First published in Bolted #1 2017.
Q: How does lubrication affect hydraulic tensioning and multi-jackbolt tensioning?
A: With the traditional method of tightening a nut, using a spanner, lubrication is very important, as there are a lot of surfaces moving against each other – the threads of the nut against the threads of the stud and the underside of the nut against the surface of the joint. Overcoming these frictional forces accounts for approximately 90 percent of the work (energy input) applied to generating the load in the joint.
When a hydraulic tensioning tool applies a clamping load to a joint, lubrication has no effect as it is applied directly to the stud and joint. A tension force is a linear force applied in an axial direction, so there is no rotation required to generate the load. This allows the nut to be turned down against the joint face under minimal friction.
As there is no friction to consider, there is no need to reduce the coefficient of friction using lubrication. Also, the lack of friction in the application permits much more accurate and repeatable results.
With multi-jackbolt tensioners (MJTs), the use of a lubricant on the main bolt thread does not affect the preload. It is advisable to use a very light film of lubricant with anti-seize characteristics to facilitate tensioner removal.
A more tangible effect of lubrication for MJTs is from the required lubricant use on the individual jackbolt threads, jackbolt bottoms of the tensioner, and washer face.
Proper use of lubrication is crucial to safeguarding repeatable and precise preload control in Superbolt installations. Superbolt mainly uses a graphite-based lubricant with a low friction coefficient and steady performance to achieve a positive impact on the preload. The MJTs are delivered with lubricant pre-applied to the installed jackbolts. Additional lubricant is included for application to the jackbolt bottoms. For subsequent installations, reapplication of lubricant is required to provide the intended performance.