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912 ULS gearbox problems.

  • James Christison
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1 year 8 months ago #13063 by James Christison
James Christison created the topic: 912 ULS gearbox problems.
Just wondering what sort of hours people are getting out of their gearbox before needing an overhaul. There seems to be a trend here of low hours ( down to 120hrs in a IS motor ) before a full rebuild. Very costly. Friction torques all seem to be ok but characterised by vibration when unloading power. We have been chasing all sorts of other things like carbys, engine mounts and props but no luck. We pull the g/box apart and find considerable wear on the dogs. Replace these and all is well. All this seems to be happening at low hours, 300 to 600 hrs. The 80 hp engines seem to last very well but not the 100's. Perhaps prop weight and setup is an issue, it does seem worse with heavier props. All these engines are running approved oil and unleaded fuel. We thought if this problem was a trend it would be mentioned on this forum but gets little mention.

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1 year 8 months ago #13064 by Roger Lee
Roger Lee replied the topic: 912 ULS gearbox problems.
Hi James,


The gearbox should easily see TBO and beyond. What kills a gearbox tends to be owners lack of attention. Usually it is what an owner does or doesn't do that is not Rotax recommended that wears a gearbox early.

These things will cause extra wear and shorten gearbox life:
carbs out of sync,
Low friction torque on the gearbox due to weak and or worn Bellville washers, (this is supposed to be checked every annual or 100 hrs. whichever comes first)

out of balance prop,
bad engine mounts,
wrong oil,
too heavy a prop, (i.e. Warp Drive with nickel edges especially if it is over 68"),
Improper starting and stopping techniques,
failing to get the gearbox serviced at 1000 hrs. (less if all you use is 100LL)


Early issues with a gearbox is a minority of owners.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 year 8 months ago #13065 by James Christison
James Christison replied the topic: 912 ULS gearbox problems.
Thanks for the reply Roger. We are very much aware of the probable reasons for early damage and most here are very careful in their operation but there are instances out of left field that occure. It would be good to know who else has problems,at what hours and how many make 1000hrs.

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1 year 8 months ago #13069 by Martin Higgins
Martin Higgins replied the topic: 912 ULS gearbox problems.
Gidday Jim
With regards to 912 uls gearbox problems, I'll pass you some actual facts that you and other readers can view and comment.
The following engines and aircraft are ones that I maintain and have done the gearbox repairs on. Very rarely do I find the friction torque out of tolerance, if I do it has been on the 80 HP 912, no vibration but hard to start problem. Most of them run on aeroshell sport plus and oil changed at 50 hrs and only run on 95 PULP. I have found the 80 HP 912 don't appear to have these gearbox problems.

Engine 1, TL sting aircraft, 3 blade wood comp propellor, 912 uls engine, problem, owner experienced excessive vibration on low power settings, turning on final, TT around the 600 hr tachometer time, friction torque as per Rotax MM, repair was to replace worn gearbox dogs, problem solved
Engine 2 Tecnam p92, 3 blade airmaster prop, 912 uls engine, same problem as above aircraft, 350 hrs flight time, replaced dogs, problem solved.
Engine 3. Fox-bat A22 3 blade Kiev prop, 912 uls engine, around the 600 hrs Tachometer time, same vibration problem, same repair and problem solved.
Engine 4. CTLS 3 blade Nueform prop, 912 is engine, 126 hrs tachometer time, severe vibration around the 4-5 k rpm 18"- 24" man pressure settings, replaced dogs, problem solved, managed to get parts under warranty.
Engine 5. Tecnam p92, 2 blade GT propellor, 912 uls, 1200 hrs flight time, 3 complete gearbox overhauls, this includes on each oh dog, gears, disc Spring washers, washers, front bearing and preload set as per Rotax MM. This ac does operate in a flying school and has its oil changed every 50 hrs with aero shell sport plus and friction torque checked at 100 hrlys. Prop dynamically balanced,95 PULP, As an experiment this engine was derated to about 80 HP by putting a simple rudder tube spacer on the cockpit throttle shaft to try extend gearbox life but gearbox started vibrating around the 400 hours flight time. Experiment failed.
Engine 6 Cub replica, 3 blade prop, 912 uls, 600 hrs, same vibration, replaced dogs, problem solved.
The other 3 Rotax 912 engines that I look after have not yet reached 100 hrs, so far no gearbox problems.

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1 year 8 months ago #13082 by Kevin Stewart
Kevin Stewart replied the topic: 912 ULS gearbox problems.
The gearbox on my 912UL failed at about 700 hours. It had been well maintained, carefully operated and had been stripped and serviced at 400 hours with new Belleville washers. The Rotax specialist who did the repair work at 700 hours said that he had only come across a worn dog and prop shaft a couple of times.

Other than on this forum I have only come across one other.

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1 year 8 months ago #13084 by Roger Lee
Roger Lee replied the topic: 912 ULS gearbox problems.
Two of the most common gearbox wear items are:

Idling too much at low rpms will beat the dogs to death in a 912ULS. When you are just sitting at idle try and stay at 2000+ rpm. That will keep engine pulses down and allow the gearbox a much smoother run. I only idle lower when I set the carb sync and when I lower the rpm to shut down. Low friction torque in the gearbox will also add to your dog wear.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 year 8 months ago #13088 by Martin Higgins
Martin Higgins replied the topic: 912 ULS gearbox problems.
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.

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1 year 8 months ago #13092 by Garrett Wysocki
Garrett Wysocki replied the topic: 912 ULS gearbox problems.
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.

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1 year 8 months ago #13097 by Kevin Kane
Kevin Kane replied the topic: 912 ULS gearbox problems.
James,

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.

I hope that helps,
Kevin

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1 year 8 months ago #13117 by Tom Kodey
Tom Kodey replied the topic: 912 ULS gearbox problems.

Kevin Stewart wrote: 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?

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1 year 8 months ago #13119 by Roger Lee
Roger Lee replied the topic: 912 ULS gearbox problems.
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)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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3 weeks 21 hours ago #17907 by Rodrigo Echenique
Rodrigo Echenique replied the topic: 912 ULS gearbox problems.
In the turbine gearboxes of the jets and helicopters we use a little ammount of calcium sulfonate grease and this is far superior than the lithium grease, it supports hi temperature and very hard service, is totally compatible with any kind of oil and is the most anti oxidizing of all the greases and it is available and inexpensive, a small 250 cc tube cost you about $20 USD

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3 weeks 20 hours ago #17908 by Roger Lee
Roger Lee replied the topic: 912 ULS gearbox problems.
calcium sulfonate grease may not be suitable for our oil and in the gearbox. Just like we aren't supposed to use friction modifiers. This isn't a jet or heilo. Different parts and fluids.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 week 6 hours ago #18031 by Leopold Schmidt
Leopold Schmidt replied the topic: 912 ULS gearbox problems.
My experience with the gearbox on an 912 --80 hp--, no slipper clutch:

1400 hrs/8 years, carb sync check every 100 hrs. at least, 3 blade prop is static balanced, oil change every 100 hrs with Shell motorcyle oil, only autofuel with 10% alcohol:

engine runs smooth, no vibration problems, no overhaul so far.

In Germany the 80 hp is much more popular than the 100 hp Rotax.

Leo

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