David-aviator
Well Known Member
The subject is an Egg gearbox failure. Does anyone have a dimensioned drawing of the actual components? Same for the flywheel assembly, with whatever sort of "damper" it might have? Third, mass moment of inertia for the propeller?
Without these things at minimum, (1) serious failure analysis is impossible, and (2) neither fans nor critics know the truth.
First off, I am no engineer, but I can make these observations about the EGG design.
Starting with the engine with the PSRU removed - the original fly wheel has been replaced with a spring loaded devise that weighs about 20 pounds. It is a fly wheel but not connected directed to the crank shaft but rather the load is carried by a series of springs in compression. If you move the prop, you can feel the springs compressing before the crankshaft moves. Its purpose is to dampen the load transferred forward to the PSRU.
The engine is mounted to a half inch aluminum plate, aproximately 30"x18" in size. The PSRU is bolted to the forward side of the plate and connected to the engine with a splined shaft about 8 inches long which slides into a splined plate bolted to the forward side of the fly wheel devise. Fore and aft movement is controlled by 2 large snap rings. Alignment is controlled by plugs in the aluminum plate and in the engine similar to the arrangment with auto transmissions except the plate is aligned to the engine with one set of plugs and the PSRU is aligned to the plate with another. An alignment tool was created to check the total alignment before the PRSU is installed. The aluminum plate is attached to the engine mount structure with bolts through bushings.
The torsion load of the prop is absorbed by the PSRU and the aluminum plate but not the engine crankshaft. The PSRU is a straight through gear reduction (planetary?) resulting in the prop being aligned with the crank shaft and turning in the same direction.
The PSRU failures to date IMHO have been caused by misalignment with the engine resulting in side loads on the aft bearing or torsion loads beyond the design limit of the bearings within the unit. Thats why GEN1 has been quite successful with the original 2.5 engine but not so with later, more powerful engines. GEN2 is simply an upgraded GEN1 with different gears and bearings. It too did not meet the load requirements or was misaligned in the process of field installation. The alignment tool may have come too late as some were installed without it. The tool is necessary to install GEN3.
As previously stated, GEN3 is a totally new beast. The gears and bearings are stronger, it has a different ratio (2.02:1 vrs 1.87:1) and has a machined case instead of cast. It is considerably more expensive to manufacture and to date has been available to upgrading customers at cost or less. The EGG factory is trying to be fair and not go broke in the process.