I have had various vibrations, at various rev ranges, since new. Recently, at 500 hours, 10500' AMSL, 5200rpm, I experienced a major vibration - as if something had come seriously loose. On doing a precautionary landing I discovered a cracked spinner. On removing it and dynamic rebalancing I discovered that it must been laid up (fibreglass) unevenly, and though dynamically balanced at 4000rpm, flexing the backplate differently through the rev range, and giving the vibration effects. There is still a range of vibration artifacts, but less so. Ever 50 hours (we run on avgas) the magplug is inspected, showing regularly decreasing fine fur, and an oil analysis is done now showing unchanging levels of elemental and micro-particle iron. I'm sure that the continuing vibrations are minor enough that they aren't about to cause any catastrophic failure in the foreseeable future - but I continue to have oil analysis done. Cooling is by oil/water heat-exchanger and thermostat in the water circuit. We run water 88C and oil 118C, winter and summer, climbing and gliding.
He said the fix includes increased oil flow to the affected area.
I just got off the phone with Lockwood. They confirmed it is a heat related issue that's causing parts to get "coked up" inside the gearbox. That would explain why mine did not vibrate at the beginning of my flight, or after I decended to a lower altitude. They did confirm there is a fix in the works...
Poor lubrication yes. Coking no. This may just be bad terminology. For oil to coke it has to reach really high temps which isn't happening in a gearbox.
Mineral Oil:
"A natural high temperature application limit of about 320°C (608°F), above which the base oil decomposes and begins to coke (turns to a sooty substance). Under severe operating conditions, such as extremely high temperatures, or in very cold operating climates, the difficulties noted above are important factors in the selection of lubricants. Very often, the only solution is the selection of a synthetic oil." Even the flash point for mineral oil is about 440F.
Semi synthetic oil coking temps are even higher.
Temps that will cause true coking for our oil would be above 600F and that would fry the engine big time.
Coking may be a poor term for the gearbox oil.
This can happen in a low grade form in the cylinder. We see this at times on top of pistons from poor combustion.
"The solid product resulting from the destructive distillation of coal in an oven or closed chamber or by imperfect combustion, consisting principally of carbon: used chiefly as a fuel in metallurgy to reduce metallic oxides to metals."
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
I have an appointment with Lockwood on May 14th. They will remove the gearbox and apply whatever “fix” is currently available. Shims and possibly a gear set. I’ll keep all posted.
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