View attachment 154495 Awhile back I posted about how my crewman had thrown the tranny into gear while the engine was revved up which immediately resulted in a heavy thumping vibration in the driveline. Since then I have replaced the prop shaft, cutlass bearing, damper plate, engine mounts and rebalanced the prop. I have also had the whole thing aligned more than once. This is a Yanmar 3gm30F with a Kanzaki KM3-A (1983) tranny. The situation is much improved but not completely gone. I still get the heavy vibration but only when in forward gear and only when the boat is under way. AND only when there is not too much resistance to the boat's headway, i.e. not too much headwind. Research has led me to a slipping drive cone in the tranny. The question is: Can a slipping drive cone cause this type of vibration? Anyone have any experience with this? I have disassembled the KM3-A transmission and all looks to be in very good condition with the exception of a mirror smooth drive gear surface and the oil was the color of a nice stout beer.
The heavy driveline vibration mystery continues
- Thread starter markwbird
- Start date
View attachment 154495 Awhile back I posted about how my crewman had thrown the tranny into gear while the engine was revved up which immediately resulted in a heavy thumping vibration in the driveline. Since then I have replaced the prop shaft, cutlass bearing, damper plate, engine mounts and rebalanced the prop. I have also had the whole thing aligned more than once. This is a Yanmar 3gm30F with a Kanzaki KM3-A (1983) tranny. The situation is much improved but not completely gone. I still get the heavy vibration but only when in forward gear and only when the boat is under way. AND only when there is not too much resistance to the boat's headway, i.e. not too much headwind. Research has led me to a slipping drive cone in the tranny. The question is: Can a slipping drive cone cause this type of vibration? Anyone have any experience with this? I have disassembled the KM3-A transmission and all looks to be in very good condition with the exception of a mirror smooth drive gear surface and the oil was the color of a nice stout beer.The remaining vibration is probably due to the age and wear of the drive train. Parts wear down and exceed the factory tolerances and wear limits and don't work as well together as they used to. You mention excessive vibration under light loads rather than under heavy loads and that is a little unusual, you may want to take a look at the governor for wear and adjustment. I would think that any vibration emanating from the clutch cone would increase with the load. There are just some vibrations in old engines and drive trains that would be too expensive to fix and if they are not extreme it is better to learn to live with them.
There is no reason this gear set should not run silky smooth.
“Mirror smooth drive gear surface”
Do you mean the concave conical surface - the one into which the shifter cone moves? Or something else? How about the shifter cone itself?
You should compare all your measurements with those published in the manual for gear lash, cone dimensions, slot width, etc.
Also the A and C model shifter needs careful attention. Yanmar changed the shifter design in the P model.
The cone surface should have little ridges flat topped ones. Also these cannot be smooth any glaze needs to go but do not reverse the cone orientation.
Consider a consult with a good transmission shop maybe.
It is still possible the vibrations source lies elsewhere something might have been overlooked.
Charles
“Mirror smooth drive gear surface”
Do you mean the concave conical surface - the one into which the shifter cone moves? Or something else? How about the shifter cone itself?
You should compare all your measurements with those published in the manual for gear lash, cone dimensions, slot width, etc.
Also the A and C model shifter needs careful attention. Yanmar changed the shifter design in the P model.
The cone surface should have little ridges flat topped ones. Also these cannot be smooth any glaze needs to go but do not reverse the cone orientation.
Consider a consult with a good transmission shop maybe.
It is still possible the vibrations source lies elsewhere something might have been overlooked.
Charles
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Had what sounds like the same problem 2 summers ago and went through extensive tests by marina technicians who could not find the source. Realigned engine, checked mounts etc. ad nauseum. Occurred only under load in fwd and was a very loud distressing thumping that sounded like the engine would burst and was impossible to ignore. At idle and under no load it was fine. Finally called in a Yanmar expert who listened and in just a few minutes stated it was clearly a slipping cone clutch. He pulled the tranny, rebuilt it, problem solved after two very frustrating and concerning months. He stated that the loud thumping and vibration resulted from the clutch slipping and then grabbing and was not an uncommon presentation when worn out.
I agree with IL but MA reports the vibration is worse when the load on the engine and transmission is light. A slipping clutch cone usually gets worse as the load gets heavier. I hate to be a pessimist but one thing to consider is to make sure that the previous jobs were done correctly, like the installation of the cutlass bearing, the balancing of the prop and the alignment of the engine. Start at basics and observe the shaft turning to see if it runs concentric, try to determine if the vibration is originated by the shaft, bearing prop assembly or the engine and transmission. Is the engine running fine in neutral, with little vibration at idle and accelerating well? No vibration at all in reverse? What is that mirror smooth drive gear that you refer to? Without actually experiencing the vibration it is hard to diagnose online. If it is a cone clutch it will continue to get worse; the issue is if this is the kind of vibration that could damage other components and requires prompt attention or can it wait until the fault becomes more evident?
Thanks all for the input. While the vibration only occurs under the conditions described (underway with moderate load) when it does happen it is heavy and would definitely cause damage if allowed to continue. I am encouraged to know that it is, at least, a possibility that it is the drive cone. If this does not do it I may have to just buy another boat and I’m not 100% sure that would fix it.
Another boat: other unsolvable problems. Problem I had also occured at dock with minimal loading which is where it was actually diagnosed. Get a Yanmar expert to look at it.
What I suspect with is happening is that the cone is slipping when the load is in the "sweet spot" When running slow or at idle it does not slip. When under heavier load the cone is making better contact does it does not slip. When running faster and the load begins to ease then the cone slips and we get the dreaded "hammer". That's my theory.
it's interesting that this problem 1st surfaced after ' my crewman had thrown the tranny into gear while the engine was revved up which immediately resulted in a heavy thumping vibration in the driveline.' yanmar cautions NEVER to do this. check out the posts on boatdiesel.com and call a yanmar GOLD dealer who can recommend the parts if you're intending to DIY. i'd also have the yanmar pro re-check the p shaft alignment and coupler faceout. a common error in some yards' replacing engine mounts is to use the same aftermarket mounts for front and rear. the yan mar specs have different stiffness for front and rear mounts for a good reason.
Sue did the same thing a few times (and I'll admit to it also 
) with our old transmission several years ago. I pulled the drive cone out and it looked fine to me. But, what do I know … after a re-build done by Mack Boring it was fine. They looked at the drive cone and could tell it would be slipping. In my experience, the slippage occurred mostly at lower RPM as I was increasing power. Once power was steady, slippage stopped, even under higher rpm.
) with our old transmission several years ago. I pulled the drive cone out and it looked fine to me. But, what do I know … after a re-build done by Mack Boring it was fine. They looked at the drive cone and could tell it would be slipping. In my experience, the slippage occurred mostly at lower RPM as I was increasing power. Once power was steady, slippage stopped, even under higher rpm.Yes, MitchM. The fact that it came on suddenly as a result of throwing it into gear has been a sticking point in solving this issue. The yard claims that the prop shaft was bent and replaced it. I did not actually see this bent shaft and after what has been a terrible experience with this yard I have reason to be skeptical. My wife brought up an interesting point: we had been experiencing intermittent vibration for awhile but always assumed it was the Martec folding prop not opening all the way. With a quick rev up we got it to stop. Maybe there was more going on. The engine mounts are Yanmar mounts and they are the correct ones. Also, I recently learned that you have to align the shaft with the boat in the water because the hull can flex appreciably in/out of the water which can affect the alignment. This was done as well. Fingers crossed.
At least two observers think the culprit is not transmission and despite your list of improvements something got overlooked.
To review the symptoms and improvements so far
Sudden onset of vibration following abrupt over- reved forward gear engagement. Polished drive gear at the clutch (cone) engagement surface presumably the forward gear. New mounts, new shaft, cutless, motor alignment, propeller balanced.
This group of facts probably rules out drive gear slippage because the polishing took a good long time and there are no other obvious transmission parts implicated. While the gear surface coincidentally needs a 600 grit scuffing that condition is not responsible for sudden onset of vibration. The condition is also not a failure to re-align on water, nor mischosen or misapplied motor mounts because these cannot explain a sudden onset either. A propeller balancing session - which suggests a fixed prop. But why balance the prop to start with? And how much balancing work was required? However, the prop is not fixed it is a Martec.
For sure diagnosis from afar is a tough job. But so far - from a differential standpoint - my money is on damaged propeller blade deployment mechanism.
Charles
To review the symptoms and improvements so far
Sudden onset of vibration following abrupt over- reved forward gear engagement. Polished drive gear at the clutch (cone) engagement surface presumably the forward gear. New mounts, new shaft, cutless, motor alignment, propeller balanced.
This group of facts probably rules out drive gear slippage because the polishing took a good long time and there are no other obvious transmission parts implicated. While the gear surface coincidentally needs a 600 grit scuffing that condition is not responsible for sudden onset of vibration. The condition is also not a failure to re-align on water, nor mischosen or misapplied motor mounts because these cannot explain a sudden onset either. A propeller balancing session - which suggests a fixed prop. But why balance the prop to start with? And how much balancing work was required? However, the prop is not fixed it is a Martec.
For sure diagnosis from afar is a tough job. But so far - from a differential standpoint - my money is on damaged propeller blade deployment mechanism.
Charles
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Charles. Thanks for the input. The Martec prop was removed and the old fixed prop was balanced at the prop shop and installed just to eliminate that possibility. I agree that the suddenness logically would eliminate cone slippage but everything else has been done and the symptoms seem to match. The key is the fact that it only happens in forward gear under way. I think a bad prop would vibrate while the boat it in the slip. I am not 100% sure of anything at this point.
OK. So we eliminate the Martec from the examination.
What things did you do when the transmission was disassembled? Did you scuff the cone engagement surfaces of the gears thus to elimanate mirror finish? Did the cone still show concentric flat topped rings? Did you align the shaft at transmission reinstall? If so what adjustments were necessary? And - an important one - how did you determine the shaft was dead center in the stern tube and held centered during the last alignment session?
Charles
What things did you do when the transmission was disassembled? Did you scuff the cone engagement surfaces of the gears thus to elimanate mirror finish? Did the cone still show concentric flat topped rings? Did you align the shaft at transmission reinstall? If so what adjustments were necessary? And - an important one - how did you determine the shaft was dead center in the stern tube and held centered during the last alignment session?
Charles
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I have just reassembled the transmission. I have not installed it yet so I don’t know if it will work. I lapped the drive cone against the forward and reverse gears, replaced the bearings and seals. All gears and bearings appear in excellent condition with no sign of wear. No leaks. Prop shaft was aligned and rechecked several times at yard and by me with a depth gauge to within 2/1000 at the flange. She runs smooth as silk when tied to the dock.
you havecertainly exhausted all thepossibilites any reasonable ownercouldpossibly do. this problem sounds to me like groundsfor a long distance phone consult with larry berlin , head tech guru at mack boring, yanmar ‘s east coast distributor . ( offer to pay his hourly rate!., .) larry was the instructor for the basic and advancedyanmar diesel courses, andis agreat guy with a wealth of yanmar knowledge...
Mark. What are your plans for centering the shaft and keeping it centered during motor mount adjustment?
Also consider this https://stevedmarineconsulting.com/the-ins-and-outs-of-shaft-alignment-part-ii/?upm_export=print
and
http://stevedmarineconsulting.com/the-ins-and-outs-of-engine-and-shaft-alignment/
Charles
Also consider this https://stevedmarineconsulting.com/the-ins-and-outs-of-shaft-alignment-part-ii/?upm_export=print
and
http://stevedmarineconsulting.com/the-ins-and-outs-of-engine-and-shaft-alignment/
Charles
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That's awesome! Thanks, MitchM for the reference. Light at the end of the tunnel. Hopefully it's not a train.