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The Roots-type scavenge pump

oil-pumpsI suppose it's the mechanical engineer in me but ever since I can remember I have always been fascinated by complex curves. It may have been the Spirograph I received as a child, rolling one circle around another to produced a series of intriguing spirals. Or it could have been the Lissajous figures describing complex harmonic motion later on at university. Either way, the complex geometries produced in such a simple way left a major impression.

The geometry of the gerotor oil pump is developed from such a simple approach. Rolling one circle around another, but in the special case when the radius of the smaller circle is exactly half that of the larger, a shape similar to that used in a Roots-type supercharger is produced.

The Roots-type supercharger is simply an air pump inserted into the intake system of an engine to force air into it. But when scaled down and fitted to a dry-sump oil system, the idea takes on a greater significance as a scavenge pump. And if the Roots supercharger is still a popular way of pressure-charging an engine, the same concept when used as a scavenge pump is perhaps more appropriate.

For its size, the Roots-type pump is the most efficient method of extracting an air-oil mix from the engine. With its large intake and output ports, and relatively cavernous chambers - varying only slightly in section throughout compared to other types of pump - the resulting flow is smooth with only a low level of pulsation.

The downside is the challenge of manufacture, so to keep the pumping efficiency high the clearances between the rotors have to be kept to a minimum without actually touching either each other or the pump wall. At the same time though, if we increase the clearance and veer away from the geometrically correct shape, the pumping efficiency will inevitably fall.

Because of this required accuracy of manufacture, the lobed rotors have to be geared together using a system of spur gears with minimal backlash. But extra gears and the problems associated with machining, together with all that extra material in the rotor, means only one thing - extra cost. With the development of cheap gerotor parts sintered to a net shape and which don't require an additional outlay of gears, it is easy to see therefore why in recent times, despite the benefits, Roots-type scavenge pumps are no longer popular.

This problem of the clearances required has a direct influence on the choice of material. Using conventional machining techniques, rotors tend to be solid and therefore, if made in scuff-resistance steel, comparatively heavy. Since the aluminium housing with a higher coefficient of thermal expansion would expand away from the steel rotor, these clearances would increase with temperature and the efficiency of the pump fall.

Ideally, therefore, rotors should be made of an alloy aluminium with a thermal expansion similar to that of the housing. This will also minimise the weight issue. Unfortunately aluminium may not be the best rotor material because, unless covered with a much harder wear-resistant coating, the debris which always passes through a scavenge pump at some time or other will very easily damage the surface.

Coatings of this type create even more expense, which could be the reason why we don't see Roots-type scavenge pumps more often.

Fig. 1 - The Roots-type scavenge pump

Written by John Coxon

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