GB2127122A - Fastener assembly for clamped wheels - Google Patents

Abstract

A fastener assembly for clamped vehicle wheel installation includes a nut (36) engageable with a wheel hub supported stud and a resiliently deformable clamp washer (30). A washer clamp surface (44) engageable with the wheel is frusto-conical and inclined to the radial direction at an initial clamp surface cone angle B. Interfacing frusto- conical bearing surfaces (48 and 46) are defined on the nut (36) and clamp washer (30) and define an initial bearing surface separation angle A at least as large as the initial clamp surface cone angle B. The nut bearing surface (48) is inclined at an angle C having a tangent not greater than the coefficient of friction at the bearing surfaces to avoid subjecting the clamp washer to undesirable hoop stresses. Substantial spring forces are achieved with a compact assembly because the axial offset of the clamp washer is substantial in relation to the radial offset. Radial clamp washer offset provides a differential in friction radii to prevent rotation of the clamp washer relative to the wheel. <IMAGE>

Description

SPECIFICATION Fastener assembly for clamped wheels The present invention relates to fastener assemblies, and more particularly to an improved fastener assembly for clamped vehicle wheel installations.

Vehicle wheels are conventionally mounted to a hub or a hub and brake drum assembly by a threaded fastener system. In one typical arrangement, threaded studs extend outwardly from the hub through stud holes in the wheels, and the wheels are held against the hubs by wheel nuts or cap nuts threaded onto the studs. In another arrangement, bolts can extend through the wheel mounting holes and are threaded into the hub or into nuts associated with the hub. The wheels may be centered in a variety of ways, as by mating of cooperating surfaces on the wheels and the wheel nuts, or by various hub centering or piloting systems.

It is essential that wheel clamp load be maintained in the mounting system to avoid movement or misalignment of the wheels on the hub. If the clamping force holding the wheel against the hub is lost or diminished, a catastrophic situation can occur in which the wheel can move relative to the hub and studs, resulting in rapid failure of the studs and separation of the wheel from the hub.

In conventional wheel mounting systems, substantial clamp load can be lost as a result of a relatively slight degree of axial deflection in the fastened joint due to factors such as creep, fastener embedment or the like. The only significant resilience in the conventional system is due to stud elongation upon tightening, four or five thousandths of an inch of elongation being typical. Thus, axial deflection of only one thousandth of an inch can result in loss of 20-25% of clamp load.

Because of the serious and possibly dangerous nature of such problems, frequent checks of tightening torque are necessary, particularly in the case of heavily loaded vehicles, dual wheel mounting systems, rough service and the like. It is not unusual that wheel nut tightening torque checks are specified as frequently as every 500 miles of use in some situations. This procedure, although necessary for safety and maintenance of the mounting system, can be inconvenient and time consuming.

It would be desirable to introduce more resilience into the wheel nut mounting system, but this is difficult due to the large clamping forces experienced and to the size restraints imposed by the environment. For example, conventional washer spring or belleville washer technology would not solve the problem because the size of the washer spring would be excessive, for example four inches or more in diameter for a clamp load of about thirty thousand pounds.

It is an object of the present invention to provide an improved fastener assembly.

The present invention is a fastener assembly for clamping one or more vehicle wheels between a hub and the fastener assembly at a predetermined clamp load, said fastener assembly comprising: a threaded fastener having a body and having a thread structure disposed generally in a cylindrical plane parallel to and surrounding the central longitudinal axis of said body; said body including a structure engageable by a tool for tightening of the fastener assembly toward a radial surface of a wheel to be clamped; a resiliently deformable clamp washer rotatable relative to said body and having a central axis aperture surrounding said cylindrical plane and interposed between said fastener body and a wheel to be clamped; a body bearing surface defined on the fastener body and a cooperating washer bearing surface defined on one side of said clamp washer and interfacing with said body bearing surface; and a clamp surface defined on the opposite side of said clamp washer; said clamp surface being conical and inclined relative to the radial direction by an initial clamp surface cone angle; said body bearing surface being conical and inclined relative to the radial direction by an angle having a tangent not greater than the coefficient of friction between the clamp washer and body; said clamp washer prior to tightening of the fastener assembly being engageable with a wheel to be clamped along a first circular line of contact between said clamp surface and the wheel and being engageable with said fastener body along a second circular line of contact between the clamp washer bearing surface and the body bearing surface; said second circular line of contact being smaller in radius than said first circular line of contact; said body bearing surface and said washer bearing surface defining an initial bearing surface separation angle at least as large as said initial clamp surface cone angle; and said clamp washer upon tightening of said fastener assembly resiliently deforming to decrease said clamp surface cone angle and said bearing surface separation angle, said clamp surface cone angle being greater than zero at the predetermined clamp load.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa fragmentary sectional view through a vehicle wheel mounting installation including a fastener assembly constructed in accordance with the principles of the present invention; Figure2 is a sectional view of the fastener assembly of Figure 1 taken along the axis of the assembly, illustrating the assembly prior to installation in a wheel mounting system; Figure 3 is an enlarged and somewhat diagrammatic fragmentary, axial sectional view of a portion of the fastener assembly and a wheel surface illustrating the components in the finger tight or non-tightened position; and Figure 4 is a graphical representation showing the relationship between clamp load and deflection in a wheel mounting system including the fastener assembly of the present invention and in a similar wheel mounting system using a conventional fastening system of the prior art.

Having reference now to the drawings and initially to Figure 1, there is illustrated a portion of a vehicle wheel mounting installation designated as a whole by the reference numeral 10 and including a fastener assembly generally designated as 12 constructed in accordance with the principles of the present invention.

The illustrated wheel mounting installation includes a hub assembly 14to which a pair of wheels 16 and 18 are clamped by a fastener system including a stud 20 and the fastener assembly 12. Although the installation 10 is illustrated as including dual wheels, it should be understood that principles of the present invention are applicable to single wheel installations also.

The hub assembly 14 includes a hub member 22 and an outboard mounted brake drum 24. The term "hub" or "hub assembly" or the like as used here should be understood to include not only outboard hub-drum systems such as illustrated wherein the wheels are clamped against the drum, but also other types of arrangements such as inboard drum mountings wherein the wheels may be clamped directly against a hub member or some other component of the hub assembly.

Depending on vehicle size, wheel size, load requirements and like factors, a given number of studs 20 are located usually in a circular pattern around the periphery of the hub 14. Only one such stud is illustrated in Figure land it should be understood that a fastener assembly 12 may be associated with each stud of the wheel mounting system. In the illustrated embodiment, the stud is semipermanently mounted by a press fit into the hub assembly 14, although other conventional types of stud mounting arrangements may be utilized. The stud, as is conventional, extends outwardly, in the axial direction relative to the axis of the fastener assembly 12, through stud holes 26 in the wheels 16 and 18.When initially mounted to the hub 14 at the time of tightening of the fastener assembly 12, the wheels 16 and 18 may be centered relative to the hub 14 in any desired manner as by a suitable piloting or alignment arrangement (not shown). It should be noted that clearance exists between the stud 20 and the wheels 16 and 18. After mounting of the wheels and tightening of the fastener 12, the clamp load applied to the wheels between the hub assembly 14 and the fastener assembly 12 serves to prevent movement of the wheels relative to the hub assembly.

The fastener assembly 12 of the present invention includes a fastener body 28 in assembly with and cooperating with a clamp washer 30. When the fastener assembly 12 is installed in the wheel mounting system as illustrated in Figure 1, the clamp washer 30 is sandwiched between the fastener body 28 and a surface 32 of wheel 18, the surface 32 being disposed radially with respect to the axis of the fastener assembly 12. In the illustrated embodiment of the invention, the fastener body 28 is a hex nut suitable for use with the threaded stud 20. Principles of the present invention are applicable as well to fasteners other than nuts such as headed bolts used in other types of wheel mounting installations. Consequently, the term fastener body should by understood to encompass not only the body of a nut but also the head portion of a shanked fastener.

For engagement with the male threaded stud 20, the fastener body 28 includes a female thread structure 34 lying generally in a cylindrical plane symmetrical about the central longitudinal axis of the body 28. To permit tightening of the fastener assembly 12 upon the stud 20, the body 28 includes a wrenching structure in the form of flats 36 of the hex shaped nut body. With fastener bodies of other forms, male rather than female thread structures may be provided, and various types of wrenching structures or driving structures may be utilized for tightening of the fastener assembly.

The body 28 and washer 30 are maintained in assembled relation by means of a flange or collar 38 extending axially from the fastener body 28 through a reduced diameter neck portion 40 of the central axial opening 42 of the clamp washer 30. The end of the flange or collar 38 is larger in diameter than the neck so that the body 28 and clamp washer 30 are maintained in assembled relationship while permitting free rotation and some movement between the body 28 and washer 30.

That side of the clamp washer 30 engageable with the wheel surface 32 is provided with a clamp surface 44. The opposite side of the clamp washer 30 is provided with a washer bearing surface 46. The washer bearing surface 46 interfaces with and cooperates with a body bearing surface 48. Each of the surfaces 44,46 and 48 is a continuous and annular surface of revolution, and has circular inner and outer peripheries. The fastener body 28 is provided with a small flange 50 to accommodate the body bearing surface 48.

The fastener body 28 and the clamp washer 30 are preferrably of a steel amenable to heat treating to a high hardness. At the clamp loads for which the fastener assembly 12 is designed, the clamp washer 30 is resiliently deformable and functions to provide substantial spring forces in the tightened condition in a wheel mounting installation such as the typical installation illustrated in Figure 1.

Clamp surface 44 of clamp washer 30 is conical in the initial condition of the clamp washer prior to tightening of the fastener assembly 12. The surface 44 is inclined at an angle B relative to the radial direction of the fastener system and thus is inclined at angle B relative to the radially disposed workpiece or wheel surface 32. As best seen in Figure 3, in the finger tight initial or non-tightened condition of the fastener assembly 12, the clamp washer 30 engages the wheel surface 32 at a circular line of contact 52 between the clamp surface 44 and the surface 32.

Washer bearing surface 46 is also conical in the illustrated embodiment of the invention and is inclined at an angle A relative to the radial direction. Body bearing surface 48 is also conical in configuration and is inclined at an angle C relative to the radial direction. Angle C is larger than angle A, and in the initial condition the body 28 contacts the clamp washer 30 along a circular line of contact 54 between the bearing surfaces 46 and 48. The surfaces 46 and 48 are initially separated by an angle D, angle D being equal to angle C minus angle A. While surfaces 44,46 and 48 are illustrated as true cones, any or all of these surfaces may be somewhat rounded or deviate slightly from a classical geometric cone, and the term cone or conical should be understood to include such variations.

As the fastener assembly 12 is tightened, fastener body 28 moves toward the workpiece or wheel surface 32 and clamp washer 30 deforms to permit this movement. As clamp washer 30 deforms, the initial clamp surface cone angle B decreases and the initial bearing surface separation angle D between surfaces 46 and 48 similarly decreases a like amount because angle A decreases while angle C remains unchanged. During this deformation, it is desireable that the fastener body 28 rotate relative to the clamp washer 30 and that the clamp washer 30 not rotate relative to the surface 32 of wheel 18. It is desired that relative rotation take place between the bearing surfaces 46 and 48 designed for this purpose and that undesireable damage to the wheel surface 32 due to rotation of clamp washer 30 be avoided.During tightening, frictional forces between the body 28 and clamp washer 30 tend to cause the clamp washer to rotate, while frictional forces between the clamp washer 30 and the wheel 18 tend to prevent movement of the clamp washer 30. Contact circles 52 and 54 are both symmetrical about the fastener axis, and frictional forces applied at these circles act through their radii. Since the radius of circle 52 is substantially larger than the radius of circle 54, the forces tending to cause rotation of clamp washer 30 are effectively attenuated and the washer remains stationary.

Fastener assembly 12 is designed to provide a predetermined clamp load in a wheel mounting system.

The initial clamp surface cone angle B is chosen so that although the angle decreases during tightening, angle B remains g reater than zero when the full designed clamp load is achieved. Since the clamp washer 30 provides a spring effect during tightening, this assures that some reserve spring effect remains at full clamp load and that there is a margin for error in case of overtightening. It is preferred that angle B decrease nearly to zero but not all the way to zero upon tightening to full clamp load, a decrease in angle of about 75% to 85% being preferred.

Angle D should be no larger than angle B so that upon tightening of the fastener assembly 12 angle D does not decrease to zero at the full designed clamp load and does not decrease to zero before angle B decreases to zero. It is preferrable that angle D be slightly larger, for example by 1 or less, than angle B so that this relationship is assured despite manufacturing tolerance variations.

The magnitude of the body bearing surface cone angle C is chosen in order to prevent the application of undesireable hoop stresses to the cone washer 30 during tightening of the fastener assembly 12. The frictional forces encountered upon tightening between the bearing surfaces 46 and 48 are utilized by the selection of angle C to prevent the application of excessive radially outward forces to the clamp washer 30 which could have the undesireable effect of causing the clamp washer 30 to loose its elasticity and fail to perform as a spring in the wheel mounting installation.

More specifically, as the fastener assembly 12 is tightened, force is applied to the clamp washer 30 by the bearing surface 48 of the fastener body 28 and is applied in a direction normal to the surface 48. This force has a radially outwardly directed component determined by vector analysis to be equal to the normal force applied at surface 48 times the sine of angle C. This force tends to cause the clamp washer 30 to be deformed radially outwardly and tends to subject the clamp washer to a damaging dilation effect or hoop stress.

Conversely, frictional forces developed between the bearing surfaces 46 and 48 tend to prevent the clamp washer 30 from dilating or moving radially outwardly. Frictional forces developed at the interface between surfaces 46 and 48 are applied parallel to these surfaces, and in the tightened condition, essentially parallel to surface 48. The component of the frictional force directed radially inwardly and preventing dilation of the clamp washer 30 can be calculated by vector analysis to be equal to the normal force times the coefficient of friction experienced at the interface times the cosine of angle C.

Thus it can be seen that the radially outwardly directed force is in opposition to the radially inwardly directed frictional force. It is desired that the frictional force be at least as great as the radially outward force to minimize the application of hoop stresses to the clamp washer 30. Consequently, the frictional force (coefficient of friction times normal force times cosine of angle C) should be greater than or equal to the radially outwardly directed force (normal force times sine of angle C). Since the normal force is a factor in both the inward and outward radial forces, that force can be factored out of the calculation and the coefficient of friction times the cosine of angle C should be greater than or equal to the sine of angle C. In other words, the coefficient of friction should be greater than or equal to the tangent of angle C.

It has been determined that with the use of steel materials the smallest coefficient of friction experienced at the bearing surface interface is about 0.15. As a result, angle C is preferrably about 8 1/250 that the tangent of that angle does not exceed the coefficient of friction.

It is important that the fastener assembly 12 not be excessively large in the radial direction so as to be accommodated in the available space in conventional wheel mounting systems. In order to achieve substantial spring forces without large radial size, the axial offset of the clamp washer 30 is substantial in comparison with the radial offset. The axial offset is defined as the distance in an axial direction between the contact circles 52 and 54, and the radial offset is defined as the distance in the radial direction between the contact circles 52 and 54. It is preferred that the axial offset distance be at least one-half of the radial offset distance in order to achieve large clamp loads within the spring capabilities of the clamp washer 30.

In a fastener assembly constructed in accordance with the present invention the following dimensions and parameters were found to achieve the objects of the invention. These are set forth here by way of illustration of a preferred embodiment of the invention and not by way of limitation: Nominal stud and nutsize 3/4 inch 3!4inch Designed clamp load ........................................ 30,000 Ib.

Axial deflection at designed clamp ........................................ load 0.025 inch Tightening torque at designed clamp load 250 ft. Ibs.

Axial offset ........................................ 0.36 inch Radial offset ........................................ 0.56 inch Angle A ........................................ 5 45' Angle 2035 Angle 8 35' Angle 2050 In Figure 4 the greatly improved clamping characteristic of the fastener assembly 12 of the present invention is graphic-ally illustrated in comparison with the typical prior art arrangement. Line 56 illustrates the relationship between applied clamp load and axial deflection typical of a prior art wheel mounting installation. It can be seen that at a clamp load of 30,000 pounds, about five thousandths of an inch of axial deflection is experienced.Consequently for each one thousandth of an inch of decrease in axial deflection due to creep or embedment of the nut resulting from factors such as vibration, temperature variations or the like, clamp load decreases by approximately 6,000 pounds possibly giving rise to damage to the wheel mounting assembly and a dangerous condition.

Line 58 illustrates the relationship between clamp load and axial deflection in a wheel nut mounting installation including the fastener assembly 12 of the present invention. Here at a designed clamp load of 30,000 pounds, the axial deflection is equal to twenty five thousandths of an inch. If one thousandth of an inch of axial deflection is lost due to fastener creep or embedment, clamp load only decreases by slightly over one thousand pounds and ample clamp load is retained to assure integrity of the wheel mounting installation. The improvement in the clamp load-deflection characteristic is due primarily to the spring effect of the clamp washer 30 and is due to a lesser extent to the fact that use of the clamp washer 30 increases the effective length of the stud 20.

Claims (7)

1. A fastener assembly for clamping one or more vehicle wheels between a hub and the fastener assembly at a predetermined clamp load, said fastener assembly comprising: a threaded fastener having a body and having a thread structure disposed generally in a cylindrical plane parallel to and surrounding the central longitudinal axis of said body; said body including a structure engageable by a tool for tightening of the fastener assembly toward a radial surface of a wheel to be clamped; a resiliently deformable clamp washer rotatable relative to said body and having a central axis aperture surrounding said cylindrical plane and interposed between said fastener body and a wheel to be clamped; a body bearing surface defined on the fastener body and a cooperating washer bearing surface defined on one side of said clamp washer and interfacing with said body bearing surface; and a clamp surface defined on the opposite side of said clamp washer; said clamp surface being conical and inclined relative to the radial direction by an intial clamp surface cone angle; said body bearing surface being conical and inclined relative to the radial direction by an angle having a tangent not greater than the coefficient of friction between the clamp washer and body; said clamp washer prior to tightening of the fastener assembly being engageable with a wheel to be clamped along a first circular line of contact between said clamp surface and the wheel and being engageable with said fastener body along a second circular line of contact between the clamp washer bearing surface and the body bearing surface; said second circular line of contact being smaller in radius than said first circular line of contact;; said body bearing surface and said washer bearing surface defining an initial bearing surface separation angle at least as large as said initial clamp surface cone angle; and said clamp washer upon tightening of said fastener assembly resiliently deforming to decrease said clamp surface cone angle and said bearing surface separation angle, said clamp surface cone angle being greater than zero at the predetermined clamp load.

2. A fastener assembly as claimed in claim 1, said first and second circular lines of contact being radially and axially offset, the axial offset distance being at least one half of the radial offset distance.

3. A fastener assembly as claimed in claim 1 or claim 2, said body bearing surface being inclined at an angle of less than nine degrees.

4. A fastener assembly as claimed in any preceding claim, said body bearing surface being inclined at an angle of more than eight degrees.

5. A fastener assembly as claimed in claim 1, said initial bearing surface separation angle being in the range of equal to or not more than about one degree larger than said initial clamp surface cone angle.

6. A fastener assembly as claimed in claim 1, said body comprising a flanged nut with said body bearing surface defined on said flange.

7. A fastener assembly as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.