Air Compressor Question

[3rdmdqm]

When the air copressor is not running, the belts are tight and rotate normally/smooth. When the air compressor kicks on, the belt that drives the compressor starts jumping around as if it is loose. It doesn't jump off the pulley or anything, the belts flex and jump in the center between the pulleys. The compressor works fine, I've just never seen a belt jump around like that before. The belt is on tight, so I don't think that's the issue. Anyone seen this before or have any ideas?

 

[stumps]

Sounds normal to me. The compressor is anything but a steady load. That means the pulling side of the belt is pulling harder and less hard as the compressor goes through a compression stroke. That causes the belt to rise and fall ever so slightly in the V groove in the pulley, and that will make the, ahh, uhmmm, err, mumble, mumble, pushing side of the belt flop..

Next time I fire up my deuce, I will look to see if it does the same. I expect it will.

-Chuck

 

[m-35tom]

you may think it is tight. how much out of line can you push it by hand? should be almost none. put on a new belt and tighten it up to specs.

 

[stumps]

An interesting thing about the front main bearing on virtually every car or truck engine: It is under a great deal of stress and wears more quickly than any other bearing in the engine.

The reason it is under such stress is the belt load. When you start the engine, it is already pulled tightly to one side by the belts, and then asked to work without oil (for 15 seconds in the case of the MF) until the oil pressure builds up. It's a wonder that we don't all have oil gushing out the front seal, due to a worn out front main bearing.

Belts should be just tight enough to service their load, and no tighter. The usual spec for a V belt calls for 3/4 inch deflection with thumb pressure in the middle of a 12 inch, or so span. The belt should feel live, not dead when you deflect it. There are gauges for measuring belt tension, but they cost more than a whole pile of replacement belts.

A little flapping motion on the output side of a belt isn't really a big deal. Do check to see if you can deflect the belt more than 1/4 to 1/2 inch with your thumb. If so, you might want to tighten it a little bit.

-Chuck

 

[Gastrap]

Its typical of a single cylinder compressor, overtightening the belt will only cause problems.

 

[LanceRobson]

The pulsation and flop is normal, provided the belt is properly tensioned and that it's the correct belt and not undersized (an undersized belt will tend to flop due to alternate stretching.

Remember that when a belt leaves a pulley it is trying to fly off into space at a tangent but it is being pulled back down to the next pulley. When running, the belt path is actually a kind of ovoid thing with a slight rise up from the first pulley and sloping down towards the next. What you're seeing is the alternating slack and tension related to the intake and compression strokes. Tighten it to the -10 TM guideline. Don't use any "general rule" kind of stuff.

The DOD bought and maintained over 50,000 of these things and for most everything, I'll take their guidance. A few of the GI guidelines are related to the way the DOD has to divide maintenance into "levels" or other unique challenges and those would be kind of goofy for us, as individuals doing all levels of maintenance to follow. This 'aint one of those times.

Other possible causes for belt flop are that the belt was overtightened at some point in it's life and has some damage to the cord or compound or that it sat without running too long and the belt has a distorted spot. If you rule these out, you should be OK.

If you've never replaced all the belts (and hoses) in the truck, I strongly recommend doing so. If any of the belts let go they often take the others with them and in their death throes they can beat on the back of the radiatior and can damage it. Also, they often get balled up on the pulley shafts and have been known to damage seals.

Having belts and hoses of known vintage goes a long way preventing a breakdown and towards keeping your peace of mind.

Lance

 

[cranetruck]

The compressor on the 8x8 has a spring loaded pulley to automatically tension the belt, the sheave halves are held together by a spring (see image) and I'm now wondering if it also effects the "flop" discussed above. I'll try to remember to check for it next time I run the engine...

 

[gimpyrobb]

Bjorn, that looks like a different animal than deuce stuff. One would have to buy the whole assembly(pulley) to convert to a spring loaded deal. It appears the two halfs of the pulley are loosely bolted together and the spring applys tension. On the deuce, the two halfs are screwed toward eachother.

 

[cranetruck]

Yeah, it's different, the pulley assembly fits on a keyed tapered shaft, which is threaded for a castle nut. Don't know if the shaft is the same on the deuce, but if it is, this pulley would fit. It uses a different tool for disassembly.

 

[kc5mzd]

I agree that the belt may have sat for along period and the cords may be broken or damaged allowing the belt to strech under load. Something I might also look at is the compressor. If it is bad it might overload your belt. Are you geting normal pressure? does it build pressure at a normal rate? If so I probably wouldn't wory about it. I would run the engine up to around 1000 -1200 rpm and see if everything smoothes out - it should.

 

[stumps]

...

Tighten it to the -10 TM guideline. Don't use any "general rule" kind of stuff.

The belt specs are in the -20 TM. The -10 tells you to look for frayed or missing belts.

The proper spec is 3/4 inch deflection, pressing with a ruler, using a finger grip on the ruler. I prefer using my thumb and eyeballing the deflection, as I always have those instruments at hand.

-Chuck

PS: I adjusted my original post to match the 3/4 inch deflection specified in the -20. They are running these belts delightfully loose compared to modern cars and trucks. A good thing given the 15 seconds without oil pressure that the front main has to survive.

 

[3rdmdqm]

Thanks for all of the info. Belt tension and deflection seems about right. Belt looks in good condition. It builds and holds pressure normally. I'm not gonna tinker with it. I'm going to subscribe to the "if it ain't broke don't fix it" theory for now and let it ride. I'll keep an eye on it, probably normal as everyone suggested, I just don't recall seeing it before. Thanks for all of the replys and information!

 

[stumps]

That's just it, the compressor is on so little of the time, that what you are likely to see after air up is done, is the compressor just idling along. If your compressor is cycling, and you aren't pressing the brakes, or running the wipers, you have a leak that needs attention. Check that your glad-hand's air valves are turned off (handle is in line with the tubing.

-Chuck

 

[3rdmdqm]

Interesting you mention that. My compressor kicks on about every 5 minutes even though nothing is running such as using brakes or wipers etc. I don't hear an air leak anywhere? Is it not normal for the compressor to cycle every few minutes when nothing is actually being used?

 

[stumps]

There is a check valve between the tank and the compressor, and the compressor has valves of its own. I can't see why it should be cycling, unless it is leaking.

If the leak was in the secondary systems, you would see the tank pressure gauge rise and fall as the compressor cycled... You probably don't, so the leak has to be on the compressor side of the pressure regulator, I would think.

-Chuck

 

[Keith_J]

The air compressor is direct drive, for every rotation of the pulley, there are 1-2 pulses where the compressor loads the belt, this causes tension to rise on one side and slack forms on the other side. Yes, the pulley is accelerating/decelerating with every rotation. Seems odd but the same thing happens with all reciprocating engines/compressors. This is why clutch disks have sprung centers.

Belts which are old get glazed with use, if they slip under load with correct tension, replace them.

On air leaks, there are a few pipe thread fittings and countless others. On your Monday morning PMCS when you are filling out your DA 2404s, grab a spray bottle of soapy water and find them. Notify all leaks on the 2404 and the mechanic might fix them . And check the drains!