- PSRU Lubrication and Cooling -
Some often-overlooked issues
It is evident that gears and bearings must be lubricated. Intuition might suggest that high-viscosity gear lubricant is required in a PSRU. However, hundreds and hundreds of trouble-free operational hours with EPI gearboxes have proven that a well-designed PSRU can be lubricated quite well by engine oil of the correct type.
It is interesting to read the solemn predictions of doom published by some of the other purveyors of propeller reduction gearboxes. One expert says it is a "requirement" that the PSRU must use different lubrication than the engine. Quoting that expert: "Sharing engine oil with the PSRU is asking for engine and/or PSRU failure in advance."
We guess that he must have done it wrong, because we have achieved THOUSANDS of hours of operation with the Mark-9 gearbox, using engine oil supplied by a high-capacity engine oil pump. We have pictures of Mark-9 gears which have been flown with 450-500 HP turbocharged engines for 700, 800, 900 hours and literally look like new.
So don't be misled by the intuition of "experts". Look instead to sound engineering theory AND to what has been successfully accomplished in the past. (That same "expert" A&P also says that planetary gears can not work in a propeller gearbox. He might be enlightened to examine the planetary reductions in the P&W 1830, 2000, 2800 and 4360, the Wright 1820, and the Garrett TPE-331.)
EPI was not the first to use engine oil for the gearbox. The Continental GTSIO-520 uses engine oil to lubricate and cool the gearbox. Although the in-service life of certain top-end components of the GTSIO-520 is somewhat limited, the gearbox reliabilitiy is quite good if the engine is flown according to the instructions.
Many of the big radial engines of the WW2 and later vintage had reduction gearboxes which also used engine oil. Oh yes, and then there was this obscure engine known as the Rolls-Royce Merlin. which powered aircraft such as the P-51. (Oh yes, and the very-heavily-loaded gears which drove the superchargers on most of those engines ALSO used engine oil for lubricatioon.)
No, it has been proven by squillions of hours of successful operation that a correctly designed gearbox can use engine oil very effectively to provide the film strength and thickness to lubricate the contact surfaces of mating gears.
If the gearbox uses rolling element bearings, a relatively limited amount of lubricant introduced into the races is optimal. In fact, it is generally the case that gearboxes using rolling element bearings would operate better if the bearings were prevented from being submerged or flooded with lubricant.
As described on the Bearing Loads page, most EPI gearboxes use hydrodynamic liquid-film bearing technology. both for the very-heavily loaded gear shafts inside, and for the very high capacity propeller thrust bearings. These bearings require a supply of pressurized oil to operate at full capacity. The oil which flows out of these bearings is sufficient for the lubrication of the gears.
An equally important (and often ignored) function of the lubrication system is COOLING of the gears and bearings.
Why? Because involute gear tooth contact is sliding motion (not rolling motion, as some believe). The amount of power which is converted to heat by tooth friction is approximately ½% per mesh for good quality, well-lubricated gears. Lesser quality gears generate quite a bit more heat.
The gears and bearings depend on the lubricant to carry away the generated heat. The temperature of heat-treated, highly-loaded gears should not get above 250°F, and the temperature of rolling element bearings should never exceed 250°F in service.
In order for a coolant to be able to carry a significant amount of heat energy away from an object, the entry temperature of the coolant must be substantially less than the temperature of the hot object to be cooled (high effectiveness), or in the alternative, the coolant flow rate must be very high (low effectiveness). In other words, in order for the temperature of the innards of a gearbox to remain less than 250°F, the oil temperature of the entering oil must be well below 250°F, and the temperature of the exiting oil must be no more than about 235°F or 240°F absolute maximum.
Those cooling requirements strongly suggest the requirement for a continuous flow of relatively cool oil. It should be clear that a high-power PSRU with its own separate oil supply cannot survive without a significant oil cooling system, which implies at least one pump and one more heat exchanger (complexity and COOLING DRAG).
As an example, consider, a good quality two-mesh gearbox transmitting 500 HP. The contact between the gears generates a heat load of nearly 13,000 BTU per hour. Depending on the specific heat and specific gravity of the lubricating oil, the inlet oil temperature, and the oil temperature rise deemed acceptable, the oil flow rate required just for cooling can exceed 3.5 GPM.
Meeting these cooling requirements is critical to maintaining the design strength of the gearbox components, and therefore critical to gearbox life. If you are considering purchase of a PSRU, ask to see test data that verify gearbox oil flow rate as well as the entry and exit temperatures, measured after a half-hour of steady-state operation at the rated power level.
Also consider that if 3.5 GPM (or more) of oil goes into the PSRU, there has to be be some suitable method to get it back out again. Gravity? Try to pour 3.5 gallons of warm ATF through a ¾" hose in one minute.