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Vibration Problems

pilotyoung

Well Known Member
I have been having vibration problems with my RV-12. It ran smoothly at cruise, but when I reduced the power from about 4600 to 3000 there would be a vibration. I tried several possible solutions: 1, did the friction test on the gear box; 2, weighed the floats to see of they were too heavy; 3, sycnhed the carburetors; 4, replaced the soft engine isolators; and 5, replaced all the muffler springs. Since those didn't fix the problem I then decided it was the Gear Box. Ronnie and Morris Smith of South Mississippi Light Aircraft agreed with my thoughts.

Today I took the gear box to them for repair. The dogs were worn and the disks were worn. All were replaced and Ronnie says my vibrations will be gone. I will be out of town next week so I won't get the gear box back on to know for sure for a week or two. But if any one is interested, here are some pictures of the worn gears and dogs.

The aircraft has 450 hours on it. I ask Ronnie is this was from abuse, bad parts, etc. He said there was no way to tell. I have flown it 120 hours and I have not let it idle at slow speeds.

I am posting these pictures in the hope it will help others understand the problem if they experience vibrations like this.







 
What was the condition of the magnetic gearbox inspection plug? Did it have excessive metal shavings on it?

I wonder if previous owner used incorrect oil not approved by Rotax for gearbox engine?
 
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Vibration

I have not seen anything other than some small fuz like particles on the magnetic plug, very small and very small quantity.

According to the logbook both previous owners used Aero Shell Sport Plus 4 oil.
 
The primary things to focus on to minimize worn gear box parts is dynamic prop. balance, carbs that are well synced in the idle speed range, and avoiding operation below 1800 RPM (on some engines,staying at or above 2000 as much as possible seems to be a good idea).

Being casual about maintaining a proper carb. sync is a sure way to cause gear box wear.
 
It's going to be interesting to see if wear on the cam angle surfaces indeed causes vibration in mid-range. I'm kind of thinking it?s a stretch...
 
cause vs effect?

Several decades ago, I was an engineer. Remnants of that life prompted my first thought to be "which came first, the vibration or the wear?" Like Piper J3, I think it's a stretch that the wear causes vibration. The converse seems more likely to me. Even if the wear did cause the vibration, what would have caused that wear in the first place? Scott's point makes me wonder about the first 330 hours you were not operating it.

On the other hand, results are results. Let us know what you feel when you get flying again.
 
Understanding how component wear in the 912 gear box causes vibration requires an understanding of what wears and how the system works.
The damping system uses a spring preload of the 3 dogs on the gear pushing against the big gear and when ever the rotation relationship of the two parts (in the posted photos) exceeds the distance between them, the dogs begin to climb the ramp. What often wears is the base of the ramp so that it no longer has a gentle climb, but the slope of the one part slams into the slope of the other. By the time this wear has occurred, some of the other parts have worn and pre-load has changed so that even at some cruise power settings the damper can be reaching the ends of its travel and lightly crashing the ramps.
This produces a vibration that can be felt, but for someone not familiar, also hard to diagnose.
 
Jim Roman,

When the gears and dogs were taken out, are you saying there was no sign of wear?

That is correct. The Rotax tech that took them out said no visible wear. Very similar to a gentleman in Alaska with a Rans S-7. Ronnie Smith has seen a few of these as well as Rocky Mountain Kitplanes. My engine was produced new in 2015 and so was another that had the same vibration problem. I have operated my engine as per Rotax operating instructions. I believe Rotax has fessed up to some gear vibration problems on the 912is but nothing yet on the 912ULS.

Jim
 
Vibration Problem

My engine was produced in 2012. When I was talking to Morris Smith, Ronnie's son, about the possibility of the gear box gears and dogs being bad, he told he he would know when he took it apart and saw the parts if it was the problem. In my case, it was obvious that there was abnormal wear.

If it had looked normal, I think we would have put it back together and not replaced the gears. But that is speculation.

The spring washers also had abnormal wear on them. They were wearing unevenly.

I ask Ronnie if he could tell if the cause was abuse or what. He said no way to know. He did allude to some defective parts along the way. He also said they have taken the dogs out of the gear box on the 912is engine.
 
vibration

Scott, when you say "Being casual about maintaining a proper carb. sync is a sure way to cause gear box wear.", can you explain what you mean. Do you mean checking and ressetting it every 100 hours? More often than 100 hours? Other inspections besides just syncing the carbs?

I would like some guideance on going forward so I will know I have maintained it properly.

Thanks.

John
 
Scott, when you say "Being casual about maintaining a proper carb. sync is a sure way to cause gear box wear.", can you explain what you mean. Do you mean checking and ressetting it every 100 hours? More often than 100 hours? Other inspections besides just syncing the carbs?

I would like some guideance on going forward so I will know I have maintained it properly.

Thanks.

John

Yes
Checking and adjusting it every 100 hrs.

My "being casual" comment was based on the large # of comments I see that mention initially running the engine without doing a synch (don't have gauges yet), "it felt pretty smooth but I sure could tell the difference after we got it synched properly, etc.

Assuring that the carb's are and remain well synched at idle and low throttle settings is crucial to gear box longevity. If at any time between the 100 hr inspections the engine seems to not be as smooth as it has in the past, it should be checked and the problem resolved ASAP.
 
Yes
Checking and adjusting it every 100 hrs.

My "being casual" comment was based on the large # of comments I see that mention initially running the engine without doing a synch (don't have gauges yet), "it felt pretty smooth but I sure could tell the difference after we got it synched properly, etc.

Assuring that the carb's are and remain well synched at idle and low throttle settings is crucial to gear box longevity. If at any time between the 100 hr inspections the engine seems to not be as smooth as it has in the past, it should be checked and the problem resolved ASAP.

So, with the 912 iS package motor on the 2018 and later RV-12iS builds, if the idle speed is kept above 1800 rpm, the gearbox causing vibrations should be a non issue anymore, due to lack of synching the carburetors? Any other considerations to be aware of to prevent gearbox wear?
 
So, with the 912 iS package motor on the 2018 and later RV-12iS builds, if the idle speed is kept above 1800 rpm, the gearbox causing vibrations should be a non issue anymore, due to lack of synching the carburetors? Any other considerations to be aware of to prevent gearbox wear?

Blade track (blades at the same pitch angle)
Propeller balance.
Excessively worn spark plugs
Etc.

Anything that could have an influence on smooth engine operation
 
Jolly,

Thanks for your list of possible causes. I worked through most of them in an effort to properly diagnose my vibrations. I finally concluded it was the gear box, and disassembly confirmed my conclusion. The dogs were worn as well as he spring washers. They were all replaced, the greabox is reinstalled and I will fly it as soon as the annual condition inspection is over, hopefully Wednesday.

The one item on his list that I would never have thought of was the wheels being out of balance. So as I get mine airplane back together, I will be eager to see the results. If it is not smooth, I will have the prop balanced, then the tires balanced. I can do the remainder of the items on the list.

But I do appreciate your providing a comprehensive list for us to use to trouble shoot.
 
Jolly,

Thanks for your list of possible causes. I worked through most of them in an effort to properly diagnose my vibrations. I finally concluded it was the gear box, and disassembly confirmed my conclusion. The dogs were worn as well as he spring washers. They were all replaced, the greabox is reinstalled and I will fly it as soon as the annual condition inspection is over, hopefully Wednesday.

The one item on his list that I would never have thought of was the wheels being out of balance. So as I get mine airplane back together, I will be eager to see the results. If it is not smooth, I will have the prop balanced, then the tires balanced. I can do the remainder of the items on the list.

But I do appreciate your providing a comprehensive list for us to use to trouble shoot.

John,

Hopefully the gearbox was the culprit. Another item for the comprehensive list is to consider re-clocking the prop one prop flange bolt hole forward or back from present position.
 
John,

Hopefully the gearbox was the culprit. Another item for the comprehensive list is to consider re-clocking the prop one prop flange bolt hole forward or back from present position.

I don't know if re-clocking the prop will have any effect. The gearbox ratio is 2.43:1 which means it is a "hunting tooth frequency" to distribute wear. The odd gearbox ratio constantly changes prop/crankshaft relationship.
 
I have been having vibration problems with my RV-12. It ran smoothly at cruise, but when I reduced the power from about 4600 to 3000 there would be a vibration. I tried several possible solutions: 1, did the friction test on the gear box; 2, weighed the floats to see of they were too heavy; 3, sycnhed the carburetors; 4, replaced the soft engine isolators; and 5, replaced all the muffler springs. Since those didn't fix the problem I then decided it was the Gear Box. Ronnie and Morris Smith of South Mississippi Light Aircraft agreed with my thoughts.

Today I took the gear box to them for repair. The dogs were worn and the disks were worn. All were replaced and Ronnie says my vibrations will be gone. I will be out of town next week so I won't get the gear box back on to know for sure for a week or two. But if any one is interested, here are some pictures of the worn gears and dogs.

The aircraft has 450 hours on it. I ask Ronnie is this was from abuse, bad parts, etc. He said there was no way to tell. I have flown it 120 hours and I have not let it idle at slow speeds.

I am posting these pictures in the hope it will help others understand the problem if they experience vibrations like this.








Is there resolution on your vibration problem?
 
Vibration Problem

I finished the condition inspection and flew the airplane Saturday. There is still a slight vibration when reducing the power between about 4,000 and 3,000 rpm. Other people have told me that is just part of the Rotax engine. But the really bad vibration is gone. The engine was running very smoothly and I was able to sync the carbs perfectly. So the airplane is ready to fly to Sun 'N Fun. I am just hoping for good weather.
 
Hope this does not open a large can of worms - but -

I am part owner of a Prop Balancer. I was having problems getting a good balance. This very light plane would just dance around if you tried to get the throttle up to max static rpm's. It caused problems trying to get repeatability. I thought I would try something since time was not an issue to get it done. Here is what I did :

I set everything up UNDER the cowl. I put the reflective tape on the rear of the spinner plate. I mounted the accelerometer as close as possible to 12 O'clock position. I set the prop so the tach light hit the center of the reflective strip. I put 2 pieces of masking tape on the top cowl exterior so I knew where the correct position of the prop was, and where the accelerometer was. I ran the two cables out the side of the lower cowl leaving the short pin out so the lower cowl could bow out enough to allow the wires out from under the cowl. I ran the two cable into the cabin thru the vent on the right side and put electrical tape over it ( because it is still cool in the Midwest ). I waited for smooth air early morning, and flew the plane with the AP on so it was steady and smooth. I was able to get 3 good 1st readings, then landed and added weight as suggested. After the 3rd flight I was able to get a 'GOOD' reading. Takes a little extra work ( maybe an hour ), but it really solved the problem. Once balanced, I was able to fly and try different RPM's and just rebooted and tried it again at other RPM's. Once away from the first desired RPM balancing, it changed a little at different RPM's, but not much. Happy so far.
 
Great information John. Couple questions:
1. How much did the accelerometer reading (ips) change from the ?in air? reading from the balance you performed on the ground?
2. How about the weight and location of the weight between the two balances, any significant change in that?
3. Which make and model balancer did you use?
4. I?m thinking you zeroed in on the rpms that you felt the vibrations and not necessarily a typical cruise rpm that we would use on the ground such as 4900?

This is very good info. I think you hit the ball out of the park one more time!
 
Well Dave - I'll try - -

1. A bunch. Had a friend help me do the "on ground" balance, and I think we both felt it was good. Took 10.5G at spinner screws to make it say 'good' again.
2. Too much time passed. Don't recall from last time.
3. ACES 1015 Sport.
4. I did balance at 5200 rpm. That is a little higher than I many times fly. 5050 - 5150 are more common for me. Once I was in the 'good' range, I tested at 5450, and 4900. I wanted to know what happened at other rpm's. It went up to and just out of 'good' which is .07 ips upper limit with this balancer. Spent a ton of time "playing" with it, but was interesting. Obviously a person trying to make a living could not spend as much time as I did.

I would like to hear from others that have watched or helped have their RV-12 balanced. On the ground with very low, or no wind, my plane still just dances around when you get to upper rpm's. I think the tail just is not seeing smooth air, and starts the 'dancing', and it will not calm down. Dave, if you have balanced yours, can you run the rpm's up high and have the plane sit fairly smoothly ? Mine never has.
 
1. A bunch. Had a friend help me do the "on ground" balance, and I think we both felt it was good. Took 10.5G at spinner screws to make it say 'good' again.
2. Too much time passed. Don't recall from last time.
3. ACES 1015 Sport.
4. I did balance at 5200 rpm. That is a little higher than I many times fly. 5050 - 5150 are more common for me. Once I was in the 'good' range, I tested at 5450, and 4900. I wanted to know what happened at other rpm's. It went up to and just out of 'good' which is .07 ips upper limit with this balancer. Spent a ton of time "playing" with it, but was interesting. Obviously a person trying to make a living could not spend as much time as I did.

I would like to hear from others that have watched or helped have their RV-12 balanced. On the ground with very low, or no wind, my plane still just dances around when you get to upper rpm's. I think the tail just is not seeing smooth air, and starts the 'dancing', and it will not calm down. Dave, if you have balanced yours, can you run the rpm's up high and have the plane sit fairly smoothly ? Mine never has.

JB, At spinner screws? Is weight bolted on prop back plate?
 
I have balanced 5 or 6 12?s, usually with the owners at the controls, but I have done my own by myself and can?t recall the dance as you say. I did one yesterday. Will find out if Mark noticed this movement. I suppose you notice this at a static rpm check of the prop?
 
JB, At spinner screws? Is weight bolted on prop back plate?

Hound, we typically install the weights under the spinner screws until we reach the best solution. When satisfied we then attach the weights to the web of the spinner backing plate.
 
Dave -

Maybe it is an issue with my plane. That is why I wanted to hear from others. When at lower rpm's, my plane will sit fairly firmly. Once I get to 4500 and up, it just moves. It will not sit still. Not actually vibrating, just moving around and not stable. My engine mounts seem very solid. Not at all soft. Flying, in smooth air, it is very smooth. I balanced my prop fairly early in its life. I remember the same thing then, but we got it done with a Dynavibe balancer. Dave, if you have a quiet air day, maybe you can take yours outside and video it with the rpm's at max static you can get.
 
Maybe it is an issue with my plane. That is why I wanted to hear from others. When at lower rpm's, my plane will sit fairly firmly. Once I get to 4500 and up, it just moves. It will not sit still. Not actually vibrating, just moving around and not stable. My engine mounts seem very solid. Not at all soft. Flying, in smooth air, it is very smooth. I balanced my prop fairly early in its life. I remember the same thing then, but we got it done with a Dynavibe balancer. Dave, if you have a quiet air day, maybe you can take yours outside and video it with the rpm's at max static you can get.


First chance I get, will do. I have your phone number and will send it to you.
 
Dave - -

If you really want to spend some time, set it up like I did and see what it reads in the air. Also, what happens at different RPM's. Just reboot ( in the air ), and test at whatever rpm's. Anxious to hear from your experience. Had an engineering friend say testing while flying is the best of all. That is where you want the balance to be best. Agree with that. Will be happy to hear ANY info you provide. Thanks,
 
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