GB2164803A

GB2164803A - Resilient means for clamping electrical equipment in housings

Info

Publication number: GB2164803A
Application number: GB08522100A
Authority: GB
Inventors: Michael Owen Mccann
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1984-09-18
Filing date: 1985-09-05
Publication date: 1986-03-26
Also published as: GB8423575D0; GB2164803B; GB8522100D0

Abstract

A housing (2) for electronic circuit boards (20) has channels (4 and 5) on opposite walls (3) between heat conducting fins (6). Opposite edges (21 and 22) of the circuit boards are urged against the fins by a spring member (30) in each channel. The spring members are made of, or include an element made of, a shape memory effect material and increase their spring rate by about five fold between the temperatures of 40 DEG C and 50 DEG C. In this way, the boards (20) can be readily inserted in the housing (2) at room temperature but are clamped firmly in place at higher temperatures, thereby improving heat dissipation from the boards to the housing. <IMAGE>

Description

SPECIFICATION Electrical equipment housings and clamping means This invention relates to electrical equipment housings and clamping means for such housings.

Various arrangements are used to dissipate the heat produced by electronic modules or circuit boards mounted in a housing. In one such arrangement the housing is provided with heat-dissipating walls which may be actively cooled by a coolant fluid, or by an air blower, or which may simply have a large thermal capacity. The edges of the module or board are held in good thermal contact with the heat-dissipating walls so that heat generated is conducted away through its edges to the walls of the housing.

The necessity of ensuring efficient heat dissipation from such modules or boards is becoming even more important as the number of components on each board increases leading to a consequent increase in power, which can be more than 50 watts.

In one arrangement, opposite edges of the module are slidable in channels mounted on the cooled walls of the housing, one side of the edges of the module being urged against the channel by means of an elongate serpentine spring in the channel.

The problem with such an arrangement is that the maximum acceptable force produced by such a spring is about 7 to 10 Ibs force (31 to 44N) since any greater force will make insertion of the module along the channel unacceptably difficult, with the consequent possible damage to the module if undue force is used for insertion. Such a spring, however, is not generally sufficient to flatten the edge of the module against the channel. As a result of surface imperfections in the module and channel, only localised contact will be produced.

In an alternative arrangement, the edges of boards are firmly clamped against the channel, such as by means of jackscrews, each of which carries wedges with opposed inclined surfaces. As the jackscrew is tightened, the inclined surfaces slide laterally forcing the edge of the board against the channel. Such arrangements can ensure reasonably good heat transfer in regions where the clamping force is applied, but they do suffer from disadvantages. Firstly, the clamping mechanism is relatively expensive, can be heavy and require lubrication. It also requires access to be provided to the clamping mechanism when a board is being inserted or removed from the housing. The need to perform an additional operation in order to insert or remove a board increases maintenance time and is an additional complication.Furthermore, if a clamp is inadvertently not tightened, there is the risk that the board will overheat with possible consequent failure or malfunction. Rigidly clamping the board in the housing brings a disadvantage in that vibration is communicated directly to the board, rather than being dissipated to a certain extent, as in the spring of the first arrangement. Severe vibration can lead to damage to connections on the board.

It is an object of the present invention to provide an electrical equipment housing and clamping means therefor by which the abovementioned disadvantages can be substantially avoided.

According to one aspect of the present invention there is provided an equipment housing adapted for supporting one or more electronic modules and for conducting heat away from said modules, the housing including at least one spring member arranged to urge a surface of a module into thermal contact with a surface of the housing, and said spring member being arranged to increase its spring rate at elevated temperatures such that the module is urged more firmly against the surface of the housing at the elevated temperatures.

In this way, at normal, room temperature the spring member allows relatively easy insertion of the module but at elevated temperatures, where efficient heat dissipation becomes more important, an increased clamping effect is produced. The resilience of the spring member still acts to damp vibration at lower temperatures.

The housing may include a pair of opposite channel members for receiving opposite edges of each respective module, the or each said spring member being located in a respective channel member, and the said surface of the housing being provided by an elongate heatconducting member. The spring member preferably extends along the major part of the length of the channel.

The spring rate of the spring member preferably increases several fold between the temperatures of about 400 Centigrade and 500 Centigrade, and may increase approximately five fold.

The spring member preferably includes an element of a shape memory effect material, and may be made substantially entirely of a shape memory effect material, or include a spring element and an element of a shape memory effect material that engages said spring element. The spring member may include a strip of elongate serpentine shape.

The element of a shape memory effect material may be arranged to contract at elevated temperatures.

The spring element may be of serpentine shape, and the said element of a shape memory effect material may be substantially straight and extend along the length of the spring member. Alternatively, the spring element may be of serpentine shape and the said element of a shape memory effect material may include at least one loop secured at each end with said spring element. In another arrangement the spring member may include an elongate member of U-shape section.

The electronic modules may be electronic circuit boards.

According to another aspect of the present invention there is provided a spring member for an equipment housing according to the one aspect of the present invention.

According to a further aspect of the present invention there is provided a spring member for urging a surface of an electronic module into thermal contact with a surface of a housing in which said module is supported, the said spring member being arranged to increase its spring rate at elevated temperatures such that the module is urged more firmly against the surface of the housing at the elevated temperatures.

A housing for supporting several electronic circuit boards, and several different clamping means for retaining the boards in the housing, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a simplified perspective view of the housing; Figure 2 is a sectional plan view of a part of the housing, to a larger scale, showing the clamping means; Figure 3 is a perspective view of one end of alternative clamping means; and Figures 4 to 6 are perspective views of three other clamping means.

With reference to Fig. 1, the electronics unit comprises a housing 2 in which are supported several electronic circuit boards 20 or other similar modules. The housing 2, which is only illustrated in a simplified form, has side walls 3 which serve to dissipate heat produced by the boards. In this respect, the walls may be cooled, such as by means of cold air or a cooling liquid flowing through the walls. Each board 20 is supported at opposite edges 21 and 22 in respective channels 4 and 5 in the walls 3, the channels being defined between parallel fins 6 which are in intimate thermal contact with one side of the edge of each board. The fins 6 are of metal, or other heatconducting material, and serve to conduct heat away from the boards 20 to the walls 3 of the housing 2.

With reference now also to Fig. 2, within each channel 4 and 5 there is located an elongate serpentine spring member 30 which forms clamping means for the boards. Each spring member 30, is a metal strip about 6mm wide and about 178mm long in its natural state. The spring member has three Vshape corrugations 31 to 33 with a pitch of about 10mm, separated from the ends by short straight end portions 34 and 35 respectively. Alternatively, the top and bottom of each corrugation may be flattened. When located in the channels 4 and 5, without the presence of a board 20, the pitch of the spring member 30 is such that it is held in slight lateral compression between opposite fins 6, so that it does not move on vibration.

The shape of the spring member 30 is conventional.

Heretofore, such spring members have been made entirely of a spring steel or similar material which has a relatively constant spring rate or resilience over the range of operating temperatures, or which produces a slight reduction in spring rate at elevated temperatures.

In the present arrangement, however, the spring member 30 is made, at least in part, of a shape memory effect (SME) material, or a similar material, in which there is a substantial increase in spring rate at elevated temperatures. More particularly, the spring rate increases by a factor of about five over a temperature range between 40 degrees Centigrade and 50 degrees Centigrade. SME materials are well known, and available from, for example, Raychem Corporation of California, USA and Delta Materials Research Limited of Ipswich, Suffolk, England. These materials can, for example, be alloys of nickel and titanium; alloys of copper and zinc with aluminium, tin or silicon; or alloys of copper and aluminium with iron, manganese or silicon.The particular material is chosen to suit the particular application, in respect of the desired spring rates and the temperature range over which the spring rate changes.

In the present application, the spring members 30 are chosen to produce a clamping force on the edges 21 and 22 of the boards 20 of about 7 to 10 Ibs force (31 to 44N) at room temperature, so that insertion of the boards 20 into the channels 4 and 5 can be accomplished without exerting undue force. At a temperature of about 40 degrees Centigrade the spring member 30 starts to become stiffer exerting a progressively greater clamping force on the edge of the boards until, at about 50 degrees Centigrade, this force is about 35 to 50 Ibs force (155 to 220N). These higher forces are sufficient to flatten the board against the respective fins 6 so that more surface imperfections on the board or fins are overcome.

The efficiency of heat transfer between the board 20 and the cooled wall 3 of the housing is largely dependent on the clamping force exerted at the edge of the board. Where this clamping force is low, the temperature difference between the edge of the board and the channel can be greater than 10 degrees Centigrade. Any reduction in this temperature difference is an important contribution to reducing the temperature of the electronic components on the board. It will be appreciated that it is at these elevated temperatures, where the components on the boards 20 are generating large amounts of heat, that it is most important for efficient heat dissipation to occur, and that at lower temperatures this is not so critical.

Various alternative arrangements of spring member are possible. For example, as shown in Fig. 3, the spring member 40 could be of shape memory effect material and of U-shape section. In this arrangement, the spring member 40 is straight and contacts the edge of the board 20, on one side, and the fin 6 on its other side, along its entire length. The spring member 40 is shaped so that its arms are urged resiliently outwardly against the board 20 and fin 6, the outward force increasing at elevated temperatures.

The spring member need not be entirely of a SME material. Three different arrangements where SME material only forms a part of the spring member are shown in Figs. 4,5 and 6.

The arrangement of Fig. 4, includes a SME element 50 in the form of a flat tape which extends along one side of a serpentine member 51 made of spring steel. The SME tape 50 stretches by up to about 8% when a board is inserted in a channel. When a predetermined temperature is exceeded, the tape 50 tries to contract and recover its original length, applying a force several times (typically five times) that applied at lower temperatures.

This arrangement has the advantage of limiting the maximum stress applied to the SME element thereby ensuring it recovers to its origi nai shape.

In the arrangement shown in Fig. 5, the SME element 60 is a straight cylindrical rod that extends through apertures 61 in a serpentine member 62 of spring steel. Opposite ends 63 and 64 of the SME rod 60 are enlarged to engage the serpentine member.

In the arrangement of Fig. 6, there are five SME elements 71 to 75 in the form of closed loops. The loops 71 to 75 are each secured at opposite ends to a serpentine member 76 of spring steel so that the loops are stretched when the serpentine member is flattened by insertion of a board in a channel. When the predetermined temperature is exceeded, the loops exert an increased force to return to their original shape.

At the elevated temperatures, above about 40 degrees Centigrade, a part at least of the resilience of the mounting of the boards with the housing will have been lost. This will result in the boards being subjected to higher vibration forces at these elevated temperatures. However, at lower temperatures the advantages of a resilient mounting will still be present. In this way, the arrangement of the present invention is preferable to previous arrangements employing some form of clamp which locks the board firmly in the channel at all temperatures.

The present invention enables the boards to be inserted into, and removed from, the housing without the need for special tools and without the need to perform locking and releasing operations. The spring member of the present invention can be of light weight, can be inexpensive and does not require maintenance. A further advantage of the present invention is that it will readily accommodate boards of different thicknesses.

Claims

1. An equipment housing adapted for supporting one or more electronic modules and for conducting heat away from said modules, wherein the housing includes at least one spring member arranged to urge a surface of a module into thermal contact with a surface of the housing, and wherein said spring member is arranged to increase its spring rate at elevated temperatures such that the module is urged more firmly against the surface of the housing at the elevated temperatures.

2. An equipment housing according to Claim 1, wherein the housing includes a pair of opposite channel members for receiving opposite edges of each respective module, wherein the or each said spring member is located in a respective channel member, and wherein the said surface of the housing is provided by an elongate heat-conducting member.

3. An equipment housing according to Claim 2, wherein the spring member extends along the major part of the length of the channel.

4. An equipwent housing according to any one of the preceding claims, wherein the spring rate of the spring member increases several fold between the temperatures of about 400 Centigrade and 500 Centigrade.

5. An equipment housing according to Claim 4, wherein the spring rate increases approximately five fold between the temperatures of about 400 Centigrade and 500 Centigrade.

6. An equipment housing according to any one of the preceding claims, wherein the spring member includes an element of a shape memory effect material.

7. An equipment housing according to any one of the preceding claims, wherein the spring member includes a strip of elongate serpentine shape.

8. An equipment housing according to any one of the preceding claims, wherein the spring member is made substantially entirely of a shape memory effect material.

9. An equipment housing according to any one of Claims 1 to 7, wherein the spring member includes a spring element and an element of a shape memory effect material that engages said spring element.

10. An equipment housing according to Claim 9, wherein the said element of a shape memory effect material is arranged to contract at elevated temperatures.

11. An equipment housing according to Claim 9 or 10, wherein the said spring element is of serpentine shape, and wherein the said element of a shape memory effect material is substantially straight and extends along the length of the spring member.

12. An equipment housing according to Claim 9 or 10, wherein the said spring element is of serpentine shape, and wherein the said element of a shape memory effect material includes at least one loop secured at each end with said spring element.

13. An equipment housing according to any one of Claims 1 to 6, wherein the spring member includes an elongate member of Ushape section.

14. An equipment housing according to any one of the preceding claims, wherein the said electronic modules are electronic circuit boards.

15. An equipment housing substantially as hereinbefore described with reference to Figs.

1 and 2 of the accompanying drawings.

16. An equipment housing substantially as hereinbefore described with reference to Figs.

1 and 2 as modified by any one of Figs. 3 to 6 of the accompanying drawings.

17. A spring member for an equipment housing according to any one of the preceding claims.

18. A spring member for urging a surface of an electronic module into thermal contact with a surface of a housing in which said module is supported, wherein the said spring member is arranged to increase its spring rate at elevated temperatures such that the module is urged more firmly against the surface of the housing at the elevated temperatures.

19. A spring member according to Claim 18, wherein the spring rate of the spring member increases several fold between the temperatures of about 400 Centigrade and 500 Centigrade.

20. A spring member according to Claim 19, wherein the spring rate increases approximately five fold between the temperatures of about 400 Centigrade and 500 Centigrade.

21. A spring member according to any one of Claims 18 to 20, wherein the spring member includes an element of a shape memory effect material.

22. A spring member according to any one of Claims 18 to 21, wherein the spring member includes a strip of elongate serpentine shape.

23. A spring member according to any one of Claims 18 to 22 made substantially entirely of a shape memory effect material.

24. A spring member according to any one of Claims 18 to 22 including a spring element and an element of a shape memory effect material that engages said spring element.

25. A spring member according to Claim 24, wherein the said element if a shape memory effect material is arranged to contract at elevated temperatures.

26. A spring member according to Claim 24 or 25, wherein the said spring element is of a serpentine shape, and wherein the said element of a shape memory effect material is substantially straight and extends along the length of the spring member.

27. A spring member according to Claim 24 or 25, wherein the said spring element is of serpentine shape, and wherein the said element of a shape memory effect material includes at least one loop secured at each end with said spring element.

28. A spring member according to any one of Claims 18 to 23, wherein the spring member includes an elongate member of U-shape section.

29. A spring member substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

30. A spring member substantially as hereinbefore described with reference to Figs. 1 and 2 as modified by any one of Figs. 3 to 6 of the accompanying drawings.

31. Any novel features or combination of features as hereinbefore described.