Norbar Case Study
Reclaimer Slewing Ring Bearing Bolts
Using ultrasonic measurement technology to improve tensioning accuracy and bolt life.
Bulk Ore Processing Circuit
The client, a large mining company in Port Hedland, Western Australia runs a busy, highly automated out-loading circuit that processes iron ore ready for transportation. The circuit includes a bucket wheel reclaimer which reclaims bulk iron ore from stockpiles ready for conveying to the Lump re-screening plant further along the circuit.
The bucket reclaimer achieves a flow rate of up to 14,000 tons per hour, so keeping the equipment operational with regular servicing is highly critical.
The Reclaimer Slew Ring Bearings
During operation, the bucket reclaimer rotates about its vertical axis, allowing a horizontal boom to move to virtually any position to reach ore. This lateral movement, or ‘slewing’ is driven by the slewing ring. The ring comprises a slewing ring bearing designed to accommodate loads acting in any direction.
These bearings are assembled via an annular arrangement of bolts that require accurate tensioning in order to perform optimally.
Servicing the Slew Ring Bearings: The Original Method
Prior to Norbar Technical Services (NTS) involvement, the slew ring bearings were serviced by two personnel, one to operate a pump and the other to tighten the bolts. The bolts themselves were tightened using either a standard torque wrench or a hydraulic bolt tensioner. While both of these methods were effective to tighten the bolts to a functional level, there was no way to accurately measure bolt tension applied to know if it was in the required tension tolerance of 320 kN ± 5%.
This method was both inaccurate and laborious leading to only 10% of the bolts being checked to see if any had relaxed in service. Consequently, the bearings experienced premature wear rates, leading to very high replacement costs and significant production losses.
Additional to the efficiency losses, this method carried an increased risk of pinch points, had cramped working conditions and line-of-fire potential.
NTS carried out a full assessment of the task. They identified that the inaccurately tensioned or relaxed slewing ring bearing bolts lead to a reduced bearing life due to accelerated wear rates. This problem would be tackled using leading technology and personnel to provide the following benefits:
- Increased bearing life
- Prevention of in-service failures/unscheduled downtime
- Significantly shorten maintenance testing routines
- Improve safety by eliminating line of fire or pinch point injury potential
- Provide a reliable, accurate way to measure the bolt tension of 100% of the bolts. It is recognised in the bolting service industry that torque has an accuracy of at best ± 25%. In practice, NTS experts have found that this is optimistic as they frequently see variation as high ± 35%.
NTS’ goal was to check 100% of bolts, rather than only the 10% previously checked. This would achieve the correct pre-load needed for every bolt, ultimately decreasing premature bolt replacement.
A Comprehensive NTS Solution
With clear goals in mind, the NTS representative created a comprehensive solution that included the following expertise and equipment:
Specialist Ultrasonic Bolting Technicians to apply their extensive specialised skills and knowledge to complete the task efficiently.
The Boltstress G6 ultrasonic monitor to accurately measure the tension applied to every bolt. Ultrasonic monitoring would be carried out on all slewing ring bearing bolts and adjustments made to achieve the correct preload. Ultrasonic measurement significantly reduces service time, as it allows the technician can quickly identify exactly which bolts need adjustment.
The time benefits of this can quickly stack up, as proven in a similar job carried out by NTS on a crane in the North Sea Oilfields. The life of statutory replacements was extended from every 6 years to every 11.
Custom slimline Hydraulic Bolt Tensioners that would allow easy access to bolts that were located in confined or difficult to access places.
Upon successful completion of the job, the following outcomes and their benefits were made evident:
Accurate — Ultrasonic bolt stress monitoring was within 2%. With this accuracy, NTS has been able to extend the statutory replacement period by almost double.
Safe and ergonomic — Elimination of pinch points, laborious lifting in confined spaces and line of fire potential due to the use of custom Hydraulic Bolt Tensioners. This also meant that bolts in difficult to access places were given the same care and attention as other bolts without causing the service technician undue physical strain or discomfort.
Fast and efficient — NTS improved servicing speeds to a mere 15 seconds per bolt. Some of the time saved during initial installation were attributed to the experience of the technicians, however the biggest time saving is in the re-monitoring phase where ten times more work can be achieved in a quarter of the time. Ultrasonic measurement means the technician can zero-in on problematic bolts and then make intelligent corrective actions in a targeted way for maximum efficiency.