Don't underestimate the complexities of bolting - Pretension your bolts


Incorrectly pretensioned bolts cause leaks, allow relative movement between components, and often result in joint failure when the bolts in question come loose or fail by fatigue or other mechanisms.

Together with good joint design, correct installation and pretensioning of threaded fasteners is fundamental to prevent failure of the fastener by fatigue and shear mechanisms.  

Correct pretension i) prevents joint separation, ii) is necessary to ensure full applied load is not carried by the bolt, iii) ensures shear loading is carried by friction between members rather than shear in the bolts (in cases where shear resistance is not the design intent), and iv) assists in preventing the bolt becoming loose in vibrating load applications.  

Appropriate design should ensure that the maximum applied load does not cause member separation.  Pretension is typically chosen to be between 70 and 90% of the bolt proof load.  Excessive pretension can cause reduction of fatigue life (important in critical design applications) and can result in plastic deformation of the bolt when the load is applied.  

Ensuring correct pretension requires appropriate design, as well as careful selection and application of the chosen pre-tensioning technique, which could include measurement of change in length of the bolt/stud, controlled extension methods (turn of nut) and torque controlled methods (need to be well calibrated for condition/lubrication).  

Without careful control, design pretension is often not achieved - causing failure. Ensure your bolts are pretensioned correctly!

Published in Technical Tips by Origen Engineering Solutions on 1 September 2014