and of the recess is between 70" and 100 .

7. A variable resistance device according to any of the preceding claims, in which the axial dimension of the chamber and the axial dimension of the rotatable member, or the part thereof positioned within the chamber, serve to limit relative movement between the rotatable member and the casing in the axial direction tending to decrease the contact pressure between the contact and said track.

8. A variable resistance device according to any of the preceding claims, in which the rotatable member has an axially positioned shaft portion extending through an aperture in the casing.

9. A variable resistance device according to claim 8, in which the shaft portion is hollow, having a non-circular internal crosssection constituting said coupling means, for rigidly coupling a turnable shaft having an end of a similar, non-circular cross-section.

10. A variable resistance device according to any of the preceding claims, in which first and second stop means are provided in or on the rotatable member and the casing, respectively, for limiting the angular turning movement of the rotatable member.

11. A variable resistance device according to claim 10, in which the rotatable member is cylindricl, the first stop means being provided in or on an end face of the cylindrical rotatable member, and the second stop means being provided in or on a confronting surface of the casing.

12. A variable resistance device according to claim 10 or 11, in which one of the stop means comprises a peg offset from said turning axis, of the rotatable member, and the other stop means comprises an arcuate slot in which the peg is received.

13. A variable resistance device according to any of the preceding claims, in which the electrical contact is mounted on the rotatable member and comprises a springy strip which presses, in said axial direction, against said resistive track.

14. A variable resistance device according to claim 13, in which said springy strip also presses against a strip of metal carried by said casing and electrically isolated from said resistive track.

15. A variable resistance device according to claim 14, in which the metal strip consists of a palladium deposit.

16. A variable resistance device according to any of the preceding claims, in which the arcuate resistive track comprises an electrically conductive film or coating of electrically conductive plastics material.

17. A variable resistance device according to claim 16, in which a surface of the material film or coating provides the arcuate track.

18. A variable resistance device for generating a voltage proportional to the angular position of a shaft relative to a reference position, the device being constructed and arranged substantially as he rein described with reference to, and as illustrated in, Figures 1 to 5 of the accompanying drawings.

19. A variable resistance device for generating a voltage proportional to the angular position of a shaft relative to a reference position, the device being constructed and arranged substantially as herein described with reference to, and as illustrated in, Figures 6 and 7 of the accompanying drawings.

20. A variable resistance device for generating a voltage proportional to the angular position of a shaft relative to a reference position, comprising a casing defining a chamber, a resistance element carried by an electrically insulating part of said casing and providing a conductive track arcuately disposed about an axis, a rotatable member at least partly positioned within the chamber and carrying an electrical contact resiliently urged in a first axial direction against said conductive track, the rotatable member being turnable relative to the casing about said axis for moving the contact along said track thereby varying the electrical resistance between the contact and an end of said resistance element, first terminal means electrically connected to said end of the resistance element, second terminal means electrically connected to said contact, coupling means for rigidly coupling said rotatable member to a turnable shaft, and axial limiting means on, or forming part of, said rotatable member and either forming part of the casing defining said chamber or being rigidly fixed with respect to the casing in said chamber, one of said axial limiting means consisting of a projection which is abuttable against the other of said axial limiting means for limiting relative movement, in said first axial direction, between the rotatable member and the casing to prevent excessive pressure between said contact and said track occasioned when connecting a turnable shaft to the coupling means, the axial limiting means being dimensioned and arranged so that they are only able to make substantially point contact with each other, said point contact being on said turning axis of the rotatable member, the axial dimension of the chamber and the axial dimension of the rotatable member, or the part thereof positioned in the chamber, serving to limit relative movement, in a second axial direction opposite to said first axial direction, between the casing and the rotatable member.

21. A variable resistance device for generating a voltage proportional to the angular position of a shaft relative to a reference position, comprising a casing defining a chamber, a carrier at least partly positioned within the chamber and being turnable about an axis, the carrier supporting a conducting strip electrically connected to an arcuate resistance element arcuately disposed about said axis, a first spring contact, carried by an electrically insulating part of said casing, pressing in a first axial direction against said strip and being electrically connected to first terminal means, a second spring contact, carried by an electrically insulating part of said casing, pressing in said first axial direction against said resistance element and being electrically connected to a second terminal means. the carrier being turnable relative to the casing about said axis for sliding the first spring contact along the strip and the second spring contact along said resistance element thereby varying the electrical resistance between the electrically connected first and second terminal means, coupling means for rigidly coupling said carrier to a turnable shaft, and axial limiting means on, or forming part of, said carrier and either forming part of the casing defining said chamber or being rigidly fixed with respect to the casing in said chamber, one of said axial limiting means consisting of a projection which is abuttable against the other of said axial limiting means for limiting relative movement, in said first axial direction, between the carrier and the casing to prevent excessive pressure between each of said first and second spring contact and said strip and said resistance element, respectively, occasioned when connecting a turnable shaft to the coupling means, the axial limiting means being dimensioned and arranged so that they are only able to make substantially point contact with each other, said point contact being on said turning axis of the carrier, the axial dimension of the chamber and the axial dimension of the carrier, or the part thereof positioned in the chamber, serving to limit relative movement, in a second axial direction opposite to said first axial direction, between the casing and the carrier.