The Importance of Torque Control
Although many methods exist to join two or more parts together, the ease of assembly and disassembly provided by threaded fasteners make them the ideal choice for many applications. And while it is the simplest and most widely utilized tensioning method, simple torque control is not a perfect tensioning method. There are a number of prominent outside factors that can affect the relationship between torque and tension such as:
► Friction
► Operator Input
► Application Geometry
► Tool Accuracy
► Joint Relaxation
It is from these factors that torque control can result in preload variance of up to 30% in repeated applications. Therefore, steps need to be taken to mitigate these effects and ensure the torque applied still satisfies the preload specifications, the most prominent of these factors being friction between the fastener and mating components. It has been determined through testing that typically only about 10% of torque applied results in tension in the bolt while the rest is used overcoming friction in the head and threads of the bolt. This means that variables such as surface condition and lubrication can have a profound effect on the relationship between torque and tension and should be strictly monitored and controlled. Because of torque’s small contribution to the final preload, it is even more critical that its calculation and application are done correctly to ensure that the bolt is tensioned correctly. Over-torqueing can cause a bolt to stretch past its limit and break, while under-torqueing can cause the fastener to become loose.
The Importance of the Operator
Because of torque’s small contribution to the final preload, it is even more critical that its calculation and application are performed correctly. Fortunately, simple efforts can be made to ensure that the bolt is tensioned correctly. Firstly, there is an often understated human element that has a profound impact on the torqueing process. Ergonomic tools that allow the operator to smoothly and comfortably apply torque will provide more accurate results. Similarly, a tool with a high resolution output gage will give greater confidence that the torque specifications have been met. The operator’s knowledge and comfortability in the process being performed can be greatly improved with the use of torque tools designed with these factors in mind. Failure to implement accurate tools and adequate operator training programs may lead to the incorrect application of preload and failure in the joint.
Installation Accuracy
We have established that many factors contribute to the torque process. From tool accuracy to environmental conditions, there is potential for your joint to be fastened to a much different force than might have originally thought. All of these factors make it more important to have confidence that the tool being used is actually applying the desired output torque. In the case of torque multipliers, the ability to measure torque directly at the output instead of calculating based on the input further reduces the associated variance in final torque. Here is an example of this concept in practice:
U.S. Patent Nos. 8,434,389 & 8,991,284
European Patent No. 2396147
Canadian Patent No. 2,752,088
Japanese Patent No. 5699338
U.S. Patent Nos. 8,434,389 and 8,991,284
European Patent No. 2396147
Canadian Patent No. 2,752,088
Japanese Patent No. 5699338
