Engine Balancing and Resonance Vibration Problem

Originally Posted by Fly

I concur with ToeCutter.
Especially in your case, you need two vibration sensors, fore and aft, to
gather info on the engine and prop. You can surmise what the crankshaft
balnce quality is. You cannot do it with one sensor alone unless you move
the sensor to different locations during subsequent runs..

Your post stroungly suggests you need to get find someone of experience to
give you good advice.
I wonder where are you located?

Wish I could help with a good answer but internet makes it difficult.
Answer is only a good as the information provided.

couple of other things:

Sounds as if the operator has a Micorvibe DSS prop balancing rig.
Their manual suggest checking the balnce at 2000 rpm.
Usually I find higher amplitudes of imbalnce and sometimes different phase
at higher rpms.

The previous balance of 13 washers was beyond the FAA approved maximum
correction.

After 600 hours, the isolation mounts can take a set, maybe even have
metal-to metal contact between the bolt and engine mount.
I recall Toecutter suggesting at least a rotation every 500 hours.

Doublecheck the ring gear installation. It will only go on in one position.
And verify the crankshaft index bushing is in the correct location for your
engine.

How are you percieving the resonance at 1500? How did you measure the freq?
have you consider that it might be an idle mixture problem?

good luck

Kent Felkins
Tulsa Ok.

Reply below in quotes:

Originally Posted by Fly

I concur with ToeCutter.
Especially in your case, you need two vibration sensors, fore and aft, to
gather info on the engine and prop. You can surmise what the crankshaft
balnce quality is. You cannot do it with one sensor alone unless you move
the sensor to different locations during subsequent runs..

"Thank you for the suggestion. It is something to learn more about and try."

Your post stroungly suggests you need to get find someone of experience to
give you good advice.

"That is why I have posted this request for information. We are learning a lot through this process. The feedback is good and opens up a number of things to think about and try. There is a problem that probably was in the engine when it was new. The prop overhaul has either added to the problem or heightened the concern. One would think the engine/prop should be smooth as silk since the engine is relatively new and the prop has been just overhauled. I spend a lot of money keeping the plane in good repair. I have owned it since 1985. When it is not right, I know it quickly."

I wonder where are you located?

"Nashua, NH"

Wish I could help with a good answer but internet makes it difficult.
Answer is only a good as the information provided.

couple of other things:

Sounds as if the operator has a Micorvibe DSS prop balancing rig.
Their manual suggest checking the balnce at 2000 rpm.
Usually I find higher amplitudes of imbalnce and sometimes different phase
at higher rpms.

"It is a chadwick-helmuth spectrum analyzer."

The previous balance of 13 washers was beyond the FAA approved maximum
correction.

After 600 hours, the isolation mounts can take a set, maybe even have
metal-to metal contact between the bolt and engine mount.
I recall Toecutter suggesting at least a rotation every 500 hours.

Doublecheck the ring gear installation. It will only go on in one position.
And verify the crankshaft index bushing is in the correct location for your
engine.

"We Will double check the ring gear."

How are you percieving the resonance at 1500? How did you measure the freq?

"The vibration is by feel as the engine RPM is reduced from 2000 to idle slowly. The vibration increases as one slows to 1500, peaks at 1500 and decrease to normal engine vibration."

have you consider that it might be an idle mixture problem?

"No, net yet, but something to consider."

good luck

"Thank you. Next step is to double check the prop at the prop shop."

Kent Felkins
Tulsa Ok.

You message suggests that the washers are bolted at the holes on the
circumference of the ring gear. IIRC, your prop model has the extended
hub. Extended hubs do not respond very well to balance correction at the
ring gear because the plane of correction is not close to the plane of
imbalnce. Sure, a balance can be achieved but the amount of correction
weight will be 30-50% more that if placed closer to the plane of imbalance.

After the prop is installed again, the final dynamic balance should be
checked above 2300 rpm. 2000 rpm readings are not valid.

Kent Felkins

Originally Posted by Fly

You message suggests that the washers are bolted at the holes on the
circumference of the ring gear. IIRC, your prop model has the extended
hub. Extended hubs do not respond very well to balance correction at the
ring gear because the plane of correction is not close to the plane of
imbalnce. Sure, a balance can be achieved but the amount of correction
weight will be 30-50% more that if placed closer to the plane of imbalance.

After the prop is installed again, the final dynamic balance should be
checked above 2300 rpm. 2000 rpm readings are not valid.

Kent Felkins

Kent,

The balancing washers were bolted on to the bulkhead not the ring gear. The ring gear has nothing bolded on it. The ring gear appears to be symmetric with bold holes. We will be checking the see if the ring gear has a preferred bolt hole orientation when the prop gets back from the prop shop.

I will post results of the prop shop findings and progress in solving all vibration problems.

Thank you and everyone’s comments. They have been most helpful as we move forward.

Allan

On Sun, 11 Sep 2005 14:42:40 +0100, AllanFuller
wrote:

snip

The balancing washers were bolted on to the bulkhead not the ring gear.
The ring gear has nothing bolded on it. The ring gear appears to be
symmetric with bold holes. We will be checking the see if the ring
gear has a preferred bolt hole orientation when the prop gets back from
the prop shop.

The ring gear support is "symmetric" with regard to the bolt holes
(technically, they are nut holes...). But, one hole should have a
slightly larger ID to match a corresponding nut pressed in the crank
flange with a slightly larger OD. The other clue for proper
orientation are the timing marks on the ring gear support.

I would hope that the person/s that are maintaining your aircraft are
aware of this.

TC

snip