Craftsman Compressor questions 5hp? replace pump?

[Honda-Guy]

I need some info here about my craftsman air compressor. The pump is bad (knocks at low psi and takes a long time to build pressure) and needs replacing but need to know which one to get.

Specs:
1992 Craftsman 60gal 5hp 2 stage air compressor

Motor - Doerr 5hp, 15amp, 230V made in USA motor (what the label says)
Pump - 2 stage 10cfm @ 90psi pump - Last time I checked it was a DeVilbliss that was subbed out and made in Italy and the parts are NLA

The questions I have:

Is it really a 5hp motor with a 15amp rating? I thought 3hp motors were 15amp and the 5hp was 20amp?

Can I get away with running a Harbor Freight pump rated for a 5hp motor or do I need the 3hp? I don't want to push the motor if it won't handle it but i'm looking for volume as its mainly used for my blast cabinet.

 

[RECox286]

I would take a look at the Harbor Freight offering. If

bought and taken home, I would open up the drain fitting

and pour some kerosene in it, then shake the beJesus out

of it and then empty the witches brew into a clean bucket.

Give the liquid a close eyeball inspection, and stir with a

magnet. Depending on the way things look, proceed with

the replacement. Not saying China does bad QA after

machining but you never know til you hit the GO switch.

Uncle Bob

 

[Citation]

So long as the motor RPM is the same as your current one (which would be a 3hp motor) a 5hp motor is fine. The pump is designed to run at a certain speed, it doesn't care if the motor is more powerful than needed so long as the RPM is correct. A more powerful motor won't help unless you bump up the pump RPM... which is likely to damage the pump.

Note that a 5 hp motor will probably have higher inrush current and thus you might need a bigger breaker/line from the fuse box to the compressor.

 

[Honda-Guy]

I don't think I was really clear, I'm needing the air pump, the motor is fine. With the current motor rating I have (I think they are over rated from Craftsman) could I get away with the bigger Harbor Freight air pump for 5hp motors or get the smaller one rated for 3hp motors?

 

[engineer2]

Is it a two stage or a two cylinder? If it is a two cylinder, I may have a pump. Can you post pictures?

 

[Honda-Guy]

Here it is

 

[md21722]

It looks like a 3HP dual stage setup to me. It is curious that they set it up this way because most dual stage run 140-175 PSI. The tank may be only good for single stage applications which may be why they limit to 150 PSI. The HF 3 HP compressor pump runs @ 1050 RPM and requires a 3.8" motor sheave. The HF 5 HP compressor pump has an operating range of 650-1050 RPM and they recommend 850 RPM. I don't have direct experience with these particular pumps, so I'm not sure if you slowed it down to 650 RPM if it would stress the motor. If you tried, it would need a 2.75" sheave, but it would be much quieter because it turns only 650 RPM. You would need an Amp clamp to measure current load. The other factor is the compressor flywheel size. If your top plate does't stand up high/wide enough off you may not be able to clear a larger flywheel without putting the pump on a base mount. You may need to buy a replacement belt depending on what adjustments are needed.

 

[engineer2]

I would agree a HF unit could probably be matched up, but you would have to measure and check specs. I have a two-stage pump I'm trying to get rid of, but it knocks too and it appears not many want to rebuild them.

 

[Honda-Guy]

Looks like unless it's industrial grade it's not worth rebuilding these little pumps. I'm not sure if the HF 3hp rated replacement pump is what I really need for blasting, might just **** it up and buy a whole new unit that's at least 15cfm and a actual 5hp motor.