Why do bolt failures occur and what can you do to prevent or address it?
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 failure that causes 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.
3 Common Causes of Bolt Failure Fatigue and Damage
Many Mills experience liner and liner bolt failure. These failures are costly and can affect the viability of some operations.
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. Bolt failure fatigue 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 and will soon result in a bolt failure.
Bolt Tension Consistency
The 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 and Bolt Failure
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.
Failure of Bolted Joints
Why do bolted joints fail?
There are several reasons why bolted joints fail. Let's take a look at some of the common reasons behind such failure:
Restricted contact between metal in most thread fasteners.
It is quite concerning that the contact existing between one metal to another is only about 20 percent. On the other hand, the remaining 80 percent is only air. This is detrimental to the thread tips due to extreme loads placed on them. As or well-machined and more expensive fasteners, the metal to metal contact that occurs is only up to 50 percent.
Premature failure is the outcome of a poorly designed fastener, faulty selection of a fastener, and heavy loads and significant impact.
Vibration is another culprit to fasteners becoming looser over time. With micromovements that result in vibration, this certainly does not do any good to the fasteners. When this arises, the fastener will easily unravel due to the friction existing from the spring washers.
It begins with an air gap in the threaded fastener. When left unaddressed, this can result in corrosion. Afterward, a seizure can occur, progressing to greater issues such as disassembly of the fasteners. In this case, it is beneficial to make use of anaerobic threadlockers that stop corrosion, as well as eliminate loosening of the fasteners. Sealants can also prevent mud, sand, and other debris from getting into and wreaking havoc to the threads.
How to Fix Bolt Failure Modes
Bolt failure is indeed a cause of alarm in the milling and mining industry because of the inconveniences and unnecessary expenses it brings.
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 unplanned downtime.
The process we follow is:
Preparation - Prior to Re-Lining
Here are the steps we perform before the re-lining process:
Calculate allowable tension for the bolts.
Determine allowable stresses for mating parts.
Set up ultrasonic data files.
Measure bolt reference lengths.
In the re-lining process, there are important techniques followed including:
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.
Bolt Failure Detection to Prevent Serious Damages
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 the total elimination of all bolt failures.
Prior to this adjustment, the bolt failure rate was one per week, each time causing an unplanned breakdown.
Boltstress Makes Bolt Failure a Thing of the Past
Ultrasonic Bolt measurement and adjustments are just one of the many services we offer to mill operators to prevent bolt failure unplanned breakdowns and costly repairs. If you're interested in learning more about our 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.