GB2117903A

GB2117903A - Thermally responsive means

Info

Publication number: GB2117903A
Application number: GB08308984A
Authority: GB
Inventors: John Thornton
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1982-04-01
Filing date: 1983-03-31
Publication date: 1983-10-19
Also published as: GB8308984D0

Abstract

A thermally compensating mounting for two elements 2, 3 (e.g. optical or quasi-optical devices) which require to lie in a given spaced co- axial relationship irrespective of temperature change and including a mounting 1 for one element 2, a mounting 7, 8, 9 for the other element 3, comprises a thermally responsive compensation means having a variable length chamber formed between the two mountings, the chamber being filled with a substantially incompressible material 5 capable of elastic deformation, the chamber being of such a length and the material having such a coefficient of expansion that in an operational temperature range any tendency of one device to move relatively to the other due to temperature change is compensated by opposite movement of said one device. In an alternative mounting arrangement (Figure 2) rotary temperature compensation is provided. <IMAGE>

Description

SPECIFICATION Thermally responsive means This invention relates to thermally responsive means, which can be used for initiating an action in response to a change in temperature, and/or, which can be used to compensate for temperature changes when such changes have an undesirable effect in apparatus operating in a range of temperatures.

Such thermally responsive means have hitherto included bi-metallic elements which use the different co-efficient of expansion of dissimilar metals, and, hydraulic fluid constrained in for axample a piston/cylinder arrangement, or in sealed bellows or capsule arrangements.

In general, these means have proved unacceptable in some embodiments, because of bulk, and sealing and fatigue problems.

In optical or quasi-optical apparatus subject to wide temperature ranges, changes in focal length have hitherto been compensated by the addition of an extra lens chosen to have compensating characteristics. Such an extra lens is found to be undesirable since the addition of an extra optical element can degrade optical performance and, moreover, adds to its cost.

It is an object of the invention to provide thermally responsive means suitable for use in apparatus where compensation for thermal effects is necessary, and/or where some response to temperature change is desired.

According to one aspect of the present invention, thermally responsive means includes first and second members, means for allowing movement with respect to one another and for forming a ciosed chamber with said members, and thermally responsive material filling the chamber, the material being a substantially incompressible solid capable of elastic deformation and with a co-efficient of expansion such that on change of temperature a desired relative movement of the first and second members is effected.

The movement allowing means may comprise mating slidable surfaces, for example a piston/cylinder arrangement. Since the thermally responsive material has an elastic characteristic, it is found to be substantially not extrudable between the mating surfaces.

A preferred material is silicone rubber.

Some examples of thermally responsive means are now described by way of example with reference to the accompanying drawings in which Figure 1 is a cross-sectional elevation of an optical or quasi-optical apparatus, having thermally responsive means for compensating for temperature changes, the cross-section including the longitudinal axis of the apparatus, and, Figure 2 is a transverse cross-section of an alternative thermally responsive means.

Referring initially to Figure 1, a mounting tube 1 has a lens 2 at one end and a device 3 at the other end which requires to be always accurately located with reference to the lens. To compensate for any change of focal length of the lens and/or any expansion or contraction of the tube 1, thermally responsive means are provided in the form of an annular chamber 4, filled with a thermally responsive material 5 of silicone rubber.

This material is substantially incompressible, a solid, and is capable of elastic deformation under compression. It also has a high co-efficient of linear expansion.

Silicone rubbers considered to be particularly suitable are: SILCOSET 100 Produced by l.C.I. of U.K.

Row 615 Produced by RTV 655 J G.E.C. of U.S.A.

RTV 655, at least, is usable in the temperature range -11 SOC to +2000C. In each case the preferred Shore hardness is in the range 40 45.

The chamber 4 is formed by necking down the tube 1 at 6 and by providing an annular flange 7 upon the device 3, slidably mating surfaces being formed at 8 and 9 respectively, the latter where the device 3 protrudes from the tube 1.

The responsive material is maintained under compression by a coil spring 1 0.

It is found that since the material 5 is constrained against expansion in both the radial and lateral senses, as drawn, a relatively large expansion results in the axial sense; thus only a relatively small working length of material is needed.

Moreover, the fact that the material is resilient ensures that little or no extrusion takes place at the slidable mating surfaces 8 and 9; thus relatively large tolerances are acceptable in manufacture.

Referring now to Figure 2, this figure illustrates rotary apparatus, for comparison with the linear apparatus of Figure 1.

An outer tube 20 has opposed radially inwardly directed axial flanges 21 which, in vane-like manner, slidably bear against the exterior surface of a co-axial shaft 22. The shaft has similar flanges 23 protruding radially outwards to bear against the interior surface of the tube 20.

Naturally, the flanges 21 and 23 are slightly out of register with one another.

Twin chambers are formed and are each filled with a thermally responsive material 24 so that changes of temperature effect relative rotation of the tube 20 and the shaft 22. The thermally responsive material 24 is similar or identical to that described with reference to Figure 1, so that extrusion at the sliding faces of the flanges and the tube and shaft is at least minimised without the necessity of specialised seals or sealing surfaces.

Although twin chambers are described, a single chamber or more than two chambers can be used.

Claims

1. Thermally responsive means including first and second members, means for allowing movement of the members with respect to one another and for forming a closed chamber with said members, and thermally responsive material filling the chamber, the material being a substantially incompressible non-fluid capable of plastic flow with a co-efficient of expansion such hmkh on change of temperature a desired relative movement of the first and second members is effected.

2. Thermally responsive means according to Claim 1, wherein the movement allowing means comprises mating slidable surfaces.

3. Thermally responsive means according to Claim 1 or Claim 2, wherein the thermally responsive material has a resilient characteristic.

4. Thermally responsive means according to Claim 3, wherein the thermally responsive material is a silicone rubber.

5. Thermally responsive means according to any one of the previous claims, wherein the first and second members form a piston and cylinder arrangement.

6. Thermally responsive means according to any one of Claims 1 to 4, wherein the first and second members form a rotor and stator arrangement.

7. A thermally compensating mounting for two elements which require to lie in a given spaced co-axial relationship irrespective of temperature change including a mounting for one element, a mounting for the other element, and thermal compensation means comprising a variable length chamber formed between the two mountings, the chamber filled with a substantially incompressible non-fluid material capable of plastic flow, the chamber being of such a length and the material having such a co-efficient of expansion that in an operational temperature range any tendency of one device to move relatively to the other due to temperature change is compensated by opposite movement of said one device.

8. A thermally compensating mounting in which the elements are optical or quasi-optical devices, wherein the mounting for one element comprises a tube with one element mounted at one end region and adapted at the other end region to slidably accept internally the mounting for the other element, the variable length chamber being formed between the two mountings which together form a piston and cylinder arrangement.

9. A mounting for optical or quasi-optical elements substantially as described with reference to Figure 1.

10. Thermally responsive means substantially as described with reference to Figure 1 or Figure 2.