Lubrication Starvation
by David Doyle, CLS, OMA I & II, General Manager, Tribology
Co-Author Gary Blevins, North American Operations Manager

Whether operating equipment is grease lubricated or is oil lubricated, lubrication starvation can hasten equipment damage and unplanned downtime. Lubrication starvation will create extreme temperatures, accelerated wear, and increased buildup of contaminants. A constant and sufficient level of lubrication is required to prevent lubrication starvation. When lubrication starvation occurs, the fluid film is no longer able to maintain full separation of moving surfaces, which leads to excessive heat and friction, wear, noise, vibration and subsequently seizure of moving mechanical parts.

Hydrodynamic lubrication relies on sufficient lubricate being present within the contact zone to establish and maintain a stable “oil wedge” to separate surfaces moving relative to each other in journal bearing etc. A drop in lubricate flow to the bearing will “starve” the wedge of sufficient fluid to maintain the required pressure in the wedge, resulting in its collapse and the bearing returning to boundary lubrication conditions, with surface asperities coming back into contact. This will resultant in increased wear, heat generation and friction losses. Pools of lubricate between asperities will provide some lubrication, however if lubricate starvation increases, the amount of lubricate remaining in the area of contact will be reduced further, increasing the surface area of direct contact, further increasing friction, with a resultant increase in wear and heat. As the volume of lubricate in the area decreases, and the temperature increases, a point is reached where thermal degradation of the remaining oil occurs and surface damage/failure increases rapidly with total failure of the bearing/component immanent.

Under certain conditions, vibration, heat, centripetal forces, oil can bleed out of the grease thickener and potentially the bearing itself leaving only the thickener in the bearing. The resultant loss in lubrication will accelerate wear in the bearing. Counter intuitively, the wear tends to be most pronounced on the cage holding the rollers in position, rather than the high load conditions between the roller and the inner and outer races. This is because the sliding contact between the cage and rollers relies on hydrodynamic lubrication with a larger volume of oil required to form separation between the sliding surfaces. While the roller to race contact experiences elastohydrodynamic lubrication, which only uses a very thin film of oil [0.2 to 0.4 micron] to separate the roller from the race. Further to this, as there is no speed differential between the race and the rollers there is no mechanical loss of material even if there is little to no oil present, provided no slippage occurs between the race and roller. If a bearing is found to have excessive wear of the cage, but there is little to no wear of the races and rollers, then oil starvation of the bearing is the prime suspect.

High-speed/load gear designs operate under a combination of hydrodynamic and elastohydrodynamic lubrication. With insufficient lubricate present, the oil film required to maintain hydrodynamic lubrication, will collapse resulting in mixed-film/boundary lubrication conditions, with a corresponding increase in friction, wear and heat generation. Further compounding the problem is with less lubricate available, there is also less anti-wear or extreme-pressure additive present to minimize damage to the gear surface under these conditions. Gear life will be dramatically reduced under these conditions.

Lubrication starvation can be attributed to many factors; restricted oil supply or pathways, inlet flow design, dry starts, low oil level in sumps, oil foaming, wrong viscosity, worn or improperly operating circulating pumps. In circulating fluid systems, were lubricate is pumped from low-pressure to high-pressure zones, can be prone to lubrication starvation due to cavitation interrupting the fluid delivery.

Monitoring with ultrasound instrumentation and temperature sensors are tools that can monitor and detect the occurrence of lubrication starvation in mechanical components. This can complement testing of the lubricant for assessment of lubricant degradation, as well as wear and increased contaminants. ALS can work with your asset reliability team and lubricant supplier to ensure a testing program that will optimize early detection and sign of problems such as lubrication starvation. For more information please contact ALS Tribology at www.alstribology.com, or any of our regional testing labs that meet your oil, coolant or fuel testing needs.