Rattling sound from ZF15M transmission in Neutral

[thematt]

Hey folks,


Recently had some engine work done and a new thrust plate installed.


I noticed some rattling that I don't believe was present before in neutral. Mechanic said it's nothing to worry about unless there's issues shifting, but was hoping someone with a similar transmission could confirm the 'neutral rattle' as I don't wanna be out of commission again for something that could be as simple as adjusting the shift cable.


Symptoms:
Neutral: Rattle from transmission or fly wheel area.


Forward and reverse: No rattle, quieter with load.


I'll try and find a way I can upload the video or sound bite so y'all can hear what I mean.


Thanks!

[thematt]

Hey folks, I uploaded the video to Reddit, hopefully it stays up long enough for y'all to review.


https://www.reddit.com/r/sailing/com...l_normal_zf15/

[GILow]

Sounds like a loose or worn damper plate to me.

[thematt]

Thanks for your reply!



The damper plate is brand new, I referred to it as a 'thurst plate' but I meant damper plate. I suppose it could be loose. Wish I had a better photo of it, but I did see it installed and looking good before the transmission went back on.


I saw in a old forum some folks mentioning ZF transmissions just rattle in neutral. Any possible truth to that?

[skipperpete]

There are some really odd driveplates on the market (Vulkan for instance) that work amazingly well under load but are noisy at idle and there are several ways to install a new drive plate that will cause it to be noisy. If the holes in the new drive plate are larger than those in the old plate it’s possible to get the plate off centre (in the absence of a pilot recess) and then there’s putting the drive plate in back to front, they are rarely symmetrical and can cause severe damage if they apply pressure longitudinally between the flywheel and the unsplined part of the gearbox input shaft.

[thematt]

Thanks for the reply, I dug up the drive plate that was installed.


R and D MARINE : Marine Damper Drive Plates


R&D 22W4, it's a non spring plate, seems similar to the vulkan you mentioned. If you check out the video, if you haven't, it's very quiet in forward with less vibration...same in reverse.



Would that be the case if the plate wasn't tight or installed wrong?

[GILow]

Originally Posted by thematt

Thanks for your reply!



The damper plate is brand new, I referred to it as a 'thurst plate' but I meant damper plate. I suppose it could be loose. Wish I had a better photo of it, but I did see it installed and looking good before the transmission went back on.


I saw in a old forum some folks mentioning ZF transmissions just rattle in neutral. Any possible truth to that?

I strongly suspect the people who think their ZF transmission is rattling are actually hearing the damper plate, not the transmission. No real justification for this belief except the fact that the transmission can often handle more than ten times the HP of the engine they are connected to.

[thematt]

Originally Posted by thematt

Thanks for the reply, I dug up the drive plate that was installed.


R and D MARINE : Marine Damper Drive Plates


R&D 22W4, it's a non spring plate, seems similar to the vulkan you mentioned. If you check out the video, if you haven't, it's very quiet in forward with less vibration...same in reverse.



Would that be the case if the plate wasn't tight or installed wrong?

I've reached out to the damper plate folks as well. thanks for your comments. It's helping narrow things down

[DougR]

Everyone refers to the part that you installed as a damper plate, when a more accurate name is a “torsional damper plate”. That is , one of the main functions is to dampen torsional vibrations between the engine and transmission. Every diesel engine produces torsional impulses on the crankshaft as a result of the compression and firing stroke, and the fewer cylinders the engine has, the more noticeable the pulses are. So a six cylinder engine produces six pulses every two crankshaft revolutions, and a two cylinder engine makes two pulses every two revolutions. Therefore the pulses on the six cylinder are more closely spaced and “smoother” than the two cylinder. We normally think of a crankshaft as rotating smoothly, but in reality the crank is really twitching it’s way around, particularly at idle speed on engines with few cylinders.

When you are running your engine at idle and in neutral these pulses (twitches) are fed into the transmission and they can cause meshed gears to get into a “clatter”, where the natural gear backlash allows one gear tooth to rattle against its neighboring meshed gear tooth. This clatter is what you are hearing, and it might be harmful to the gear tooth, or it might be harmful to the splines on the transmission input shaft.

When you shift the transmission into gear, the power is now fed all of the way thru the box to the shaft and prop, and the resistance of the prop against the water pins all the gears together and gets rid of the backlash, so the clatter noise disappears, because the backlash disappeared.

So how to fix it....? The way to resolve this problem ( not necessarily easy) is to install a torsional damper which can absorb the torsional spikes at idle but still be strong enough to transmit full engine torque at full throttle. Usually this means a damper with two or more resistances.
For example, many spring damper plates are equipped with two or three different spring strengths. Rather soft springs to soak up the torsional spikes at idle and stiffer springs to transmit full torque at w.o.t. Many manufacturers of small marine engines use the Centa DS dampers. (DS =dual stage). Softer rubber pads to absorb torsionals at idle and more firm rubber or even pin against stop for full power.

The selection of this ideal damper is typically the responsibility of the person or company who marries the gearbox and the engine. If I had to guess I would say that the R&D damper which was installed is not well matched to the engine and transmission characteristics that you have. Maybe R&D can offer additional guidance.

[thematt]

Wow Doug, you are a legend. Thanks for the very thourough explanation! I'll reach out and see what R&D says.


Sounds like it the mean time I should minimize time in neutral, or would this be a don't run the engine moment for you?

[DougR]

Wouldn’t hurt to avoid lengthy periods of idle in neutral.....

And it might help if you can raise the idle rpm. This changes the pulse frequency and might avoid exciting components.

[thematt]

From what I could find on the specific model of damper plate, it's a 100ft lbs plate, while my engine W50(41HP) peak torque is 85ft/lbs, and 75 continuous.



Appologies for the ignorant question here, but sounds like the plate is just to beefy for my engine, is that accurate?


I found an an online product selector from R&D and it spit out the exact model I had installed. The plate I had removed was about to go, could it be the transmission just needs a rebuild?


I'll try increasing idle a bit. Right now it's 730-50 rpm, perhaps there's a sweet spot near by.

[DougR]

It could be, but I doubt the 100 ft.lb. damper size is too beefy for your engine, as it provides an adequate safety factor. However it might be that the “high deflection” model damper in the same torque capacity would have been a better choice.
It’s better suited to light flywheel engines that may be more torsionally “active”.

What type of damper was installed originally?

[skipperpete]

That’s an excellent description of damper plate function DougR.
Since I was recently involved with a damperplate disaster, I can suggest a few quick checks that you can make. The problems can be that
1. The drive plate is off centre on the flywheel ( very rare )
2. The splines on the gearbox shaft are worn badly ( reasonably common)
3. The hub of the plate is sitting hard against the flywheel ( in the recess)
4. The splines in the plate are bottomed out on the gearbox shaft spline.
You can check only one of these without removing the gearbox, #4. If this situation exists, the crankshaft endfloat ( measurable at the front pulley usually with a dial indicator) ceases to exist and if the interference is severe, bad things happen to both the drive plate and the crankshaft. Light interference would be similar to the load that an automotive clutch applies to crankshaft when you push the pedal down but heavy interference will really damage the thrust bearing on the crank.
Or you might just have a noisy gearbox, most mechanics would check 1-4 as part of the job.

[thematt]

Thanks SkipperPete and DougR!


"What type of damper was installed originally?"
I really have no idea here, I'll have to ask the mechanic.


I heard back from the manufacturer, who watched the video I linked, and they had mentioned a high deflection dampener being an option as well. I think I'll trying increasing idle RPM a bit to see if it helps. Currently idling at 730 ish.


SkipperPete, #4 is a whole new concept for me. Am I understanding an easy way to check this is there should be a little fore and aft play at the crackshaft pully?


For option 1,3 and 4, Would this noise not be present in forward and reverse too then?



Thanks again for all the help