Transmission Rattle

[Dean Strong]

Kevin,I've always had something between a rattle and grind at very low RPMs with my Yanmar 2GM20F (1990 Hunter 30, w/PSS shaft seal), but it has always disappeared at anything above 1000 RPMs. Last time out,however, it continued, and was loud enough to resonate through the hull (I kept thinking I heard something while in the cockpit, so I went up on the foredeck, I put my ear to the deck, and could hear it). When I went below, and lifted the access cover on the aft side of the engine, I couldn't hear it---its not an engine noise, but something in the drive line.Today I ran the boat briefly, and experimented at the dock. No sound in neutral. Loud and clear in forward (forgot to try reverse).I've been thinking cutlass bearing, but don't really know what they like when they go bad. I went to the PYI website to see how to pull the prop shaft in preparation of replacing the cutlass bearing. There I read about about "gear chatter," "gear rattle," and/or "gearbox rattle." This really describes the sound I have always heard, but as I said, now it has grown worse and can be heared at cruising speed (2700-2850 rpm). The engine only has 700 hours on it, though it was replaced by the former owner who apparently blew the original. The transmission looks original.So, here are some questions:1) If I haul the boat, and the cutlass bearing is bad, can I just unbolt the shaft from the transmission and let it slide out through the shaft seal? 2) What wears or goes bad in a transmission that would cause "gearbox rattle" and why would it get worse? What am I looking at here?Any help or directions would be great.

 

[Steve Dion]

Have it done by the yard.

Dean:I don't know about your specific boat, but normally you only need a special tool to press the cutlass out of the strut. They remove the prop, slip on the new bearing reset the allen screws and it is done. This normally takes the yard about an hour to an hour and a half (depending on the set screws).It is also a good time to have the engine alingment checked if you are having problems.

 

[Kevin]

long winded response

Hello Dean, Thanks for the note. Does the rattle take place in reverse? Does it take place if you tie the boat to the dock and run it gear? Each of these questions may help to start looking at one particular area verses a multitude. If the cutlass bearing is bad or worn out I would think that you would see the support structure of the bearing affecting the high end Rpm’s at first and migrating down the Rpm scale as time goes on. Not the opposite as you are describing. To remove the shaft from the coupling and vessel, you will need to remove the set screws from the rotor and slide it off the shaft as the shaft is being removed out of the boat. I’m not sure about your model of boat but I would think that it is likely the rudder could be in the way with the propeller installed. If all you are attempting to do is replace the cutlass your yard may have ability of doing so with the shaft in place. It is however, when chasing down a problem of vibration, suggested to do a fit and face of the shaft and coupling along with confirming the shaft is true. I’m sure there are a few items that could cause gear clatter in a transmission but the most common is your damper plate. The damper plate is a device that does have a life expectancy. I’m aware of some commercial fishing fleets that change out the damper yearly. What affects the damper is the frequency at which the system is cycled. A cycle being the damper plate being compressed/uncompressed. As the cylinder fires the torque produces a very large spike driving the flywheel in rotation. As the pistons in the motor rise the developed torque of the motor begins to reduce. This reduction may drop so low as to allow the running gear, ie prop shaft propeller rotation…, to literally attempt to rotate the flywheel through the gear box. This results in a damper plate flexing in order to absorb that deflection or drive train inertia. If we look at the current damper the springs may have gotten to a cycle time that the flex is no longer 3 degrees but 1 degree. This will result in the transmission to bang or rattle. This rattle is the back of the gear teeth making contact to the back of the opposing gear during the low end production of torque when the weight of the running gear is and has the most inertia. Chang chang chang… These noises are mostly seen at low range Rpm’s, idle and Rpm’s leading up to cruise. As time and cycles move on the problem increases to include higher ranges of Rpm’s to the point where the damper is basically a ridged part no flex and a noise throughout the full range of Rpm’s. Worst case is the damper fails and looses the springs and now nothing is connecting the transmission to the flywheel. I know that when I was growing up on the farm we used a round stick, poor mans stethoscope!!! J But I remember my shop teacher always telling us that the noise will travel on hard mounted systems that could pass the noise to the far end of the system. The bearing in the crank would be noisy at the CV. In my opinion I would either replace the Damper with original equipment or go to a high end high deflection damper. Its difficult-easy to change depending on the room available. I still think that if a balance or out of column vibration is setting in your high end Rpm’s will act up first verses the low end. If you are still awake, good luck. Kevin

 

[Frederick Pierce]

Hi, DEAN yanmar has a website for problems its WWW.YANMARHELP.COM I look at it very often its very helpful you tell them your problem and they will answer I have a 3GM30F on my oday 28 give it a try maybe they could help Frederick Pierce WWW.YANMARHELP.COM