Why bother with wrenches?

By Richard Wright, Chairman, Wright Tool September 20, 2006

Why bother with wrenches?Wrenches are almost never a part of a finished product, nor are they much of a concern of a design engineer unless he notices that a fastener is located in the manner that will make wrenching very difficult or impossible.Of course, some very cost-conscious companies have found that they can reduce the cost of assembly and the cost of maintenance by the appropriate selection of fasteners and by providing space for efficient access.

Although unit construction, adhesives, welding and fastener redesign to use fewer stronger fasteners reduce the number of fasteners, they can never eliminate it because fasteners are required for joining subassemblies, providing access for inspection and maintenance, and for the ability to replace portions of the product.

Another reason for considering wrenches and their associated fasteners is that this is an area that has not received much study in the past, which means it is more likely to provide opportunities for improvement or cost reduction.

Wrenches and fasteners must work together (see Figure 1) to accomplish the job of either installing or removing fasteners.Good engineering requires specifying the installation torque of tension bolts to ensure their performance because bolts that are either over-tightened or under-tightened are prone to failure.Furthermore, removal torque is usually more than installation torque — often twice as much — that may result in wrench failure, which can cause injury or an expensive delay.

Historically, wrenches and fasteners have not been thought of as a system, resulting in a lower-grade of performance and thus providing opportunities for improvement.Figure 1 shows the five components of the wrench/fastener system, which is most clearly illustrated by a socket wrench.The wall of the wrench has been counted as a separate part from the rest of the wrench.That is critical because most socket wrenches and other types of wrenches fail through the breaking of the wall.

The faces of a wrench only transmit force, but the wall of the wrench must transmit torque and is subject to high circumferential stresses if not properly designed and mated to the other components.There are five possible modes of failure in the causality chain between the mechanic applying force and the fastener.In researching the various possibilities for improving wrench performance and reducing their size, Wright Tool has produced failure in every one of the five modes to learn how to increase strength without making wrenches larger, which is important in today’s designs.

The original design of the fasteners’ engaging surfaces was square or hexagonal, which worked quite well but were bulky and expensive. For lack of design information, the fasteners were substantially over-sized.So the wrenches did not have to deal with high torques.

Twelve-Point Wrenches

Twelve-point wrenches were developed as a more convenient way of engaging with a fastener and are preferred by mechanics for that reason.Most mechanics also know that they are not quite as strong as 6-point wrenches.If the fit is snug, they are as strong but because of manufacturing clearances on both fasteners and wrenches and wrench wear, line contact tends to be the rule.Line contact not only can damage both wrench and fastener but it can combine with the high circumferential stresses in sockets to produce areas of failure.

Wrench manufacturers can help this problem by making subtle departures from geometric designs.The best wrench design is one that produces a load pattern that avoids damaging the corners of the fastener or the corners of the wrench and, thereby, decreases strength and reduces life.The maximum strength and least wear when turning 12-point fasteners is produced by the shapes in Figure 2.Advanced 12-spline type design wrenches are stronger on both 6-point and 12-point hexagonal fasteners by 6% to 10%.

The lower left corner of figure 2 shows three different wrenches applied to a 12-point, double-hex fastener.The relationships would be the same on a 6-point hexagonal fastener.The traditional double-hex wrench contacts only the corners of the fastener resulting in high contact pressure and a tendency to deform the fastener. Depending on the clearance between wrench and fastener, the location of the point of contact will vary.The high contact pressures will tend to cause a socket wrench to expand.If the point of contact is close to the inner point of the wrench, the wrench may slip on the fastener and shear the tip of the fastener teeth.

Figure 3 shows the strength of the spline wrenches compared to the 12-point and 6-point wrenches.Twelve points are not twice as good as 6 points because the torque is limited not only by the teeth but also the ability of the socket to withstand the increased circumferential tension, which is, of course, proportional to the load.Because socket wrenches were not increased in diameter when the manufacturers went from 6 to 12 points, there is no strength in the body of the sockets to maximize load transmission.

This failure to mate the design of the wrenches and fasteners is clearly shown in Figure 3, where the loads for 12-points are substantially better than a 6-point fastener but not double the load as one might expect.The premium wrench profiles do help.

While these differences may seem subtle, the difference between success and failure may be just a few percentage points.