A lubrication solution from Royal Purple, distributed exclusively in South Africa by wear-control specialist Filter Focus, has been specified for the flagship China Star bullet train project.
The China Star project, which aims to develop China’s own bullet-train technology, was inaugurated in 2001 by the State Development Planning Commission and Ministry of Railways, in conjunction with the Zhuzhou Locomotive Company.
The bullet trains will be powered by 1 250 kW motors from Zhuzhou that use specially-insulated, resin-coated roller bearings supplied by NSK Bearing Company of Japan. It is essential that the bearings be insulated in order to prevent damage to the raceways and rolling elements caused by electrical arcing.
Each motor incorporates a 170 mm (NH219) grease-lubricated bearing and a 280 mm (NU326) thrust bearing, lubricated by the same oil as the drive gear. The lubricants for the traction motors were selected based on tests carried out at NSK’s dedicated traction test facility in Tokyo. A total of 26 different greases were evaluated, including the latest formulations for high-speed bearings.
The greases were subjected to high temperatures and speeds in order to detect any leaking, as well as to ascertain adhesion and grease life. Based on its superior performance during the testing, a general-purpose lithium complex grease thickener was selected as the standard traction motor bearing grease for the China Star project.
However, the greased motor bearing was subsequently found to overheat at speeds in excess of 160 kph, resulting in high vibrations and reduced bearing life. Despite various adjustments being made to the traction motor, the NSK team was unsuccessful in arriving at a mechanical solution.
The breakthrough came when the team focused on the grease itself, which provides a thin film of lubricant that the bearing depends on for its proper operation. It was suspected that the grease’s drop point was being exceeded, the temperature at which the thickener in the grease can no longer hold the lubricating oil in place. Thus the oil runs out of the bearing, leaving only the thickener behind. This resulted in extensive damage to the bearing.
The China Locomotive Research Institute began a global search for a grease that would allow the traction motor to run at higher speeds. It eventually settled on Royal Purple’s Ultra-Performance Grease No. 2, an aluminium complex EP grease. This is a high-performance synthetic grease with advanced additive chemistry to reduce bearing temperature and vibration.
Royal Purple was subsequently asked to recommend lubricants for the other components of the traction motor. It suggested its multi-purpose industrial oil with Synerlec® for the 280 mm bearing and drive gear, and its maximum-load synthetic lubricant for the gear coupling.
High-speed motor tests were conducted with the new lubricants. Not only was a new Chinese rail speed record of 321.5 kph set, but all of the bearings still looked like new after 80 000 km of high-speed testing.
“The China Star project achieved rapid success because it was determined at an early enough stage that the bearing problems were due to lubrication deficiencies. By focusing on a total lubrication solution from Royal Purple, as opposed to a costly and time-consuming mechanical redesign, it was able to quickly exceed all speed and reliability goals,” Filter Focus Chief Operating Officer Craig FitzGerald concludes.
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