Effects of lubrication when reusing fasteners
What happens to preload in the joint during reuse?
It is common in many applications to reuse the same nuts and bolts as long as they are in good condition after operation. What happens to the joint preload during reuse is, however, not always considered.
During tightening, metal faces between male and female threads, as well as between the bolt head/nut and substrate, grind against each other causing wear to occur. The result is higher and more scattered friction.
Friction scatter is inherent in every bolted joint upon each installation and can be quite large when installing without any lubrication. When fasteners are reused and tightened to a specific torque, more of the applied torque is required to overcome the friction and less is utilised to obtain the required preload. For each subsequent reuse the preload becomes lower and more inaccurate. This can cause issues when the designer has specified a preload range for the application to maintain sufficient clamp force to overcome the external forces on those joints. Finally, the preload may not be sufficient to withstand the working load applied to the joint, which subsequently fails.
By lubricating the fastener before every use, not only can friction be reduced to improve torque-to-load ratio, friction scatter can also be reduced to improve preload accuracy. The graph here illustrates the torque-to-load behaviour between a dry installation and a lubricated installation. While the preload scatter at a given torque is quite random, you can influence the variability of that scatter by this simple modification.
The prescriptions of the manufacturer regarding lubrication have priority and must be followed. It may be necessary to reduce the torque to avoid damage to the bolt or the clamped parts.
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