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Kokanee 15-06-2020 15:04

Vibration damper failures
I repowered a couple years ago, and installed a Vetus M4.45 (42hp) with a ZF25MA gearbox. At the same time I installed a flex coupling between gearbox and driveshaft.

All was good for a while. Drivetrain nice and quiet, but a rattling started to develop when in gear at idle. It sounds like it was coming from the gearbox. It slowly got worse with a very loud rattling at anything under 1600 rpm.
At 250 hours I replaced the Centa vibration damper. These dampers have rubber "bumpers" rather than springs to control the vibration. The bumpers were heavily worn

All was good for a while but slowly exactly the same situation deveIoped. Loud rattling when in fwd or rev gear at low rpm. At 500 hours, I again contacted Vetus.

This time they suggested installing a traditional spring type damper plate. I've installed this and the rattling is still abnormally loud with the brand new damper plate.

Any suggestions on what to do? Do I just have a poor match between engine and gearbox? (gearbox is next size up from what was recommended)
Change the gearbox? Remove the flex coupling? The alignment has been checked several times.

slug 15-06-2020 15:20

Re: Vibration damper failures
Dampers last a long time if your engine shaft alignment is correct

Check your alignment

Your prop shaft creates thrust

This thrust pushes the engine forward

If your engine mounts are to soft or not up to the task , alignment suffers when subject to thrust

a64pilot 15-06-2020 15:47

Re: Vibration damper failures
Could be a couple of things, one is mass of the prop, what kind of prop do you have? Is is particularly heavy?
How about your idle, if it’s low that can contribute to early wear as well. The flex coupling I don’t think should contribute, but I guess it could, why do you have it?
Then to be honest it could also be the engine itself, some engines have what amounts to powerful low RPM pulses, that is actually a typical Diesel. The purpose of a dampener plate is to absorb those pulses that exist in any piston engine but more so in a diesel, the lower the RPM the more prelevant these pulses are.
The dampener plate really only comes into play when your in gear of course so try to avoid low RPM in gear.

Actually now that I think about it I can see how a flex coupling could contribute as it flexes then releases the energy it initially absorbs, it could get out of phase with the flex plate and that could result in a harmonic feed back that could amplify the engine pulses.

The flex plate is of course meant to be what is a flexible link between the pulses coming off the crankshaft to the transmission, maybe a second flexible link could cause issues?

Kokanee 15-06-2020 17:25

Re: Vibration damper failures
Thanks for the suggestions. Here's a bit more history and info.

Slug - I checked the alignment numerous times; I've even temporarily misaligned it a bit just to see if it made any difference. Still the same.
The engine mounts were replaced with recommended Vetus when the re-power was done.

A64 - The Austral prop is a fairly standard 3 blade bronze 17" diameter x 11"pitch.

I agree that diesels always have a pulse that can get out of balance. The old Volvo MD17 had a heavy flywheel to deal with it. The new lighter diesels are more susceptible.

I added the flex coupling|70mm.15|4 when I re-powered just because I wanted the engine a bit further forward and didn't want to replace the driveshaft. Moving the engine back will be a PITA but I may see if I can pick up a solid steel spacer to replace the flex coupling.

Thumbs Up 15-06-2020 17:50

Re: Vibration damper failures
You could have input shaft spline wear. It is a big issue with certain configurations. Something is incompatible and causing abnormal wear. But the "all was good for awhile" clue... think you are better off with the Centa which might have been used to solve the incombatability. Can they rebuild the Centa with new bumbers? Maybe it has a guarantee.

DougR 15-06-2020 18:47

Re: Vibration damper failures
In general, within the marine engine business, the selection of the flywheel torsional damper is the responsibility of the individual or company who assembles the package. Normally this means the engine manufacturer or packager.

It’s normal for the engine manufacturer to run a series of TVA/TVCs (torsional vibration analysis/torsional vibration calculations) when matching an engine/flywheel damper/ gearbox combination. This analysis is a study of the total mass elastic data of everything that rotates within the drive system. This includes accessories driven from the front of the crankshaft, the engine itself, the flywheel moment of inertia, the flywheel damper, the gearbox, the propshaft coupling, prop shaft, and propeller. All of this data is fed into a computer program to assist with the analysis.

In the case of say, an 80’ commercial trawler with 1000 h.p. engine, 6:1 reduction gearbox, 8” diameter prop shaft 20’ long and a 4 blade prop 70” in diameter, it can be critically important to get the analysis correct. If it isn’t correct, torsional spikes in the drive train can break crankshafts, or shear an 8” propshaft, or wreck a transmission.

In the case of a small sailboat engine, the stakes aren’t quite so high, but it’s still important for safety, reasonable component life and low noise levels. The fact that you noted a high degree of wear on the Centa damper components tells me that the engine is probably what I would call torsionally “active”, and the damper is absorbing a good bit of energy. As mentioned earlier, all of this can be a combination of the mass of all the components in your system, and it can also be something as simple as an idle speed set too low, or lack of ventilation in the flywheel housing, and the damper is getting too hot.

I would start by going back to Venus and discussing it with them. I can guarantee you they have seen the problem before. I would also contact Centa and ask for their advice/help. When you talk to them have all of your system data at hand....such as engine and gear box model, damper model, approximate shaft coupling size, drive saver info,shaft diameter and length, prop diameter, number of blades, manufacturer and model, etc. Centa has lots of experience with this and they have the technical ability to do a torsional analysis. The best option, of course, is to get Vetus to do this leg work for you, as they are ultimately responsible for the compatibility of the package.

Jim Cate 15-06-2020 19:14

Re: Vibration damper failures

Good post, Doug!


Kokanee 16-06-2020 00:49

Re: Vibration damper failures
Thanks for the input, gents.
I've had plenty of discussion with Vetus. They've been helpful and I have supplied the two replacement dampers without cost. Vetus has discussed the issue with Centa to come up with the original package. I opted for a gearbox a step up from their standard package for the 42 hp diesel. After 2 failures, Vetus decided to go with their standard spring damper which has been consistently noisy since installed.

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