Norbar Case Study
Stacker Reclaimer Slew Ring Bearing Bolts – Using ultrasonic measurement technology to improve tensioning accuracy
The current issues
Most sites will rely on torque tightening or hydraulic bolt tensioning (stretching) to control the bolt load of slew ring bearing bolts.
There are many factors to take into consideration when using the torque tightening method to manage bolt preload. A major drawback is that small changes in frictional conditions can lead to very significant shifts in bolt preload, as most of the torque in play is used to counter friction.
Typically, friction between the threads and faces absorb 90% of the torque applied to the nut, consequently only a small portion of torque will translate into useful bolt load.
Hydraulic Bolt Tensioners don’t suffer from frictional losses but the difference between the applied force and residual bolt tension is not consistent nor is it fully understood. In a nutshell the nut threads are free running when the tensioner force is applied but when the pump pressure is released the nut threads take up stress causing the threads to deflect. This factor is not well documented or proven in anyway, often the residual bolt tension is significantly less than the design pre-load.
If the preload force is not even throughout the bearing this will reduce the service life of the bearing, increase vibrations in the machine and cause wear to the surrounding parts.
Using ultrasonic measurements, we take a reading on each individual bolt and tighten to the exact required bolt load. After the initial tightening process, we then take another ultrasonic reading which will identify if there has been any interaction or bolt relaxation. At this point we can make any necessary adjustments to ensure all bolts remain at the desired bolt load. Our process removes the guess work and confirms the correct bolt load has been achieved.
- Ultrasonic measurements provide a reliable, accurate way to measure the bolt tension of 100% of the bolts.
- Quick and easy means of re-monitoring bolt tension. All readings are saved and bolts can be re-monitored every 6 or 12 months to ensure there are no defects or bolt relaxation occurring in service.
- We have an engineering team that can custom design slimline tensioners for tricky tight spaces.
- We can carry out testing for any current bolt failures and from the results provide recommendations.
- We have 32 years of dedicated ultrasonic bolting experience starting back in June 1991.
- Experience working with stackers, reclaimers and ship loaders.