General

The PSRU (Propeller Speed Reduction Unit) is essentially a single gear ratio transmission system that allows the engine to operate at it's optimal rotational speed whilst allowing the propeller to operate at it's optimal rotational speed. Most auto engines, excluding those of North America design, are designed to operate using relatively small displacements but with relatively high engine speeds to achieve the necessary power level in today's competitive automotive marketplace. The use of small displacement, high revving engines promotes a significant weight reduction over the large displacement counterpart, however the it is necessary to match the engine to the propeller. To this end a single ratio typically in the order of 2:1 has been found to be the optimal choice.

There are an number of vendors offering various methods of PSRU ranging from Toothed Belts, Chains, spur gears, and planetary designs. Every vendor has a good story to justify his/her design. I bought my PSRU a number of years ago from Ross Aero in Tucson, Arizona. I wanted a PSRU that was coaxial with the crankshaft which was my reason for this choice.

The Design

Lou Ross had some reservations about the power I would be producing compared with his more typical customer with the Subaru Legacy conversion and as a result he made me a special using a six planet set in a 7075-T6 cage. In hindsight the six cylinder engine using the same displacement per chamber as the 2.2 litre Legacy is a more benign condition since the power pulses are identical from both engines the power pulses just come more frequently and with more overlap. The resulting mean torque is 50% greater in the six cylinder but the torque perturbations are reduced and it is these perturbations that excite the torsional resonances. The figure below illustrates the concept in an oversimplified form.

The ROSS AERO PSRU is pictured below and attached to the SVX engine. In this photograph the drive is configured almost exactly as it arrived from Lou Ross. The only modification at this point was the removal of some aluminium material at the top and bottom of the flywheel casing where there was to be no stress and it only represented a weight penalty.

Breaking open the PSRU reveals the working parts, namely the planetary drive. The helical cut of the gear system is clearly visible on this photograph. Under load conditions the input shaft is drawn towards the output shaft to which the propeller is attached. A thrust race, indicated in the photograph below, takes this load quite satisfactorily. When in a decent the propeller drives the engine and the input shaft is forced to move in the direction of the engine. The ROSS design does not provide a true thrust bearing to support this load, moreover the load is taken by a conventional sealed ball race contained within the lightweight flywheel. This bearing is there to accommodate the slight differential rotational rocking motion encountered as a result of the torsional isolator mounted on the flywheel. A resonant condition is set up when operating near idle when the engine drives the propeller followed by the propeller driving the engine. There is considerable chatter at that point and the input shaft is hammered back and forth between the thrust race illustrated below, and the conventional bearing previously described. The bearing begins to break down and the end movement increases only exacerbating the problem.

The FIX

The solution to this problem is relatively simple but requires some machining. Essentially it is necessary to fabricate a thrust bearing arrangement on the front side of the input shaft. To this end it is necessary to remove the oil seal and replace the oil seal with the machining illustrated in the centre of the photographs below. The ring on the left is simply to captivate the Oil Seal in the event that a pressure build up tries to blow the seal out. The right hand photograph indicates new part in place. Notice also, I have installed a temperature sensor to measure PSRU oil temperature and have added an additional oil return. This oil return has been installed to overcome the oil pressure build up that has been encountered by some builders as a result of gear pump effect of the planetary drive. The new bearing now reacts against this newly machined part thereby taking the load off the bearing contained within the flywheel.

The final assembly is shown in the photograph below. Notice the two blue plastigauge strips. These strips are compressed when the assembly is torqued  together. After disassembly the blue strips will be significantly wider that shown below. The new width of the strip is then compared with a gauge to determine the end float within the structure. In this case the end float was measured at 0.007".

The job is finished off with the addition of a 7" propeller extension.

Last Updated:    Thursday August 31, 2006