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.
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 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.