Reviving this thread after installing my third dog hub/gear set in 700 hours. This one was an emergency installation in the field, 400 miles from home. The gear set that I replaced had 214 hours on it. Just Aircraft Highlander with a 912ULS. I'm using a three blade Kiev propeller, and using the Rotax prescribed method, its Moment of Inertia is well below the maximum. The wear I'm getting is where the dog hub and gear mate. I have three propellers for this aircraft and have had gearbox problems with all three. The Kiev is the lightest, so I've been using it exclusively since my last gearbox problem. All three propellers that I have used on this aircraft were dynamically balanced.
I use unleaded fuel except when I'm on a trip. I would estimate that I use av-gas 15% of the time.
I've changed out the bellville washers, bearings and of course the dog hub and gear set. The spring tension is shimmed carefully to Rotax specifications.
I've sent the clutch to Lockwood to have it checked and to have the torque adjusted. It did not require any adjustment, but they took it apart to inspect and reassembled it.
I change oil at less than 50 hour intervals, using Aeroshell Sport Plus 4 and Rotax filters.
Idle is set low (~1700) on this engine so that I can get the power out of it for landing, but by using the throttle, I never let it idle on the ground at less than 2300 RPM. When it starts, I immediately go to 2500 RPM.
I perform the gearbox friction test every annual condition inspection and it always passes. The last time I did the check was 20 hours before the gear set started vibrating.
I check carb sync every annual, it never takes more than minor adjustments.
I rebuilt the carbs per the scheduled maintenance checklist.
Even though there were no signs of sagging, in desperation, I've changed the motor mounts twice hoping to eliminate a source of vibration.
Symptoms of my gearbox problems are vibration that I can feel in my feet. The vibration starts when I change propeller loading, usually when unloading the prop, such as when pulling power at the end of a climb, when starting a descent or in moderate turbulence. I can manage the vibration somewhat by pulling power to around 4500rpm and slowly bringing it back up to 5400RPM. During the most recent episode, the vibrations got so strong that it partially dislodged the left air filter due to hitting an oil line support bracket a couple of inches above the air filter.
It's not the propeller, idling too low or leaded fuel. At this point, I suspect the notoriously rough engine start and shut-downs of this engine. Even with the soft start module, this engine shakes like a Harley at a stop sign when it starts.
Changing the gears every 200 hours does not seem reasonable to me and is bad news when it happens far from home. I've had it with this problem and am thinking of either welding the dog hub to the gear set or selling the aircraft and going to an airplane with a direct drive engine.
Incidentally, my friend has a Rans S20 with a Rotax 921ULS and he had to replace his gear set and dog hub after 200 hours of operation.
Any suggestions would be greatly appreciated.
