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912 ULS gearbox problems.

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2 years 2 months ago #13063 by chrisag
912 ULS gearbox problems. was created by chrisag
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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2 years 2 months ago #13064 by Roger Lee
Replied by Roger Lee on 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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2 years 2 months ago #13065 by chrisag
Replied by chrisag on 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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2 years 2 months ago #13069 by meflyrv6
Replied by meflyrv6 on 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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2 years 2 months ago #13082 by kstewart
Replied by kstewart on 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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2 years 2 months ago #13084 by Roger Lee
Replied by Roger Lee on 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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2 years 2 months ago #13088 by meflyrv6
Replied by meflyrv6 on 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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2 years 2 months ago #13092 by garrett
Replied by garrett on 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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2 years 2 months ago #13097 by Advantage
Replied by Advantage on 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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2 years 2 months ago #13117 by tomkk
Replied by tomkk on 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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2 years 2 months ago #13119 by Roger Lee
Replied by Roger Lee on 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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6 months 2 weeks ago #17907 by Rodeche
Replied by Rodeche on 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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6 months 2 weeks ago #17908 by Roger Lee
Replied by Roger Lee on 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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6 months 2 days ago #18031 by Leopold
Replied by Leopold on 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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5 months 2 weeks ago #18120 by Rodeche
Replied by Rodeche on topic 912 ULS gearbox problems.

Roger Lee wrote: 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, maybe you have never seen a helicopter gearbox, here is one and it also have a clutch and is similar to the Rotax but a lot more complex because it has a lot of planetary gears and the calcium sulfonate grease have prooved to be the best and maybe Rotax must do some tests unless they want to sell a lot of gearbox parts.

Rotax sugested to use Isoflex that is a garbage and is very acid.
Kevin above sugested no to use Isoflex and use instead Lithium grease, now if you want the best protection for your gearboxes use calcium sulfonate grease, at least do a search about the properties of both kind of grease Calcium Sulfonate and Lithium and then decide what is best.

I just repaired my gearbox that had a lot of wear in the dogs and used the calcium sulfonate grease, you think that I want to risk my plane or my life with a product that can be bad?

Don´t be closed to improvements that has been proven in superior machines like certified turbine helicopters

Regards


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5 months 2 weeks ago - 5 months 2 weeks ago #18121 by Rodeche
Replied by Rodeche on topic 912 ULS gearbox problems.
Mi gearbox failed at 160 hrs and when I took it appart the dogs on the gears had a lot of wear to have only 160 hrs. I machinned my two gears and let the same finish and tolarances I posted many pictures but I can´t find that post but here are some before and after pictures after I machinned the gears lobes again.

It was just a test and a risk because if I have to change the gears and dogs I don´t loose anything trying to fix it and today I tested the engine and the vibration dissapeared!!

I was able to fix my old gears and did a very good job.

I don´t advise to make it if you don´t know what are you doing and I have all kinds of CNC machine centers and tools to make this job, just to prove that it can be done.

Now my advice is don´t trust Rotax for the momentum of the propeller, even with a propeller inside the Rotax numbers my gearbox was damaged, I used a Woodcomp in flight adjustable and the weight of this propeller made the Rotax gearbox to fail and cause a severe vibration at only 160 hrs. so use the lightest propeller you can if you don´t want to damage your gearbox.

Regards



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5 months 2 weeks ago #18122 by RobSeaton
Replied by RobSeaton on topic 912 ULS gearbox problems.
What failed? I don't see any broken parts? or is this an english translation issue, and you mean "worn"?? The dogs are sacrificial parts and will always show some wear.
I don't know about the lubricants but I think there is a big difference between a turbine transmission and a piston engine gearbox? Does a turbine transmission have any torsional vibration?

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5 months 2 weeks ago #18123 by Roger Lee
Replied by Roger Lee on topic 912 ULS gearbox problems.
Hi Rodrigo,
I don't think this has anything to do with prop weight. Too many of these props being used around the world without any issues on the 912 engines.

I flew helicopters for 6 years. Your helicopter gearbox oil isn't used for the engine like a Rotax. Don't compare. There is no similarity between a helicopter and Rotax gearbox. The forces involved are different. The Rotax is closer to a motorcycle than a helicopter. You must use an oil that is compatible with the engine and the gearbox. The dog gear wear can come from several things, but you haven't addressed those. Things like did you keep the carbs synced, was the prop blades in track, were the prop blades dynamically balanced, the way you start and stop the engine, the oil you use, ect.. These are only a couple of things that can cause excessive gearbox wear and there are others. A 912ULS engine doesn't wear out a gearbox in 160 hrs. unless something either wasn't done to the engine that should have or something was done that shouldn't have been done. I have never seen a 912ULS gearbox wear out in 160 hrs.
A couple of your parts almost look like they have been exposed to water? If so then running an engine too cold would allow this because it wouldn't burn off contaminates.

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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5 months 2 weeks ago #18129 by Rodeche
Replied by Rodeche on topic 912 ULS gearbox problems.

Rob Seaton wrote: What failed? I don't see any broken parts? or is this an english translation issue, and you mean "worn"?? The dogs are sacrificial parts and will always show some wear.
I don't know about the lubricants but I think there is a big difference between a turbine transmission and a piston engine gearbox? Does a turbine transmission have any torsional vibration?


I said my gearbox failed not broked, any part can fail and it don´t need to be broken, it failed to work propperly and this is a fail.

Yes, the dogs are sacrificial parts as well as de pistons, rings, crankshaft, cam, valves, etc. etc. they are not suposed to last forever but in a aicraft engine that have a TBO of 1500 or 2000 hrs the gearbox is not sipòsed to fail at 160 hrs.

And your last question if the helicopter gearboxes have any torsional vibration? they not only have torsional vbibration, also resonance from the frecuancy of the blades, stress of lift, stress of the torque of very large blades and they last in some helicopters more tan 2000 hrs and this are also sacrificial parts

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5 months 2 weeks ago #18130 by Rodeche
Replied by Rodeche on topic 912 ULS gearbox problems.
Hi Roger,
This is exactly the problema, this plane has been maintaned by me and I can tell you that I did everything!
The propeller is a three blade Woodcomp in flight adjustable and now I changed it to a Bolly Prop of carbón fiber and very light because my engine is not the only Rotax 912 with this problem, all the Pipistrel Virus planes that uses the same prop have the same problem.

I check my carburators every oil change and at least I verify the sincronization, my propellers are balanced in a profesional propeller balancer to cero tenths of a gram at the tips, I uses for some time Mobil 1 sinthetic sugested by Rotax and I only use high octane auto gas, at 100 hrs I changed the oil to Aero Shell Sport Plus 4, and I change my oil every 50 hrs, I use a full changed battery with lot of power to start the engine and to turn off the engine I always set to iddle, then turn off one ignition circuit and in one or two seconds the second magneto, I think I have done it correctly.

The engine have never been exposed to wáter or coast and never in cold weather and as you can see I found some corrosión inside the gearbox and this was caused by the Isoflex that is very acid, the fuel pump lobe had corrosión, I resurfaced it and polished to like new condition and now my gearbox is working like new and I think I didn´t do any abuse to the engine in fact I maintain this engine with change of oil and spark plugs at shorter intervals because the oil is the life of any engine.

Here in the air club are one guy that have a Pipistrel with a Rotax 912 of 80hp, he flies every day all day and his engine is perfect!

Why is no notorius that the 80hp engine have a much better and reliable gearbox?
If the gearbox is the headache of Rotax why they don´t make a forged and stronger gears with dogs and if the clutch is giving trouble why not use the non clutch gearbox in all the engines?

In this same post above you can read that in New Zeland many gearboxes are failing at very low hours, I think mine is not the only one.

Regards

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