GB1573923A

GB1573923A - Thermal fuse device

Info

Publication number: GB1573923A
Application number: GB778876A
Authority: GB
Original assignee: Dunlop Ltd
Current assignee: Dunlop Ltd
Priority date: 1976-02-27
Filing date: 1976-02-27
Publication date: 1980-08-28
Also published as: FR2342444B1; FR2342444A1; DE2707403A1

Description

(54) AN IMPROVED THERMAL FUSE DEVICE (71) We, DUNLOP LIMITED, a British Company of Dunlop House, Ryder Street, St. James's, London S.W.I, 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: The present invention relates to an improved thermal fuse device suitable for incorporation in the rim of a pneumatic tyre and wheel rim assembly to relieve tyre pressure in the event that the temperature of the assembly rises to a potentially dangerous level.

It has been known for many years that it is desirable, especially in the case of aircraft wheels, to provide a safety device to prevent explosion of the tyre as a result of its exposure to a high temperature, for example generated by an associated brake. The need for such a safety device becomes greater as aircraft are braked from higher speeds and more efficient brakes produce greater heat, especially in exceptional conditions such as when take off is aborted and the now stationary wheel is subject particularly in its upper region to heat radiated by and conducted from nearby brake discs having high energy absorption characteristics.

British Patent Specifications Nos.

878,973, 963,392, 975,559 and 1,036,138 and U.S.A. Patent Specification No. 3,157,219 disclose various proposals for utilising an eutectic plug which will melt at the critical temperature allowing the air pressure of the tyre to be relieved through a passage vacated by the plug. In Specification No.

878,973 the problem of "creeping" of the eutectic material is referred to and the solution offered is to locate the eutectic plug in a passage with a 40 taper. Although an eutectic material is one which has a sharply defined melting point there is a danger that in use the eutectic plug will be raised repeatedly to a temperature near to its melting point causing a temporary loss of solidity which may be sufficient to cause progressive extrusion of the plug from the passage under the influence of air pressure in the pneumatic tyre. This problem is again referred to in British Patent Specification No. 1,036,138 which discloses a device in which a visual indication is given when "creep" has occurred beyond tolerable limits.

"Creep" of the eutectic plug can cause it to be deformed, without ever fully changing from the solid to the liquid state, until a tiny hole is formed through the eutectic plug or between it and the wall of the passage, through which pressurised air from the tyre can leak. The effect of passage of air past the plug is, however, to cool and harden the eutectic plug with the result that the hole is not enlarged and may even be temporarily reclosed with the result that the tyre is slowly and progressively deflated without the fuse device ever operating in the intended manner to avert danger.

This major problem in the use of such thermal fuse devices is to some extent overcome in arrangements such as described in British Specifications Nos.

963,392 and 975,559 wherein the eutectic plug is followed through the air passage by a plunger or piston which has a higher melting point than the eutectic plug. It has been proposed that the plug should take the form of a rod extending transversely across the air passage below the piston and not filling the air passage, the opposite ends of the rod being engaged in apertures opening to the passageway. Such arrangements go some way toward overcoming the leakage problem in that no air will escape until the eutectic plug has been liquified or deformed sufficiently to allow the plunger to be expelled from the air passage, ensuring that there will be no air leakage unless and until the device operates rapidly to deflate the tyre.

However even such arrangements as have a piston following the eutectic plug through the air passage will eventually fail through "creep" of the eutectic material if the device is frequently brought near to the melting temperature of the eutectic without the danger situation which the device is intended to safeguard ever arising. One of the factors contributing to this in prior art proposals utilising a plunger sealing the air passage behind the eutectic plug is that the eutectic plug spanning the air passage through the device has a relatively large unsupported area on its downstream face so that it can progressively deform in the air passage. Another principal drawback of prior art devices is that since the eutectic plug is located in the air passage there is also the danger that air under pressure venting through the passage will cool and resolidify eutectic material remaining in the passage which may at least partially obstruct the air flow and inhibit the desired rapid deflation of the tyre.

To summarise, it is desirable that the device should not function except in the presence of a predetermined, critical temperature whereupon it should function to deflate the tyre as rapidly as possible.

It is a principal object of the present invention to provide an improved thermal fuse device.

In accordance with the present invention there is provided a thermal fuse device for deflating a pneumatic tyre in response to a predetermined increase in temperature, the device comprising a housing having a passage for venting air from the tyre, a control element in the housing movable between a position closing and a position opening the passage and biassed by pneumatic pressure in the tyre, when the housing is attached thereto, towards the opening position, a latch element positioned to obstruct movement of the control element to the opening position, a plug of a fusible composition positioned to obstruct movement of the latch element and an outlet discreet from said passage through which the fusible composition can flow when melted without being cooled by air venting through the passage, the control element moving between positions fully closing and fully opening the passage in response to displacement of the latch element by the control element permitted by melting of the plug.

In the device of the immediately preceding paragraph retaining means for the plug is preferably provided by a chamber having a restricted outlet for the fusible composition when melted, said outlet opening at a position remote from the portion of said passage through which the control element moves from its closing to its opening position. Preferably the outlet is of reduced cross-sectional area relative to the chamber so that the chamber communicates with the outlet over a step or shoulder having a flat surface abutted by the plug and opposed to the direction of flow of the melted composition from the chamber through the outlet.

The latch element preferably extends between the chamber and a recess in the control element, the latch tending to move into the chamber under the influence of the bias of the control element when permitted to do so by molten fusible composition vacating the chamber through the outlet.

The fusible composition is preferably an eutectic metal, by which is meant a metal having a sharply defined melting point.

The outlet for the melted fusible composition may be located in any chosen position in the device such that melted fusible composition will not remain in the said venting passage to be cooled and hardened by venting fluid after the control element has moved to open the passage.

Preferred embodiments of the invention will now be described with reference to the accompanying diagrammatic drawings, in which: Figures 1 and 2 are similar longitudinal sectional views of respective thermal fuse devices in accordance with the invention.

In the embodiment of the invention illustrated in Figure 1 a thermal fuse device 8 comprises a T-shaped housing I IA having a longitudinal bore 12A crossed by a transverse bore 13. The branch 14 of the housing 1 lA is externally screw threaded at 15 ta engage in a screw4hreaded aperture (not shown) of an aircraft wheel rim on which a tubeless pneumatic tyre is mounted so that the bore 12A in the branch 14 will be in communication with the cavity defined by the rim and the inner surface of the tyre.

Freely slidable in the bore 12A is a control element in the form of a piston or plunger 20A so dimensioned that the piston 20A extends across the junction between the bores 12A and 13. At one end the piston 20A is surrounded by an O-ring 21 in an annular recess in the piston, the seal of the O-ring 21 between the piston 20 and the bore 12A in the branch 14 serving to prevent leakage of air past the piston 20A from the bore 12 into the bore 13.

At the free end of one branch 22 the transverse bore 13 is reduced in diameter to provide an outlet opening 17A communicating with the bore 13 over a step or shoulder 1 8A and between this and the piston 20A is located a billet 19A of eutectic material and a latch member in the form of a metal ball 23. The ball 23 extends between the chamber for eutectic plug 19A defined by the portion of the bore 13 in the branch 22 and a recess '4 cut into the piston 20A and opening to the end of the piston 20A located at the end of the bore 12 remote from the branch 14.

When the plug 19A of eutectic material is in its solid state it prevents movement of the ball 23 into the chamber in the branch 22 which in turn prevents movement of the piston 20A downward from the position shown to be expelled from the bore 12 by air pressure in the associated tyre. However, the inner end of the recess 24 in the piston is in the form of an inclined cam surface 25 which, under the influence of air pressure behind the piston 20A, will cause displacement of the ball 23 into the chamber of the branch 22 when the material of the plug 19A is liquified. By flowing out of the opening 1 7A the eutectic permits the ball 23 to move into the chamber, whereupon air pressure behind the piston 20A expells it from the bore 12A to allow deflation of the tyre through bores 12A and 13.

In use the device 8 is mounted in an aircraft wheel and the eutectic material of the billet 19A is chosen to have a melting point which corresponds with a temperature which is potentially dangerous for a tyre mounted on the wheel. If such a temperature is reached the material of the billet 19A will be converted from the solid to a liquid state and the piston 20A will then displace the eutectic material through the opening 1 7A as it moves under the influence of air pressure in the tyre. As soon as the 0ring 21 clears the inner end of the bore 12A in the branch 14 of the housing the pressurised air in the tyre will be vented through the transverse bore 13 and, as soon as the piston 20A is discharged from it, from the open end of the bore 12A.

Since the eutectic material flowing through the opening 17A is isolated from pressurised air venting from the tyre by the piston 20A it will not be cooled and hardened by the air flow. The flat surface 18A confronting the billet 19A will tend to prevent the billet progressively loosing its shape as a result of being brought repeatedly to a temperature near to its melting point so that the device 8 will not function to discharge air from the tyre unless the material of the billet 19A is fully changed from the solid to the liquid state.

The alternative thermal fuse device 9 illustrated in Figure 2 resembles the device 8 of Figure 1 in possessing a metal ball 23A serving as a latch between the housing 11 B and the piston 20B but here the housing has a single bore 12B in which the piston 20B is movable from the end 27 to the end 28 of the bore by air pressure in the associated tyre. A chamber 29 is formed in the piston 20B and a plug 30 of eutectic material is located in the chamber to be abutted by the ball 23A, the chamber 29 having a diameter approximately equal to that of the ball 23A and having the reduced diameter outlet branch 31 which, in use of the device, opens to the end of the piston 20B exposed to air pressure within the associated tyre. The dimensions of the plug 30 and ball 23A are chosen so that the ball 23A extends out of the chamber 29 into a recess 32 of semicircular section in the housing 1 lB surrounding the piston 20B, the dimensions of the recess 32 being such that in the assembled condition as shown the centre of the ball 23A is located within the chamber 29.

In use the device 9 of Figure 2 is mounted in an aircraft wheel rim by passing the shank 33 of the housing through an aperture of the rim (not shown) until the rim is engaged by the flange 34 of the device to form an airtight seal.

The end of the piston 20B having the opening 31 is thus subject to air pressure in the tyre mounted on the rim tending to force the piston 20B out of the end 28 of the housing but the piston is prevented from so moving by latching of the ball 23A in the recess 32. If the plug 30 of eutectic material is liquified the ball 23A can move inwardly of the chamber 29 under the cam action on the ball of the recess 32 until the piston 20B is free to be moved by the air pressure out of the bore 12B.

It will be noted that the relationship of the outlet 31 to the interior of the chamber 29 is such that mere deformation of the plug 30 before it becomes liquid is resisted by the surrounding surfaces of the piston 20B and of the ball 23A. It will also be noted that the outlet opening 31 is considerably smaller in cross-sectional area than the bore 12B and is spaced from the wall of the housing 11 B defining the bore 12B by the end surfaces of the piston 20B in which the outlet 31 is formed. The effect of this arrangement is that liquid eutectic material discharged from the opening 31 accumulates on the piston end without extending right across the bore 12B. More importantly, however, since the outlet 31 is in the end of the piston 20B presented toward the tyre cavity there will at no time be a rapid flow of discharging air over eutectic material on the piston end so long as the piston 20B remains in the bore 12B Each of the above thermal fuse devices therefore has the advantage that the outlet 17A or 31 for melted eutectic material is distinct from the passage 12, 13 or 12A, 13 or 12B through which air will be vented from a tyre and vented air will not flow over melted eutectic material tending to cool and harden the same. In each embodiment, also, the eutectic material is contained in a chamber which will tend to prevent progressive deformation of the plug or billet of eutectic material and which will allow escape of the eutectic material through the outlet 17A or 31 only when the eutectic material has been liquified.

WHAT WE CLAIM IS: A A thermal fuse device for deflating a pneumatic tyre in response to a predetermined increase in temperature, the device comprising a housing having a passage for venting air from the tyre, a control element in the housing movable between a position closing and a position opening the passage and biassed by pneumatic pressure in the tyre, when the housing is attached thereto, towards the opening position, a latch element positioned to obstruct movement of the control element to the opening position, a plug of a fusible composition positioned to obstruct movement of the latch element and an outlet discreet from said passage through which the fusible composition can flow when melted without being cooled by air venting through the passage, the control element moving between positions fully closing and fully opening the passage in response to displacement of the latch element by the control element permitted by melting of the plug.

2. A device as claimed in claim 1, wherein retaining means for the plug is provided by a chamber having a restricted outlet for the fusible composition when melted, said outlet opening at a position remote from the portion of said passage through which the control element moves from its closing to its opening position.

3. A device as claimed in claim 2, wherein the outlet is of reduced cross-sectional area relative to the chamber so that the chamber communicates with the outlet over a step or shoulder having a flat surface abutted by the plug and opposed to the direction of flow of the melted composition from the chamber through the outlet.

4. A device as claimed in claim 2 or claim 3 wherein the latch element extends between the chamber and a recess in the control element or housing, the latch element tending to move into the chamber under the influence of the bias of the control element when permitted to do so by molten fusible composition vacating the chamber through the outlet.

5. A device as claimed in any preceding claim, wherein the fusible composition is an eutectic metal.

6. A thermal fuse device constructed and arranged substantially as herein described and as shown in Figure 1 or Figure 2 of the accompanying drawings.

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

Claims

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

progressive deformation of the plug or billet of eutectic material and which will allow escape of the eutectic material through the outlet 17A or 31 only when the eutectic material has been liquified.

WHAT WE CLAIM IS: A A thermal fuse device for deflating a pneumatic tyre in response to a predetermined increase in temperature, the device comprising a housing having a passage for venting air from the tyre, a control element in the housing movable between a position closing and a position opening the passage and biassed by pneumatic pressure in the tyre, when the housing is attached thereto, towards the opening position, a latch element positioned to obstruct movement of the control element to the opening position, a plug of a fusible composition positioned to obstruct movement of the latch element and an outlet discreet from said passage through which the fusible composition can flow when melted without being cooled by air venting through the passage, the control element moving between positions fully closing and fully opening the passage in response to displacement of the latch element by the control element permitted by melting of the plug.
2. A device as claimed in claim 1, wherein retaining means for the plug is provided by a chamber having a restricted outlet for the fusible composition when melted, said outlet opening at a position remote from the portion of said passage through which the control element moves from its closing to its opening position.
3. A device as claimed in claim 2, wherein the outlet is of reduced cross-sectional area relative to the chamber so that the chamber communicates with the outlet over a step or shoulder having a flat surface abutted by the plug and opposed to the direction of flow of the melted composition from the chamber through the outlet.
4. A device as claimed in claim 2 or claim 3 wherein the latch element extends between the chamber and a recess in the control element or housing, the latch element tending to move into the chamber under the influence of the bias of the control element when permitted to do so by molten fusible composition vacating the chamber through the outlet.
5. A device as claimed in any preceding claim, wherein the fusible composition is an eutectic metal.
6. A thermal fuse device constructed and arranged substantially as herein described and as shown in Figure 1 or Figure 2 of the accompanying drawings.