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Lubrication systems used in gear trains are intended to serve two distinct purposes: (i) provide the quantities of oil to gear mesh contact interfaces to allow formation of a healthy elastohydrodynamic fluid film and (ii) help remove heat generated at gear mesh contact interfaces. Failure of the lubrication system in either of these tasks often results in a temperature induced contact failure, called scuffing. This study investigates the effectiveness of various lubrication methods in preventing scuffing. A new high-speed gear test set-up is developed specifically for investigating the scuffing performance of high-speed, high-load helical gears operating at realistic oil temperature conditions. The objective of this study is to experimentally characterize the scuffing performance of the helical gears as a function of various lubrication methods and parameters defining each method. Sets of scuffing experiments are performed using the test methodology developed and lubrication methods that successfully prevented scuffing of the gears are identified. The test matrix includes two different automotive drivetrain lubricants and different lubrication methods of forced (jet) lubrication, dip lubrication and mist lubrication. The test specimens consist of gears having three surface finishes, (i) ground-honed gears which were identified as the baseline, (ii) super-finished gears and (iii) phosphate coated gears. The effects of parameters such as jet flow rate, jet velocity and impingement depth on scuffing are investigated and tabulated using the results from the jet lubricated tests. The impact of gear micro-geometry and edge-loading effects on scuffing initiation are also investigated.
In this study, a test methodology was developed to induce debris to the gear tooth profiles during the operation of a gear pair. This methodology was applied to a number of spur gear specimens for different types, quantities and sizes of debris particles. The extent of surface damage due to application of debris was documented and related to the sizes, types and quantities of debris applied. A high-speed and high-temperature test machine was used to put gears with varying severity of debris damage through a staged scuffing test to investigate the influence of such damage on scuffing outcome. While selected damage sites monitored during staged scuffing tests did not exhibit any progression to be identified as initiation sites for scuffing failure, gears with no or little debris damage were shown to pass the scuffing test while gears with heavier debris damage scuffed consistently. As such, the results of this study show conclusively that there is a direct correlation between severity of the debris damage and resultant scuffing performance of the gears.
Abstract: In this study, a number of spur gear tests were performed under high-power and high-temperature conditions representative of certain aerospace gearing applications. As the first type of tests, long cycle tests of 100 million cycles were performed at set operating speed, load, and temperature conditions. The second type of tests, load-staged scuffing tests, implemented an incrementally increased torque schedule under constant speed and oil temperature conditions. Two different gear tooth surfaces were considered in these tests: hard ground surfaces representative of rough, as machined gear surfaces and chemically polished gear surfaces that were an order of magnitude smoother than the ground surfaces. The primary failure mode of concern was scuffing of the contact surfaces due to temperature build up. The impact of surface roughness amplitudes, contact stress, and oil inlet temperature on scuffing failures were investigated. Effects of ramp up procedures for the speed and torque, as well as the introduction of a break-in test stage were also investigated to show that they are critical to the scuffing performance of gears.
Gear teeth experience contact conditions that vary continuously as they pass through the meshing zone. Thus, not only the sizes but also the positions of surface defects become critical to their scuffing survivability. High-speed gearbox cost and reliability can be improved by quantifying these features and determining their impact on scuffing performance. Pursuant to this, representative defects in the form of scratches are applied in two batches to the contacting surfaces of high-quality spur gear specimens. These, along with an undamaged baseline gear pair, are then tested through a staged scuffing matrix incrementally increasing the operating load, speed, and lubricant temperature. Metrological procedures developed to quantify scratch parameters and track surface damage are used initially and throughout testing to document evolvement of the surfaces. It is concluded that (i) larger scratches generally decrease scuffing performance, (ii) the location of scratches is critical to scuffing performance; scuffing was never observed in areas where sliding velocities were low, (iii) increased wear and heat generation are observed on defects in high-sliding regions, and (iv) wear and tribo-film formation improve the scuffing performance of scratched gears. In addition, thermal elastohydrodynamic lubrication simulations are performed to confirm that increasing scratch width and surface sliding velocities have the most influence on increasing the lubricant flash temperatures.