Liner Bolts

  • Eliminate liner bolts failures

  • Reduce Shutdown durations

  • Eliminate re-torquing

  • Special techniques not requiring bolt end machining

Liner bolts failures are extremely common. There are several reasons why Liner bolts fail (break or loosen off) and the vast majority of these can be eliminated if the pre-load and maintenance are correct.

Liner bolts failures can cost you production losses and be expensive to repair. We have developed systems, services and products that prevent these failures but also reduce the existing shutdown durations and eliminate re-torquing. Find out more

Case Study: Grinding Circuit Re-Lines

Overview

We have been providing complete bolting solutions for over 28 years to the Mining, Refining, Hydrocarbon, Power Generation, Marine, Motor Vehicle, Construction and Fairground Industries.

The foundation of our knowledge base has been built by measuring the actual bolt tension achieved by any tightening method. This has given rise to a knowledge base that has driven the solutions that we now provide, the two key factors are:

  • Ultrasonic Bolt Tension Measurement

  • Joint Analysis & Management

Discussion

The process of re-torquing after a re-line is very common. It has been a necessity to reduce the number of failures.

In order to find the solution we first had to fully understand all the contributing factors.

 

Root Cause of Failures

Pre-load too low

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

Consistency

Consistency is a bigger factor than most realize. The relationship between torque tightening and bolt tension in good conditions is generally regarded as +/- 25%. Add rough surfaces, interaction and short clamp length to diameter ratios and this becomes significantly higher. Our experience is 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

We have found that the relaxation on liners is particularly high. The mating surfaces between the lifters and liner bolts are rough being cast and forged. Added to this the tapered engagement multiplies any relaxation. 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.

liner-bolts-1

Liner bolts fail (break or loosen off) for several reasons but our extensive experience demonstrates that 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. The reasons for this can in turn be divided into 3 groups, inadequate original tension, inconsistent tension and high relaxation.

To eliminate this we measure and adjust every bolt to precisely the required bolt pre-load. Many companies fear that this will slow down the process but because our people do this constantly they are very quick and we reduce the time taken.

Our solution process

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

liner-bolts-2

Typical Results (this is from a real case)

These unadjusted 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.

liner-bolts-3

liner-bolts-4

Other points to consider

1. Tooling is often bought to cover a variety of applications, there is often a quicker and more accurate option if specifically engineered tools are used. We have a dedicated design team that provide lighter, safer and stronger tooling.

2. Bolt threads are often burred, damaged, corroded or have slurry ingress. This causes galling and changes the friction, effecting the residual tension.

3. Short grip to diameter ratios (less than 5:1) are often problematic (see explanation below)

Why short bolts fail?

Manufacturers of tooling make claims and counter claims concerning accuracy of any given method but the biggest contributor to failure is the joint design. One of these common designed failures is short bolts, or more accurately short clamp length to diameter ratios.

So why is this the case?

1st, 10° angle of turn is virtually impossible to see when tightening (one 36th of a turn) but that small amount can have a significant effect on bolt tension. Typically, a bolt 500mm clamp length will be stretched over 1.25mm, whereas as 150mm clamp length only requires 0.45mm. 10° equates to 0.125mm elongation which is a 30% error on the short bolt but only 10% on the longer version.

2nd, re-iterating the relaxation graphic shown above, loss of elongation due to relaxation is likely to be 0.4mm on any fishtail arrangement, or 32% on the longer bolt but 90% on the shorter version. Consequently shorter bolts often fatigue fail due to pre-load that is too low.

Conclusion

We have 28 year's experience with the technology and 9 years specifically working on Mill Liner bolts. We are 100% confident that we can both eliminate failures and remove the re-torque process.

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