- PSRU Bearing Loads -
Not from a catalog!
Many PSRU’s use rolling element bearings (ball, roller & needle) to support the shafts. As part of the design process, it is prudent to calculate the expected life of each rolling element bearing in the context of the PSRU design load model.
The prediction of the expected life of a rolling element bearing in a specific application involves a bit more analysis than simply plucking a load rating from a bearing catalog. The dynamic load rating listed in bearing catalogs is defined as the load at which 90% of a large population of apparently-identical bearings will survive one million cycles. A shaft turning at 2250 RPM will produce one million cycles in 7.4 hours. Not exactly an aircraft-quality life expectancy.
Many rolling element bearing manufacturers publish detailed life-load analysis procedures. These procedures enable a designer to predict how many hours a desired percentage of apparently identical bearings will survive with a specified load at a specified RPM.
As usual with fatigue predictions, the analysis is statistical. The usual industrial design standard (known as the "L-10 life") allows a 10% failure rate within the specified design life. A 5% failure criterion is more appropriate for aircraft design. Achieving the 5% rate requires 1.64 times greater bearing capacity than the 10% rate. A 2% failure rate requires 3.0 times greater capacity than the 10% rate.
The bearing life calculations which the manufacturers publish take into account factors including:
- The combined effect of applied radial and thrust loads,
- Pitchline velocity,
- Lubricant viscosity,
- Bearing load ratings, and
- Desired probability of survival.
(See PREDICTING BEARING LIFE for more information on this subject.)
These calculations have been implemented in a computer program which EPI wrote for doing bearing life analysis. This analysis procedure uses the combinations of loads, speeds and the percentage of occurrence defined in the design load model to calculate the projected life of a particular bearing in a specific application.
Certain bearing manufacturers (SKF, for example) have, as of early 2004, made available on their websites, the same set of calculations for predicting "L-10" bearing life. Our program and the SKF website programs produce the same results when given the same input data, but our program includes the ability to calculate life ratings for higher survival probabilities (L-5, L-4, L-3, L-2 and L-1).
Although this analysis is neither obscure nor complex, it is interesting to note that in many PSRU’s it is apparently ignored. In one PSRU, advertised as suitable for over 750 HP, we found bearings with an approximate design life of 150 hours when analyzed under the 500 HP LOAD MODEL described on a previous page.
NOW, HAVING GIVEN A BASIC EXPLANATION OF THE LOAD CAPACITIES OF ROLLING ELEMENT BEARINGS, IT IS IMPORTANT TO NOTE THAT EPI PSRU's DO NOT USE ROLLING ELEMENT BEARINGS on either the propshaft or the input gear shaft (where the large loads are) because ........... rolling element bearings which are sufficiently robust to provide satisfactory calculated life factors in a realistic PSRU load model are unacceptably large and heavy.
Instead, EPI PSRU's use HYDRODYNAMIC BEARING TECHNOLOGY for input and propeller shafts and for thrust absorption. These are the same type of bearings that support the crankshafts and connecting rods on essentially every piston engine the world has produced in any quantity for the past century. The loads which crankshaft bearings must survive, especially in compression ignition ("Diesel") engines are far more severe than the steady loads typically applied in a gearbox application.
This technology has been exhaustively proven in hundreds of thousands of hours of flying experience with the bearings in the Continental GTSIO-520 gearbox, not to mention squillions of hours of automotive, industrial and marine applications. Further, in addition to all the crankshaft bearings in the countless Lycoming and Continental aircraft engines in existence, all the thrust bearings in those engines are hydrodynamic. These pressure-lubricated hydrodynamic bearings provide significantly greater capacity than comparably-sized rolling element bearings. Therefore, the EPI design bearing life at severe load levels is conservatively calculated to be well over 2000 hours. Inspections of in-service EPI PSRU's suggest a much longer life, despite the uninformed speculations of more than one self-appointed authority.