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Condition Assessment of Drafting Roller CirclesROB ANGUS
Associate, WBM Pty Ltd
PETER RYAN FRY
Director, WBM Pty Ltd.
Dragline roller circles (slew bearings) and the associated structures in the tub and the revolving frame are components that cause many dragline owners significant downtime and high maintenance cost. Diagnostic tests, such as the Spall Detection and Roller Load Tests, provide a rational basis for action for controlling costs and downtime and planning repair schedules.
The WBM Spall Test determines and documents the current condition of the load bearing surface area of the roller circle. In contrast, the rate and cause of deterioration is determined by the load in the rollers, measured using a Roller Load Test. This paper looks at these different techniques for assessing dragline roller circles and provides a guide to when they are appropriate.
KeyWords: Dragline, Spall Test, Roller Load Test, Roller Circle, Slew Bearing.
Draglines are large mining machines used to remove overburden in open cut mines. Figure 1 presents a photograph of a dragline operating in a Central Queensland coal mine. Draglines typically have working weights of between 2,700 to 5,000 tonnes with typical bucket capacities of between 30 to 100 m3. The largest dragline that is currently operating in the world is in Australia and has a working weight of approximately 7,200 tonnes with a bucket capacity of 109 m3.
Draglines have a slew bearing, referred to as a roller circle, which allows the machine to rotate about the vertical axis. The roller circle is essentially a very large and expensive rolling element thrust bearing. The diameter on these bearings is typically around 15 metres and the hardware costs are of the order $900,000 to $1.2 million, excluding installation and down time costs.
|Fig 1: Photograph of dragline in open cut coal mine.|
Despite their size, the roller circles behave as precision bearings and small errors in the geometry can result in high loads. Operators need to manage the maintenance of the slew bearings to achieve the optimal life and reduce costs. This paper looks at two non-destructive tests developed by WBM to assist operators manage and improve the life of the slew bearings of draglines.
Other papers have looked specifically at the Roller Load Test (Morrison, 1982; Morrison and Beale, 1980; and McGriffin and Proud, 1988), however, this paper looks at both the Spall Test and the Roller Load Test and discusses when it is appropriate to use these different techniques.
As discussed above, the roller circle is a precision bearing and small errors in geometry can result in high loads. High loads in the roller circle often result in fatigue cracking of the structure supporting the roller circle. Often this is managed by regular weld repairing of the cracks, requiring high maintenance costs and machine down time.
|Fig 2: Photograph of roller circle derailment.|
The major mechanism of failure of the actual roller circle is surface fatigue or spalling resulting from the applied cyclic loads. During normal operation, sub-surface cracks form in the rails and, less often, in the rollers. A spall is formed when these cracks reach the surface of the rail or roller and a piece of the metal breaks loose. A dragline can operate for a long period with spalled rails, although there are some risks involved. If the inner edge of the rails has a large enough spall, the rollers can ride up onto the rails and cause a derailment of the bearing. Figure 2 presents a photograph showing a roller circle derailment incident that occurred in South Africa.
Other failure mechanisms of the roller circle include cracking of rails or rollers. Cracking of rollers has generally occurred due to overstressing as a consequence of the roller bore being oversized. Cracked rails are usually the result of high loads and/or poor support geometry.
WBM have been involved with draglines since the 1970's and have developed a number of techniques to assist operators with the management of dragline roller circles. The two main techniques discussed in this paper are the Spall Test and the Roller Load Test. Each of these tests provides valuable, but very different, information regarding the roller circle.
The Spall Test provides information about the current condition of the roller circle rails. It provides an accurate record of the remaining load bearing surface of the rails. The Spall Test does not directly provide predictive information about the rate of deterioration, although, by conducting the Spall test periodically, the rate of deterioration can be monitored. The Spall Test also does not provide any information regarding the cause of premature spalling.
The Roller Load Test provides information about the condition of the roller circle that will affect the life of the bearing and supporting structure. It identifies any problems that may result in reduced life and quantifies the maximum roller load, which is used to provide a prediction of the time until the onset of spalling using a load-life curve developed by WBM.
Each of these techniques is discussed in more detail below.
3.1 The Spall Test
As discussed above, draglines can be operated for a substantial period with spalling of the roller circle present, although there are some risks. By monitoring the condition of the rails it may be possible to operate a spalled roller circle for a considerable time before replacement is necessary.
Some operators use manual techniques to measure and monitor spalling. This usually involves crawling around the machine and probing for spalling on the rails using a metal rod. There are a number of problems with this approach, including:
The WBM Spall detection system provides an efficient, accurate condition assessment of the surface of the rails. The extent, depth and distribution of surface spalls are determined across the full width of the rails.
|Fig 3: WBM Spall Detection equipment installed in Dragline.|
Figure 3 presents a photograph showing the WBM Spall detection equipment installed on a dragline. The spall detector is fitted into the space created by removing a single roller from the circle, while the battery powered recording equipment is mounted on the roller circle side plates. The entire width of the rail is scanned simultaneously, using inductive proximity transducers that measure the distance to the steel. In this method the probes only detect where the material is actually missing from the surface, providing an accurate assessment of the load bearing capacity of the rails. Since the transducers are non-contacting and there is an air gap between the probes and the rail, it is unnecessary to remove rail lubricant or to apply coupling fluid.
Output from the probes is digitally processed and presented as colour plots for quick and easy interpretation of the spall detail. Figure 4 presents a photograph of the output from a spall test. Figure 5 shows photographs of segments of the spalled rail from front rail number 1 in Figure 4.
|Fig 4: Results OF WBM spall Test.||Fig 5: Photographs of spalled rails from Front Rail number 1 in Figure 4.|
The advantages of the WBM Spall test include the following:
Spall detection provides an accurate assessment of the surface condition of the roller circle rails, which allows the operator to manage the remaining life of the rails.
3.2 The Roller Load Test
WBM developed the Roller Load Test in the 1970's in response to significant maintenance problems experienced in the early draglines built in Australia. These problems included premature roller and rail failure, as well as cracking in the supporting diaphragms in the tub and revolving frame. WBM has carried out Roller Load Tests on almost all the large draglines in Australia and South Africa, and many draglines in the USA and Canada, a total of over 155 machines worldwide.
The main objectives of the Roller Load Test are to evaluate the quality of the roller circle construction, identify problems in the upper or lower rail geometry, measure the maximum roller load and estimate the life of the bearing to the onset of spalling. The test also detects other problems such as mistracking of the rollers or oversized rollers, which can also lead to increased loads and reduced roller circle life.
The roller load measurement system is shown schematically in Figure 6. A standardised test procedure is used to ensure the transducers are installed at the correct locations. Strain gauges are installed on the circumferential diaphragms (the plates supporting the roller circle) in the revolving frame (structure above the roller circle) and the tub (structure below the roller circle). These strain gauges are used to assess the geometry of the upper and lower rail paths and to choose the most highly loaded roller.
|Fig 6: Schematic diagram showing WBM Roller Load test set-up.|
The roller load transducer is a precision displacement measurement device that is installed in the bore of a test roller. The test roller is installed to replace the highest loaded roller and used to measure the roller load distribution and maximum roller load. The maximum roller load will control the fatigue life of the roller circle.
The life of the roller circle to the onset of spalling can be predicted based on a load-life curve developed by WBM. This curve was originally based on theory and has been fine-tuned based on the data collected by WBM over more than 20 years.
A range of problems can be detected using the Roller Load Test. The majority of these problems can be corrected to increase the expected life of the roller circle. The Roller Load Test provides a rational basis for deciding cost benefit of corrective action.
|Fig 7:Roller Load Test showing incorrect lead-in taper on rear rails.|
Figure 7 presents the results of a Roller Load Test that detected an incorrect lead-in taper on the upper rails of a dragline. (Many draglines do not have a complete raceway on the upper rails. A lead-in taper on the rails adjacent to the gap in the rails is required to reduce the loads). The following are examples of problems that can be detected using the Roller Load Test:
When the Roller Load Test reveals deviations like these, corrective action can be taken. Some of these problems have resulted in WBM developing other techniques to assist dragline operators manage their roller circles. For example, the in-situ rail grinding machine and the grouting technique for installing rails (Herringe and Fry, 1992).
As discussed above, the Roller Load Test and the Spall Test provide different information about the roller circle. The Spall Test provides information about the current state of the roller circle. Whereas the Roller Load Test provides information about the rate of deterioration (and causes of deterioration) of the roller circle. Consequently, the tests should be used at different stages in the life of the dragline/roller circle.
Spall Tests are most appropriate for machines that have accumulated significant service hours on the original or a replacement roller circle - circles in which the rail surfaces may have developed spalls. The test is used to "map" the spalling, so that the owner can see the location and extent of spalling. This test is not appropriate for new or recently replaced rails.
The following table shows the circumstances under which Roller Load and Spall Tests are appropriate.
|Situation||Roller Load Test||Spall Test|
|Appropriate Times for Roller Load and Spall Tests.|
Dragline roller circles and the associated structures in the tub and the revolving frame are components that cause many dragline owners significant downtime and high maintenance cost. Diagnostic tests, such as the Spall Detection and Roller Load Tests, provide a rational basis for action for controlling costs and downtime and planning repair schedules.
These tests provide valuable but different information to the operators about the roller circle. The Spall Test provides information about the current state of the roller circle. Whereas the roller load test provides information about the rate of deterioration (and causes of deterioration) of the roller circle. Consequently, the tests should be used at different stages in the life of the dragline/roller circle. This paper has discussed the differences between these techniques and when it is appropriate to use these techniques.
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