Strange Belt Wear

[SabreToothedEngineer]

Out in the middle of the Strait of Georgia after fishing, my engine, a Yanmar 3GMF (1983) wouldn't start due to known starter wiring issues (I'll fix that soon, I promise), so I looked under the hood to turn the crankshaft a bit - sometimes that helps. What I saw was a nearly broken drive belt (unrelated to the starting problem, but for once I was glad the engine wouldn't start). Luckily, I carry a spare belt (and spare pretty much everything else).

This photo is the OD. This next one is the ID:

The rest of the belt is in perfect condition.

Whatever happened was in one local spot, meaning the issue travelled around with the belt. If it was an issue with the pulleys (sheaves), the problem would be all over the belt perimeter. My suspicion is that something stuck to the belt and pushed it up at one spot, and it shaved itself off at the guard over the alternator. Once it wore enough it stopped doing that but was so thin by that time that the inside started wearing. Whatever was stuck to the belt was long gone by this time, so there is no evidence of the cause.

Has anyone ever had something like this happen before, and any advice on how to avoid it? I'm baffled by what could have done this. It would have had to be pretty thick to stick it up enough to wear a good 3 mm off the OD. There was another ~1mm off the ID, too, but that would have been after the fact.

Ideas appreciated.

 

[dlochner]

That does seem strange. The wear would have to be very localized and that suggests the wear occurred when the motor was not running.

It might also be a defect in the belt, something in the manufacturing process that weakened that section of the belt.

Or it might have been damaged when it was installed. Perhaps the installer didn't loosen the alternator sufficiently and pried it on to a pulley. The damaged area then wore more than the rest of the belt.

Is there a lot of belt dust on the motor?

 

[SabreToothedEngineer]

There is nothing in the area that could spin when the motor is not running, and the damaged area is clearly shaved off. The only potential culprit is the guard over the alternator; there is nothing else anywhere close that could have done it. It had to be something stuck to the belt making it stick out, otherwise the wear would be all over, not just in one place. The belt ran for two years without problems (and I inspect both belts fairly regularly; in fact, I just re-tensioned it a couple of weeks ago, and there was no such wear at the time).

 

[dLj]

Got rodents in you boat?

dj

 

[smokey73]

The belt ran for two years without problems (and I inspect both belts fairly regularly; in fact, I just re-tensioned it a couple of weeks ago, and there was no such wear at the time).

There might be your first clue. It didn't wear that way for two years and shortly after you re-tensioned it you have this unusual wear. Not questioning your ability to properly adjust but is it possible that you left a tool or something that contacted the belt and damaged it on that outer surface? Just asking. When something goes wrong that has been okay before the first question is always "what changed."

 

[Ward H]

I’d think the guard would show a “polished” edge if it was in contact with the belt. As @dLj mentioned, chewing by rodents seems more likely.

 

[SycloneDriver]

There might be your first clue. It didn't wear that way for two years and shortly after you re-tensioned it you have this unusual wear. Not questioning your ability to properly adjust but is it possible that you left a tool or something that contacted the belt and damaged it on that outer surface? Just asking. When something goes wrong that has been okay before the first question is always "what changed."

I was thinking something along the same lines.
What if the "cords" in the belt failed after re-tensioning. It would stretch and wear more at the point of failure.

 

[dlochner]

I was thinking something along the same lines.
What if the "cords" in the belt failed after re-tensioning. It would stretch and wear more at the point of failure.

Was having the same thought. Was the belt looser than usual when it was found?

If this is the work of a rodent, there should be teeth marks on it, are there any?

 

[SabreToothedEngineer]

First, housekeeping: no rodents are involved. The OD has been cleanly scraped, and the ID is simply worn. No tooth marks.

New information:

When I tensioned the belts (August 21st, according to the photo metadata) I reached within about 1/8" of the end of the tensioner travel. I concluded at the time that I had one or more worn sheaves (pulleys). I've seen this industrially in my working life several times on belt drives. I took a couple of photos to assist in buying new sheaves, which I have bought but not yet installed.

I looked at the photos I took, and the belt was worn then. It was mostly hidden by the alternator cover, and although I didn't notice it at the time, you can see it in the pictures. The close-ups are cropped from larger photos - I wouldn't have missed it at the time if I'd taken them that close up. The reason I used up all the tensioning travel was that the worn bit was right on the alternator sheave/pulley.

The alternator cover doesn't seem close enough, but there is absolutely nothing else around. The tail of a hose clamp looks like it's close, but in actual fact it's nearly an inch away.

I'm still baffled by what could have stuck to the belt to lift it high enough to contact the alternator cover (or something else?).

 

[jssailem]

My guess,
If it was rubbing on something I would expect to see damage over the whole of the belt.

It looks more like the belt is coming apart in one place. Either the belt was unknowingly damaged during the las tensioning, the belt had a defect, or it just reached the end of life.

New belt, tension properly and go out sailing would be my plan.

 

[dlochner]

I think the alternator cover is the culprit, what I can't explain is why the wear is localized.

Last year I upgrade the alternator and belts with a serpentine belt. It kept shredding belts. Eventually (and many expensive belts later) I figured out that when running at speed for a while the belt would heat up and stretch a little. This was just enough to cause some bounce in the belt, i.e., it did not run in a perfectly straight line, rather it wiggled up and down. This extra movement was enough to case the belt to rub against an engine component. Over time the belt work and came apart. This was compounded when I tried tensioning the belt more. The adjusting bolt could not withstand the pressure causing the alternator to slide forward a bit.

Changed the adjusting bolt system and the problem went away.

 

[SabreToothedEngineer]

My guess,
If it was rubbing on something I would expect to see damage over the whole of the belt.

It looks more like the belt is coming apart in one place. Either the belt was unknowingly damaged during the las tensioning, the belt had a defect, or it just reached the end of life.

New belt, tension properly and go out sailing would be my plan.

Exactly. The belt moves. If it was rubbing on something stationary, the wear should be all over. Ditto for normal wear. This wear is all in one place; the rest of the belt looks brand new, not even any visible wear on the normal wear surfaces. Something got stuck onto one point on the belt and caused that one spot to contact something else, almost certainly the alternator cover (because there is nothing else to touch).

 

[Hayden Watson]

My guess,
If it was rubbing on something I would expect to see damage over the whole of the belt.

It looks more like the belt is coming apart in one place. Either the belt was unknowingly damaged during the las tensioning, the belt had a defect, or it just reached the end of life.

New belt, tension properly and go out sailing would be my plan.

That was my thought as well. I had almost the same damage to my belt a couple of weeks ago. That is how a belt will fail.

 

[Ralph Johnstone]

I've seen this industrially in my working life several times on belt drives. I took a couple of photos to assist in buying new sheaves, which I have bought but not yet installed.

Sheaves don't usually wear too much except if they're subject to heavy corrosion. The alt. sheave looks to be in good condition, the main sheave looks good and the water pump sheave inside can't be seen but the exterior looks good. I would suspect the slack in the belt causing the tension adjustment to reach its limit to be the result of the damage to the belt and NOT sheave wear. If you use the same length belt again, I would expect the tension adjustment to return to its original position.

New belt, tension properly and go out sailing would be my plan.

I'd put my money with John and forget the new sheaves.

 

[Morgan donor]

I see no reason the belt cover should be so close to the belt. Asking for trouble... with any cyclical whipping, the belt will hit the cover! What does that shroud do anyway? Funnel air into the ALT?
As always, when things stop working, go to the last thing you "fixed". So your belt adjustment is suspect. Sorry. Let's not over think.
Either the sheaves are not in alignment or the belt was too loose. Or, Sheaves don't always turn concentrically. The belt can be right, turn the crank 90 deg. and it can be loose (or tight)
I put a Gates green belt on my Perkins, got rid of an improperly sized sheave on the ALT. Confirmed the belt and sheaves were properly aligned... Never touched it for 10 years. 50-75hrs a year.

 

[dlochner]

I put a Gates green belt on my Perkins, got rid of an improperly sized sheave on the ALT. Confirmed the belt and sheaves were properly aligned... Never touched it for 10 years. 50-75hrs a year.

The same belt continues to run today. The new owners have not changed the belt on @Morgan donor's boat. But I might mention them that it may be time to do a little preventive maintenance. (The boat is in my marina and docked next to mine.).

 

[RoyS]

It looks to me that your belt failed all by itself. Compare with your spare belt and see if there is a manufacturing joint in the outer lamination where the belt failed. Is your belt the correct gauge for your sheaves? The belt seems to be running too deep in the sheaves. You can purchase inexpensive sheave gauges to determine correct belt gauge and sheave wear. Does the belt contact the bottom of the sheave groove? If the belt delaminated at a manufacturing joint a flap would develop and be torn off upon contact with your alternator guard. BTW there is significant belt dust visible in your close up photo.

 

[DayDreamer41]

The belt has reached the EOL, change, tension and go sailing.
I would guess the belt lost its ability to go round and round, was stationary for the short duration and it toasted itself on the crank pulley at which point there must have been that obvious odor of burning rubber, which leads me to believe that you have Covid and have lost your sense of smell..... the last statement is optional.....

 

[Ralph Johnstone]

The belt has reached the EOL, change, tension and go sailing.

which leads me to believe that you have Covid and have lost your sense of smell

Excellent observations, both mechanical and medical.

@SabreToothedEngineer , you're just making life difficult for yourself. Follow the group wisdom here.

 

[jssailem]

you're just making life difficult for yourself.

To say it another way (Shakespearian inspired)...
'The Skipper doth protest too much, methinks'