Do you need stronger wrenches?
Because of the pressure for higher performance and lower cost for virtually every product made, bolted joints are now made to work harder and their failure could be very damaging.
Over the past few years, fasteners have grown stronger, and the use of high-strength fasteners has increased. Through the widespread use of torque wrenches, fasteners are being tightened to higher torques.
Different users will get radically different performance results from a wrench in terms of strength and service life, depending on how it is used and what it is used on. By far, the most important distinction is between using a wrench for assembly or disassembly.
Because assembly involves tightening new, clean, lubricated fasteners to a specified torque, it is relatively easy to select a wrench or test whether or not a wrench is adequate. Even if there is no torque specification, the torsional strength of the fastener sets an upper limit to the torque the wrench will encounter.
Complicated products such as machine tools are disassembled and reassembled regularly, and high removal torques can be encountered during disassembly. However, the fasteners will still be clean and rust-free, and not be seized because of long service and vibration.
Even in assembly operations, different makes of wrenches will have different service lives depending primarily on their metallurgy and accuracy of manufacturing. Wear is the normal failure mode. However, if the wrench is allowed to become seriously worn, it could damage fasteners. The wrench might also break because of stress concentrations produced by poor contact between the wrench and fastener.
Field tests of removal torques show they are typically in the range of 100%-300% of specified installation torques. Loss of lubrication, seizing, and corrosion are likely causes. If fasteners are loaded beyond their elastic limit in installation, service, or otherwise damaged, additional torque is required to overcome a change in thread pitch due to bolt stretch.
How can a plant engineer know that available wrenches are adequate? He can rely on his experience with similar fasteners on similar products in similar environments. But this is not sufficient for new products or new environments. Fasteners are being turned with wrenches designed and built before today’s fasteners were developed.
If any one of many factors is changed, the increase in bolt tensile strength or specified torque may be beyond the capacity of available wrenches. The result can be unexpected wrench failure.
Fastener manufacturers are not much help since they typically do not recommend torques because of all the uncertainties involved. The most practical solution is to have wrenches with a substantial reserve of strength to overcome the problem of uncertainty.
Wrench manufacturers cannot simply make wrenches that are heavier because they might not fit in the space provided by past designs or new designs based on old wrench dimensions (Fig. 1).
The ANSI standard for a fastener (Fig. 2) might have a proof torque value of 5000 in.-lb. The ultimate strength of a typical socket wrench might be 6000 in.-lb. These data suggest that the wrench would have ample strength to remove the fastener. However, this is not so, because the testing of wrenches is done with maximum-size mandrels. It is unlikely that the actual fastener would be the size of the mandrel.
The mandrel’s hardness of Rockwell 56C, or higher, is above that of the fastener. Fasteners are softer than the wrench and yield under high-load conditions, reducing their load capacity.
If the points of the fastener are not fully formed, more capacity is lost, when compared to the precisely shaped test mandrel. Sockets with a 6000 in.-lb test under ideal conditions would be likely to have a strength of no more than 4800 in.-lb when tested with a softer fastener. They might also have a strength as much as 3800 in.-lb when tested on a mandrel corresponding to the low end of the fastener size tolerance range.
All of these tests are made on new wrenches. If a reasonable adjustment for wear on the wrench is made, 4800 becomes 4300 in.-lb and 3800 becomes 3400 in.-lb. This would be under conditions in which most mechanics would consider a reasonably good fit between the wrench and fastener.
If the wrench were allowed to wear, the amount of torque that it can transmit to a fastener continues to drop. A reasonable value for minimum wrench performance would be about 3400 in.-lb. This value is not guaranteed, and it is based on approximations and assumptions, but it is realistic.
A wrench torque of 3400 in.-lb is below the maximum value that might be the required removal torque. However, if conditions are favorable, the wrench will work with no problem. On the other hand, if the condition of the fastener or the wrench is unfavorable, then problems will be frequent. If the mechanic is using a worn wrench, it may need to be replaced with a new wrench to do the job.
If a wrench is too badly worn, the fastener contact area will be damaged and might not be usable with a new wrench. Other factors to consider are the number of times the load will be encountered and the number of times the job will be performed. If high loads are imposed frequently on the wrench, it could fail in fatigue. Fatigue failures are cracks caused by overloads.
The fact that many assumptions were needed to produce this example shows the difficulty and pitfalls in trying to calculate required and available strengths. It also demonstrates why wrench manufacturers do not predict wrench strength for field conditions.
What does this mean to the average mechanic? Very little, after replacing worn wrenches. He can only hope that the product designer and the fastener manufacturer have taken his plight into consideration and avoided fasteners that will have wrenching problems in the environment where they will be used.
One major area of concern is 12-pt fasteners with small heads. These are made of high-strength materials and require high torques. The small head requires a smaller wrench, which is not as strong.
If top-of-the-line wrenches are used and allowances are made for the 12 points of engagement, compared to 6 points of engagement, the strength will still be at least 20% less. If the 12-pt configuration is not well formed, or the fit is poor, the available torque will be lower.
The standards for 12-pt wrenches were developed many years ago and did not take into account higher-strength materials or smaller-head fasteners. Spline fasteners have been used, but even they do not get back to the strength level of big, hex-head fasteners.
The next generation
New designs of wrenches are capable of transmitting twice as much torque as older wrenches in the same space. Plant engineers do not have to wait for upgrades to troublesome fasteners from Grade 5 to Grade 8 or from Grade 8 to 12-pt, because they will fit in the same space. Unfortunately, spline fasteners are not stock items. They can do it with less sensitivity to wear using thin-walls and short engagement lengths. But just as new software will not run on old computers, these new-style wrenches require a new-style fastener head.
New designs are available for both externally wrenched and internally wrenched fasteners (Fig. 3). The fact that standard wrenches will not fit high-performance spline fasteners may be an inconvenience, but it is very important for maximum safety, because they do not have the strength to turn those fasteners.
These shapes are not nearly as simple as traditional fasteners because they take advantage of today’s available technology to design and manufacture sophisticated shapes for higher performance at reasonable cost (Fig. 4).
If you have any questions about wrenches contact Mr. Wright at 330-848-0614, firstname.lastname@example.org , or visit the web site wrighttool.com . Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, email@example.com .