BACKGROUND TO THE INVENTION

THIS invention relates to a fail safe brake assembly.

With a view to increasing the speed at which personnel and materials can be transported in underground mines it has been proposed to use light delivery vehicles and trailers. However given the inherent dangers of underground mining operations, the severe inclines which can be encountered and the limited space within which such vehicles must operate, it is considered necessary for the vehicles, including the trailers, to be fitted with fail safe brakes, i.e. a brakes which will automatically apply a braking action to the vehicle/trailer in the event of a failure of the normal service braking equipment.

One fail safe brake system suitable for use with heavy duty vehicles is described in the specification of ZA 88/8491. However the system described in this specification is relatively bulky and as a result is considered unsuitable for use in smaller light delivery vehicles and trailers. SUMMARY OF THE INVENTION

According to the invention there is provided a fail safe brake assembly for a vehicle wheel, the assembly comprising:

a pressure plate,

brake discs at least one of which is stationary and one of which is arranged to rotate with the wheel, the discs being arranged to be acted upon by the pressure plate,

an hydraulic service brake system which includes a first hydraulic cylinder arrangement operable to cause the pressure plate to drive the brake discs into engagement with one another and thereby apply a service braking action to the wheel, and

a fail safe brake system including:

mechanical spring means operable to cause the pressure plate to drive the brake discs into engagement with one another and thereby apply a braking action to the wheel, and

a second hydraulic cylinder arrangement which resists the operation of the mechanical spring means when hydraulic pressure supplied to the second hydraulic cylinder arrangement does not fall below a predetermined value,

the first and second hydraulic cylinder arrangements being arranged concentrically with one another about a rotational axis of the wheel.

Other features of the invention are set forth in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings.

In the drawings:

Figure 1 shows an axial view of a fail safe brake assembly, according to the invention, suitable for use in braking the wheel of a light delivery vehicle;

Figure 2 shows a cross-sectional view at the line 2-2 in Figure 1 ;

Figure 3 shows a cross-sectional view of the brake assembly fitted to a hub of the vehicle; and

Figure 4 shows a view, corresponding to that of Figure 3, of a fail safe brake assembly suitable for use in braking the wheel of a trailer.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Figures 1 to 3 illustrate a brake assembly according to the invention which is suitable for use in braking a wheel of a vehicle such as a light delivery vehicle.

Figure 3 illustrates a hub assembly of the vehicle. In this Figure, the numeral 10 indicates a half shaft or axle of the vehicle which is supported for rotation in a tubular housing 12 by bearings 14. Mounted fast to the shaft 10 is a rim 16 carrying threaded studs 18 by means of which a wheel of the vehicle (not shown) can be mounted to the hub. • Referring also to Figures 1 and 2, the numerals 20 and 22 indicate a front cover and housing respectively of the brake assembly of the invention which are secured to one another by cap screws 24 (only one visible).

The brake assembly comprises a pressure plate 26 and brake discs including non-rotating discs 30 and discs 28 which rotate with the hub. Located within the housing 22 is a first hydraulic cylinder arrangement, composed of an annular cylinder 32 around the central axis of the hub, which provides a service braking action.

When normal service braking of the vehicle is to take place, the cylinder 32 is supplied with hydraulic fluid under pressure. This will normally be in response to depression by the driver of the vehicle of the service brake pedal in the vehicle cab. The cylinder 32 accommodates an annular piston 34 which acts on the pressure plate 26 under the influence of the pressurised hydraulic brake fluid. The pressure plate in turn drives the brake discs 30 and 28 into frictional engagement with one another, thereby applying a normal braking action. The magnitude of the braking action which is applied is dependent on the fluid pressure which is in turn determined by the operator's action, i.e. the extent to which the brake pedal is depressed.

When service braking is no longer required, the fluid pressure in the cylinder 32 is relieved by the operator, normally by releasing the brake -pedal. Frictional engagement between the discs 28 and 30 is released as relative rotation between them takes place.

The brake assembly of the invention also includes a fail safe or emergency brake system which automatically applies a braking action in the event of loss of brake fluid pressure. The fail safe brake system includes a series of disc spring assemblies 36 spaced apart on a pitch circle centred on the central hub axis. Each spring assembly 36 consists of a stack of annular disc springs 38 seated in a pocket 40 in the housing 22 and located on a centralising pin 42 secured to the housing by a cap screw 44. The spring stacks are normally in a compressed state and each of them acts on the inner end of an elongate pusher 46. The outer ends of the pushers 46 act on the pressure plate 26.

The brake assembly includes a second hydraulic cylinder arrangement comprising an annular cylinder 48 accommodating a piston 50. The piston 50 has a radially inwardly projecting piston extension 51 which bears on the inner ends of the pushers 46. The spring assemblies 36 act outwardly on the pressure plate, via the piston extensions 51 and pushers 46, in a sense to cause the pressure plate to drive the discs 30 and 28 into frictional engagement with one another, thereby to apply a braking action.

This action of the spring assemblies is normally resisted by the action of the second hydraulic cylinder arrangement. The cylinder 48 is supplied continuously during normal operation with hydraulic fluid at a predetermined service pressure. The piston 50 is urged inwardly by the hydraulic pressure, with the result that the extension 51 acts inwardly against the spring assemblies. The normal service pressure is sufficient to hold the spring assemblies 36 in a condition in which no braking action is applied. However should the fluid pressure drop below a predetermined value as a result, for instance, of loss of hydraulic fluid in the brake system, the inward force applied to the spring assemblies 36 by the piston 50 is insufficient to resist the spring assemblies with the result that the braking action is automatically initiated.

Figure 4 illustrates an embodiment of the invention used to brake a vehicle such as a trailer. Components corresponding to components seen in Figures 1 to 3 are indicated by the same reference numerals.

In Figure 4 the numeral 60 indicates the stub axle and the numeral 62 the wheel hub. There is again a single pressure plate 26 arranged to act on discs 30 and 28, but in this case it will be noted that the pressure plate acts in an inward rather than an outward direction. It will also be noted that in this case the disc springs 38 of the spring assemblies 36 are not located on centralizing pins corresponding to the pins 42 in the first embodiment.

Another distinction between the embodiment seen in Figure 4 and that of Figures 1 to 3 is the absence of pushers corresponding to the pushers 46. In this case, the piston 50 is connected to a radially inwardly extending connecting member 64 against which the associated spring assemblies 36 act. The connecting member 64 carries an integral extension 66 which acts on the pressure plate 26.

During normal service braking, the piston 34 acts inwardly on the pressure plate to generate the required braking action. At the same time, the piston 50 acts outwardly on the spring assemblies to hold them an inoperative state. If hydraulic pressure reduces to an unacceptably low value, the spring assemblies act on the connecting member 64, causing the piston extension 66 to act on the pressure plate and apply a fail safe braking action.

Both embodiments of brake assembly are compact and accordingly suitable for incorporation in the hub structure of a light delivery vehicle or trailer. The compactness of the assembly is in each case partially attributable to the fact that the first and second hydraulic cylinder arrangements are arranged in a concentric relationship. Furthermore there is only a single pressure plate on which both the first hydraulic arrangement and the spring assemblies act. In this regard it will be noted that in the first embodiment, the entire cylinder 32 is located radially inwardly of the cylinder 48 whereas in the embodiment of Figure 4, compactness is enhanced even further by the fact that the cylinder 32 is located between the cylinder 48 and the piston extension 66.

Either of the embodiments described above could operate with dry or wet friction discs, the latter being more suitable for higher speed applications.