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Squealing Compressor

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Mar 9, 2010
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I have a Craftsman “Paint Sprayer Air Compressor†that has suddenly started to squeal whenever the motor is running. I’ve done a reasonable amount of troubleshooting and adjustments over the 20 plus years I’ve had the compressor, but this is a new one.
The motor turns just fine and the ribbed drive belt wants to go, but the compressor pulley hesitates, causing the belt to give off a loud squeal. After a second or so, the compressor pulley will start to rotate, but the belt is still slipping and chirping. This slip/chirp scenario lasts through the cycle.
The compressor oil level is fine and I can turn the compressor drive pulley by hand without undo effort. The belt tension feels reasonable and there is no real adjustment for tension, save for shifting the motor, anyway.
I welcome any suggestions as to the cause and cure of this annoying issue.
 
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The belt may be worn (almost certainly is worn now that it has been slipping). You might try cleaning the belt and adjust the tension (take up the slack in the motor mounts).

The belt likely got some oil splashed on it so cleaning the belt might put everything right.

Be sure to clean the pulleys also.
 
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I have a Craftsman “Paint Sprayer Air Compressor†that has suddenly started to squeal whenever the motor is running. I’ve done a reasonable amount of troubleshooting and adjustments over the 20 plus years I’ve had the compressor, but this is a new one.
The motor turns just fine and the ribbed drive belt wants to go, but the compressor pulley hesitates, causing the belt to give off a loud squeal. After a second or so, the compressor pulley will start to rotate, but the belt is still slipping and chirping. This slip/chirp scenario lasts through the cycle.
The compressor oil level is fine and I can turn the compressor drive pulley by hand without undo effort. The belt tension feels reasonable and there is no real adjustment for tension, save for shifting the motor, anyway.
I welcome any suggestions as to the cause and cure of this annoying issue.

After 20 years I'd spring for a new belt or some belt "dressing", but questions occur: When you turn the pump pulley by hand, do you rotate it 2 full turns? Does you pump take longer to refill the receiver than it used to? If you meet some real resistance when turning through a full compression cycle or if it takes longer to fill the tank, you may have an outlet valve problem that causing the pump to work against its own compression in the cylinder which can cause to pump to stall. You may have an unloader valve which is not bleeding the internal pump pressure after it cuts off. This makes for hard starts and added stress on the belt.

To check all of this you need the power off (preferably unplugged), and the tank depressurized and open. There are rebuilding kits for most air pumps (good luck with Sears however), but if you find a real problem, you'll likely do better just replacing the pump. Depending on how much motor power you've got (real, not "peak" or other BS) Northern Tool and Harbor Freight both sell replacement oil-lubed belt-driven pumps.
 

Bill Boehme

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The belt is worn out -- plain and simple as that! Don't waste time or money with belt dressing as that is not the cure and you can get a belt for not much more than he cost of a can of belt dressing.

How do you know when a belt is worn out? A v-belt is designed to run on the sidewalls. When it wears to the point that the bottom of the belt makes contact with the bottom of the pulley, it is way past worn out. When a belt starts running in the bottom of the pulley, several things will begin to happen:

  • The gripping friction that a belt normally has from the wedging action of running on the sidewalls will suddenly be gone and this means that the belt will begin to slip.
  • A slipping belt will glaze the rubber
  • It will run hot
  • All of the above will cause the belt to squeal as it slips and wastes energy.
  • It is also likely that you will see the steel in the motor pulley turn blue -- it actually can get hot enough to anneal the steel.
  • slopping belt dressing on a worn out belt is not going to address any of the above problems. In fact, there is almost no good use for belt dressing except in a very few specialized applications -- things like pinch rollers are an example, but that has nothing to do with belts.
I hope that you have not had the same belt for 20 years -- that is about four belt lifetimes. Even if not mechanically worn out, an old belt will fail from "ozone cracking". When rubber gets hot, it reacts with ozone which makes the rubber brittle. Eventually, fine fine cracks develop in the rubber because it is too hard to flex -- therefore, it slips and squeals.
 
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First, thanks to all for the replys. I'm certain I have replaced the belt on a number of occasions over the years but I don't remember these symptoms. So, even though the belt dosen't look worn, I'm going to replace it and see if that cures the problem.
 

Bill Boehme

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It is often to tell just by looking at a raw edged belt if it is worn out unless it has become hard and glazed on the sidewalls. Another way that you can sometimes tell if it is worn out is by looking at the narrow bottom edges of the belt. If it looks like there is wear on the ribs from running on the bottom of the pulley grooves, that means it has worn down the sidewalls to the point that they are not carrying much of the load anymore. It is more difficult to detect wear on poly-V belts because the ribs are small, but the same thing that applies to standard v-belts also applies to them. If your compressor poly-V pulleys are aluminum, there is a slim chance that the pulleys have worn, but that is not very likely unless something has happened to necessitate an abnormal amount of torque to run the compressor pump.
 
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The fix for stopping a fan belt from squealing was to use a bar of soap. Just hold it on the running belt for a couple seconds. If the problem is fixed it is almost certain the belt is worn and needs replacing, or keep a bar of soap in the glove box.
 
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