Gidday Roger
This gearbox issue is a well know problem with the New Zealand and Australian Tecnam agents, Rotax are blaming the GT Tontini propellers saying that they are heavy, well they are not, they have total mass of around 3.4 kg and also have a low moment of enertia when checked with the method Rotax recommend. It could well be the material the GT prop is made from, possibly a resonance harmonic problem that is causing premature wear, I don't know, just a thought.
Of all the Rotax 100hp 912 uls in our area, non have made it to 1000 hrs, see the above example engines. They all have different operators, different aircraft, different props, so I am of the opinion that it is not the operators. Of all these 912 engines non have failed the friction torque check.
The wear on the dogs is only on the drive side of the dog.
Please note that I'm not here to bag Rotax as I think they are an extremely reliable engine, I've personally spent around 3000 hrs behind them, I'm only stating what has happened in our part of the world, we have had nil engine failures, only other common problems we have had are ignition modules, voltage regs and fuel pumps but everybody has got home safely.
I had my gearbox serviced at Lockwood 396 hours the eccentric that drives the fuel pump replaced it was galled up from the previous old style pumps and a cleaning/friction torque dione.
There is a trend on the iS engines wearing gearbox components out at a faster rate.
I've replaced numerous components on at least 10 912iS Sport gearboxes. The dog gears wear, but also the splines on the propeller shaft and that is where the vibration comes from when pulling back power so make sure you check the splines. The Rotax manuals say to lubricant with Kluber Isoflex, don't use that garbage. Rotax is supposedly going to change from Isoflex in the near future. The Isoflex tends to harden in the gearbox and does not allow the clutch assembly to slide freely on the prop shaft spines causing advanced wear in the splines, dog gear, and dog hub. The advanced wear is not caused by Isoflex alone, but it speeds up the damage.
Like I said, I have done many gearbox services on iS engines. Rotax has honored 100% of all the warranty claims I've submitted for the repairs, even on engines well past the 200 hour warranty period. Now I change the prop shaft (oil seal and bearing require replacement when removing prop shaft), gearset, dog hub, springs, split rings and sometimes the bushing and thrust washer if they're worn. It seems to keep the gearbox running smoothly before needing to come off again, and Rotax is paying for the parts and labor.
I would suggest replacing the gearset, dog hub, and propeller shaft, use lithium grease instead of the Isoflex and you should be good for about 300 hours.
The gearbox on my 912UL failed at about 700 hours.
Don't want to hijack the thread but what symptoms does a failed gearbox show? Seems like some level of vibration is at least one of them. Is that the predominant one?
Vibration is the predominant one. There are a couple of things that can cause this. The phenolic washer disintegrates which causes the belleville washers to loose their tension, old bellville washers that haven't been replaced and the gearbox re-shimmed, worn dogs, worn gears, prop strike not attended to properly, sitting at a low idle for long periods over time and wrong oils.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
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