Safety-Based Wrench Selection

      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.

      Typically, removal torque will be between 100% to 200% of installation torque, which gives some guide for removal torque.  However, values as much as 300% of installation torque, and even higher, are not unusual.

      One approach to torque removal is to provide a wrench with torsional strength exceeding that of the fastener.  This also allows the user to twist-off bolt heads, which is often a faster way of disassembly.  But the twist-off torque sets an upper limit of the useful wrench strength and the user must be careful to avoid injury or failure.  

      If space permits, the best way to remove fasteners is with an impact gun.  These produce a series of shock blows, which are more effective in loosening a fastener and have less tendency to break the wrench because steel is stronger under impulsive loading than under steady loading.  The sockets made for impact gun usage are stronger than those for hand usage, but they still have a strength limit.  

      For low-strength fasteners, it is practical to provide a wrench that is stronger than the fasteners.  But wrenches that were originally designed for low-strength fasteners do not have the strength to twist off 12-point or other high-tensile fasteners.  They simply were not designed for such loads.  To increase strength of the wrench by increasing the diameter of the wrench may produce a wrench that is too large to work within the available space.  Also, the teeth of the wrench can fail.

      The best solution lies in skillful design of the bolted joint.  The bolt should be chosen so they do not have to be tightened beyond what can be safely removed after a long period of service. Twelve-point wrenches may be used where space is limited.  These should be turned with spline wrenches for higher torque.  Spline fasteners are substantially better than 12-point wrenches but cannot be turned with ordinary 6- or 12-point wrenches and are more expensive.

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.

      The wrenching surfaces on many fasteners are made near the small end of their size range and the corners may not be sharp.  These factors reduce the ability of the wrench/fastener coupling to transmit torque.  When fasteners are on the small side, the amount of contact area is substantially reduced and the load is concentrated on or near the corners of the fasteners.

      Because wrenches are typically much harder than fasteners, the fasteners will deform under conditions of heavy loading.  The amount of deformation is very dependent on the fastener hardness.  The result is that there is no such thing as a single value of strength for a wrench.  It depends very much on the circumstances.

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.

      These test values are made with little clearance between wrench and fastener, which is not typical of service.  They are also made with mandrels that are harder than the wrench and much harder than even high-strength fasteners.  The result is values that are not useful for determining working strength.  On the other hand, high-quality wrenches do more than just meet the standard.

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.
1-330-848-0600
www.wrighttool.com