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Unknown Air Compressor

I "inherited" a medium-sized air compressor with an unusual feature. The rig consists of a ~20 gal tank with a faded label that says Sears Roebuck, with a Leeson 2HP motor, and what I guess is a 2 cylinder compressor (painted green). The compressor has a block bolted onto the head (partially covering the felt intake filter) with 2 piston rods that protrude through the filter and rest on (or very close to) the reed valves. There is a 1/8" pipe fitting on the block leading to a chamber above the pistons. It appears as though the pistons are meant to be forced down (by air pressure), presumably to unload the pressure from the pump.

Assuming that is correct, how is the pressure ported to this block when the compressor cycles off? The tubing attached to the block is cut off and crimped shut. If my guess is not correct, what else would this feature be used for?

The pump connects to the tank through a check valve, with no unloader visible anywhere (it is controlled by an old Penn pressure switch, without an integral unloader). There is no sound of air exhausting when it shuts off, but it does sound like something is bleeding off the pump pressure slowly. At any rate, if I unplug the unit at, say, 100 psi, and plug it back in right away, the motor seems to have no trouble starting the pump.

Any thoughts on the origin of this pump, or its operation?

TIA, Joe (correct return addy is jdella-feraATmmmDOTcom)

Reply to
Joe
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Good guess. It's called a unloader alright.

The most common use of unloaders now days is on gas powered compressors. It allows the engine to run all the time without over pressuring the tank. I have a seen a very few electric compressors with unloaders but in at least one case that seemed to be because the company made gas powered compressors and just decided to stick a electric motor on one.

At one time unloaders where used on electric compressors during startup to help the motor get up to speed. This is still true on some large compressors.

If it starts with no trouble I probably wouldn't mess with it. It would be better if there was a unloader of some form but it may not be absolutely required if the motor starts up fine under load.

If you absolutely want to hook the unloaders up the easiest would be with a solenoid valve to the tank pressure. You'd want a three way valve that would pressurize the unloaders when power was off and dump the pressure from them through a orifice (to create a delay before the compressor engaged) when energized.

It's possible that it was originally a gas powered compressor or came off one. For those they use what's called a load genie which is basically a relief valve with it's output piped to the unloaders.

Wayne Cook Shamrock, TX

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Reply to
Wayne Cook

I had a two cylinder, two HP compressor with the suction unloader you describe. That was because it also had about a two gallon tank. It needed to be very portable and an unloader was a lot easier on the motor than an electric switch.

bob g.

Wayne Cook wrote:

Reply to
Robert Galloway

I have one of these, the compressor came from a company in Prinston, IL and was marketed by Monkey Wards, is a V-2 design 10 cubic feet at 90 psi, tank is from Sears.

Was built to spray aircraft dope on restored and recovered airframes. Installed the head unloader instead of a pressure switch as it surged when the compressor started and pulled too much electricial load when starting the compressor.

Put it together in 1967 and it's still running.

Hugh

Reply to
Hugh Prescott

Thanks to all who replied. I hadn't considered that it might be off of a gas-powered compressor. Although the compressor starts OK as is, I thought of rigging up a very small "tank" (maybe a short piece of 4" pipe) that will be dumped into the unloader through a solenoid valve when the pressure switch trips. Even though it doesn't seem to be needed, I might try it just for the halibut.

BTW, the original pipe from the compressor to the tank was cracked (it looked like it was a 1/2" chrome plated water supply line); I replaced it with 1/2" refrigeration tubing, looped into 3 coils to absorb vibration. Will the copper tubing work harden and fail, or is this an OK idea?

Joe

Reply to
Joe

The copper will fail, but it'll take time. The compressor sounds similar to many used on air-brake systems used on heavy trucks, buses and earthmoving equipment, except for the felt filter. The block you describe is an unloader, and the compressor was intended for continuous running. The unloader receives a pressure signal from a governor, a valve that senses tank pressure and opens when the preset pressure is reached. The pins hold the inlet valves open and prevent compression. When tank pressure falls below the cut-in point, the governor exhausts the unloader and the compressor goes back to work. An awful lot of compressors are replaced because they won't pump. Mechanics often don't know the system and spend $500 on a replacement compressor instead of $20 on a governor. A sticky governor will apply or release the pins slowly and the valves peen them, wrecking both pins and valves. The governor is designed for snap-action to avoid this, but they're often mounted right on the compressor and the heat dries up the lube on the o-rings, making them stick.

Dan

Reply to
Dan Thomas

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