THE IMPORTANCE OF THE WRENCH-FASTENER INTERFACE

By Richard B. Wright, Chairman, Wright Tool Co.

The tightening or removal of a fastener almost always involves the application of torque that is transmitted from a wrench to a fastener. What is unusual about this coupling is that the two parts are almost invariably designed and manufactured independently. Furthermore, the mating parts are almost never chosen by the respective manufacturers, but rather by a user or mechanic, who may or may not choose wisely. There are several points to consider regarding the interaction between wrenches and fasteners as well as what can go wrong, and what can be done to prevent wrenching problems.

A Wrench Should Fit The Fastener, But Does It?

The most obvious point is that there is a good fit. Still, most people do not realize that 30% of the ultimate strength can be lost in the difference between a snug fit and a moderately loose fit. Many mechanics use wrenches until they simply fail to turn fasteners. The obvious result in this is damage to the wrenching corners of the fastener by a worn wrench. A likely consequence is that the corners of the fastener may be damaged to such an extent that there is insufficient bearing area-and even a good quality wrench will be unable to turn the fastener. In cases such as this, the fastener will need to be drilled out. While it is possible for this to happen on an assembly line, it is more likely to occur during maintenance, resulting in additional lost productive time. Using poorly made fasteners where the corners are not filled will cause the same result.
For simplicity sake, the discussion that follows will focus on socket wrenches and the corresponding fasteners, but the principles can easily be translated to other styles of wrenches and other styles of fasteners.

Hexagons And Compensating Shapes

Does a hexagon wrench mate with a hexagon headed fastener? Technically, it does not, because there is clearance due to manufacturing tolerances of both the wrench and the fastener. Therefore, when the wrench is placed on the fastener and turned to make engagement, the wrench engages on the corners of the fastener, which produces undesirable stress concentrations in both the fastener and the wrench, and tends to deform the corners of the fastener. The wrench rotates relative to the fastener-producing camming forces that are capable of splitting the socket wrench. Quality tool manufacturers now produce designs such as Flank DriveÒ and Wright DriveÒ that provide relief so that the wrench does not bear on the corners of the fastener, which distributes the pressure more widely over the wrenching surfaces. This compensates for some of the clearance and increases wrench strength by up to 12%. However, if the clearance is too much because of undersized fasteners or the excessive wear on either the wrench or the fastener, this benefit is lost. These designs work best in the 12 pt. configuration.

Are 6 or 12 Pt. Wrenches Better For Hexagonal Headed Fasteners?

If fasteners are well formed and toward the upper end of their size range, 12 pt. wrenches have more ultimate torque strength and there is less chance of the corners of the fasteners being damaged. If the fasteners are badly corroded or poorly formed, 6 pt. wrenches turn them better because of the slightly smaller diameter. If the socket is driven by an impact gun, a 6 pt. socket must be used because the repeated blows of the impact gun cause metal flow in the 12 pt. design of sockets.

For very small fasteners, manufacturing tolerances make the hex wrench a better choice. For a 1/4" drive, only single hex sockets should be used, except, of course, for 12 pt. fasteners.

12 Pt. Fasteners

Twelve point fasteners have approximately twice the contact area of 6 pt. fasteners when used with 12 pt. wrenches, resulting in approximately 70% more strength if the socket has a sufficiently thick wall. However, the most common mode of failure for sockets is splitting. The socket must not only transmit the torque to the fastener, it must also serve as a torque tube to bring the torque from the handle to the fastener. Historically, 12 pt. wrenches were designed for single hexagon fasteners to provide more possible positions of engagement; to provide additional wear surfaces; and, more recently, to take advantage of the improved geometry made possible by modification of the traditional 12 pt. design. But, in many cases, the wall thickness was not increased, although additional torque is possible with the 12 pt. fastener configuration and is often required by higher strength materials used in those fasteners.

Taking this into consideration, a wrench manufacturer contemplating this situation is tempted to increase socket wall thicknesses, but is tempered by the fact that there are many fasteners already in place, or designs that have already been made, that do not provide wrench clearance for thicker-walled sockets. Fortunately, these higher strength fasteners are usually installed using a torque wrench at the manufacturer's torque recommendation, which provides a margin of safety. Problems can occur if fasteners are over-tightened, because removal torques will be proportionately higher and may be greater than the wrench-fastener coupling can deliver-either because of failure within the coupling, failure of the wrench as a torsional transmitting device, or even failure of the twist off from the bolt. Also, torque can be greatly increased by corrosion or by thread damage.

Because the required torque to produce a designed preload is very dependent on lubrication, a change in the lubrication factor can produce not only preload problems but wrench failure problems. This is the reason why the torque required to remove fasteners varies so much in practice. Today, wrenches and fasteners work reasonably well together because of the safety efforts provided by both wrench and fastener manufacturers. However, as the complexity of designs escalates, and, perhaps, the wall thicknesses of wrenches are squeezed, safety factors may be lost. This is hard to predict in advance because of the many variables, not only in installation torque, but more so in removal torque.

Which Drive?

Most sizes of socket wrenches are made in three drive sizes to provide the user a choice of strengths and outside diameters. For example, a 3/8" opening socket is made in 1/4", 3/8", and 1/2" drive. A 7/8" opening socket is made in 3/8", 1/2", 3/4" and, at times, 1" drive. For medium strength work, the middle size is normally used because it is most convenient and the handles provide the approximate amount of leverage needed. For high-strength bolting, only the largest possible drive should be used because the thicker walls provide more strength and the longer handles safely provide more leverage. The smallest drive should be used only when necessary, because of close quarters, and should be done with the realization that the tool may not provide enough strength.

Metric Versus Inch

It may seem odd to mention that the metric-inch confusion is a significant problem-but it is. Because clearance is such a major factor in determining wrench-fastener performance, the mistaken use of metric wrenches on inch fasteners or inch wrenches on metric fasteners almost always creates a problem. Since some of the sizes are very close, mechanics that simply go by their sense of feel may not realize that there is more clearance than there should be because of an inch-metric mismatch. To overcome this problem, the fastener should be measured in millimeters, and the measurement should be compared to a table provided by a wrench manufacturer that indicates the best possible fit, whether it be inch or metric for that particular fastener.

Spline Fasteners

Twelve-spline wrenches will transmit a bit more torque than double-hex wrenches to 12-spline and double-hex fasteners. The difference is not great and depends on the circumstances. The biggest advantage of the 12-spline over a double-hex configuration is it does not have the radial forces tending to split the socket and, therefore, can be made with a thinner wall than a double-hex socket of the same strength. Similar to other special configurations, it has the advantage that a mechanic will not be tempted to risk injury by using a lower strength wrench. While the most popular spline design has been in use for approximately 30 years, newer manufacturing methods have created a 15-spline design with approximately 25% more strength.

Internal Wrenching vs. External Wrenching

Wrench designers have discovered that a very sharp cornered hexagonal wrench that provides a very close fit to the socket head screw opening is essential in order to deliver the required torque. Paint can make the problem much worse. However, it is essential to get the paint out of the corners if the required torque is going to be developed. Currently, there are proprietary systems of special wrenches and mating fasteners that provide significantly improved performance. It appears that higher torque transmission is possible with externally wrenched fasteners rather than internally wrenched, even when the outside diameter of the socket is compared to the head diameter of the socket head screw.

Article published in American Fastener Journal, November/ December 2001

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About the Author:
Richard B. Wright is chairman of Wright Tool Company, a leading manufacturer of professional-quality sockets and wrenches. Located in Barberton, Ohio, Wright Tool manufactures more than 3,600 tools for the industrial, contractor, and MRO markets.

Mr. Wright holds a degree in Mechanical Engineering from the California Institute of Technology and an MBA degree from the University of Pennsylvania Wharton School. A licensed engineer, Mr. Wright holds several patents in the field of hand tools and electrical instrumentation.