60 gal air compressor bogs down

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bud16415

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The motor is: 3.1 RHP, 7 Peak HP, 3450 RPM, 56 Frame, 208/220 Volts, Single Phase, 5/8" Shaft

The compressor is a single stage, belt drive Sanborn 60 gal compressor.

The compressor is about 16 years old and gets regular light usage, e.g., 1/2 to 1 hour of usage per month.
That’s a pretty common motor used on a lot of compressors normally they call it a 5HP 56 Frame. I think they can be had new for around $200, and I know it would be nice to know it is the problem before spending that amount. To replace the whole thing with a 2 stage might be $1500.



I would see if you have a local shop to look at it.

Good luck.
 

oldognewtrick

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Try looking through this.

 

BuzzLOL

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Sounds like same compressor as one of mine except mine has a 20 gallon horizontal tank... It has a beautiful gold "5 HP" plaque on it but is about 3.1 real HP. The motor HP plate just says "special". Like yours it's from back when the Govt was allowing lies about air compressor HP. Over the years I've seen actual 2 HP compressors called 2 1/4, 2 1/2, 3, 3 1/2, 4, 4 1/2, 5, 6, 7, even as high as 7 1/2 HP before the semi-crackdown! Of course anything over a real 2 HP would melt down a 120 volt plug/outlet. Shop vacs were also dishonestly labeled. Higher HP rating = higher price. Mine's black because I bought it 30-35 years ago as a 'Rocket' brand which is a rebuilt Samborn. It's 2 cylinder single stage. It's single stage because both cylinders are the same size and quickly do the same thing in parallel to 120 psi. IF it were dual stage it would likely have one large cylinder that compresses the air to a certain pressure and then a smaller cylinder that compresses the air to a higher pressure like 175 psi. It also would likely be a real 5 HP and be hard wired rather than plugged in.
About 3 or 4 months ago mine would only go to about 60 psi and then the belt would start making squealing noises and a bad smell as it was burning up as it started slipping. I thought the motor adjustment slots were all used up but discovered by loosening a shield holding bracket that was in the way I could give it one last tightening of the belt. When it starts slipping again, I either get a new flat belt or lengthen the adjustment slots. I think the belt does still slip momentarily from motor high starting torque when it's first turned on.
Your outlet valve(s) to the tank could be leaking back into the cylinders, this would prevent building up above a certain amount of pressure because more air couldn't get into the system, but that shouldn't really cause the motor to struggle, it just wouldn't make enough pressure. The motor should be able to handle the lighter load though as the lower pressure air moves back and forth.
 

RegularGuy

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Thank you for your thoughts, Buzz! Last night I did this test: I took off the air intake filter. I put 60psi of pressure into the tank and then shut the compressor off. I then opened the intake check valves with my finger. No air came out. This is my thinking: if the output check valve(s) between the compressor and the tank were bad (leaking) then when there is pressure in the tank (with the compressor off) and I manually open the intake check valves, shouldn't the tank's pressure leak out the intake check valves? This is because, if the output check valve(s) were bad, then the tank's pressure would go back from the tank into the compressor and would then go out the intake check valves when those valves are opened (and the compressor is off). Does this experiment seem to rule out the output check valve(s) being bad? Your thoughts, Buzz and everyone?
 

bud16415

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The valves on the compressor in/out are to hold pressure within the compressor pump, one cylinder full at a time. The check valve before the tank is to hold pressure in the tank when the compressor is off and also during the cycling. There can be some leakage within the compressor around the pistons and valves as long as the net effect is compressed air is making its way past the tank check valve. The compressor itself isn’t 100% efficient. And isn’t intended to hold tank pressure.

When the compressor shuts off it should hold pressure for a long period of time. I have mine plumbed to three places in my shop and outside and I have a few small leaks in my lines. When mine shuts down it will fire back up in about an hour. For that reason I turn mine off when I leave the shop. I used to always forget so I wired a light over the compressor to be on when the compressor is on. Now when I leave the shop even though the compressor isn’t running I see the light and it reminds me to flip the switch. I have thought about fixing the leaks and also adding another main air shutoff valve before the manifold to my lines, but never got to it.
 
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