When a major open cut coal mining operation undertook an initiative to improve the service life of its walking draglines, their main objective was to meet prevailing operating conditions and improve productivity. While the mechanical and operational improvements delivered some positive results, the overall success of the programme was still limited by some critical application bearings. CBC was engaged to evaluate potential to improve bearing reliability and operating life for these identified applications.
CBC recommended the use of the NTN “EA” long life bearings. Following technical appraisal, these bearings were put on a controlled comparison trial with the objective of doubling the interval between replacements – matching related durability improvements and delivering substantial cost savings.
According to Ross Lee, Technical Manager, Strategic Partnerships – Bearings at Motion Asia Pacific, the bearings had not been used in a dragline machinery operation prior to this. Historically, they had proven service performance in rolling mills, continuous casters, pellet mills, exciter gearboxes, and dump truck wheel and final drive applications.
He explains how the trial came to be. “This customer operates a large fleet of draglines for overburden stripping. In their endeavours to improve productivity, as well as operating costs and efficiencies, the mines operator set about looking to extending the time between major shutdown maintenance, involving a number of reliability enhancements. After statistical analysis of key components on the machines, they also pinpointed specific bearings where premature failure was a key performance inhibitor. So, they approached CBC to seeking a solution that would not require costly changes to mating gearing and mountings,” says Ross.
“We knew that NTN’s EA bearings had extensively demonstrated good results in demanding industrial applications, here and overseas, so we proposed this NTN long life material,” he adds.
NTN EA long life bearing data and capabilities were then also independently evaluated by customer appointed consulting engineering firm before the agreed trials were conducted on two different draglines.
Ross says the EA bearings were suited to this application because of their greater fatigue cracking resistance, wear strength and peeling resistance characteristics, all of which translate to a longer operating life.
“The NTN EA material composition starts with a specialty case hardening steel. The two-stage heat treatment process is comprised of conventional case carburising, plus compound carbon nitriding. Carbon and nitrogen are diffused in the surface layer, stabilising the requisite amount residual austenite to toughen the surface layer,” Ross expands. “This provides increased fatigue crack resistance caused by surface-initiated damage. Moreover, the nitrogen enhances the resistance to temper softening.”
Another reason the NTN EA bearing was put forward, Ross explains, was because of its resistantance to distress in the most demanding applications.
“Where exposed to hard debris contamination, EA long life material exhibits higher retained hardness and thermal stability and increased residual compressive stress to compensate for tensile stress concentrations forming around surface indentations, what inhibits fatigue crack propagation. This in turn extends the rolling contact fatigue life of the bearing, under conditions of high dynamic loading, diminished and/or contaminated lubrication, and high fitting stresses.”
Further, Ross says the supplied bearings were also identical in boundary dimensions and internal design type to the original bearings in the equipment, so the customer did not have to make any modifications to their mating machinery components, thereby avoiding the cost of additional parts, modifications, and perpetuated spares inventory.
“Of course, the EA bearing material will still wear, it will still fatigue, but it will dramatically slow the rate of this damage occurring,” Ross says.
The trial was lengthy and other variables were involved, which meant the two draglines needed to be shut down for unrelated critical repair work to be done after 6- and 9-years’ service respectively. These shutdowns presented the customer with an opportunity to disassemble machinery and inspect the trial bearings.
Ross says post removal, inspections were carried out by the customer and CBC which demonstrated the EA bearings were all in very good condition with no discernible damage and fit for continued operation. Inspections of the conventional comparison bearings showed cases of advanced fatigue spalling and ring cracking damage.
“The NTN EA bearings were not re-fitted for continued service due to extended shutdown work scope implications. However, based on historical bearing life data, the clear indicator is that the period between bearing replacement can be doubled. This corresponds to the calculated Life Adjustment Factor of 2.3 for the EA material,” says Ross.
Likewise, the total cost savings were not fully determined despite the customer realising significant benefits since switching to the EA bearings.
“The customer now formally recognises the benefits of the bearings based on trial results, and specifies their use where draglines are overhauled for intended long term service intervals.”
Ultimately, word got out to the dragline manufacturer, and now one OEM has also nominated these bearings into machine parts. Another Australian mining company has also achieved similar life improvements applying EA bearings in the same applications, on a different manufacturer dragline.
“Because these types of trials take such a long time to materialise, it’s difficult to get a cost saving report on them. But I will say this: if your objective is avoiding bearing failures and therefore unplanned stoppages in a critical machine valued at around $100 million, and if you are extending the interval between complete overhauls of these machines by two times, then the cost implications are pretty massive,” Ross concludes.
Performance features of EA bearings
The below is based on operating life and strength comparisons of EA bearings compared to standard carburised bearings:
• Operating life using lubricant mixed with foreign contaminant is more than 5 times
• Operating life using clean lubricant is more than 2 times
• Peeling strength is 3 times (rate of incidence ⅓)
• Wear strength is 2 times (wear rate is ½)
• Fret strength is 1.3 times (wear rate is about 80%)
• Operating life when fitting stress is high equals 3 times
Reference: NTN corporation brochure EA bearing: https://www.ntn-snr.com/sites/default/files/2017-03/en_large_size_long_operating_life_bearing.pdf