Twin V-Belt vs "matched set" of belts

I have managed to get just about everything fixed on my '87 JD 310c after several months of repair/waiting for parts.
After firing it all up,and fixing a few hydraulic cylinder issues, it easily came to life, and I was ready to put the hood back on.

But v-belt wobble has me a bit worried.

I thought I had these cranked down pretty well, but the section of belt between the water pump/fan and the main drive pulley flops around quite a bit. Not as bad as it was before doing all the work (e.g. that +- $600 main pulley/balancer replacement, ant the complete coupler assemblies, spline, and hydraulic pump). The other pulley points (main pulley-to-alternator, and alternator-to-water pump) which are shorter appear to be just fine.

An older thread mentions the use of a side-by-side dual belt instead of two single belts. Is this a better solution? I bought 2 belts from a dealer for the 310c, but neglected to look for "matching" in the description (yet another error I made).

Replacing these is a real pain. Not sure if I should just crank everything down tighter, and wait for winter; I believe that I am throwing pieces of flex-ring in the Stanadyne pump, causing that ball valve to clog, and will likely send it out for rebuild in a few months. But I just need to get some actual work done now before winter.

My gut tells me to just run it for a couple months, and then put a twin belt (or at least a "spare" twin belt) on there at pump rebuild, when the hood will have to come back off anyways.

IIRC-belts don't say matching anymore. According to belt manufactures the process of making V-belts has changed so two belts of
same brand and part number are suppose to work.

For the OP, are the belts relatively the same tension as installed? Just push on them with your finger. Check different spans in case one came to rest with the tension on the top and the other one has the tension underneath. But as long as the tension is reasonably similar, it should be good enough.

20 years ago I had this argument at the parts counter. They said they all match now and gave me one from shelf stock and one that was ordered in, from very different lots.

Well I put them on the truck and one was banjo tight and the other had like an inch of depression with your finger. I took them back.

To solve the problem, what I will do now is go to a parts house that has a lot of belts on the shelf and ask for them to bring all of them in my size to the counter. They have lots of meaningless numbers printed all over the back and many of these numbers are small and printed multiple times per belt, one above the other. Most of them will be identical one one belt but different across lots. So find a couple belts that have all the same small numbers on the back to each other. The final thing is they will often have a sequential number printed 3 or more times per belt. It will look something like

74
75
76
77

If you can find a couple of belts where the rest of the numbers are all the same, and those sequence numbers are close, you will have a matched pair, probably.

Cat belts are the only ones I know of sold as a matched set.

Able to tilt the fan pulley???

MarcNH said:

They are both tightened at the same point: the alternator. The old ones were rather floppy. I ordered ones that were supposed to fit, and assumed they were the same length. I am not sure how much play there ought to be.
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My question is, when you tighten them with the alternator, do they both feel like they have the same tension when you press on them?

If they feel about the same, good enough, run it.

MarcNH said:

I thought I had these cranked down pretty well, but the section of belt between the water pump/fan and the main drive pulley flops around quit

Click to expand...

MarcNH said:

There is a small amount of play in the fan pulley. You can just feel it tilt if you try. I am not sure if that is enough to cause my problems.

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Sounds to me like the bearing carrying the fan is in trouble

Kobelco ireland said:

Sounds to me like the bearing carrying the fan is in trouble

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and that can get expensive fast if the fan flies through the radiator.

Otherwise, if they feel the same pressed by finger in the middle of the longest span, then run them until they need to be tightened the first time, then run them a little more, and then see how close they feel. Should be fine, but it's nice to get consecutive numbers like Birken Vogt described.

Take the Cummins 855 for instance-Accessory to fan hub both wide belts-rarely do both of them tighten the same even on a matched set.
DT466 the same issue. Belts can be close but only matched if cut one after another from same piece of belt stock. Even then it depends on
how straight the alternator/ac is mounted-off half a degree and one belt will be tighter than the other. Or how deep the groove is worn in a
pulley.

My favorite one though-a engine keeps throwing belts----crank damper.

Industrial maintenance practice involves checking all the pulleys for wear....many occasions an OK looking pulley will not appear so good when the gage is applied to the belt groove - and when the pulleys are replaced, the belt slipping flapping squealing etc as well the belt life is magically improved.
I mention this as a point of discussion and will also admit I have not had to replace any pulley on an engine although maybe I should (have)....

Kobelco ireland said:

Able to tilt the fan pulley???

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Yes. If I push down on the fan end, it moves slightly and makes a "click" or "ding." I am assuming the bearing in there needs to be replaced (which is part of the rebuild kit).

If there is an alternator rebuild kit, I might do that too.

Still thinking about the twin belt.

Truck Shop said:

Take the Cummins 855 for instance-Accessory to fan hub both wide belts-rarely do both of them tighten the same even on a matched set.
DT466 the same issue. Belts can be close but only matched if cut one after another from same piece of belt stock. Even then it depends on
how straight the alternator/ac is mounted-off half a degree and one belt will be tighter than the other. Or how deep the groove is worn in a
pulley.

My favorite one though-a engine keeps throwing belts----crank damper.

Click to expand...

So I am guessing that the water pump/fan pulley is a factor. The generator might be doing the same thing, but it is not as easy to check (e.g. the fan gives it some leverage).

Truck Shop said:

Take the Cummins 855 for instance-Accessory to fan hub both wide belts-rarely do both of them tighten the same even on a matched set.
DT466 the same issue. Belts can be close but only matched if cut one after another from same piece of belt stock. Even then it depends on
how straight the alternator/ac is mounted-off half a degree and one belt will be tighter than the other. Or how deep the groove is worn in a
pulley.

My favorite one though-a engine keeps throwing belts----crank damper.

Click to expand...

Makes me wonder why there would not be a idler pulley, like they put (or used to put) in dryers.

MarcNH said:

Yes. If I push down on the fan end, it moves slightly and makes a "click" or "ding."

Click to expand...

Loosen the fan belts and then check water pump bearing play. I'll bet it is shot.

But do note that even with all new stuff you will have some belt deflection when running. Fairly normal on those old donkeys.

The reason for using banded belts( there proper name) is where normal belts twist in the pulleys and are mainly used on woodchippers, forestry mulchers etc. I have never seen them used to drive ancillary equipment on an engine though. Most banded belts have a very different V section to a normal fan belt.

Swap the belts side to side and see what happens.

ih100 said:

Swap the belts side to side and see what happens.

Click to expand...

That was the last thing that I tried before just bolting everything else together and dropping the hood back on. Minimal change, if any. It was worth a try though.

Looks like the rebuild kit is pretty cheap, but I am more inclined to just swap it all out with an aftermarket one (+- $320) at the start of winter. There will undoubtedly be a few other things to do by then.

Everything is all back together now, and I am scheduled for tree cutting/stump removal tomorrow (provided it actually does cool down). No extra parts, so I think it must be good to go!