Norbar Articles

3 Common Causes of Bolt Failure and How To Avoid Them

July 20, 2021
Most mill operators are familiar with (and regularly go through) a process known as re-torque. Re-torque is an expensive exercise resulting in lost production, but is believed to be necessary in order to avoid bolt failures that cause unplanned shutdowns. It doesn’t have to be this way however, by eliminating the root cause of the failures we can eliminate the need for re-torque all together. Costly Bolt Failures Many Mills experience liner and liner bolt failures. These failures are costly and can affect the viability of some operations. TCS Boltstress Ultrasonics has eliminated these at several mine sites and mills. Liner bolts fail (break or loosen off) for a number of reasons but the vast majority of these can be eliminated if the pre-load is correct. Fatigue failure is the most common mode; this is almost invariably due to pre-load that is too low. This can in turn be divided into 3 groups: inadequate original tension, consistency and high relaxation.

Pre-load Too Low

The interfaces between the bolt, liner & shell are rarely in a smooth or machined condition, consequently the friction is greater than standard calculations consider. This results in low average bolt tension.

Bolt Tension Consistency

Consistency of Bolt Tension is a bigger factor than most realize. The relationship between torque tightening and bolt tension in good conditions is generally regarded as +/- 25%. Add in rough surfaces, interaction and short clamp length to diameter ratios and this becomes significantly worse. Our experience demonstrates that the variation becomes more like +25% – 100%; it is rare when we measure all bolts that we don’t find at least one that is completely loose.

High Relaxation

The relaxation on liners is particularly high, the mating surfaces between the lifters, liner bolts and liner plates are roughcast and tapered. Any small embedment of high spots will be multiplied by the slope angle. By way of an example, if 0.1mm embedment occurs on a 1:4 taper that will result in 0.4mm in lost elongation. That is a likely scenario when both sides of the slope are considered.

Our Solutions

To solve and eliminate the effects of these common problems, we use Ultrasonic bolt tension measurements to prove the achieved pre-load and then adjust the torque settings to a qualified and considered level. This allows us to reduce shutdown durations, eliminate failures thereby avoiding un-planned down time. The process we follow is:

Preparation – Prior To Re-Line Start

  • Calculate allowable tension for the Bolts
  • Determine allowable stresses for mating parts
  • Set up ultrasonic data files
  • Measure bolt reference lengths

During Re-Line

  • Allow re-line crew to tighten 1st set/row of bolts
  • Measure residual tension
  • Calculate average tension, review and adjust tooling pressure settings
  • Re-test on subsequent rows

Typical Results (this is from a real case)

The results below show the normal variation that torque tightening generates. The average is well below the required pre-load. The variation is + 5% / -75%, which is consistent with other work we have completed.
The graph below shows the final result after adjustment of each bolt. The outcome was total elimination of all bolt failures.
Prior to this adjustment, the bolt failure rate was one per week, each time causing an un-planned breakdown. Conclusion Ultrasonic Bolt measurement and adjustments are just one of the many services we offer to mills and industry to prevent unplanned breakdowns and costly repairs. If you’re interested in learning more about our 25 years of bolt ultrasonics experience and how our torque services can help to improve your profitability, reduce re-line durations and eliminate unplanned shutdowns, contact us today.  One of our consultants will be able to help you with one of our unparalleled solutions.