Fastener pre-stressing joint
Fastener joints induce static pre-stresses into a fastener that clamps the joint in a magnitude equal and direction opposite from bending stresses induced in the fastener by application loading. Such joints include angling the bolts seats of a connecting rod bearing cap inwardly, creating a relief at the bolted interface between the cap and the rod adjacent to the main bore of the connecting rod or skewing the threaded holes in the rod body inwardly as they extend from the bolted joint faces.
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This claims the benefit of U.S. Provisional Patent Application No. 60/516,488 filed Oct. 31, 2003.
Not applicable. FIELD OF THE INVENTIONThis invention relates to fastened joint design, such as a bolted joint, and in particular to a joint that pre-stresses a fastener to result in a more uniform stress in the fastener at maximum application load.
BACKGROUND OF THE INVENTION Bending stress is induced across the shank 11 of a fastener such as a bolt in the plane of bending when the joint is non-symmetric and when the fastener loading is not on the centerline of the fastener.
The bending stress in the fastener in
The load being carried by the fastener is related to the average stress in the fastener. In
Referring to
The word “fastener” as used herein is any type of fastener having a shank that is subjected to tensile forces when applied to a joint, such as bolts, rivets, rods (threaded, pinned, welded, etc.), screws, etc. The word “bending stress” refers to a non-uniform stress across the fastener shank. This invention includes the use of a nut in the joint system, which could act like a bolt head and therefore “head” of a fastener includes a nut, a bolt head or screw head, a rivet head or rivet flange, etc.
SUMMARY OF THE INVENTIONThis invention provides a bolt joint that at the maximum loading conditions in the service application of the joint the maximum stress will be reduced across the bolt shank. It does this by the joint inducing a bending stress in the fastener shank in the plane of bending of the application bending stress when the fastener is assembled to the joint. The bending stress induced by the joint is substantially inversely proportional to the bending stress induced in the plane of bending by the maximum application load that the fastener shank is subjected to in service so as to reduce the maximum stress when the maximum application load is applied.
By so doing, the invention also reduces the cyclical mean stress to which the fastener shank is subjected. This is especially useful to increase the fatigue life of the fastener.
In a useful aspect of the invention, the bending stress induced by the joint is of a magnitude and direction to produce a substantially uniform stress distribution across the fastener shank in the plane of bending when the maximum application load is applied, to obtain the full advantage of the invention.
In one form of the invention, the joint has a seat that the fastener bears against to induce tension in the shank and the seat is skewed at an angle other than 90 degrees to an axis of a fastener hole in the parts through which the shank extends. The seat is angled in a direction so as to induce bending stresses in the fastener opposite in direction to the bending stresses induced by the maximum application load. Thereby, the bending stresses induced by the joint cancel the bending stresses induced by the application load to reduce the maximum application load on the shank of the fastener and to reduce the cyclic mean stress to which the fastener shank is subjected.
In another way of practicing the invention, the joint has joint faces that face one another and are held together by the fastener, a portion of the joint faces defining between them an unsupported gap that induces bending stresses in the shank of the fastener opposite in direction to bending stresses induced by the maximum application load.
In another ways of practicing the invention, a hole that extends in the parts and receives the fastener shank has a first portion in one of the parts and a second portion in the other part, with the first portion skewed relative to the second portion so as to induce bending stresses in the fastener opposite in direction to bending stresses induced by the maximum application load.
These different ways of practicing the invention can be practiced alone or in any combination with one another.
In an especially useful form, the joint is a joint in a connecting rod connecting a bearing cap to a rod portion of the connecting rod. A bearing cap joint is an especially useful application of the invention because the fastener shank is subjected to a cyclic bending stress by the cyclic motion of the connecting rod, such that prestressing the fasteners using the invention can reduce the maximum application stress and the cyclic mean stress in the fastener shanks.
The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to
The schematic stress charts of
Uniform stress distribution at maximum application loading can be accomplished in any number of ways. Currently, typical connecting rod bearing cap joints are made as illustrated in
One way to practice the present invention would be to skew each joint bolt seat 36 to the bolt hole 37 and threaded hole 39 centerline 38 by some small amount, chosen based on the maximum application loading that is to be cancelled or offset. Typically, the angle would be less than one degree, for example 0.125 degrees, depending on the magnitude of application loading. The angle must also be in the correct direction so that it cancels the bending stress at the maximum application (dynamic) loading condition, which is induced by the joint and application load. This is illustrated in
If in
Another way to create a uniform stress across the bolt shank 11 in the plane of bending at maximum application load is to make the joint faces, where they face each other near the center of the main bore 40, at a small angle to each other tapering outwardly so as to create a small unsupported gap 48 between each set of the joint faces in the area adjacent to the bore 40. This is illustrated in
Yet another way to create a uniform stress in the plane of bending across the bolt shank 11 at maximum application load would be to create the centerline 38A of the threaded hole 39 at a small angle to the bolt hole 37 and (unbent) bolt 10 centerline 38B as illustrated in
Preferred embodiments of the invention have been described in considerable detail. Many modifications and variations to the preferred embodiments described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiments described.
Claims
1. In a joint between at least two parts clamped by a fastener having a shank in tension that holds the parts together, the improvement wherein the joint induces a bending stress in the fastener shank in a plane of bending when the fastener is assembled to the joint, the bending stress induced by the joint being substantially inversely proportional to a bending stress induced in the plane of bending by a maximum application load that the fastener shank is subjected to in service so as to reduce the maximum stress when the maximum application load is applied.
2. The improvement of claim 1, wherein the bending stress induced by the joint is of a magnitude and direction to produce a substantially uniform stress distribution across the fastener shank in the plane of bending when the maximum application load is applied.
3. The improvement of claim 1, wherein the joint has a seat that the fastener bears against to induce tension in the shank and the seat is skewed at an angle other than 90 degrees to an axis of a fastener hole in the parts through which the shank extends in a direction so as to induce bending stresses in the shank of the fastener opposite in direction to bending stresses induced by the maximum application load.
4. The improvement of claim 1, wherein the joint has joint faces that face one another and are held together by the fastener, a portion of the joint faces defining between them an unsupported gap that induces bending stresses in the shank of the fastener opposite in direction to bending stresses induced by the maximum application load.
5. The improvement of claim 1, wherein a hole that extends in the parts and receives the fastener shank has a first portion in one of the parts and a second portion in the other part, wherein the first portion is skewed relative to the second portion so as to induce bending stresses in the fastener opposite in direction to bending stresses induced by the maximum application load.
6. The improvement of claim 5, wherein the second portion is threaded.
7. The improvement of claim 5, wherein the first portion is adjacent to a fastener seat that is substantially perpendicular to an axis of the first portion.
8. The improvement of claim 1, wherein the joint is a joint in a connecting rod connecting a bearing cap to a rod portion of the connecting rod.
Type: Application
Filed: Oct 28, 2004
Publication Date: Jun 7, 2007
Applicant: GKN SINTER METALS, INC. (Germantown, WI)
Inventor: Henry Knott (Ypsilanti, MI)
Application Number: 10/577,534
International Classification: E04F 15/14 (20060101);