compressor issue

[Henrik]

I've got an old dentists air compressor I bought used a couple of years ago. It's been an awesome compressor, in that it is oil free, has an air drier etc built in, so perfect for plasma and spraying.

It used to build up to 8 bar of pressure, but now all of a sudden it doesn't go further than 5 bar.

This is the compressor in question:

I've replaced all the reed valves in the pump, but it still doesn't go above 5 bar.

Also, I cannot feel any air escaping anywhere around the pump, so any idea what I should check next?

If I can't get it working, it looks like I might have to go for one of those wolf 3hp belt driven compressors of amazon, but obviously if I can save 250 odd pounds that'd be a better option

 

[awemawson]

Does it have an input air filter - if so is it blocked?

 

[normspanners]

HI
is the crank open to atmosphere, can you detect any coming out of there ? also does it pump from 1 cyl to the next etc, to slowly up the pressure ie multi stage that could give you a clue.
also you can switch it on from 'cold' and monitor the various heads for temp rise, that can give a clue as to which is doing what
N

 

[normspanners]

just another thought does the motor sound the same as it did ? can you could check the revs ? maybe the run capacitor is faulty ( if one is fitted).

 

[Henrik]

it does have an input air filter, but I've replaced that too (and even tried running it without).

It does sound very similar to what it used to sound like, maybe a tad bit louder, but nothing out of the ordinary.

I'm going to make a last ditch attempt to fix it by taking it all apart to see if there's anything obviously awry inside the actual pump itself, otherwise it's off to amazon.

 

[normspanners]

louder? maybe worn brgs not allowing pistons as far up therefore lowering the compression ratio?

 

[pedrobedro]

Could it be a faulty gauge ?

 

[Henrik]

no, because it doesn't turn off, just sits there at 5 bar and the cut off pressure is never reached.

 

[normspanners]

your picture does not show how the air gets from the heads into the rec.
also is it piston or diaphragm? 5 bar is roughly 2/3 of your 8 bar which would indicate one of the 3 'cyls' is not pumping ?

 

[KemppiFrog]

your picture does not show how the air gets from the heads into the rec.
also is it piston or diaphragm? 5 bar is roughly 2/3 of your 8 bar which would indicate one of the 3 'cyls' is not pumping ?

But.. would it not just take a bit longer to reach pressure if one was out?
And, oil free. If not diaphragm, which I doubt at 8 bar, how are the pistons sealed? That might be the prob.

 

[normspanners]

I was thinking the cyls look very short to be pistons,

But.. would it not just take a bit longer to reach pressure if one was out

not always if it is multi stage the first stage feeds the next higher pressure cyl, and so on, like a turbo and so it goes up, I have 240v diving type compressor if high stage goes down it will not make the 2000 psi even if you let it run all day.
Multi stage is an easy on the motor way of getting quick high pressure.

 

[Wallace]

There's a couple of photos on the Dencomp website of the cylinder and pistons here. Being for dental use I thought it might be a wobble type and oilless but it has a conventional piston and connecting rod with a sealed big end bearing, the cylinder bore looks to be brass/bronze too.

 

[Henrik]

Managed to fix it It was my own numptyness that was at fault again...

So I replaced all the reed valves as per the original post, but I forgot to stick one of the valve-plate to "head" gaskets on, so the cylinder was leaking air back into itself, so to speak. The gaskets are some weird teflon looking things, and they leave a residue behind that looks just like the gasket itself, so I didn't notice it earlier.

I.e. the original problem was a broken reed valve (causing leakage between pressure / inlet chamber in the "head" of one cylinder), which I fixed, but then I created an identical problem on another cylinder by not replacing the gasket...

Anyway, thanks for the posts, without the encouragement I would most probably have let it sit in the corner of the garage and eventually taken it to the tip

 

[normspanners]

well found that man, yeh real short stroke plus air passes through heads /block to where ? it still could be multi stage with the pots being shimmed up to lower then raise comp ratio pot to pot, and the last stage is faulty /worn piston etc, looks like a real cheepo thing must have only cost a grand or so!!!btw check the seats of the reed valves, checking temp rise in cyls from cold is a way of monitoring which pot is doing what

 

[Wallace]

Glad you fixed it Henrik, it would be a real shame to skip that machine. Norm, these are not really cheapo compressors, they are designed to supply clean, oil free dry air for dental work and quite a precision built unit compared to a normal reciprocating compressor for industrial use or spray painting. I dread to think how much a Dencomp 12 is with 3 x 2.5hp motors!

 

[normspanners]

Norm, these are not really cheapo compressors

looks like a real cheepo thing must have only cost a grand or so!!!

I was Joking---------

 

[Wallace]

I was Joking---------

I missed the punchline!

 

[normspanners]

I missed the punchline

no probs
Henrik glad you got it fixed, I fixed the local garages compressor, where the wife works, a few months ago , it was a brand new CLARK pump, ran for two days and seized, machine mart said you will have to send it back under warranty it will take 2/4weeks a garage without a compressor for a few weeks !!
It turns out the head gasket had blown between the pots passing the air from one to the other. It got hotter n hotter till the pistons seized !! a quick hone clean up the rings n pistons and made the gasket out of a piece of copper sheet annealed it, rebuilt and its run virtually flat out since. I have got him a proper old broom wade now to replace it with motor for £200, he is now going to keep the clark as a back up chances are it will never get used now those old broom wades just keep on pumpin

 

[Henrik]

I love compressors now that I've had one for a while, even if its a little baby one. I originally bought it purely to use with the plasma cutter, but golly gosh it's been useful for all manner of things from chipping away car sound deadening to running an impact wrench to clearing out oil passages and spraying.

I can't imagine working in the garage without a compressor now, I use it almost every time I'm in there.

Everyone should have one, more important than welder

 

[Wallace]

Henrik, your compressor would be perfect for airbrush and custom work as it would hardly have to work for a living. Does the tank have a water drain?

Norm, fair play to you. Knocking up a gasket these days is beyond a lot of people let alone doing one in copper and annealing it! If the machine has run flat out since it was probably ordered undersize in the first place and run above the duty cycle.

Regards, Mike.