# How Does This Switch Work?



## abrogard (Jun 4, 2012)

My cheap little compressor has stopped switching itself off and on when it reaches the set pressure.

The on/off switch mechanism seems very simple. The contacts are closed and the machine operates when one pulls the red knob up. When you push it down the contacts open and the machine switches off.

That's fair enough.

But how does the pressure sensing/switching mechanism work? I can't figure it out.

There are two pipes going up to the switch mech from the tank. A small one and a big one.

The small one leads to a little plastic rod that can be pushed UP towards a brass hinged plate at the bottom of the switch mechanism. UP. Not down. When the machine is operating that plate is in the UP position.

Pushing it UP wouldn't seem to do anything.

And the large pipe leads to a large brass spring and it's equally hard to see what it might do.

For the life of me I can't figure out how it works. 

GMC, a chinese company, 'Global Machinery Company', went bust in the financial meltdown. They're finished now. But they sold many thousands of these low cost machines and I'm hoping someone has run across them and knows something about how they work.

Perhaps the same mechanism is currently in use is some other Chinese made machines on the market today.

I have attached a couple of pics to this post and I have some more pics of the mechanism up on Picasa at:
https://picasaweb.google.com/abrogard/Compressor?authuser=0&authkey=Gv1sRgCJW_-aDo_7zj9gE&feat=directlink

ab


----------



## cedgo (Sep 29, 2011)

Hello.
Looks like the large pipe is for the pressure relief, so you won't end up with a bomb.
The switch itself is an "off the shelf part", any pressure switch should work.
But you need to make sure the motor is good or any switch won't do you any good.


----------



## abrogard (Jun 4, 2012)

Thanks for that, cedgo.

I finally figured out the problem. There's a wire spring in there and it came unhooked. I've got two of these compressors and the same thing has happened to both. Obviously a weak design.

It's interesting, to me at least. The spring works like a seesaw. Without a fulcrum. It is attached at one end to the metalwork above the small diameter pipe and at the other end to the metalwork above the large diameter pipe.

Normally, when working, the 'large pipe' end is low and the other end high. So the spring is nearly vertical and keeps the 'small' end up - and 'UP' is when the thing is switched on.

But when the large pipe end rises, as it does when the pressure in the tank rises to the design pressure, then the spring becomes more horizontal and can no longer exert any upward force on the small end.

This enables part of the switch to drop down by gravity, breaking the contact, for there is this allowance for movement within the thing.

So it switches off.

Then the pressure falls with usage in the main tank and the situation reverses itself.

The 'large' end of the spring goes down, it exerts pressure upwards on the small end again, closing the contacts. 

In this way it switches on and off even though switched on all the time.

Be good if it worked well but it obviously does't. Main problem I think is the lack of sufficient 'hook' on the ends of the spring where they go through the metalwork.

Reminds me of a space shuttle being lost for want of an 'O' ring.


So I was thinking of somehow finding these things in China and getting a replacement switch - hoping a modern version of the switch is improved and doesn't have this problem.

But now you say any pressure switch would work. But fitting one? Surely I'm not going to find a switch that'll fit the pipe diameters and locations I've got there?

Or is there some kind of standardization or something?

Or do I run pipework from my existing pipes to the new switch?

How's it work? 

Excuse my ignorance.




p.s. Yes, the motor is good. The whole thing was working fine until a couple of months ago. Then this happened. It stopped switching itself on and off. But the pressure relief works fine. When I have to use it I switch it off myself when the pressure is up, use some air, switch it on again, etc... If I'm late getting there it blows off the air itself...


----------



## cedgo (Sep 29, 2011)

Could you lok at other compressors at a retail outlet to see how they are plumbed?
You should be able to just put a new switch on without a problem.
Pipe fittings are cheap and available about anywhere.


----------



## pawon (Dec 26, 2011)

The big pipe with big block with the pressure gauge looks like a safety overpressure relief valve? Can't really see. Anyway, it also 'senses' the tank pressure from that big pipe.

The small pipe is definately a blow-off valve for when the compressor shuts down.
When the compressor stops, the air in the pipe between the compressor and the tank is blown down to make for an easy start up when the compressor starts up again. See here for more info: Air compressor pressure switch

Anyway, you can replace it with any standard pressure switch with integrated blow-down valve, as long as you can fit it on the pipes.

Make sure that the tank still has a safety valve installed somewhere to protect it from exploding.


----------



## abrogard (Jun 4, 2012)

Thanks for that, guys. I meant to come back to the thread and post an update but forgot. I did get another switch. Amazingly I was able to get precisely the right part online from eBay, sent to me from China, for only $10 !
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170542764941&ssPageName=STRK:MESCX:IT


Crappy design as I think it is apparently it is widely, widely used in China and hence so many of them available so cheaply on eBay. I fitted the part and it works fine.

You get the manifold with it, too. Totally excellent value. My first searches took me to all kinds of 'western world' pressure switches all costing hundreds of dollars. 

So I'm happy with all of that. Figured out how it works and all, think I've learned a little about compressors. 

But that 'blow down' line. I've still got to come to grips properly with it...


----------



## pawon (Dec 26, 2011)

Great that you found the exact right parts, it makes it all so much easier..

About the blow down line... it's actually quite simple... 

when the compressor starts up, the motor is having a hard time to make everything come to life, it will draw more than 3 times the current (amps) during startup.

Now there is the pipe that connects the compressor pump with the tank. The blow-down valve releases all air pressure inside this pipe to atmosphere when the air compressor stops. When you stop the compressor you hear "ssssshhhhhhhh" for a few seconds. That is the blow-down valve doing it's work.

Now when the compressor starts up again, there is no pressure in pipe directly after the pump. So it is much easier for the compressor to start up. If the blow down valve malfunctions, pressure remains in this pipe. When the compressor starts up, it is having a hard time to get everything moving, as it has to work against this pressure.

Here's a link with some more info: Air compressor unloader: how does it work?

Actually, most compressors won't even be able to start against this pressure. Others will try but will trip the breaker, as it draws a too high current for too long.

One more thing: there is a check-valve between the pipe and the tank. This way only the pipe is blown-down and not the whole tank.

Alright, enjoy your now working again air compressor!


----------

