How to measure - bolt fastening and Force clamp

One of the original and most commonly used method of attaching parts together is by means of a threaded fastener, however the threaded joint and its fastening is a complicated matter. Factories, production lines, manufacturing plants all use a wide assortment of fastener and assembly systems. From the small and meticulous word of the watches assembly industry requiring accuracy and low torque, to heavy duty equipment requiring high performance, all utilize an array of fastener solutions that need to provide crucial operational performance. Fastening threaded joints correctly, the first time, is a challenge. A common problem that many industries are facing is caused by fastener loosening due to inadequate (inaccurately or incorrectly) tightening and which can result on a complete product failure.

At ANDILOG Technologies, we understand that clamp load and torque measurements can be critical, so we want to bring you a large range of solutions measurement devices and indicators, and also share with you our knowledge of what tightening means.

What is tightening?

When you tighten with a torque wrench, you are in effect measuring the amount of friction being generated by the mating surfaces in the joint and as the clampload is increased so is the torque. You can see that there is some relationship between torque and clampload. This relationship is determined by the amount of friction in the joint as well as the clampload being developed. The problem is that you cannot measure or predict what the absolute value of the friction is in the joint. We can however, minimize the effect of friction by using lubricants (among grease, graphite, teflon, plastics).

Why measuring torque can help?

The most popular and oldest assembly tools and way of ensuring that an assembled bolt complies with an assembly specification is torque. It is easy to implement, measure and control. The major problem related to this method is that the fastener clampload generated as the result of an applied torque is dependent upon fastener design and the prevailing frictional conditions. Despite these problems, it is still the most accepted way of auditing a tightening assembly.

Measuring during R&D stage

Most Engineers already know the importance of a high preload in maintaining joint integrity. For torque controlled tightening, achieving the right preload is dependent upon the correct tightening torque being specified.

Without the proper analytical tools and information, specifying the correct torque can be problematical. Whether you are a Service or a Design Engineer, you frequently need to know what the correct tightening torque is. For many types of threaded fasteners, this information is either not available, or not readily available. Here are two examples of measurements that you do:

Measure the Torque and Tension: Determination of the torque-tension relationship for a threaded fastener allowing the appropriate tightening torque to be determined. Such tests will allow the nut factor (sometimes referred to as the torque coefficient or k factor) to be determined and the overall coefficient of friction. By completing several similar tests, the variation in the torque-tension relationship, due to friction variation, can be established for an application. 
Discover how to measure the fasteners loading force with our Through hole load cell

Torque-Angle and Torque to Yield Tests: Tests can be performed on actual assemblies to obtain torque-angle graphs that can be used to establish the torque needed to reach the yield strength of the bolt. Such graphs can be used to establish the appropriate torque-angle specification. Torque-angle test information can also be used to assist in assessing the structural integrity of an assembly. Discover our drive rotary Torque and Angle Gauge

Measuring during production assembly

There are two approaches used to audit installation torque. The first is dynamically, which, by the use of in-line transducers attached to the tightening tool; the installation torque is measured directly. The second approach is by an operator or inspector measuring the torque after the installation has been completed.

Dynamic Torque or “in process” auditing: The dynamic method gives results which are independent of operator reading accuracy and will tell engineers how well the tools are performing on the line. This is the torque actually applied to the fastener during run-down. The powered torque tools are link to rotary drive torque sensor. Because this method allows the automatic storage and retrieval of tightening data it can be an important tool in statistical process control. Learn more on our dynamic torque gauge

Torque Auditing after Assembly: The residual torque should be checked as soon as after tightening operation as possible, in a slow manner in order that dynamic effects on the torque gauge are minimized. The torque readings are dependent upon the coefficients of friction present under the nut face and in the threads. (Note: The torque values can vary by as much as 20% if the bolts are left standing for two days). Discover our static torque gauge.

Are those two method equivalent?

Fastening experts warn that residual torque and dynamic torque are not identical, and of course tightening and loosening torque are going to be different (tightening involves dynamic resistance and loosening involves static resistance – the difference being keeping something moving vs. getting something to move). Residual torque is often lower than dynamic torque. In many cases, the joint relaxes after being fastened.

You want more?

You are using fastening tools on your assembly line, their performance should still need to be checked periodically and you may want to conduct periodic spot checks to confirm the accuracy and quality of your daily tools: torque analyzer.

Testing a tool before using it on the line will tell engineers if the tool can do the job if all other variables are in control.

The tests will show the tool's accuracy, or its ability to hit a target torque.
The tests will also reveal the tool's repeatability.

Learn more on our Torque analyzer range