Low cost compressors

[MG David]

I have been using a friend's compressor for some time and now feel I should return it and buy my own. The one I have been using has a 24L tank and I think it is 1.5hp. The pump has oil in it.

I am looking to buy a compressor a little better than this and had in mind 50L tank and 2hp motor. In that size range some are described as "oil less". Is that good or bad?

My guess is an oiled pump will have alonger duty cycle. Over the years I have had a number of electric drills, sanders and immitation dremmels fail because I have used them for too long at a stretch.
Uses will be a spay gun (normally one panel at a time), schultz gun, die grinder etc. From the specs on the machine mart site 8 cfm 6.2 bar seems to be enough.

 

[rtbcomp]

The oil-less ones are diaphram types and tend to be direct drive, which are noisy.

Go for a belt driven, piston type with cast-iron liners/cylinders for longevity and relative quietness.

 

[shenion]

The best to worst:

Belt-driven oiled pump - quiet and reliable. Can be repaired as there re rebuild kits for pumps and motors are standard.
Direct-drive oiled pump - Usually throwaway as non-standard parts, loud
Direct-drive oiless. - Throwaway, usually very loud, no balancing in pump.

Direct drive is usually Chinese.

My current tank was of a oiless, direct drive compressor. It broke the connecting rods after the oiless bearings went; so got it cheap. Was probably less than 3 years old. I took the motor and pump off my 20+ year old, belt-driven compressor and mounted it on it. STill runs almost like new.

You get what you pay for. Go for CFM and a good duty cycle, you can always add more storage.

My last compressor was 6.8 CFM @ 90 PSI. Worked ok and was an oiled, direct drive. Was a Cambell-Hausfeld and is still running today in my dad's garage (rarely used though.) I had it running non-stop for hours as it could not keep up with my air toold.

See if you can find specs for xxx CFM @ 90PSI (6 bar.) That is a real nuumber unlike the displacement they quote over there (you never see displacement in the US.) Here the motor HP are a outright lies though.

My current one is 9.9CFM @90PSI. Works well but I would like to double that for blasting. As is, can run on a medium tip and just keep up.

Note: there are oiless pumps for things like dental offices and other specialty applications. They are usually very expensive, low CFM and would not be confused with the low-end oiless.

 

[pedrobedro]

Schultz gun?

He knows nossing (Hogan's Heroes)
Compressor and quiet are not really words associated with each other unless you count the Hush Power using fridge type compressor pumps. If you see a direct drive at the right price new don't be put off it by the bit more noise it makes. I've had one for 18 years and apart from oil it has not cost me anything extra. It gives 14cfm and 150psi. about 9cfm fad. It will run a Schutz (German for protection) gun but a die grinder makes it grunt and the machine you have in mind will really struggle with one. You will be doing a 3 minute stretch and waiting for five while it fills up the tank. You need big numbers to run tools efficiently and it's cheaper and quieter to burn out a few electric tools at 600w than to buy and run a big compressor.

 

[Sam(GTD)]

The compressor should be the most durable part of your air system and should be able to outlast (including rebuilding) all of your airtools.

Many friends have bought the cheap oil free direct drive type (typical size 24 litre) and the do break down very quickly with no chance of repair.

I would say 'you get what you pay for', but in regards to economy it dosn't make sense to buy a cheap compressor if you use air tools a lot because you'll need to buy another a few years down the line, if you buy a proper belt driven/cast iron pump/decent motor compressor then it is worth servicing and repairing for the next decade or two....

If you know what you are doing second hand is the way foward on a budget
For £160 i got a 12cfm (if not more, no rating), twin pot cast iron (sherry of london) + hawker siddeley motor, with little use as it had been stored for 10 years.
Absolutely bulletproof.

And i wasn't lucky, you can see plenty of deals if you look hard, just know what you're looking for:

-Good quality cast iron pump which is high capacity low RPM (these cheap alloy pumps on direct drive cheap ones cycle much faster are poorly balanced and shake themselves to death)
-Good quality motor, you can usually tell this from the weight.....the heavier the better.

The rest isn't really important at all, pressure switches are cheap to replace, big tanks are cheap to install (be it second hand or converted propane cylinders)

 

[zardos]

The best to worst:

Belt-driven oiled pump - quiet and reliable. Can be repaired as there re rebuild kits for pumps and motors are standard.
Direct-drive oiled pump - Usually throwaway as non-standard parts, loud
Direct-drive oiless. - Throwaway, usually very loud, no balancing in pump.

Adding

Hydrovane - very very quiet and reliable. Can be repaired, can be used without a receiver.
Just note that they are quoted as Free air delivery (FAD) CFM not displacement CFM as per most piston compressors. e.g. 8CFM FAD is about 12 CFM displacement.

But very expensive unless you pick up a second hand one.
£400 got me a Hydrovane 502 (8.5 CFM FAD @ 10 Bar)

 

[brightspark]

Adding

Hydrovane - very very quiet and reliable. Can be repaired, can be used without a receiver.
Just note that they are quoted as Free air delivery (FAD) CFM not displacement CFM as per most piston compressors. e.g. 8CFM FAD is about 12 CFM displacement.

But very expensive unless you pick up a second hand one.
£400 got me a Hydrovane 502 (8.5 CFM FAD @ 10 Bar)

400 quid id want 2 for that price and some change

 

[tigler]

Also, take a look at the compressor FAQ section.

 

[MG David]

I have noticed that Machinemart have direct on line starters. I am not sure what they are for. Are they to stop a surge on startup of something like a compressor popping the circuit breaker?

 

[rtbcomp]

I have noticed that Machinemart have direct on line starters. I am not sure what they are for. Are they to stop a surge on startup of something like a compressor popping the circuit breaker?

They just connect the motor to the mains via an overload coil, so if the motor draws too much current they switch it off. This is to protect motors which don't have their own re-settable trip.

There is no surge protection in MM ones at least.

Some types have a no-volts release coil, so if the mains fails the machine won't start up when power is restored.

 

[MG David]

Looking at the machine mart website they quote the CFM output as displacement. They also quote a CFM requirement on the air tools they sell. Is it reasonable to assume they are quoting those as displacement or is that too logical?

 

[eil]

Looking at the machine mart website they quote the CFM output as displacement. They also quote a CFM requirement on the air tools they sell. Is it reasonable to assume they are quoting those as displacement or is that too logical?

On the tools its the air the tool consumes in FAD @ its working pressure normally around 90psi, which means you need a compressor FAD that can at the very least match the consumption of your chosen tool. As has been previously stated subtract about a third off the compressor displacement to give you a ball park figure of FAD. Say 14cfm displacement would be around 9cfm FAD. the free air delivery is the one that is really important.

 

[tigler]

Also bear in mind that a compressor that only delivers what your tool or tools demand will be running constantly to keep up, not good. Rule of thumb is the compressor should be able to deliver 50% more than the demand.