“Watch when you are sizing your air cylinders…” says Norgren.

They write, “oversizing an actuator by one bore size can result in a 50% increase in the cost of compressed air required for the application. If care is taken in this initial step of the design process, every component upstream of the actuator will have a better chance of being correctly sized and applied”.

Sound advice, I’d say.

Here’s more information on air actuators.

Compressor motor just hums, and then blows breaker

This issue:

The compressor worked just fine until I shut it off for about 1 minute and tried to turn it back on. The pump motor makes a humming or buzzing sound, not the usual pump sound. If I leave it buzzing for too long (maybe 5 seconds) it trips the breaker, so I don’t do that for fear that I’m burning something up internally. I unplugged it, drained air pressure, and let it cool down overnight for 12 hours. The problem still exists. There were no sounds or physical indications that the pump or compressor had a problem before shutting it off when it reached 100psi.

It was plugged directly into an outlet without an extension cord. I’ve been using it in this manner for 3 years without problems. I’ve tried several outlets, (GFCI, non-GFCI, and unused circuits), but there is no change. It is a 25 gallon, Sears/Craftsman, 175psi air compressor. It has a red lever switch for OFF and AUTO positions. I haven’t located any reset switch.

Are there internal fuses? Is there a reset switch? Any other ideas or something I can check?

Thank you.

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Bill says…

First, thanks for writing in.

Second, thanks too for such a good description of the issue.

From the sounds of it, you either have a failed capcitor or a failed motor.

Since compressor motors (along with all AC electric motors) require an “inrush” of energy to start, a capacitor is used to “jump start” the motor, since there’s not enough amperage in the typical home circuit to start the motor.

If the capacitor fails, then the motors can’t start, and it sits and hums until it pulls too many amps and pops the breaker.

Could be the motor, but I’d bet on the capacitor. Since the capacitor is cheaper to replace than a motor, if you want to do it yourself, start there. If you are taking it to a shop, they can tell you pretty quickly which is causing your problem.

Cheers,

Bill

If your compressed air valves are plugging up…

If you notice that your compressed air control valves are getting plugged up all the time, it may be that you are using spool type air valves.

These have a sliding spool inside the valve body that redirects the air stream as the spool moves along, opening some air passages and closing others.

In a spool valve the air flow passages are sometimes quite small. If your airstream is dirty, then particulates can build up in the valve, either blocking air flow or causing the spool to stick so that the valve doesn’t shift.

You really should clean up your air, but if that’s not to be, then consider using a poppet style air valve that has relatively larger flow paths and less of a tendency to plug up. Also, poppet style valves tend to shift better when contamination is present in the valve.

A small sand blast application question.

I’m thinking about buying a small, home use sand blast cabinet that says it requires 10 cfm at 100 psi.  I have a 26 gal compressor that is rated for 6.4 cfm at 90 psi.

Would I be correct in thinking that I could probably make this work but only for short periods of time; in other words, if I start blasting the compressor will put out 10 cfm at 100 psi but only for a limited time.

Then the compressor will turn on and try and bring the pressure back up.

Since it’s not quite big enough, it will never be able to continuously operated at the required pressure/flow, but could be operated for short periods of time then pause to let the pressure build up again.

We are only talking about sand blasting some rusty trailer wheel hubs after all.

Thanks, Charles
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Bill says yes…

Hey Charles. Thanks for writing in.

I have a little 1.5 HP air compressor, and I use it to sandblast all the time.

You are correct in that, with a small compressor,  you will only be able to blast for a few seconds before the compressor kicks in.

If your compressor is rated for 6.4  CFM at 90  PSI, then that’s what it will give you.

The issue is, a blast gun will use much more than 6.4 CFM, even at 70 or 80 PSI.  It will drain your tank so fast that the pressure will drop below the point where the sand blasting is doing anything.

That’s why you have to wait for it to catch up. To get the pressure back to the point where the sand will actually take off the rust or paint.

Cheers,

Bill

Air cylinder mounting issue.

Whenever possible, use a clevis style mount when installing an air cylinder.

You can have a rear clevis, and / or a rod clevis.

The use of these devices will help compensate for an alignment issue between the air cylinder and the tooling when the air cylinder is cycling.

If the tooling is not aligned correctly with the axial motion of the cylinder, sideloading can result. Sideloading will result in premature cylinder wear and early air cylinder failure.