How Ultrasonic Testing of Liner Bolting in Grinding Mills Eliminate Retorque and Prevent Unplanned Shutdowns

Ultrasonic bolt tension testing is a highly accurate method of determining what bolt tension tightening methods are really achieving. Torque figures are at best an estimate and don’t provide any data. Ultrasonic Bolt Tension Testing also detects any defects in used or new bolts.

Grinding mills, for instance, are massive capital-cost equipment that require proper maintenance to ensure their performance and operation. However, once they halt performing for whatever reason, this leads to significant loss in revenue. The length of time required to get it back on operation can take several hours, days, or even weeks, depending on the severity of damage.

Additionally, grinding mills have complex components that can be costly to troubleshoot and replace. This is why it is pertinent to improve the condition of crucial parts such as liners, bolts, nuts, and bearings to extend the lifespan and minimize the need for frequent repairs and replacement.

Ultrasonic testing of liner bolting in grinding mills help to eliminate retorque and unplanned shutdowns. Keep reading to learn more about these processes and why these are essential in your industry.

Eliminating Retorque and Unplanned Downtimes with Ultrasonic Testing and Liner Bolting

Retorque refers to the costly process that leads to lost production. Yet, it is an essential method in reducing the likelihood of bolt failures that result in unplanned shutdowns.

It is common for mills to undergo liner bolt and liner failures. When these happen, it can lead to steep costs while impacting operation.

However, there is a way to eliminate the need for retorque, which is through ultrasonic testing.

Liner bolts have a tendency to loosen off or break over time because of several reasons. Nevertheless, these issues can be eliminated by using the correct preload. By doing so, it reduces the chances of fatigue failure caused by pre-load that is too low.

A low preload is largely due to greater friction when the interfaces between the shell, nyloc nut, liner and bolt are forged and cast consequently giving rise to bolt tension that is too low . This is why the bolt tension becomes lower than torque figures suggest.

Additionally, the bolt tension consistency should be maintained at a desirable value. When other conditions are in place such as short clamp length, interaction, and rough surfaces, this aggravates the issue at hand.

With all these things in mind, ultrasonic testing helps in solving and eliminating the results of the conditions mentioned above. An ultrasonic bolt tension measurement proves the obtained preload, followed by an adjustment of the bolt tension to a desirable level.

By doing so, shutdown durations are reduced and failures are eliminated. Consequently, this prevents unplanned shutdowns and keeps normal operations going.

There are several processes involved in this technique such as:

  • Calculating the allowable tension for bolts

  • Identifying the allowable stress for the mating parts

  • Setting up ultrasonic data files

  • Measuring bolt reference lengths

These are steps required before initiating the re-line process. During re-line, these are the methods involved:

  • Allowing the re-line crew to tighten the first row of bolts

  • Measuring residual tension

  • Calculating average tension, followed by a review and adjustment of the tooling pressure settings

  • Lastly, re-testing on subsequent rows will be done


Ultrasonic testing of liner bolting in grinding mills are necessary for eliminating retorque and unplanned shutdowns. Moreover, these are proactive techniques that will spare you from having lost production and to pay for costly repairs, which can significantly impact your bottom line.

For more information on ultrasonic testing and liner bolting in grinding mills, contact Boltstress today.

Avoid using the wrong socket. Download our free Bolt to Nut comparison chart.