It’s a question that is often misunderstood. For many the obvious answer “would be to enable them to be removed at a later date”. Although a definite benefit, the real reason is to ensure consistent tightening of the fasteners.
When asked “what tool should be used to properly tighten a fastener” most would respond “a torque wrench”. While the torque wrench is one of the most practical ways to control fastener tension, its important to understand that it does not measure tension… it measures torque or twisting force, and that can lead to problems when trying to preload fasteners if we are not careful.
A threaded fastener works by applying clamping force to the components that are being fastened. This is accomplished by turning the screw thread. The action of the screw thread, preloads and stretches the bolt. The amount of preload is critical…too much and the bolt may exceed its elastic limit (stretch), too little and the bolt maybe subject to alternating forces and fatigue (very common). Yes that’s right… most bolts fail due to fatigue caused by under tightening!. If want to be very precise when tightening a faster then we must measure the preload directly, and there are ways of doing that. However they often complicated and expensive and are not the most practical. That leaves us with the torque wrench.
Here’s the problem… When turning the bolt some of the twisting force goes into overcoming friction of the threads and importantly the nut and washer interface. Even if the applied torque is meticulously controlled there are many, many, variables at work that determine how much friction is present. These include fastener, materials, class of thread, type of thread, thread condition and many others. The end result is that despite our best efforts with the torque wrench the clamping force applied is always inconsistent. Of course if a torque wrench is not used even more inconsistency occurs. A hands-on exercise from our popular Essential Craft Skills series illustrates this.
Each of the 15 students is asked to properly tighten a 3/8 x 16 grade 5 fastener using a standard combination wrench. Once they have tightened the bolt the clamping force applied is then measured and recorded. The results are always pretty amazing.
This particular fastener is designed to apply 4940lbs of clamping force. Classroom results usually vary from 6500lbs to 1500lbs!
That’s a variation of close to 100% of the desired clamping force! Interestingly the vast majority of attempts result in under tightening the bolt by a significant amount.
When using a (calibrated) torque wrench to perform the same task the results improve a little and the scatter is reduced to 30-40% of the desired clamping but the numbers still vary from 6000lbs to 4200lbs.
By lubricating the fastener the results are further improved and scatter is reduced to 10%-15% which is about as good as can be achieved using torque control methods.
So to summarize
- When tightening a fastener ALWAYS use a controlled method.
- Unless specifically told not to, always lubricate the fastener.
- Lubricate the threads, and head/washer to get consistent results
- Use the lubricated torque value
For more details on calculating slip and how to prevent it please contact Reliability Solutions