Vibration Problems

pilotyoung · #1 02-28-2019, 10:35 PM

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.

Piper J3 · #2 03-01-2019, 12:08 AM

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?

pilotyoung · #3 03-01-2019, 08:38 AM

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.

rvbuilder2002 · #4 03-01-2019, 09:05 AM

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.

Piper J3 · #5 03-01-2019, 10:36 AM

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...

dbhill916 · #6 03-01-2019, 11:32 AM

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.

romaja · #7 03-01-2019, 11:53 AM

Below is my post from several months ago. Magnetic plug clean. My carbs were always synced and dynamic balance was done from the start. No signs of gear wear. Once the gears were replaced the vibration was gone. Albeit, at a considerable expense.

Link:
http://www.vansairforce.com/communit...=912+vibration

rvbuilder2002 · #8 03-01-2019, 01:04 PM

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.

pilotyoung · #9 03-01-2019, 02:39 PM

Jim Roman,

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

romaja · #10 03-01-2019, 04:11 PM

Quote:

Originally Posted by pilotyoung

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