From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Engine lubrication systems, bearing pressures, and the effects of temperature on oil.
| Identifier | ExFiles\Box 141\2\ scan0146 | |
| Date | 31th May 1936 guessed | |
| 2. If any of these lubricating system designs were unsound, a major change in design, rather than simple modifications might have been expected in six years, through large changes in horsepower, and in two cases in R.P.M. also. In each instance the horsepower per cubic inch of engine displacement which affects the heat rejected to the oil, has increased materially, from 11 to 15%. It is not within the scope of this paper to deal at length with bearing materials, bearing pressures, oil supply pressures, filters or strainers, oil viscosity or oil viscosity slope. The task is rather to determine from information available, largely supplied by engine manufacturers, what constitutes an ideal lubrication plan for any type of engine. Divorcing the problem for the moment from its many complications, it is seen that some reasonably successful systems provide oil under substantial pressure to most of the friction surfaces in the engine, while other systems, equally successful, either supply oil to most of these surfaces under a small gravity head, or deliver it under relatively low pressure. Bearing pressures unquestionably cover a wide range, from the small high compression vehicle engine, where the pressure of the gases of combustion exerts the maximum pressure on the bearings, to the large aircraft engine under terminal velocity dive conditions where the bearing pressures due to inertia loads are double those of normal flight R.P.M. In a search for a common ground of comparison between systems, it seems evident neither oil pressure to the bearings or the magnitude of bearing pressure maintained, provides a common denominator for all types of systems. Methods of guiding the oil between the bearing and the journal, type of filter or lack of one, design of oil screen and pump, can all be studied without finding a common feature. There is one factor these widely divergent systems have in common as an effect on the bearing surfaces and the oil it is hoped to keep between them, and that is heat or temperature. Some critical maximum temperature will carry any oil to a viscosity too low for effective fluid film lubrication of bearings. A critical maximum temperature will be found, limiting the ability to keep oil in the top compression ring groove, except as dusty carbon. At the opposite end of the scale of the lubrication keyboard, critical minimum temperatures may limit circulation due to high viscosity or may find cold sluggish oil, trapped in piston ring grooves, burned to carbon before consistent oil movement is established. Somewhere in these few octaves of this heat instrument, the lubrication system designer may make harmony or discord, according to his skill. Assuming the selection of oil of the correct viscosity for the engine requirement, failure in the lubrication system occurs when:- 1. The oil supply is shut-off for any reason and there is no longer a continuous flow between the friction surfaces. 2. The oil begins to vaporize in contact with friction surfaces, losing its cohesion, due to high temperature. (Figure 3) | ||