- Propeller Governor Drives -
Problems with crossed helical gears
Very few PSRU’s have a drive system for a propeller governor integrated into the design. Often, the governor drive mechanism is an optional toothed belt add-on. On EPI PSRU's, the propeller governor drive and plumbing are tightly integrated into the reduction unit design, using a pair of crossed helical gears to drive the governor and a proven reliable servo-pressure transfer mechanism from the (fixed) housing to the (moving) propshaft. (Crossed Helical gears are meshing helical gears on non-parallel shafts, similar to the gearset on an engine camshaft which drives the perpendicular oil pump and distributor.)
That being said, here is a strong caution: IF a PSRU you are contemplating uses a crossed helical gear system for the governor drive, be especially wary. The field of crossed helical gear design is very specialized, and many gear shops will not even attempt to design or analyze a crossed helical system. Even though the teeth of such a system may look quite robust, the actual contact load which the gears can transmit is extremely limited, due mostly to the nature of the tooth interface: high-velocity, sliding, point-contact.
The correct design of these systems is critical. Materials which work well in spur and helical gears on parallel shafts will not survive more than a few hours in a crossed helical application.
Consider the fact that a standard, single-acting governor such as found on most light singles and twins (Woodward 210xxx series, Hartzell and MT prop governors) requires less than 3/8 HP to drive at 220 PSI output pressure. That is 12 pounds-inches of torque at 2000 governor RPM and less than 9 pounds-inches of torque at 2700 governor RPM. If the pitch diameter of the driven gear in the governor drive mechanism is 1.25", for example, then 12 pounds-inches of torque generates 19.2 pounds of contact force at the gear faces. That’s not a very big load, but it can destroy a badly-designed set of crossed helicals in just a few hours.
When the prop governor drive of a typical single-engine installation (pressure to INCREASE pitch) fails in flight, the prop goes to its low pitch stop and the engine, relieved of most of its load, goes into extreme overspeed unless the pilot is VERY quick about closing the throttle.
In the typical twin engine situation, (pressure to DECREASE pitch), the prop goes to its high-pitch stop, which is typically far too much load for the engine to pull, so the engine speed quickly slows, and unless the pilot is quick to diagnose the situation and REDUCE the power setting, the engine can quickly experience severe detonation and subsequent engine destruction. What is the natural reaction to a slowing engine? ADD POWER. Hmmm.
In one failed PSRU we inspected for the unhappy owner, the geared prop-governor drive completely wore away the teeth and failed in flight after about 15 hours of use. (Because of the rev-limiter in the electronic engine control, the engine didn't destroy itself, but it tried.) The governor drive gears were a poorly-designed set of crossed helicals, made from a very high-quality but inappropriate material, driving a standard Woodward governor. We redesigned that governor drive, and replaced the failed gearset with a properly designed set of gears, using the correct material, and it has flown for hundreds of hours showing no apparent wear.
Another related area you should investigate is the mechanism the PSRU uses to transfer governor pressure (about 320 PSI max) between the fixed housing and the moving the propshaft. We have seen implementations which had major strength and / or life expectancy problems. Another common problem with this transfer system is sealing mechanism used to transfer servo pressure from the (static) housing to the (rotating) propshaft. Seals which will survive in that environment of high pressure and high surface velocity require special engineering. Be especially wary if the design uses O-rings as dynamic seals. They typically don't last very long.