Reducing downtime with the right fasteners and wrenches

The simplest way of reducing wrenching problems is to recognize that all wrenches are not created equal in either design or manufacture.

Wrenches work together as a system to transmit torque between engaging surfaces and the body of the wrench. Traditional wrench and fastener designs produce large radial forces that tend to either split the wrench or shear the teeth off either the wrench or fastener under heavy loads, according to Richard B. Wright, chairman of Wright Tool Co., Barberton, Ohio.

Various factors interact to transmit torque. They function as a chain and the weakest link determines the strength of the combination. In the typical repair situation, the fastener is already in place and the mechanic has the problem of removing a fastener that has been worn and perhaps even stretched or otherwise damaged. Since nothing can be done with the fastener, the best thing to do is to use the best possible wrench.

All wrenches are not created equal in either design or manufacture, says Wright, who is a licensed engineer who holds several patents in the field of hand tools. Furthermore, he adds, even the best wrenches wear and lose strength; engagement is not as good with a worn wrench as it is with a new wrench.

The strength of wrench/fastener combinations, which Wright says is what we are talking about when we talk about the capacity to remove a fastener, varies over a range of 330%. This, of course, covers a wide variety of fasteners and wrenches that have substantial differences in cost.

To improve the strength, you can change the wrench or change the fastener — which is best done by the product designer.

It is amazing what can be done without changing the fastener itself. For example, almost everyone uses a common 12-point wrench to turn 12-point fasteners in aerospace. But, if they use a spline wrench, they can apply up to 20% more torque. Spline wrenches are expensive but the increase in a mechanic’s productivity can justify it. A more economical solution can be to use a special mass-produced product, which can provide 11% more strength than a standard wrench because it is made to fit better and wear longer.

The next step is to change the fastener as well as the wrench. In many cases, this can be done during maintenance when a troublesome fastener is removed and replaced with a higher-grade fastener. This does not require engineering approval. Either an engineer or at least a good handbook should be used to specify the correct tightening torque and a torque wrench should be used to obtain that torque.

Fastener options including using a reduced fastener head height, smaller and stronger fasteners, hollow head cap screws or six-point fasteners with rounded corners.

Torque important

Torque wrenches should always be used for installation of high-strength fasteners or bolting for transportation. It is good practice to always use a torque wrench for final tightening because many problems with removing fasteners are simply the result of the fastener being tightened beyond its yield point to near the ultimate strength.

After the fastener has been in use for a while, the lubrication at the installation point goes away and there is a tendency for the metal to seize, so that removal torque typically becomes between 120% and 200% of the installation torque. If the fastener was installed beyond its yield strength to close to ultimate strength, it will likely break instead of turning.

For more information from Wright Tool Co., use the reader reply number.