Wheel nut assemblies

A wheel nut is formed of a nut body and a seating washer, each having a cooperating engagement surface. The washer engagement surface is spheroidal and of non-constant spherical radius of curvature, and the washer additionally includes a flat bearing surface for engagement with a flat work surface. The washer is formed of a softer material than is the nut body so that, when the wheel nut is tightened against the work surface and about a substantially perpendicular stud projecting therefrom, the spheroidal surface of the seating washer deforms from its initial contact with the nut body surface at only the outer periphery of the washer to contact with the nut body along the entirety of the washer engagement surface.

FIELD OF THE INVENTION 
The invention relates to wheel nuts primarily (but not exclusively) to hold 
wheels of automotive vehicles in position on a supporting rotating 
structure. 
PRIOR ART 
Traditionally a vehicle wheel is secured in correct position on the hub by 
providing a conical seat around the wheel stud and a corresponding bearing 
face on the wheel nut, so that as a series of these nuts are tightened the 
wheel is held automatically centered in relation to the studs. While this 
arrangement is satisfactory for lighter vehicles, it has been found 
desirable for heavier vehicles to locate the wheel centrally on its hub by 
means of a spigot or the like, and to use the wheel studs and nuts merely 
to hold the wheel on the hub and to transmit torque between the wheel and 
hub. 
British Pat. No. 1,389,976 (Erich Neumayer) describes one form of wheel nut 
which has been used in commerce. A nut body bears on a conical seat in a 
collar. 
British Pat. No. 2,051,285, belonging to the assignee of the present 
inventor, describes an improved form of two part wheel nut, where the 
bearing surface of the nut body, and the seat in the washer, are 
complementary part spherical surfaces, to allow for misalignment of the 
stud axis in relation to the wheel surface. British Pat. No. 2,065,818 
(MacLean-Fogg Company) shows a two part construction where initially the 
washer has a concave bearing surface, but where the pressure of the nut 
body deforms the washer so that it sits flat on the washer surface. 
OBJECTS OF INVENTION 
In spite of the various improvements, it has still been difficult to 
achieve proper contact between the nut body and washer seat, so as 
properly to distribute the high stress. Relationship between torque 
applied on tightening the washer nut and the tension in the stud has been 
uncertain due to frictional irregularities. There has always been a 
tendency for nuts to loosen in warm weather and over tighten when the 
weather is cold. It is the object of the invention to overcome these 
difficulties, at least to some extent. 
BRIEF SUMMARY OF THE INVENTION 
With the above objects in mind, the invention provides a wheel nut adapted 
to bear against a flat work surface and comprising an internally threaded 
nut body and a seating washer, the nut body having a convex bearing 
surface and the washer a co-operating concave seating surface both said 
surfaces being spherical in character whereby to allow freedom of movement 
between nut body and washer to accommodate misalignment of a co-operating 
stud which is nominally perpendicular to said flat surface, characterized 
by the washer having a flat bearing surface to bear on the flat surface, 
being of softer material than the nut, and having its seating surface 
initially in contact with the nut body at the outer periphery of said 
surface and in the tightened condition of the nut making contact over its 
whole area by virtue of elastic deformation of the washer. 
The construction described allows a more even contact between the bearing 
surface of the nut body and the seat in the washer. This, coupled with the 
relative softness of the washer leads to reduced, and more closely 
determined coefficient of friction, so that the torque required to achieve 
a given tension on the stud is less than for prior arrangements. The 
washer in compression beneath the nut body acts as a spring washer and 
takes on a locking function.

DESCRIPTION OF PREFERRED EMBODIMENT 
The wheel nut illustrated comprises a nut body 1 with hexagon flats 2, top 
3 and internal threads 4 all of standard configuration. The nut body 1 has 
also a bearing surface 6 of part spherical shape, centered at a point 7 on 
the nut axis, and, internally of the surface, a collar 8. 
The wheel nut includes also a seating washer 10 with a flat bearing surface 
11 to bear on a flat wheel surface 12 surrounding a stud (not shown) 
extending at a nominal right angle thereto. The washer 10 has a spheroidal 
seating surface 13 to receive the bearing surface 6 of the nut body 1. The 
washer 10 has also a frusto-conical interior surface 14. When the nut body 
1 is assembled with the washer 10 the collar 8 is expanded outwardly so as 
to hold the washer captive. 
The washer 10 is formed by coining to have the shape shown in FIG. 2. The 
spheroidal surface 13 is formed by a punch having a true spherical head of 
the same diameter as that of the nut body bearing surface 6 but going 
towards its inner edge the surface 13 is progressively more recessed than 
a truly spherical surface would be, assumed centered on the washer axis. 
In the section shown, opposite sides of the washer surface 13 are seen 
centered at points 16, 17 which are offset to opposite sides of the axis. 
On assembly of the nut with a wheel stud, before any stress is applied, the 
nut body 1 bears on the washer 2 as shown in FIG. 3. Initial contact is 
made at the outer periphery 18 only of the surface 13, and the gap between 
nut body and washer increases going inwardly. 
When the nut is tightened against the wheel surface 12 the nut body 
elastically deforms the washer to produce full area contact between the 
two. Any misalignment of the stud is accommodated by relative movement of 
the engaging nut and washer surfaces before full clamping force is 
applied. 
When the nut is removed, the washer regains its original shape, ready for 
the next application. 
The nut body is preferably made of medium carbon steel, hardened and 
tempered. Washers, particularly those of special profile, are commonly 
made by forging. By contrast the washer 10 of this preferred embodiment is 
made by parting off a length of thick-walled mild steel tube in the 
as-drawn condition and coining it to the shape illustrated, without 
further treatment. The washer, therefore is work-hardened to some extent 
but will have a much lower hardness than the nut body. 
When the surfaces 6, 13 of nut and washer are in full contact the harder 
surface 6 on the nut body will make the less hard surface 13 of the washer 
conform to it. In effect the nut beds into the washer. It will be 
appreciated that the grain formation in the washer is essentially 
longitudinal, and not as it would be in a forged washer. The conditions 
therefore favour reliable contact between the surfaces and it has been 
found that there is an appreciably lower coefficient of friction in the 
construction described than in comparable prior art nuts. 
The reduced coefficient of friction leads to a lower torque requirement for 
a given clamping force, and to a more reliable relationship between torque 
and clamping force. Because of the elasticity of the washer more angular 
movement is required to tighten or loosen the nut, with a given clamping 
force.