Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to fast~ner assemblies,
and more particularly -to an improved fastener assembly for
clamped vehicle wheel installations.
The present application concerns subject matter
related to co-pending Canadian application Serial No. 365,714
filed on November 28, 1980.
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 by 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 signi-
ficant resilience in the conventional system is due to stud
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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 desireable 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 res-
traints imposed by the environment. ~or 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.
Among the important objects of the present invention
are to provide an improved fastener assembly, particularly a
fastener assembly useful for clamped vehicle wheel installa-
tions to provide a fastener assembly capable of resisting
loss of clamp load in use; to provide a fastener assembly
overcoming the problems and dangers resulting from loss of
clamp load due to fastener creep or embedment or the like in
conventional systems; to provide a fastener assembly providing
3n increased resilience in the fastened system in a reliable and
economic manner and without overlarge fastener components; to
provide a fastener assembly capable of providing substantial
resilience without damage to the components of the assembly
upon tightening; and to provide an improved fastener assembly
for clamped wheels overcoming disadvantages of fasteners and
fastener assemblies used for this purpose in the past.
In brief, in accordance with the above and other
objects of the present invention there is provided a fastener
assembly for clamping one or more vehicle wheels between a
hub and the fastener assembly at a predetermined clamp load.
The fastener assembly includes a threaded fastener having a
body and a cylindrical, axially oriented thread structure,
the body being provided with a configuration engageable by a
tool for tightening of the fastener assemblyO The assembly
also includes a resiliently deformable clamp washer rotatable
relative to the body and having a central aperture surrounding
the plane of the cylindrical thread structure. A body bearing
surface is defined on the fastener body and interfaces with a
cooperating washer bearing surface defined on one side of the
clamp washer. A wheel engaging clamp surface is defined on
the opposite side of the clamp washer.
The clamp surface formed on the clamp washer is
generally conical and is inclined relative to the radial
direction by an initial clamp surface cone angle. The body
bearing surface is also generally conical and is inclined
relative to the radial direction by an angle having a tangent
not greater than the coefficient of friction between the clamp
washer and fastener body, thereby preventing the imposition
of hoop stress to the clamp washer upon tightening of the
fastener assembly.
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Prior to tightening of the fastener assembly, the
clamp washer is engageable with a wheel to be clamped along a
first circular line of contact and with the fastener body
along a second circular line of contact between the clamp
washer and body bearing surfaces. The circular lines of con-
tact are radially and axially offset, and the axial offset
distance is substantial in relation to the radial offset dis-
tance in order to achieve large spring forces in a compact
arrangement.
The body bearing surface and washer bearing surface
derine an initial bearing surface separation angle, and that
angle is at least as large as the initial clamp surface cone
angle. Upon tightening of the fastener assembly, the clamp
washer resiliently deforms, and it is assured that a separa-
tion angle is maintained between the washer and the body
bearing surfaces. The clamp washer is designed so that upon
tightening of the fastener assembly the clamp washer resil-
iently deforms to decrease the clamp surface cone angle as
well as the bearing surface separation angle, with the clamp
surface cone angle being greater than zero at the predeter-
mined clamp load so that upon full tightening substantial
resilience is present in the system.
The present invention together with the above and
other objects and advantages thereof will best appear from
the following detailed description of a preferred embodiment
of the invention illustrated in the accompanying drawings
wherein:
FIG. 1 is a fragmentary sectional view through a
vehicle wheel mounting installation including a fastener
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assembly constructed in accordance with the principles of the
present invention;
FIG. 2 is a sectional view of the fastener assembly
of FIG. 1 taken along the axis of the assembly, illustrating
the assembly prior to installation in a wheel mounting system;
FIG. 3 is an enlarged and somewhat diagrammatic
fragmentary, axial sec~ional view of a portion of the fastener
assembly and a wheel surface illustrating the components in
the finger tight or non-tightened position; and
FIG. 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 con-
ventional fastening system of the prior art.
Having reference now to the drawings and initially
to FIG. 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 desig-
nated as 12 constructed in accordance with the principles of
the present invention. The illustrated wheel mounting instal-
lation includes a hub assembly 14 to which a pair of wheels
16 and 18 are clamped by a fastener system including a stud
20 and the fastener assembly 12. Although ~he installation
10 is illustrated as including dual wheels, it should be under-
stood that principles of the present invention are applicableto 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
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the wheels are clamped against the drum, but also other types
of arrangements such as inboard drum mountings wherein ~he
wheels may be clamped directly against a hub member or some
other component of the hub assembly.
Depending on vehicle size, wheel size, load require-
ments 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 FIG. 1, and it should
be understood that a fastener assembly 12 may be associated
with each stud of the wheel mounting system. In the illus-
trated 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 FIG.
1, the clamp washer 30 is sandwiched between the fastener
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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 200 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 be 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 40 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 wi~h
the wheel surface 32 is provided with a clamp surface 44.
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The opposite side of the clamp washer 30 is provided with a
washer bearing sur~ace 46. The washer bearing surface 46
interfaces with and cooperates with a body bearing surface
48. Each of the surfaces 44, 45 and 48 is a continuous and
annular surEace 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 480
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 deormable
and functions to provide substan~ial spring forces in the
tightened condition in a wheel mounting installation such as
the typical installation illustrated in FIG. 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 ~astener
system and thus is inclined at angle B relative to the radially
disposed workpiece or wheel surface 32. As best seen in FIG.
3, in the finger tight initial or non-tightened condition of
the ~astener assembly 12, the clamp washer 30 engages the
w'neel surface 32 at a circular line of contact 52 be~ween 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 conigurat.ion 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
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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 illus-
trated as true cones, any or all of these surfaces may besomewhat rounded or deviate slightly from a classical geometric
cone, and the term cone o~ 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 sur~ace 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 rota-
tion of clamp washer 30 be avoided. During tightening, fric-
tional 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
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washer 30 are effectively attenuated and the washer remains
stationary.
Fastener assembly 12 is designed to provide a pre-
determined 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 greater
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
overtighteningO 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 ~han 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 æero. 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 o 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 ~ndesireable eect of causing the clamp washer
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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
lS 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 pre-
venting 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 rictional
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 fric-
tion times normal force times cosine of angle C) should be
greater than or equal to the radially outwardly directed force
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tnormal 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 1/2 so 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 54O It is pre-
ferred 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 para-
meters 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:
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Nominal stud and nut size... ~...... O.... ...3/4 incn
Designed clamp load.... ~,............... O. 30,000 lb.
Axial deflection at
designed clamp load.............. ,.... ..Ø025 inch
Tightening torque at
designed clamp load................... ...250 ft.lbs.
Axial offset............................ ...0,36 inch
Radial offset........................... ...0 D 56 inch
Angle A.. D~ 545'
Angle B~ 235'
Angle C................................. ...835'
Angle D................................. ...250'
In FIG. 4 the greatly improved clamping characteristic
o~ 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 ~ive thousand~hs of an
inch of axial defl~ction is experienced. Consequently for
each one thousandth of an inch of decrease in axial deflec-
tion 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 r~lationship 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
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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 t 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 lengt'n of the stud
20.
While the invention has been described with reference
to details of the illustrated embodiment, it should be under-
stood that such details are not intended to limit the scope
of the invention as defined in the following claimsO
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