Gearbox rattling

[Steve DAntonio]

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.

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?

A bad stern tube? Can you elaborate? Do you mean shaft bearing? If it's getting worse it could very well be a worn bearing, that usually creates a hum or rattle. Clearance between the bearing and the shaft is established in the ABYC standards, for a 1.5" shaft, for instance, it is 0.004" - 0.009". This can be measured with a feeler gauge.

You mentioned "How much up/down or side to side play should I have at my prop shaft flange?" Do you mean when it is disconnected from the transmission? If so, that's entirely a function of the stuffing box arrangement, which limits shaft motion to a degree, and the gap between it and the flange. For a short shaft like yours, not much.

It is possible the shaft is making contact with the stern tube, while still being properly aligned, I've seen this several times. In most cases it will generate a noise and/or vibration, as well as wearing away the tube. You'd need to disconnect the stuffing box and look at the shaft where it exits the tube to know if this is occurring. Ideally that's best done while hauled, but it could be done in the water, in a pinch, if you aren't too squeamish about some water coming in. You might also be able to feel it making contact with the flange disconnected, but that may be difficult if you have a stiff stuffing box hose. If it's a dripless seal and a more pliable hose then that's easier to make that determination.

 

[Steve DAntonio]

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.

If the flexible coupling had no effect I'd ditch it for two reasons. One, they are failure prone, and two, it's designed to absorb slight misalignment between flange faces (if you ask it to do too much it overheats and fails), not any significant amount of torsional vibration. The flexible coupling mentioned in the ZF manual, otherwise known as a damper plate, is as it says, located between the transmission and the flywheel, not between the shaft and trans couplings. Oh one more reason, I've never noticed any difference in noise or vibration what so ever aboard vessels equipped with flexible couplings of this sort.

Additionally, adding it, unless you shortened the shaft, has extended the clearance between the aft end of the stern bearing and the leading end of the prop hub. That clearance should not exceed one shaft diameter (one and a half at most). If it's now greater that adds stress to the shaft in this area, and accelerates shaft bearing wear.

I've removed (broken examples of) these on many occasions and installed a shim in their place in cases where the shaft had been shortened. PolyFlex, btw, is located in Brisbane, Australia. I've been to their factory, they are a squared away outfit that makes some very nice polyurethane motor mounts, I just don't care for flexible couplings.

If the shaft has been shortened to accommodate them, it's impossible to carry out a proper alignment, in which both coupling faces must meet, without temporarily removing them and inserting a purpose-made precision shim (Spurs makes these).

Regardless of who makes them, I've seen too many flexible couplings fail, and they become an excuse for many mechanics and DIYers to get sloppy with alignment, as they rely on them to take up the gap.

On the subject of ZF trivia, it's a fascinating company, Zeppelin history and all. The company is actually owned by the city of Fredrichshafen, I visited their factory a few years ago in preparation for writing this article. http://stevedmarineconsulting.com/wp-content/uploads/2014/03/ZF-Marine-PBB-134-1.pdf

 

[Nomad Willy]

AusCan,
From your text it sounds like the only time your boat rattles is after it's run for some time.
IMO the rattle is comming from the gears permitted by the lowered viscosity of the warmed up gear oil. Run it for a couple of hours and the rattle will be even worse.

Because everything wasn't right and I had very soft Yanmar mounts w a 1' shaft it wobbled around and knocked on the stern tube a bit. Been there done that but it had nothing to do w the low speed gear rattle. And the harmonic vibration wasn't part of that either.

For me anyway it was just low viscosity lube oil in the gearbox. The only thing that had a significant affect on the rattle was increasing the viscosity of the oil. I used a mix of 90-140 gear oil and high vis synthetic oil. Synthetic because it decreases it's viscosity less when warmed. AND the oil cooler I cooked up and installed. Quiet gears in the end. But I sold the boat w the harmonic vibration still doing it's thing.

Now I have a very quiet and smooth BW VD. Love it.

 

[AusCan]

Ski -
I may have become paranoid about gearbox noise. If I listen too hard at boat noises my mind hears all kinds of disasterous things happening.

Steve- Thanks for your in depth reply(s).
I didn't shorten my prop shaft to fit the flex coupling. The new Vetus engine was much shorter than the old Volvo MD17. The prop shaft is still within 1/4" of its original position.
The harmonic hum I hear has been fairly constant since I bought the boat. I've replaced engine, gearbox, and prop, so the only things left are the shaft and the shaft bearing. I think the harmonic hum may be getting worse or it may be just me listening harder.

Eric
There is some truth to what you say about the viscosity. I fired up the boat today (a colder day) and idled in gear. No rattle at all.
The Vetus people weren't so willing to try a heavier oil in the gearbox while its under warranty.