Yanmar 3GM30 alternator drive belt

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1corinthian

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After 6 months of ownership I believe I have discovered why the alternator drive belt suffers excessive wear . There appears to be significant misalignment between the crankshaft and alternator pulleys ; so far I have ascertained that the bolt attaching the alternator to the engine bracket appears to be a very loose fit . I am contemplating reaming out the fixing holes on the alternator and fitting bushes to restore the holes to the original 8mm dia . I would be interested to hear if anyone has carried out a similar exercise .
 
Once the alternator is tightened up I would be surprised if it does not align with the plane of the belt even if the holes and/or bolts are the wrong size.

The usual reasons for misalignment is that an new alternator has been fitted and the pulley has a slightly different placement on the tapered/woodruffed shaft. Easily sorted by removing the pulley and grinding off a sliver or adding a shim .... I've done it many times.

The usual reason for belt wear is a belt of too small a width being fitted. If the outer flat side of the belt is below the level of the outer circumference of the alternator pulley then it is the wrong belt. If it's a mm or two proud of the pulley edge that's fine.

Richard
 
The alternator pivots on one long bolt running through a cast ali bracket, to set the belt tension?
I recall having to saw a little off the end of that bracket to get the alignment right!
Maybe someone has changed the alt?

Is the alt shaft not parallel to the crank, or is the alt pulley in a plane parallel to where it needs to be?

I got much better belt life with 'Gates' cogged belts, my new alt gave the belts a hard time.
 
A quite new 2GM20 that once I owned ate alternator drive belts until I realised that the crankshaft pulley was corroded on the faces of the Vee.
A session with emery cloth cured the problem.
 
lw395 said:
I got much better belt life with 'Gates' cogged belts, my new alt gave the belts a hard time.
Click to expand...

Definitely +1.

Lots of black dust in engine compartment from standard Yanmar belt. Not only did Gates toothed belt actually fit nicely into the very bottom of the the pulley (the Yanmar one didn't - hence dust) but it allowed more lateral flexibility, so it may solve OP's problem.
 
rudolph_hart said:
Definitely +1.

Lots of black dust in engine compartment from standard Yanmar belt. Not only did Gates toothed belt actually fit nicely into the very bottom of the the pulley (the Yanmar one didn't - hence dust) but it allowed more lateral flexibility, so it may solve OP's problem.
Click to expand...
Standard V belts are not intended to sit on the bottom of the V. The drive is taken by the flanks of the belt. When they touch the bottom they are worn out.
 
1corinthian said:
After 6 months of ownership I believe I have discovered why the alternator drive belt suffers excessive wear . There appears to be significant misalignment between the crankshaft and alternator pulleys ; so far I have ascertained that the bolt attaching the alternator to the engine bracket appears to be a very loose fit . I am contemplating reaming out the fixing holes on the alternator and fitting bushes to restore the holes to the original 8mm dia . I would be interested to hear if anyone has carried out a similar exercise .
Click to expand...

I can't see how over sized mounting bolt holes would cause misalignment with the crank pulley.

In what way is the alternator pulley misaligned ? Is it forward of the crank pulley, or not forward enough, or is it not parallel ?
 
I got much better belt life with 'Gates' cogged belts, my new alt gave the belts a hard time.[/QUOTE]

+2 Seems to be quite a common issue with the Yanmar engine
 
Had similar problem , when though all usual checks , and solution was simple , engineer took pulley off polished it ( some minor corrosion) end if belt wear . Told me very common on Yanmar pulleys.
 
lw395 said:
The alternator pivots on one long bolt running through a cast ali bracket, to set the belt tension?
I recall having to saw a little off the end of that bracket to get the alignment right!
Maybe someone has changed the alt?

Is the alt shaft not parallel to the crank, or is the alt pulley in a plane parallel to where it needs to be?

I got much better belt life with 'Gates' cogged belts, my new alt gave the belts a hard time.
Click to expand...
I have found the same on my MD22.
Stu
 
vyv_cox said:
My Yanmar 3GM30F has done 2500 hours, on only its second pair of belts. Keeping the pulleys rust free is the answer.
Click to expand...

Also not using slow idle with a big alternator charging big, fairly flat batteries.
Using a faster idle until the current drops a bit helps a lot.

The original alternator gave no belt problems, but took longer to charge the batteries, while also running a fridge.
 
vyv_cox said:
My Yanmar 3GM30F has done 2500 hours, on only its second pair of belts. Keeping the pulleys rust free is the answer.
Click to expand...
Me too, although I’d say 20years and only needed two belts. I cleaned the surfaces of the pulleys and removed the dust/ wear problem. Not changed a belt in over 10 years, ar a guess, 1200hrs. I keep looking at the two of them and wonder if I’m doing something wrong in not replacing them regularly. :)
 
we had the same problem - the alternator V pulley had quite a different section to the engine pulley. changed it, fine since. so i guess the alternator had been changed at some point i guess?
 
I had a dust problem with a small Beta (original alternator, I believe) despite carefully cleaning up the pulleys; I do believe that a cogged belt helps by causing less side wall distortion as the belt traverses a small pulley, and I would not go back to a plain one.
 
I think the cogged belt bends more easily, so grips small pulleys more tightly and also wastes less power.
Anyway it worked for me and was not expensive.

It's quite hard to find small pulleys for vee belts, most modern car alternators are polyvee belt driven, partly because simple vee belts are not ideal when you need more power and want to turn the alternator faster.
Also to drive more things from the same belt of course.
 
To add to the above my 3GM belt suffered excessive wear that turned out to be a combination of corrosion pitted pulley surfaces and pulleys out of alignment. The former was partially cured by emery cloth with the pulley spinning in an electric drill, the other by shimming. The corrosion problem never really went away and if I'd kept the boat I'd have considered a toothed belt conversion which would solve both problems.

To be fair it was the water pump belt that gave the most trouble - for the same reasons.
 
Thank you all for your collective opinions on this problem . The crankshaft and alternator pulleys are neither in the same plane or even parallel . The alternator pivots on the long through bolt not only in a radial direction but also longitudinally . As a result tensioning the belt skews the pulley out of line . I am proposing to ream out the holes to the same diameter and replace the long bolt such that this movement is eliminated .
 
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