Gearbox rattling

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AusCan

AusCan

Guru
Joined
Jul 15, 2012
Messages
3,218
Location
Australia
Vessel Name
Kokanee
Vessel Make
Cuddles 30 Pilot House Motor Sailer
I re-powered about 2 years ago with a 42 hp Vetus diesel (Mitsubishi based) and a ZF-25A gearbox.

The gearbox is a ~7 degree downangle design and a 1.98:1 ratio. The ZF-25A is one size up from what Vetus was originally offering with this engine.

The gearbox was always a little bit rattly when in gear at low rpm (850-1000rpm), although just recently it seems to gotten slightly worse (rattling from 850- 1200 rpm). The rattle is louder than the actual engine noise. Over 1200n rpm it is nice and quiet.

I've heard a variety of opinions from "this is normal" to "change from ATF to a 10/40 engine oil". A search on the web suggests this is a common problem with many gearboxes, but opinions vary considerably as to what to do about it, or if this is anything more than a nuisance noise.

Any suggestions from the TF forum?
 
Could be the damper plate failing. It attaches to the engine flywheel and drives the gear input shaft often using springs for pulse and shock absorption. Eventually the springs fail and break. That can cause the noise and if left will fail completely leaving you without drive.
One possibility.
 
C lectric said:
Could be the damper plate failing. It attaches to the engine flywheel and drives the gear input shaft often using springs for pulse and shock absorption. Eventually the springs fail and break. That can cause the noise and if left will fail completely leaving you without drive.
One possibility.
Click to expand...

Could be, but I've never heard of one failing when new. (the noise was there right from the start) Less than 300 hours on the engine & gearbox now.
 
At low rpm the gears just aren't loaded up and the sound is gear slap when the gears are torqued up they load up and the tolerance between the gears reduces so dose the noise .
 
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Had a relatively new one loose a punch out and thee punch out balled up and created all kinds of funny noises at different RPMs.

Not likely if your sound is constantly similar at the same rpm as you vary them...but a weird one for me.

Maybe yours isn't failing bit loosened up somehow?
 
Spring type torsional couplings are wear items. As they age, their performance degrades. A ZF 25 I think is a mechanical gear and it will be a noisy beast behind a four banger at low rpm.

Those gears are usually spec'd for sailboats where the noise is not an issue for various reasons.

There is gear rattle from the natural torsional reversals from the engine, but also fwd torque is what keeps the clutches in the gear engaged. The torsional reversals alternately engages and disengages the clutch, and that wears the mechanism out.

Try to avoid running where it rattles. Eventually you will get poor engagement in fwd.

Where noise would be an issue, hydraulic gears are spec'd with rubberized torsional couplings.

That's why they developed the "Velvet Drive" and trademarked it. Soft shifts and a quiet drive.
 
I think Ski nailed it. A mechanical gearbox like yours can be noisy under certain circumstances.

There is even a disclaimer in the latest ZF 25 A factory brochure:

Torsional Vibration and Torsional Couplings

The responsibility for ensuring torsional vibration compatibility rests with the overall propulsion system integration responsible party. Compatibility check of torsional vibration must include excitations induced by engine governor. ZF cannot accept any liability for gearbox noise or for damage to the gearbox, the flexible coupling or to other parts of the drive unit caused by torsional vibrations. Contact ZF for further information and assistance.

And from the ZF Repair Manual: ZF25A, ZF25, ZF45A, ZF45-1:

Troubleshooting

14. Chattering transmission noise mainly at low engine speed

14.1 The engine or propeller generates torsional vibrations in the drive unit which produces a hammering noise in the transmission.

14.1 Mount a flexible coupling with a different stiffness factor between en-gine and transmission; a coupling with a higher stiffness factor might be sufficient. Otherwise analyze the torsional vibrations to find out the re-quired stiffness for the coupling.

If your damper plate is OK (no loose springs), perhaps a flexible coupling might help mitigate some of the noise. Just a thought:thumb:
 
With the Kansaki gear that I had on my Yanmar 3cyl 12-15 yrs ago I only had the problem when the gear oil got fully warmed up. Comming back to the marina I was embarrassed. Sounded like the trans was falling appart.

At first I used heavier oil. Hardly made any difference. Then I used ATF and 90W gear oil mixed. Better.

Eventially I installed an oil pump and an oil cooler. Used the drain hole and the fill hole for circulation. Used a mix of gear oil, synthetic gear oil and some ATF. That did fine. Almost quiet.

Ran the boat for about 4 years after that and suffered no downsides. Other than the expensive oil pump. It was a 32V unit all made of bronze. It ran slow of course but that was ideal. Do'nt remember what I used as a cooler. I think it was a very small unit like a typical oil cooler. Probably hooked it up w a seawater intake hose. No more clatter though.
 
I think Volvo had a rash of ZF 25 (they rebrand them MSS 25) failures behind their D2s. Not sure if it was infant mortality or what. I have the same box with a different ratio, and no issues yet.
 
I think I saw an upgrade to use a different damper plate. I should mention that I have a flexible coupling that the PO installed, that I kept during the repower. I think he did that when he took out his previous gear behind the BMW engine. But he trolled a lot.
 
Thanks for the suggestions, guys.

Yup - I'm in agreement about the noise being caused by torsional vibration.

I did add a flexible coupling when I did the re-power. Hard to say if it has helped the issue as I installed it when the package was new.


The gearbox also came with a cooler so it doesn't see very high temperatures, although it is quieter idling out of the marina in comparison to idling back in. I am thinking that the noise could be slightly higher recently just due to hot weather.

I guess this is one of the negatives of new lightweight engines. I should have kept the big heavy flywheel off the old Volvo MD17 and bolted it on the Vetus :)

Here's the flex coupling I have installed.
 

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Northern Spy said:
I think Volvo had a rash of ZF 25 (they rebrand them MSS 25) failures behind their D2s. Not sure if it was infant mortality or what. I have the same box with a different ratio, and no issues yet.
Click to expand...

I did read about some ZF issues when deciding on which gearbox to use. In the end I decided to go with the down angle of the 25A to keep the engine somewhat level. Vetus was recommending the 15A but I decided to go one step up to the 25A (bigger is always better?). Maybe that wasn't such a good idea.

I also noticed that the ATF capacity of the ZF-25A is much less than the ZF25.
 
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Auscan,
I had a much more flexable coupling than the orange guys. And I changed my damper plate to one that had white plastic wedges instead of springs.
Sorry I should have mentioned that. Both the aforementioned items worked good. I think it was an RD coupling.

Re the 25A to 25A that probably was a good move. You may only have half the ratteling that I had.
 
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Maybe I will shop around and see what is available for damper plates. I can then pull the gearbox and have a look at it. If it looks fine, i'll have a spare which probably isn't a bad idea.
 
I had a ZF without a flexable coupling, connected to my 6,5 L Gm V8 diesel, and it rattled at lot, under 800 rpm, so I avoided to drive lower than 800 rpm, worked for me in the six years, I drove the boat. This was in my Coronet 31, and I'm shore it won't be a problem for the next owner. Lowest speed without noise, was 4,7 knots.if I wanted to drive slower I djust put her out of gear.
 
C lectric said:
Could be the damper plate failing. It attaches to the engine flywheel and drives the gear input shaft often using springs for pulse and shock absorption. Eventually the springs fail and break. That can cause the noise and if left will fail completely leaving you without drive.
One possibility.
Click to expand...



Bingo!

I pulled the gearbox today and had a look at the back of the damper plate. It was a Centa DS damper plate. They don't have springs or roller pins like some. Just rubber bumpers. One set for low power dampening and another set for high load dampening. Guess what... the low power bumpers were flogged out.

The Vetus dealer will have a new damper plate for me tomorrow under warranty. I should be ready to go again by the weekend. Woohoo!
 
AusCan said:
Thanks for the suggestions, guys.

Yup - I'm in agreement about the noise being caused by torsional vibration.

I did add a flexible coupling when I did the re-power. Hard to say if it has helped the issue as I installed it when the package was new.


The gearbox also came with a cooler so it doesn't see very high temperatures, although it is quieter idling out of the marina in comparison to idling back in. I am thinking that the noise could be slightly higher recently just due to hot weather.

I guess this is one of the negatives of new lightweight engines. I should have kept the big heavy flywheel off the old Volvo MD17 and bolted it on the Vetus :)

Here's the flex coupling I have installed.
Click to expand...

I am interested in your split coupling attaching to the prop shaft behind the red flex coupling, who makes that?

I am planning to get one like that
Are they available for velvet drives?

Buck Algonquin is one type I found, does yours completely split in half?
splitcoup.jpg
 
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The "split" coupling is a good way to go. It does not split completely, but has a cut through it that allows it to flex and squeeze the shaft when you tighten the big bolts. Much better fit than the alternative solid couplings.
 
AusCan,
I think you said in another thread that you fish. That usually means trolling and that's very hard on damper plates. You may want to consider plastic wedge type dampers. May be better for fishing.
I don't slow idle so have no need for anything but the usual springs.

Do you have Poly Flex engine mounts? I do and mine are over 10 years old and have considerable rust on the cast iron bases. You may want to take steps to keep yours from rusting.
I think the Poly Flex mounts do very well with vibration. You agree?
 
Eric,

Yes I do fish, and that may be one of the reasons the damper plate wore out prematurely, although there was some rattling noise right from when the engine/gearbox was new.
It may have been a faulty damper plate.

I may have damaged it when I got some rope wrapped around the prop, or a less than perfect alignment may have contributed. Its hard to say.

At this stage, I'm replacing it with the same, (Vetus are replacing it under warranty). If it fails again, we'll try something else. I still have another year of warranty.

According to the Centus literature, the 2 stage design of these damper plates are specifically made to stop low rpm rattling.

http://www.centa.info/data/products/35/int/cf-ds-09-04.pdf


btw- no - don't have Poly Flex engine mounts. Just the standard Vetus mounts. The base on mine are rusted already as well.
 
Ski in NC said:
The "split" coupling is a good way to go. It does not split completely, but has a cut through it that allows it to flex and squeeze the shaft when you tighten the big bolts. Much better fit than the alternative solid couplings.
Click to expand...

They do grip well. My prop shaft is unusual in that it doesn't have a keyway at the gearbox end, so the coupling relies only on the squeeze to stop it from slipping. No sign of slippage so far.
 
Today I removed the bell housing so I could pull off the damper plate. A bit of a PITA because I had to jack up the engine and remove the rear engine mounts, starter motor, and fuel pump & filter assembly. I managed with minimal skinned knuckles.

The damper plate has four wedge shaped bumpers which compress as load is increased. One of these was split which caused misalignment and wear on the others. I'm guessing the split bumper may have occurred soon after it was installed as the gearbox was a bit rattley since new.
 

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Very interesting, Mr Deckrot.

Hopefully my ZF is all good. It's just that damper plate that's not playing the game.

I'm holding off replacing the damper plate with the same model until the Vetus & Centa engineers review the failure and decide whether an upgrade may be more optimal.
 
OK - The damper plate has been replaced with the same Centa model DS. The old one was completely thrashed.
I aligned the prop shaft to within .002" gap at the gearbox coupling coupling.

I started her up and took it for a test run. It idled along in gear very quietly and smoothly, better than it ever had. I slowly took it up to 2000 rpm, then to 2400 rpm . It was generally quiet, but I could hear a slight harmonic vibration which pulsed every second or so. Slowing the revs to 1800 slowed the pulsing.

After 30 minutes, I dropped back down to an idle, and listened. The harmonic pulsing stopped at under 1500 rpm, but I now had a very slight rattle from the damper plate again. :facepalm::banghead:

The harmonic pulsing has always been there since I owned the boat. The sound isn't obvious but it is getting slightly worse.

I am now thinking the root cause of this may be a bad stern tube.
My prop shaft is only about 3 feet long. I'm guessing the stern tube is only about 8-12" long. (no strut, cutlass bearing etc).

How much up/down or side to side play should I have at my prop shaft flange?
 
Some gear rattle is normal, even with everything new and perfect.
 
Surely you'll have a cutless bearing at the water end of the shaft tube. It will have the same rules for slop that any other would have.
 
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