GB2088503A

GB2088503A - Hydraulically controlled brake protecting against overheating of the hydraulic fluid

Info

Publication number: GB2088503A
Application number: GB8136345A
Authority: GB
Original assignee: Valeo SE
Current assignee: Valeo SE
Priority date: 1980-12-03
Filing date: 1981-12-02
Publication date: 1982-06-09
Also published as: FR2495253B1; GB2088503B; DE3147933A1; IT8112696A0; FR2495253A1; IT1145994B

Abstract

A hydraulically controlled brake comprises a friction element (12A) adapted to be clamped against a track (11A) on a rotatable element e.g. a brake disc (10) by a piston (18) urged by hydraulic pressure generated in a control chamber (20). To protect the fluid in the chamber (20) from overheating, at least one mass (26; 27) of fusible material is disposed in a cavity (28;29) adjacent the chamber (20), to absorb heat, generated during braking, by changing its physical state. As shown cavities (28;29) are provided respectively in brake yoke (16) and piston (18). Piston (18) has an additional core of heat insulating material (30); a partition (31) separates core (30) and cavity (29) and Figures 2, 3 show alternative partitions. Fusible material may be alloy of 58% bismuth and 42% tin by weight. <IMAGE>

Description

SPECIFICATION Hydraulically controlled brake protecting against overheating of the hydraulic fluid The present invention relates to a hydraulically controlled brake comprising at least one friction element, for example a brake plate in the case of a disc brake, adapted to be clamped against a track on a rotatable element by the thrust of a piston member mounted to slide in a cylinder member, in response to hydraulic pressure being generated in a control chamber defined by the piston in the cylinder.

At the moment of braking, the heat generated by the friction of the friction element against the rotating track causes a rise in the temperature of the piston and consequently of the hydraulic fluid contained in the control chamber and in contact with the piston. The hydralicfluid, consisting usually of oil, is in danger of losing its properties and undergoing a phenomenon called "vapour lock", when its temperature exceeds a specific value, for example 15000. It is therefore important that the heat generated by the friction does not cause a rise in the temperature of the hydraulic fluid above such a value.

It is an object of the present invention to provide a hydraulically controlled brake of the type mentioned above, in which the hydraulic fluid of the control chamber is protected against an excessive rise in temperature.

According to the invention there is provided a hydraulically controlled brake comprising at least one friction element, a piston member slidably mounted in a cylinder member, a control chamber defined by said members, the friction element being adapted to be clamped against a track on a rotatable element by the thrust of the piston member in response to hydraulic pressure being generated in the control chamber, and at least one mass of a material, which exhibits a change of physical state at a predetermined temperature, disposed in a cavity in at least one of the two members, the cavity being located in the vicinity of the control chamber, whereby heat generated by frictional engagement of the frictional element and the track during braking is absorbed by the mass of material during its change of state.

This predetermined temperature is advantageously close to a temperature beyond which the hydraulic fluid is in danger of losint its properties, particularly a temperature close to 1 500C.

If, at the moment of braking, the heat generated by the friction of the friction element against the rotating track causes, in the vicinity of the control chamber, a rise in the temperature up to a moderate value below 1 50 C, the hydrualicfluid is not in danger of losing its properties. The mass of material does not change its state.

If, on the other hand, the heat generated by braking causes a rise in temperature, in the vicinity of the control chamber, beyond the predetermined value, particularly 1 50 C, the mass of material is allowed to change its physical state. During this change of physical state, the temperature of the mass of material remains constant, for example close to 1 50 C, while the mass of material absorbs the heat originating from the braking region. In this way, the temperature of the hydraulic fluid in the chamber is not in danger of rising beyond 15000 and this hydraulic fluid is not in danger of losing its properties. Such a protective effect continues for as long as the mass of material has not changed its physical state completely.It is therefore sufficient to select a sufficiently large mass of material to ensure that this is so in the case of the strongest and most prolonged braking operations which are assumed to occur during normal operation.

In a non-limiting example, the material showing a change of physical state consists of an alloy which has a percentage by weight of 58% bismuth and 42% tin.

The mass of material preferably passes from a solid phase to a liquid phase during its change of physical state at said predetermined temperature and is enclosed in a cavity made in the brake in the vicinity of the control chamber, for example in the cylinder member or else, preferably, in the piston member.

The piston advantageously possesses a cupshaped part having a cylindrical side wall engaged in the cylinder and a bottom wall adjacent the control chamber, and the mass of material is accommodated in the piston in the vicinity of the bottom wall of the cup.

The piston may be provided with a core made of heat-insulating material which is accommodated in the cup above the mass of material and which has an outer face by means of which the piston is able to push on the friction element.

By means of this arrangement, the hydraulic fluid in the control chamber is protected against an excessive rise in temperature, not only because of the presence of the mass which can exhibit a change of physical state, but also because of the insulating core.

A thin partition preferably separates, in the cup, said mass of material and the insulating core.

Embodiments of the invention are described below by way of example, with reference to the accompanying drawing in which: Figure lisa general diagrammatic view, in cross-section, of a brake according to the invention, the cylinder body and piston of which accommodate masses of fusible material; Figure 2 is a sectional view of an alternative form of piston; Figure 3 is a sectional view of another alternative form of a piston.

Reference will be made, first of all, to Figure 1 which relates by way of a non-limiting example to an applicationof the invention to a hydraulically controlled disc brake, especially for a motor vehicle.

The disc 10 of the brake is shown in Figure 1, and comprises two opposite rotatable tracks 11 A and 11 B which are adapted to engage frictionally two friction elements 12A and 12B comprising brake plates.

Each plate 12A, 12B comprises a lining made of a friction material 13 and integral with a support 14.

Each plate 12A, 12B is allowed to rub against the corresponding friction track 1 1A, 1 IB of the disc 10 by means ofthe free surface 15 of the lining 13 which is opposite the support 14. The force is transmitted to each plate 1 2A, 1 2B via its respective support 14 so asto clamp it against the disc 10.

The brake comprises a yoke 6 which straddles the disc and in which the plates 1 2A and 12B are mounted. The yoke 16 comprises a cylinder member 17 in which a piston member, designated generally by 18, is mounted to slide.

The piston 18 has an outer face 19 by means of which it is able to push on the support 14 of the plate 12A. The piston 18 defines in the cylinder 17 a control chamber 20 which is filled with hydraulic fluid, for example oil. A passage 21 communicates with the chamber 20 and through this passage the chamber 20 can be pressurised.

The yoke 16 also comprises a reaction face 22 against which the support 14 of the other plate 1 2B can be broughtto bear.

The plates 12A and 12B are thus adapted to be clamped against the rotatable tracks 11Aand 11B of the disc 10 under the thrust of the piston 18 mounted to slide in the cylinder 17, in response to pressure being generated, via the passage 21, in the control chamber 20.

The piston 18 comprises a cup-shaped piece 23 which has a cylindrical side wall 24 engaged in the cylinder 17 and a bottom wall 25 adjacent the control chamber20. In the example illustrated in Figure 1, the cup 23 is made of cast steel.

At the moment of braking, the heat generated by the friction of the plates 1 2A and 1 2B against the rotating tracks 1 1A and 11 B of the disc 10 causes a rise in the temperature of the hydraulic fluid located in the control chamber 20. This hydraulic fluid, which usually comprises oil, is in danger of losing its properties and undergoing a phenomenon called "vapour lock", when its temperature exceeds a specific value, for example 150"C. It is therefore important that the heat generated by the friction does not cause a rise in the temperature of the hydraulic fluid above such a value.

For this purpose, the brake incorporates, in the vicinity of the control chamber 20, at least one mass, for example two masses of material 26 and 27 in the case of Figure 1, of a material showing a change of physical state at a predetermined temperature, in particular close to 1 50 C, and absorbing, during the change of state, heat resulting from braking.

In the example illustrated, the material of the masses 26 and 27 is of the fusible type and passes from a solid phase to a liquid phase during the change of physical state at the predetermined temperature, in particular close to 1 50 C.

The material of which the fusible masses 26 and 27 are made consists, for example, of an alloy which has a percentage by weight of 58% bismuth and 42% tin.

The masses 26 and 27 of material are enclosed respectively in cavities 28 and 29 in the brake in the vicinity of the control chamber 20.

In the example illustrated in Figure 1, the cavity 28 in which the mass of fusible material 26 is accommodated is made in the yoke 16 comprising the cylinder member. This cavity 28 has, in cross-section, the general shape of a horseshoe, so as to surround the control chamber 20.

The cavity 29 in which the mass of fusible material is accommodated is formed by the hollow interior of the cup 23 in the vicinity of the bottom wall 25.

The piston 18 also comprises a core 30 made of heat-insulating and mechanically resistant material.

This core 30 is accommodated in the cup 23 above the mass of material 27 and extends beyond the up 23 so as to present externally the face 19 by means of which the piston 18 is able to push on the plate 12A.

The heat-insulating and mechanically resistant material of which the core 30 is made is a brake friction lining material comprising fibres, fillers and a binder. More particularly, this material comprises, for example, a percentage by weight of 30% glass or asbestos fibres, 50% fillers such as calcium carbonate and 20% of a binder such as phenolic resin.

There may be seen at 31 in Figure 1, a thin partition, for example made of sheet metal, which separates, in the cup 23, the mass 27 of fusible material and the insulating core 30. In the example of Figure 1,the partition 31 has a convex shape and rests on an annular shoulder 32 of the side wall 24 of the cup 23.

To make the piston 18, the fusible material 27 is introduced into the bottom of the cup 23, the partition 31 is placed on the shoulder 32 and the insulating core 30 is fitted. For this purpose, this core 30 is moulded in situ in the cup 23 or else is preformed and force-fitted in this cup 23.

For braking, pressure is developed in the control chamber 20 via the passage 21, thus pushing the piston 18 against the plate 12A and ensuring that the disc 10 is clamped between the plates 1 2A and 12B.

Only a small amount of the heat generated by the friction of the plates 12and 12B against the tracks 1 lA and 11 B of the disc 10 is transmitted to the bottom wall 25 of the cup 23 because of the insulating core 30 which forms a heat shield.

If the temperature in the vicinity of the control chamber 20 remains below a value at which properties ofthe hydraulic fluid are likely to be impaired, particularly 150 C, the fusible masses 26 and 27 remain in the solid state. In any case, the hydraulic fluid is not in danger of losing its properties or undergoing a phenomenon called "vapour lock".

If, on the other hand, braking is sufficiently strong or prolonged for the temperature in the vicinity of, the control chamber 20 to be likely to exceed the dangerous value, particularly 150"C, despite the presence of the insulating core 30, the fusible masses 26 and 27 will then pass from a solid phase to a liquid phase.

During this change of state, the temperature of the fusible masses 26 and 27 remains constant, whiie these fusible masses absorb the heat generated by braking. In this way, the temperature of the hydraulic fluid in the control chamber 20 is maintained at a value which does not exceed 150"C and which thus enables the hydraulic fluid to retain its properties and not to undergo a phenomenon of "vapour lock".

Reference will now be made to Figure 2 where the arrangement is similar to that which has just been described with reference to Figure 1, but in which the shoulder 32 is omitted. The partition 31 has a skirt 33 which runs along the cup 23 on the inside and which surrounds the mass of fusible material 27. The partition 31 is provided with a concave shape. In the example of Figure 2, the skirt 33 is engaged forcibly in the cup 23.

In another alternative form (Figure 3) where the partition 31 has, for example, a convex shape, as in Figure 1, the skirt 33 is shown as fixed to the cup 23, for example as a result of engagement on an annular projection 34, such as a welded ring or the like, of the inner wall of the cup 23.

Claims

1. A hydraulically controlled brake comprising at least one friction element, a piston memberslidably mounted in a cylinder member, a control chamber defined by said members, the friction element being adapted to be clamped against a track on a rotatable element by the thrust of the piston member in response to hydraulic pressure being generated in the control chamber, and at least one mass of a material, which exhibits a change of physical state at a predetermined temperature, disposed in a cavity in at least one of the two members, the cavity being located in the vicinity of the control chamber, whereby heat generated by frictional engagement of the frictional element and the track during braking is absorbed by the mass of material during is change of state.

2. A brake according to Claim 1, in which said predetermined temperature is close to a temperature beyond which the hydraulic fluid in the control chamber is likely to lose its properties.

3. A brake according to Claim 2, wherein said predetermined temperature is close to 1 50"C.

4. A brake according to any preceding claim, wherein the material of said at least one mass comprises an alloy which has a percentage by weight of 58% bismuth and 42% tin.

5. A brake according to any preceding claim, wherein the material of said at least one mass passes from a solid phase to a liquid phase during its change of physical state at said predetermined temperature and is enclosed in said cavity.

6. A brake according to any preceding claim, wherein said mass of material is accommodated in the cylinder member.

7. A brake according to any of Claims 1 to 5, wherein said mass of material is accommodated in the piston member.

8. A brake according to Claim 7, in which the piston member comprises a cup-shaped part, having a cylindrical side wall engaging the cylinder member and a bottom wall adjacent the control chamber, said mass of material being accommodated in the cup-shaped part in the vicinity of said bottom wall.

9. A brake according to Claim 8, wherein the piston member is provided with a core made of a heat-insulating and mechanically resistant material which is accommodated in the cup-shaped part above said mass of material and which has an outer face by means of which the piston is able to push against the friction element.

10. A brake according to Claim 9, wherein a thin partition separates, in the cup-shaped part, said mass of material and said insulating core.

11. A brake according to Claim 10, wherein said partition rests on a shoulder of the side wall of the cup-shaped part.

12. A brake according to Claim 11,wherein said partition has a skirt which extends around the inside of the cup-shaped part and which surrounds said mass of material.

13. A brake according to Claim 12, wherein said skirt is fixed to the cup-shaped part.

14. A brake according to any of Claims 10 to 13, wherein said partition has a convex shape.

15. A brake according to any of Claims 10 to 13, wherein said partition has a concave shape.

16. A hydraulically controlled brake substantially as hereinbefore described, with reference to and as illustrated in Figure 1 or Figure 1 as modified by Figure 2 or by Figure 3 of the accompanying drawings.