Micro and Macro pitting background
Micropitting and macropitting are rolling contact fatigue failure mechanisms which occur in cyclically loaded machine components such as gears, cams and rolling element bearings.
Micropitting is a fatigue phenomenon resulting in microscopic pits forming on the surfaces in contact. These are produced by asperity scale plastic flow caused by repeated cyclic contact stresses. The pits are typically less than 100 microns wide.
Macropitting is a surface fatigue phenomenon which causes large pits to form on the surfaces in contact. These result from surface or subsurface initiated cracks propagating into large scale pits. Macropitting tends to occur in more heavily loaded contacts than micropitting.
The PCS Micropitting rig (MPR) and test specimens have been designed to reproduce the lubricated contact conditions which can lead to either micro or macropitting.
The MPR is a computer controlled three-contact disc machine in which there are three ‘counterface’ rings of equal diameter positioned apart with a smaller diameter roller located in the middle and in contact with all the rings. This arrangement allows the test roller to be subjected to a large number of rolling contact cycles in a short period of time and hence significantly reduces testing time. At a typical entrainment speed of 3.5m/s, the central test roller will experience approximately one million contact cycles per hour.
The rig has an on-board processor, which allows the speed, slide-roll ratio, temperature, and load to be automatically controlled. Two servo-controlled motors are used to control the speeds of the rings and the roller separately, therefore allowing any combination of slide-roll ratio and entrainment speed to be set. Since the rig is computer controlled, it is possible to perform both simple and complicated test steps under precisely controlled conditions, allowing the effect of lubricant compositions on micropitting, macropitting or failure load testing to be studied.
MPR Specimens in situ − 3 counterface rings and a central roller
A dip lubrication system is used to supply lubricant into the contacts. The test requires a relatively small volume (150 ml) of oil into which the two lower rings are dipped. An electric cartridge heater is used to adjust the temperature of the test chamber, which in turn heats up the test lubricant. A temperature probe is inserted into the test chamber with the tip of the probe close to the contact region.
MPR rig with lubricant prior to testing
The load is applied by means of a motorised ball-screw, acting through a loading arm. Strain gauges are attached on the loading arm to measure the applied load. The rig is equipped with a piezoelectric accelerometer which is used to measure the vibration in the contact. Once a macropit propagates on the test roller, the increased measured vibration level is detected by the control system and the test is stopped automatically. The number of cycles to failure (“Life”) is recorded.
MPR loading system
The test head design allows a test to be stopped at any time, the load removed and the rotation stopped. The central roller can then be withdrawn for inspection, without disturbing either the lubricant or the three rings. The roller specimen can then be placed back in position and the test can continue. This is particularly useful in the study of both micropitting and macropitting, where physical examination of the specimen is essential.
The rig has comprehensive safety features. A test will be automatically stopped when any preset limit for temperature, vibration level, or friction is exceeded. The temperature limit uses the lubricant bulk temperature to trigger the shutdown of a test. In addition, there is also a safety alarm temperature sensor inside the test head. Vibration and friction coefficient limits are detected using an accelerometer mounted on the test head and a torque sensor respectively.
The test can also be stopped manually either by selecting ‘Stop test’ button in the software, which is used to run the test and monitor the test condition or by pressing the ‘Emergency Stop’ button in front of the rig .
The on-board microprocessor system incorporates a watchdog system which will shut down the test should the PC experience a failure.