Norbar Case Study

Vibrating screen exciter motor hold down bolts / Elimination of exciter motor bolting failure

The root causes of bolt failure:

Exciter motor hold down bolts fail 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 failure; this is almost invariably due to pre-load that is too low.

The reasons for this can be divided into three groups: inadequate original tension, inconsistent tension and high relaxation. When the clamping force is too low to overcome the vibration, the exciter launches off and the remaining bolts yield. Often with significant bending/necking.

From our experience we have seen at some sites the corner bolts tend to have a higher failure rate as these corner fasteners have a higher stress concentration. Another important factor is the mating surface between the mech beam and exciter which must be in good condition.

exciter hold down bolt failure
Exciter Bolt Tensioning

Our Solution:

The key to our method is ultrasonically measuring each individual bolt and then adjusting it to the precise required bolt load. A typical procedure is below: • Record an ultrasonic baseline (loose) reference reading for each bolt. • Follow the OEM tightening procedure with the tightening sequence, bolt lubrication and bolt torque. As each bolt is tightened our transducer will be on the bolt end taking a ‘live’ reading to see what elongation the current procedure is achieving. • Allow the bolts to sit (relax) for the stipulated timeframe from OEM. Take another ultrasonic measurement to record the bolt relaxation that is occurring. • Further tighten the bolts as per OEM procedure and record a final elongation measurement to determine what the actual achieved bolt load is. Provide a report and recommendations from the results.

Ultrasonic Bolting

The Technology:

The Boltstress bolt tension monitor uses time-of- flight (TOF) to precisely determine bolt tension.

An ultrasonic signal is sent down one end of the fastener and the time it takes to return to the transducer is measured. This measurement is recorded as a reference measurement (unloaded fastener length).

As the bolt is tightened the velocity of sound reduces and fastener will elongate. The change in properties of the bolt before and after preload paired with the bolt geometry, is used to accurately determine the amount of tension within the installed bolt.

Ultrasonic Bolting

The Advantages:


Ultrasonic testing has been established as the only accurate method to measure bolt tension in the field.

Over torquing can push a fastener to yield. Under torquing can cause the bolt to fatigue and the joint to fail. Joint failure for either of these two reasons can be dangerous and costly. It is critical to know the actual achieved tension of a fastener.


All ultrasonic readings are saved which enables us to carry out future re-monitoring to be able to determine if there is any bolt relaxation occurring in service or any bolt defects occurring.


We have 32 years of experience with the technology, we come to your site to carry out testing for any bolting failures you are experiencing. From the measurements and results we can make recommendations and provide a report. We have successful solved many bolting failures in applications range from stackers and reclaimers, grinding mills both liner bolts and structural bolts, heat exchangers, power generation turbines, ore car rotary dumper ring rail and rack bolts, wind towers, vibrating screens, ship loaders and berths, coal rotary breakers, crushers, large bolted storage tanks, F1 racing engine conrod bolts and more.
ultrasonic bolt tensioning

The Outcome: The implemented solution provides the following results:

Reduced Downtime

Less Bolt Failure

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Ultrasonic Accuracy

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Experienced Personnel

Learn more about our torque tools and other products:

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Tensioning Equipment