Method of making threaded fasteners

The present invention relates to a method of making threaded fasteners and, more particularly to a method of making bolts or studs having an external thread form.

It is a problem that when a bolt is tightened into yield the actual clamp load produced is dependent upon the material strength, effective stress area and the torsional stress in the bolt which itself is a function of the thread friction.

When existing bolts such as those described in GB-A-2 187 791 are used the actual clamping load produced on tightening to produce yield will vary and this can lead to failure in the components being clamped or secured together. For example, if the bolts are used to secure a cylinder head to a cylinder block insufficient clamping load will result in failure of the cylinder head gasket.

It is an object of the present invention to reduce the variation in clamp load produced when a threaded fastener such as a bolt is tightened into yield.

GB-A-1,392,513 discloses a method of producing a single fastener from a workpiece. It does not disclose the drawing, testing and cutting steps required by claim 1 of the present application. The advantage of those steps is that, while each wire has a relatively constant tensile strength and stress area, those parameters can vary significantly between wires. If the grooves for each wire are made to suit that wire, fasteners produced give a more uniform clamp load when they are tightened to yield.

According to the present invention there is provided a method of making a threaded fastener including the steps of:

   drawing from a material of suitable material properties a wire of appropriate diameter;

   testing at least a length of the so formed wire to determine the tensile strength thereof;

   cutting the wire into blanks of appropriate length according to the desired length of threaded fastener being made;

   forming an appropriate external thread form on a portion of each of said blanks; and

   forming at least one substantially circumferential groove in a shank portion of each of the blanks the depth of which groove is such that the effective stress area thereby produced is less than the minimum effective stress area of the external thread form and is such that the yield of the threaded fastener being made occurs in use in the region of the or each groove at a predetermined applied load.

Preferably the depth of the or each groove is such that the diameter of the shank at the base of the or each groove is in accordance with the relationship:$$\text{D =}\sqrt{\frac{\text{F}}{\text{σ}}}\text{x K}$$ where

   D is the diameter at the base of the or each groove (m)

    is the tensile strength of the wire (N/m²)

   F is the load (N) required to produce yielding of the fasteners in the region of the or each groove

   K is a constant which is a function of the stress concentration factor appropriate to the shape of the groove and the type and composition of the material used to make wire.

The constant K can be readily determined by experiment and requires no inventive effort.

Preferably a length of wire is tested from each end of the wire.

The at least one substantially circumferential groove may be formed simultaneously with the external thread form, or may be forced prior to or subsequent to the external thread form.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which:

• Figure 1 is a side view of a bolt according to a first aspect of the present invention;
• Figure 2 is an enlarged and exaggerated view of the area 'A' on Figure 1;
• Figure 3 is a graph illustrating the strength distribution of lengths of wire from a single cast within an overall acceptable range; and
• Figure 4 is a graph showing the elongation of the wire with increasing load.

Figures 1 and 2 show a threaded fastener in the form of a bolt 11 having a shank 12 and a head 14. The shank 12 has an external thread form 13 near to one end and is terminated by the head 14 on the other end. A portion of the shank 12 towards its juncture with the head 14 has a number of circumferential grooves 15 formed in it to produce a ribbed formation.

The depth 'd' of each of the circumferential grooves 15 is such that the minimum diameter 'D' of the shank 12 in the region of the grooves 15 produces an effective stress area which is less than the effective stress area produced in the region of the thread form 13 by the minimum thread diameter 'X'. The actual dimension 'D' is determined in accordance with the relationship$$\text{D =}\sqrt{\frac{\text{F}}{\text{σ}}}\text{x K}$$ where:

   D = diameter at the base of each groove (m)

   σ = tensile strength of the wire (N/m²)

   F = load (N) required to produce yielding of the bolt in the region of the grooves

   K = constant which is a function of the stress concentration factor appropriate to the shape of each groove and the type and composition of the material used to make the wire.

By measuring the tensile strength of the wire from which the bolt is to be made it is possible to determine an appropriate value of 'D' co produce yielding at a predetermined clamp force.

It is therefore possible to compensate for slight changes in material strength and thereby reduce the variation in clamp load produced by loading any such bolt into the elastic region.

The primary manufacturing steps include drawing from a material of suitable properties such as carbon steel a wire of appropriate diameter, and testing a short length of the wire to determine its tensile strength at yield. Tests show that while the strength of the wire may vary between minimum and maximum acceptable strengths, the tensile strength at yield of lengths of wire from a single cast fall within a much narrower band. This is shown in Figure 3 where the strength distribution is typified by a narrow band within the range of acceptable strengths. The test for tensile strength is normally carried out by testing a sample from each end of the wire.

The wire is then cut into blanks of appropriate length according to the length of the bolt being made and each of the blanks then has an appropriate thread form rolled onto it. At the same time as rolling the thread form, or as a separate step, the grooves 15 are produced by rolling, the depth of each groove being controlled to produce an effective stress area that will produce yield of the bolt in use in the region of the grooves at a predetermined applied load.

The number of grooves 15 to be rolled is not critical, but preferably the number of gooves should be at least six, and ideally as many grooves as possible should be formed. A greater number of grooves minimises the risk of stress fractures and allows the bolt to be re-used repeatedly because work hardening in the region of the grooves occurs during removal of the bolt after it has been secured and a larger number of grooves permits the bolt to be removed more times before the region of every groove is work hardened with the result that any further yield will take place in the region of the thread and not in the region of the grooves.

Not only does the formation of grooves in the bolt permit re-use of the bolt without yield occurring in the thread, but it also enables the overall yield strength of the bolt to be controlled within tighter limits. Figure 4 is a graph showing the elongation of the wire with increasing load. The bottom curve A shows the behaviour of wire having the minimum acceptable yield strength while the top curve B shows the behaviour of wire having the maximum acceptable yield strength. The intermediate dotted line C shows how a wire having the maximum acceptable yield strength can have grooves rolled thereon so as to reduce the yield strength of the wire close to the minimum acceptable yield strength. The distribution of yield strength for such a grooved wire is also shown in Figure 3 in a dotted line and explains why the average strength is required to be greater than the minimum strength.

By testing the strength of each batch of wire it is possible to establish the average strength of the wire and thus to determine the depth of groove 'd' to establish a minimum diameter 'D' of the shank of the groove in order to bring the average strength of the wire down to the dotted line C. In this way the tolerance on the yield strength of the wire can be reduced and the variation in clamp load produced when a bolt is tightened into yield can be reduced.