The present practice in selecting wrenches is based on experience of what works and what doesn’t. This can be learned only be experiencing failure, which may result in injury, damage to equipment, or unnecessary downtime.It would seem to be a simple matter to define the strength of fasteners and wrenches so that the user could make an intelligent selection without risk. However, it turns out to be much more complicated because of the multitude of factors involved. These include the difficulty of defining the shape of the fasteners in so far as they affect "wrenchability," and the degree of wear of the wrench. There are over two dozen significant variables, many of which are difficult or impossible to measure. In addition, the interaction is impossible to define. Some of this lack of information can be covered by a suitable safety factor, a common practice in mechanical design. But, if safety factors are too large, the recommended choices will be so large as to be impractical. Current practice has been to use wrenches that are not strong enough to do all required jobs. The odds have been good enough that this has worked most of the time. However, equipment builders sometimes provide too little space for wrenches so that those of a safe size may not fit. Some of these fasteners require very high torque out of relatively small wrenches. There are not only variations in wrench quality, but much wider variations in fastener design and manufacture. Some of the factors to consider in designing or selecting a safe series of wrenches and fasteners include: maximum tightening torque, maximum removal torque, minimum torsional strength, and torques from wrench specifications.
Maximum tightening torque
This is the bolt tensile strength-to-yield multiplied by the K factor appropriate for new, lubricated fasteners; the diameter; and a percentage safety factor. Different values might be required for various plated finishes, or a maximum value could also be used. In addition, there are published torque tables for general use and specific torque values for specific applications by aircraft manufacturers and others.
Maximum removal torque
There are no published tables for removal torque because it must be whatever it takes to do the job. Most new fasteners are either plated or coated with oil which protects against corrosion and provides some lubrication during installation. Because of the eventual loss of lubrication, as well as seizing and corrosion, the removal torque is usually higher than installation torque by a substantial amount.
Minimum torsional strength
This is the other side of the wrench/fastener match. The value used for wrench strength must be based on actual conditions including wrench and fastener tolerances and wear.
Torques from wrench specifications
The ASME, SAE and ISO standards all provide torsional test values for a wide variety of wrenches. These are proof loads for testing purposes and made to be applied only once. They do not provide for fatigue or even a few cycles of load or for wear.
The Wrenching Solution
Choosing the best wrench for optimal safety should always be of prime concern. One step that helps is using the largest possible drive size for socket wrenches. Another is to replace worn wrenches and fasteners. Still another is to replace fasteners that have proved difficult to wrench with fasteners having more "wrenchability." These would include fasteners with larger heads or spline fasteners in place of 12-point fasteners. With grade 8- and 12-point and other high-strength fasteners more care must be used in the selection and use of the wrench/fastener coupling. The goal should always be to have the wrench stronger than the fastener.
Richard B. Wright is Chairman of Wright Tool Co, a manufacturer of s more than 3600 tools for the industrial, contractor, and MRO markets. A licensed engineer, Wright holds several patents in the field of hand tools and electrical instrumentation.
Wright Tool Co.