This invention relates to wheel nuts and more particularly concerns wheel nuts of the type having a captive washer.

Modern vehicles, particularly commercial heavy goods vehicles are designed to carry heavier loads at greater speeds and this must be taken into account when designing wheel nuts. In particular, accurate location of the wheel is desirable in order to minimise any unbalance of the wheel and the arrangement must be such as to be capable of withstanding high axial loads. Modern heavy goods vehicle wheels are located on a centre hub and axial movement is prevented by a series of radially distributed threaded studs to which the wheel nuts are fitted. The use of captive washers has been found to be desirable as a high torque must be applied to the wheel nuts in this arrangement.

British Patent No. 1 389,976 describes a wheel nut having a captive washer. An internally threaded nut head is provided with an axially extending neck with a pressure plate (washer) captively fitted around the neck. Opposing equi-inclined bearing surfaces are provided on the pressure plate and a collar which extends radially outwardly from the nut head.

It is an object of the present invention to provide an improved wheel nut of the type referred to.

According to the present invention we provide a wheel nut having a captive washer, said wheel nut comprising an internally threaded nut head provided with a radially outwardly extending shoulder, the shoulder and the washer being provided with a co-operating formation which serves to retain the washer in position on the nut head. Preferably the co-operating formation comprises an intumed lip which engages in a co-operating peripheral recess in the shoulder.

Advantageously, the washer and the nut head are each provided with a conical bearing interface which extends at an

angle of 16° to 25° to the radial plane of the nut. Desirably, the angle is substantially 20°.

In a preferred form of the invention the bearing interface extends substantially from the outside diameter of the thread 5 to the outside diameter of the shoulder.

Reference is now made to the accompanying drawings, in which:

Figure 1 is a side elevation with part cut-away of a wheel nut according to the invention; lδ Figure 2 is an end elevation of the wheel nut shown in Figure 1; and

Figure 3 is a cross-section on an enlarged scale of part of the nut shown in Figures 1 and 2.

The nut 10 comprises a nut head 11 having an internally 5 threaded bore 12 and hexagon flats 13 on its outer surface. _. The nut head 11 has a radially outwardly extending shoulder 14 with an external peripheral recess 15..

The shoulder 14 is provided with a conical bearing interface 16 for engagement with a conical bearing interface 17 0 on the washer 18. The washer is rendered captive on the nut head 11 by means of a continuous intumed lip 19 which engages in the recess 15.

With the washer retained by intu ed lip 19 and with the conical bearing interfaces 16 and 17 at approximately 20° to 5 the radial plane, the resultant thread length through the nut head 11 is a very high proportion of the overall length of the nut, thus ensuring high torque capacity.

As torque is applied from spannering on the hexagon flats 13, the hoop stresses induced in washer 18 react inwards 0 through the section of the nut head 11 to improve the thread holding ability.

With the conical bearing interfaces at approximately 20° to the radial plane, there is a 6% increase in face area from that available for a radial face with similar inside and 5 outside diameters. Taken together with the bearing interfaces extending from the outside diameter of the thread 12 to the outer diameter of the shoulder 14 the interface area is as

large as possible within the constraints of the practical physical size of the nut.

The larger area gives correspondingly lower unit pressures as torque is applied, and gives a higher torque capacity before the interface seizes up. During the manufacturing process lubrication may be applied to the interfaces to assist in the prevention of seizure.

Furthermore, the intumed lip 19 and recess 15 provide a labyrinth to prevent external elements entering to inhibit the lubricated surfaces.

In order to test the effectiveness of the labyrinth in preventing external elements inhibiting the lubricated interface surfaces the following test was carried out.

Torque was applied to a nut on a threaded stud and a measured clamp load was plotted against torque applied to the nut. A straight-line graph was obtained. The nut was then removed from the stud and subjected to 24 hours of salt spray and a degreasing agent. Torque was then applied to the nut on a stud as before and it was found that the plot of clanp load against torque followed the same line as that obtained before the salt spray and degreasing steps.

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