GB2275321A - Locking element for a thermostatic valve - Google Patents

Abstract

A locking element 56 for a thermostat has attachment means comprising a body portion 64 and an end portion 63 for attaching the locking element to the thermostat. Locking means comprising a tab 61 extend from the body portion and 64 lock a displaceable valve 20 of the thermostat 10 in an open position when the valve 20 has been displaced a pre-determined amount. The pre-determined displacement is a displacement between the normal operational temperature displacement and the failure temperature displacement of the thermostat. The end portion may be attached to the displaceable valve and the locking means may engage a lower bridge 16. <IMAGE>

Description

Title: FAIL-OPEN LOCKING ELEMENT AND THERMOSTAT INCORPORATING SAME FIELD OF THE INVENTION This invention relates generally to the field of thermostats such as may be used in liquid-cooling systems and in particular relates to the type of thermostat that incorporates a temperature sensitive valve which varies the liquid flow rate according to temperature.

BACKGROUND OF THE INVENTION Thermostats of the type used for temperature regulation in liquid-cooling systems are typically disposed in a closed flow loop of conduits which also include a pump and a heat exchanger such as a radiator or similar devices. Such thermostats usually incorporate a valve which may be opened according to the temperature of the cooling liquid. By varying the opening of the valve, control of the flow of liquid to the heat exchanger or radiator can be effected whereby the temperature of the liquid can be regulated. Typically the valve is displaced by a temperature sensitive plunger which acts against a spring which biases the valve to the closed position. In the closed position no liquid flows to the heat exchanger or radiator.However, when the temperature of the liquid exceeds the desired value the plunger displaces the valve into an open position, against the spring, to allow a desired amount of liquid to flow through the heat exchanger or radiator. In this manner the liquid temperature can be controlled.

Failure of the thermostat, for example by the valve being stuck in the closed position, results in a loss of temperature control. In this circumstance the liquid is not directed to the heat exchanger or radiator and as a result the cooling properties of the system are lost. Inevitably the system intended to be cooled becomes damaged due to prolonged overheating. In an automobile engine, for instance, permanent damage can occur when the cooling system has failed.

In automobile thermostats, the temperature sensitive plunger is typically controlled by a thermally expansive composition formed, from among other things, of wax. Thermostats may malfunction for other reasons however, including corrosion to the plunger components. In an extreme overheating situation, the thermostat may be damaged, for example, by the loss of the thermally expansive wax. Regardless of why extreme overheating occurs, thermostats are typically located close enough to the parts being cooled to be damaged by the overheating.

However, after extreme overheating occurs the thermostat itself will be damaged resulting in difficulty in determining whether the thermostat was damaged prior to the overheating (and thus was a contributing factor), or whether the thermostat was merely damaged as a result of the overheating. Determining if there was an intrinsic failure of the thermostat is important in apportioning fault, since the cost of repairing or replacing the damaged system, such as an automobile engine, can be very high.

One way to resolve this difficulty is disclosed in U.S. Patent No. 4,883,225 (Kitchens). This patent discloses providing a thermostat that includes a fusible alloy in a temperature sensitive valve. When the thermostat is subjected to an extreme overheating, the fusible alloy melts causing the valve to be permanently displaced in an open position. The fluid is therefore directed to the heat exchanger even though the thermostat has failed.

The difficulty with this prior approach however is that a thermostat incorporating such a fusible alloy is very difficult to manufacture. In particular, the components of the plunger need to be manufactured under very precise tolerances, which are difficult if not impossible to achieve in mass production. Furthermore, expensive materials are required and additional steps are needed in the manufacturing process in order to assemble this device.

BRIEF SUXNRY OF THE INVENTION What is desired is a way of assuring that the thermostat does not fail in the closed position, after it has been overheated and damaged. Preferably any solution should be simple, inexpensive and work without impeding the normal functioning of the thermostat. Also, the solution should not require difficult or impossible manufacturing standards.

According to a first aspect of the present invention there is provided a locking element for a thermostat having a displaceable valve, said locking element comprising: (a) attachment means for attaching said locking element to said thermostat, said locking element being positioned to act between said displaceable valve and the remainder of said thermostat; and (b) at least one locking means located on said attachment means for locking said displaceable valve in an open position when said displaceable valve has been displaced a pre-determined amount.

According to a second aspect of the present invention there is provided a thermostat for liquidcooling systems, comprising: (a) a flange having an opening; (b) a valve for sealing said opening, said valve being displaceable from said -opening for allowing the passage of liquid through said opening; (c) means for supporting said valve in said opening; (d) means for displacing said valve from said opening in response to a temperature variation; and (e) at least one locking element having i) attachment means for attaching said locking element to said thermostat, said locking element being positioned to act between said displaceable valve and the remainder of said thermostat, and ii) at least one locking means for locking said displaceable valve in an open position when said displaceable valve has been displaced a pre-determined amount.

LIST OF FIGURES Figure 1 shows a cross-sectional view of a thermostat containing a first embodiment of a locking element in accordance with the present invention along the lines 1-1 of Figure 2; Figure 2 shows an end view from above of the embodiment of Figure 1; Figure 3 shows a perspective view of the first embodiment of the locking element of Figure 1; Figure 4 shows an enlarged view of a locking element in accordance with the present invention as it flexes under contact with a valve.

Figure 5 shows a cross-sectional view of a thermostat containing a second embodiment of a locking element in accordance with the present invention along the lines 5-5 of Figure 6; Figure 6 shows an end view from above of the embodiment of Figure 5; Figure 7 shows a perspective view of the second embodiment of the locking element of Figure 5; and Figure 8 shows a diagram of valve disk displacement relative to temperature.

DEvTT-Rn DESCRIPTION OF THE PREFERRED EMBODIMENT A thermostat is indicated generally as 10 in Figure 1. The thermostat 10 is a high flow type of thermostat used in automobile cooling systems. While the preferred embodiment relates to automotive thermostats, it will be appreciated that the present invention is not limited to such thermostats, and may be used in many other types of thermostats which operate in a similar manner.

The thermostat 10 consists of a generally circular flange 12 surrounded by a valve supporting means comprising an upper bridge 14 and a lower bridge 16. As can be seen in Figure 2, the upper bridge 14 spans an opening 18 which is generally circular. The upper bridge 14 is cut away on opposite sides to form a further opening 19 on either side of the upper bridge 14. In the preferred embodiment of the Figures, the upper bridge is formed in one piece with the flange 12, whereas the lower bridge 16 is a separate piece fastened onto the flange 12.

Figure 1 also shows a temperature sensitive valve indicated generally at 20 which comprises a pin 22 contacting the underside of the bridge 14 at one end and housed within a body 24 at the other end. The body 24 contains a rubber boot 25 containing a temperature sensitive displacement means 26 as shown in the broken away portion of Figure 1. The temperature sensitive displacement means 26 responds to the temperature of a surrounding fluid as further described below. It will be appreciated that the left sides of Figures 1 and 5 show the valve 20 in a closed position, and the right sides show the valve 20 in a fully open position. These positions are described further below.

Also attached to the body 24 is a valve disk 27 which is secured around an end cap 28 of the body 24. The underside of the valve disk 27 is contacted by one end of a spring 32. The spring 32 is supported at the other end by lower bridge 16. Preferably, in the installed position, the spring 32 is under some compression whereby the valve disk 27 is urged into engagement with a shoulder 34 which surrounds the opening 18. The shoulder 34 provides a good sealing surface and if desired, the valve disk 27 may further include a rubber sealing gasket (not shown). Also shown is a jiggle pin 40 which is located in an opening 42 formed within the valve disk 27 and which is described in greater detail below.

The lower bridge 16 is typically secured to the flange 12 at either side by points 44. Tabs are extended through the flange 12 and are then sheared off to form a solid attachment between lower bridge 16 and flange 12.

The lower bridge 16 extends downwardly on either side below the flange 12 along arms 46 which connect to a bottom member 48. Bottom member 48 in turn has an opening 50 which is large enough to accommodate the body 24. In this manner, the body 24 is free to move axially along the axis of line 52 in the direction of double-headed arrow 54.

Locking elements according to two embodiments of the present invention are generally shown respectively at 56 in Figures 1 and 3, and 57 in Figures 5 and 7. In both embodiments, the locking elements 56, 57 have an attachment means generally indicated at 58 and a locking means generally indicated at 60.

In the first embodiment of Figures 1 and 3, the locking element 56 has an end portion 63 and a body portion 64. The end portion 63 includes a pair of apertures 66a through which are inserted the attachment tabs of the lower bridge 16. The end portion 63 is urged firmly against the flange 12 to secure the locking element 56 when the lower bridge 16 is secured to the flange 12 by points 44. The end portion 63 may also be secured by welding, brazing or other suitable ways of attachment as will be known to those skilled in the art. To securely lock the valve 20, it is preferred to provide at least two locking elements 56, one attached to each arm 46 of the lower bridge 16.

In the second embodiment of Figures 5 and 7, the locking element 57 has an end portion 65 and body portions 67. The end portion 65 and body portions 67 have a similar function as the corresponding elements in the first embodiment. The end portion 65 is semi-annular and rests against the bottom member 48 of the lower bridge 16. The spring 32 urges the end portion 65 against the bottom member 48 to hold the locking element 57 in place. There are two preferred configurations of end portion 65, namely, one with an opening to allow the locking element 57 to be assembled on to the thermostat after the thermostat is fully made, or the bottom portion can be formed as a closed circle which is assembled under the spring as the thermostat is assembled.As seen in Figure 7, the locking element 57 has two body portions 67 which are adapted to extend on opposing sides of the valve 20 to securely lock the valve 20.

For both embodiments, the body portions 64, 67 of the locking elements 56, 57 are shaped to generally follow the profile of the adjacent arm 46 of the lower bridge 16. The body portions 64, 67 are located outside of a path 68 that the periphery of the valve disk 27 follows when the body 24 moves along the axis 52 in the direction of double arrow 54.

In both embodiments, the locking means 60 comprises a tab 61 that extends at an angle inwardly and downwardly from the body portion 64, 67 into the path 68 of the valve disk 27. Alternatively, the locking means 60 could be an aperture defined in the body portion 64, 67 that is adapted to receive the periphery of the valve disk 27 to lock the valve 20 (in such a case, the body portion 64, 67 would extend into the path 68 of the valve disk 27 to enable the locking means 60 to operate). In a further alternative, the locking elements 56, 57 could be attached to the underside of the valve disk 27, and the locking means 60 could be spring biased against the arms 46 of the lower bridge 16. In such a case, the spring biased locking means 60 could extend through an aperture in the arm 46 to lock the valve 20.

In the embodiments depicted in the Figures, the body portion 64, 67 is spaced from the arm 46 of the lower bridge 16 at the point where the tab 61 is located. As shown in Figure 4, the space allows the body portion 64, 67 to flex outwardly towards the adjacent arm 48 to accommodate the passage of the valve disk 27 as it moves along the path 68. When the valve disk 27 moves downwardly past the locking means 60, the body portion 64, 67 springs back to its original position and the tab 61 blocks the return path 68 of the valve disk to prevent the valve disk 27 from being urged by the spring 32 back to its closed position.

As shown in Figures 3 and 7, the locking element 56, 57 is formed from a single piece of resiliently flexible material, such as stainless steel or other material. The tab 61 is cut from the body portion 64, 67 and bent inwardly into the path of the valve disk 27.

Alternatively, the locking elements 56, 57 could be formed from a rigid material, such as steel, and combined with a spring to provide the spring biased locking means 60 described earlier. For automobile thermostats, the locking means 60 should be capable of resisting a force of up to 50 kilograms. The flexing capacity of such thermostats should be between 3 and 10 kilograms approximately.

For both embodiments, the tab 61 terminates in an engaging surface 70 that is spaced a pre-determined locking distance DL from the shoulder 34. The engaging surface 70 defines the point at which the valve disk 27 becomes locked. The engaging surface 70 engages the valve disk 27 to prevent the valve disk 27 from returning to the closed position. The pre-determined locking distance DL corresponds to the distance the valve disk 27 is displaced when the thermostat 10 has been subjected to a predetermined extreme temperature TL as described further below.

The thermostat 10 is installed in the liquid cooling system of a vehicle. For example, the thermostat 10 may be installed on the upper return line of the cooling system in line with the cooling liquid so that the cooling liquid has to flow through and around the thermostat 10 in order to circulate. The return line then feeds into the top of a radiator structure or heat exchanger which acts as a way of dissipating excess heat carried by the cooling liquid. A line then is fed from the bottom of the radiator to a water pump or the like which typically forces the liquid back into the engine block where the cooled liquid is used to cool portions of the engine block (by absorbing heat). As will be appreciated by those familiar with engines, in other systems, the thermostat may be placed in the bottom and the circulation will go in the opposite direction from the bottom to the top.

The thermostat 10 is situated in the cooling line in such manner that the body 24 is exposed to the heated cooling fluid. It is preferable for the body 24 to be fully immersed and therefore the valve 10 may be located somewhat below the highest liquid level of the coolant in the cooling system, on the engine side.

When the motor or vehicle is running, heat is generated in the engine block which in turn is passed into the liquid coolant. As the temperature of the coolant rises, the temperature rises around body 24. This has the effect of causing temperature sensing means 26 to expand.

As temperature sensing means 26 expands, the pin 22 is forced axially outwardly of the body 24. The end of the pin 22 engages the underside of the bridge 14 which in turn causes the body 24 to move axially in the direction of arrow 54. This unseats the groove 30 of the valve disk 27 from the shoulder 34 allowing cooling fluids to pass through opening 18 and out openings 19. The fluids then are permitted to return to the radiator or heat exchanger.

The purpose of the jiggle pin 40 is to provide a form of pressure relief across the valve 20. In the event that the liquid coolant heats up too quickly, for example, a pressure differential can be built up under the valve disk 27. Such pressure is permitted to escape past the jiggle pin 40 before it builds up to an extent which would prevent the pin 22 from pushing the valve to an open position. As will be appreciated, if there was a great pressure differential between the two sides of the valve disk 27, this would require much more work by the pin 22 to open the valve which is undesirable.

Figure 8 provides a diagram showing the displacement of the valve disk 27 relative to changes in temperature.

As shown, the valve disk 27 is displaced under normal working conditions from its initial position Di at initial temperature Ti up to a distance Dw at a maximum normal working temperature Tw The range between temperatures Tj and Tw is the normal working zone of the thermostat and will vary according to the design characteristics of the thermostat.

As the temperature exceeds Tw, the valve disk 27 is further displaced, and then will reach the predetermined locking distance DL at the pre-determined extreme temperature TL. The range between temperatures Tw and TL is an overheating buffer zone for the thermostat.

While the temperatures in this zone exceed the normal working temperatures of the thermostat, they are not considered to be high enough that permanent damage is likely to occur to either the motor, or the thermostat itself.

At the pre-determined extreme temperature TL the thermostat is being subjected to extreme overheating and at this point there may be a risk of damage to the thermostat and to the motor. Further increases in temperature above the pre-determined temperature TL cause a further displacement of the valve disk 27 up to a failure distance Df where a failure temperature Tf is reached. At this point, the internal components of the thermostat 10 will absolutely fail. For conventional automotive thermostats, failure typically occurs when the internal rubber boot 25 containing the temperature sensitive wax 26 becomes damaged at temperature Tf. When the thermostat has failed, the pin 22 no longer acts to force the valve disk 27 axially outwardly from the body 24 and consequently the spring 32 urges the valve disk 27 upwardly towards the closed position against the shoulder 34.However, since the valve disk 27 has been displaced beyond the pre-determined distance DL the locking means 60 prevents the valve disk 27 from returning to the closed position.

The pre-determined locking distance DL is thus set between the maximum normal working distance Dw and the failure distance Df. Preferably, the overheating buffer zone is provided to account for minor, non-detrimental, temperature increases above the working zone. However, once DL has been reached, the engaging surface 70 will engage the valve disk 27 to prevent the valve disk 27 from returning to the closed position.

It will be appreciated by one skilled in the art that the operating parameters of the thermostat 10 will vary according to the particular operating parameters of the cooling system for which it is designed. By way of example, a table of operating parameters for a sampling of differently rated automotive cooling system thermostat is provided below (with tolerances shown in brackets). It should be noted that for such thermostats full opening of the valve 20 is typically required at the maximum normal working temperature Tw The corresponding maximum normal working displacement distance Dw is typically between 7 and 8 mm (as indicated below), although the average working displacement typically is less, approximately one half of the maximum, about 3.5 to 4 mm.This forms an average working zone as shown in Figure 8 as 90.The limits of this zone will vary depending upon particular engine characteristics. At 92, another zone is shown which represents a beginning to overheat condition.

Ratings 710C 1l 77 C 82 C 880C 910C (+2 C) (i2 C) (t20c) (#2 C) (+20C) (#2 C) (#2 C) (#2 )(#2 C) (#2 C) D1 (mm) 0 0 0 0 O T1 ( C) 71 77 82 88 91 Dw (mm) 7 to 8 7 to 8 7 to 8 7 to 8 7 to 8 (approx) Tw ( C) 81 87 92 98 101 DL (mm) 9.5 9.5 9.5 9.5 9.5 (approx) TL ( C) 93 85 90 96 99 Df (mm) 11 11 11 11 11 (approx) T1 ( C) 135 135 135 135 135 It can now be appreciated how the present invention operates. Under normal working conditions, the valve 20 will operate in a conventional manner to control the flow of coolant through the opening 18. As the operating temperatures increase beyond the normal working temperatures, the valve will continue to operate. When the operating temperatures exceed the pre-determined locking temperature TL (at which point extreme overheating is occurring), the locking means 60 blocks the return path 68 of the valve 20 to lock the thermostat 10 in an open position. Thereafter, if the thermostat is subjected to such extreme temperatures that failure occurs, the locking means 60 prevents the valve 20 from returning to seal the opening 18. Thus, unlike conventional thermostats, the locking means 60 ensures that the valve remains open before, during, and after the thermostat fails due to overheating.

It will be appreciated that the preferred locking is achieved by two locking means, one on each side of the valve 27. Even once locked, the locking means may be released, by a simple use of a screwdriver point or the like.

It will be appreciated that the foregoing description is in respect of a preferred embodiment of the invention and that various modifications are possible within the broad scope of the appended claims. Some of these modifications have been discussed above and others will be apparent to those skilled in the art. For instance, the locking element 56, 57 may be used with many types of automotive thermostats, including by-pass thermostats, or may be used for any heating systems in which temperature control is applied. The locking element 56, 57 may be adapted for use in other types and configurations of thermostats or heat controllers not necessarily directed to automotive cooling systems.

Claims (18)

1. A locking element for a thermostat having a displaceable valve, said locking element comprising: (a) attachment means for attaching said locking element to said thermostat, said locking element being positioned to act between said displaceable valve and the remainder of said thermostat; and (b) at least one locking means located on said attachment means for locking said displaceable valve in an open position when said displaceable valve has been displaced a pre-determined amount.

2. A locking element as claimed in claim 1, wherein said pre-determined amount is located in the range that said displaceable valve is displaced when subjected to temperatures between the maximum normal working temperature of said thermostat and the failure temperature of said thermostat.

3. A locking element as claimed in claim 1 or claim 2, wherein said locking element is resiliently flexible relative to said displaceable valve in order to flex as said displaceable valve is being displaced by said pre-determined amount.

4. A locking element as claimed in claim 1, 2 or 3, wherein said attachment means has an end portion for attaching said locking element to said thermostat, and at least one body portion for carrying said locking means.

5. A locking element as claimed in claim 4, wherein said thermostat has upper and lower bridges for supporting said displaceable valve, and wherein said end portion is attached to said lower bridge.

6. A locking element as claimed in claim 5, wherein said body portion is positioned generally parallel to a path of displacement of said valve and wherein said locking means comprises a tab rigidly connected to said body portion, said tab protruding into said path of displacement.

7. A locking element as claimed in claim ó, wherein said tab extends downwardly into said path of displacement relative to said upper bridge.

8. A locking element as claimed in 5, 6 or 7, wherein at least one aperture is defined in said end portion for attaching said end portion to said lower bridge.

9. A locking element as claimed in any preceding claim, wherein said locking element is formed from a single piece of resiliently flexible material.

10. A locking element as claimed in claim 5 or claim 6, wherein said end portion is adapted to rest against the bottom of said lower bridge, and wherein two of said body portions extend from opposing sides of said end portion.

11. A locking element as claimed in claim 4 or claim 5, wherein said body portion is positioned to intersect a path of displacement of said valve, and wherein said locking means comprises an aperture defined in the part of said body portion that is located inside said path of displacement.

12. A locking element as claimed in claim 5, wherein said end portion is attachable to said displaceable valve, and wherein said locking means are adapted to engage said lower bridge.

13. A thermostat for liquid-cooling systems, comprising: (a) a flange having an opening; (b) a valve for sealing said opening, said valve being displaceable from said opening for allowing the passage of liquid through said opening; (c) means for supporting said valve in said opening; (d) means for displacing said valve from said opening in response to a temperature variation; and (e) at least one locking element as claimed in any preceding claim.

14. A thermostat as claimed in claim 13, wherein said supporting means includes an upper bridge and a lower bridge spanning said opening on opposed sides of said flange.

15. A thermostat as claimed in claim 14, wherein said displacing means includes a pin contacting the underside of one of said bridges, at one end, and being extendibly housed within a body at the other end, said body containing a temperature sensitive means for extending said pin; and wherein said valve includes a valve disk which is rigidly attached to said body and is capable of sealing said opening in said flange when said pin is retracted relative to said body; and further comprising a spring between said valve disk and the other of said bridges for urging said valve disk into sealing engagement with said opening.

16. A thermostat as claimed in claim 13, 14 or 15, wherein two of said locking elements are attached to said lower bridge on opposing sides of said valve.

17. A locking element substantially as described herein with reference to Figs. 1 to 4 and 8 or Figs. 5 to 8 of the accompanying drawings.

18. A thermostat substantially as described herein with reference to Figs. 1 to 4 and 8 or Figs. 5 to 8 of the accompanying drawings.

18. A thermostat substantially as described herein with reference to Figs. 1 to 4 and 8 or Figs. 5 to 8 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A locking element for a thermostat having a displaceable valve supported by upper and lower bridges, said locking element comprising: (a) an end portion for attaching said locking element to said thermostat; (b) at least one body portion connected to said end portion, said body portion being adapted to extend generally parailel to a path of displacement of said displaceable valve; and (c) a tab rigidly connected to said body portion, said tab protruding into said path of displacement to lock said displaceable valve in an open position when said displaceable valve has been displaced a pre-determined amount, said pre-determined amount being located in the range that said dispiaceable valve is displaced when subjected to temperatures between the maximum normal working temperature of said thermostat and the failure temperature of said thermostat; wherein said locking element is resiliently flexible relative to said displaceable valve in order to flex as said displaceable valve is being displaced by said pre-determined amount.

2. A locking element as claimed in claim 1, wherein said end portion defines an attachment flange that is adapted to fit in a seat defined in said lower bridge, said attachment flange being sandwiched in said seat between said upper and lower bridges to attach said locking element to said thermostat.

3. A locking element as claimed in claim 2, wherein at least one aperture is defined in said attachment flange, said aperture being adapted to receive an attachment tang for securing said upper and lower bridges together.

4. A locking element as claimed in claim 1, 2 or 3, wherein said tab extends downwardly into said path of displacement relative to said upper bridge.

5. A locking element as claimed in any preceding claim, wherein at least one aperture is defined in said end portion for attaching said end portion to said lower bridge.

6. A locking element as claimed in claim 1, wherein said lower bridge comprises a pair of arms connected by a bottom member, and wherein said end portion defines an attachment flange that is urged against said bottom member to attach said locking element to said thermostat.

7. A locking element as claimed in claim 6.

wherein said attachment flange is generally ring- shaped for seating against said bottom member.

8. A locking element as claimed in claim 7, wherein said ring-shaped attachment flange has an opening defined in its perimeter to permit said locking element to be slidably attached to said lower bridge.

9. A locking element as claimed in claim 6. 7 or 8, wherein said attachment flange is sandwiched between said bottom member and a spring connected to said displaceable valve.

10. A locking element as claimed in any of claims 6 to 9, wherein a pair of said body portions are connected to said end portion, said body portions being positioned to extend on opposing sides of said displaceable valve to permit said locking means to lock said valve on said opposing sides.

11. A locking element as claimed in any of claims 6 to 10, wherein said body portion is positioned to intersect a path of displacement of said valve, and wherein said attachment flange comprises an aperture defined in the part of said body portion that is located inside said path of displacement.

12. A locking element for a thermostat having a displaceable valve supported by upper and lower bridges, said locking element comprising: (a) an end portion for attaching said locking element to said displaceable valve; (b) at least one body portion connected to said end portion; and (c) a locking means located on said body portion, said locking means being adapted to engage said lower bridge to lock said displaceable valve in an open position when said displaceable valve has been displaced a pre-determined amount, said pre-determined amount being located in the range that said displaceable valve is displaced when subjected to temperatures between the maximum normal working temperature of said thermostat and the failure temperature of said thermostat; wherein said locking element is resiliently flexible relative to said displaceabie valve in order to flex as said displaceable valve is being displaced by said pre-determined amount.

13. A locking element as claimed in any preceding claim, wherein said locking element is formed from a single piece of resiliently flexible material.

14. A locking element substantially as described herein with reference to Figs. 1 to 4 and & or Figs. 5 to 8 of the accompanying drawings.

15. A thermostat for liquid-cooling systems, comprising: (aj a flange having an opening; (b) a valve for sealing said opening, said valve being displaceable from said opening for allowing the passage of liquid through said opening; (c) an upper bridge and a lower bridge spanning said opening on opposed sides of said flange for supporting said valve in said opening; (d) means for displacing said valve from said opening in response to a temperature variation, said displacing means including a pin contacting the underside of one of said bridges, at one end, and being extendibly housed within a body at the other end, said body containing a temperature sensitive means for extending said pin, and said body being rigidly attached to said valve for sealing said opening in said flange when said pin is retracted relative to said body; ; (e) a spring located between said valve and the other of said bridges for urging said valve into sealing engagement with said opening; and (f) at least one locking element as claimed in any preceding claim.

16. A thermostat as claimed in claim 15, wherein said valve comprises a valve disk.

17. A thermostat as claimed in claim 15 or 16, wherein two of said locking elements are attached to said lower bridge on opposing sides of said valve.