How to Replace an Air Compressor Pump

How to Replace an Air Compressor Pump

We are talking here about Piston Air Compressor pumps here. Screw Compressors have safety cut-outs to protect them against low oil.

Sometimes a Piston Air Compressor pump will fail, but still be repairable. i.e. A cylinder head not tightened down, allowing air to escape. Or a spring valve inside the head not working properly. These are cheap fixes as the Major Brands have repair kits at a fraction of the cost of a new pump or complete compressor. However, if your luck is not in, and the pump has had a catastrophic failure and is past the point where it is economically repairable, then the next cheapest fix is to replace the pump.

The Two main reasons that piston Air Compressor pumps catastrophically fail are:

1. Compressor Oil not there. Causing it to seize.

“We’ve got no air” an employee tells his boss. “We can’t spray the car that had to be finished today because the Pump has seized on the Air compressor and the Electric motor keeps blowing a fuse when we try to start it.”

When we have had to repair these “No Oil” pumps, the amazing thing is even though the pump is many years old, the oil in it is Brand new and clean. This would suggest that the employees have failed to check the oil on the dipstick or siteglass for a long time, and only topped the oil up with clean oil after the damage has been done, to avoid getting the blame.

2. Overheating Compressor Pump Causing it to seize.

A properly maintained Piston Air Compressor pump can last for many years, but overheating shortens the life of it drastically. For this reason they are built with a fan blade structure on the flywheel which blows cool air over the pump itself when running.

The Pump will pump air whether it is running clockwise or anti clockwise, the only difference is that if it is running the wrong way, then it will not be getting the cold air blowing over the pump as it should do, and this will have unfortunate consequences and shorten the life of the pump. This is not a problem when using single phase electric motors, but with a 3 phase electric motor, you have a 50/50 chance of it running the wrong way, in which case you can correct it by merely swapping 2 of the phases over and the motor will run the other way.

So now lets replace the compressor pump

Firstly Disconnect the compressor from the Electricity supply and remove the belt guard.
The electric Motor is held on with 4 bolts. You will notice then that the bolts that hold the motor are in elongated slots in the “saddle” (the flat bit) of the tank (air receiver). Do not remove the bolts, but merely slacken them off and slide the motor towards the pump. This will cause the drive belt(s)to go slack and the belts can be removed at this point.

Now carefully disconnect the air delivery pipe from the cylinder head by completely undoing the air delivery pipe olive compression fitting that is holding it. Undo the 4 bolts that hold the Compressor Pump which can now be removed and discarded.


Fitting the new Compressor Pump

Even if you have bought exactly the same replacement pump, it is unlikely that the new pump footprint will be exactly the same as the old one, so you will probably have to re-drill new holes in the saddle. The metric/imperial problem

So, after cleaning the area where the pump has to go, slide the electric motor forward and put the new pump in position. Now put the belts on. You will need another pair of hands at this point. The pump position should be that the electric motor slots will still allow the belt to be tighten up a bit more once the Compressor Pump is firmly bolted down. And, very important, make sure that the pump pulley and electric motor pulley are parallel to avoid rapid belt wear.

Mark where the 4 holes need to be and drill them. Then bolt the new pump onto the saddle and put the belts back on and re-tension them by pulling the electric motor backwards. i.e. away from the pump.

Now reconnect the Air Delivery Pipe. That is the pipe (usually made of copper) that goes from the pump cylinder head to the tank. This pump does not go directly into the tank but through a Non Return Valve.

If the Air Delivery pipe does not line up, see if turning the cylinder head of the pump helps. If not, then you are going to have to make a new air delivery pipe. Copper pipe is notoriously difficult to bend. It flattens out when you try to bend it, and you need special tools. Plumbers and Gasfitters bend copper pipe in their work, so they are the best people to ask for help.

We are nearly there. With the new pump now bolted down, and the belt(s) correctly tensioned.  Now replace the belt guard.

Before starting, check the oil level via the dipstick or siteglass. Although these units are tested by the manufacturer it is important that they are refilled/topped up or checked for the correct amount of compressor oil before use.

This is because if you have had the pump delivered by carrier, the pumps must be drained of oil before the pump stockist sends to you via Parcel Force or any other carrier, oil filled items which could damage other of the carrier’s customers parcels.

Air Compressor oil is SAE 30 oil with no additives.

Maximum sizes of Air Compressor Pumps to Motors

2HP (1.5 Kilowatts) Motor = 9cfm (cubic feet per minute)

3HP (2.2 Kilowatts) Motor = 12cfm to 14cfm (cubic feet per minute)

4HP (3 Kilowatts) Motor = 18cfm (cubic feet per minute)

5.5HP Three phase Motor = 23cfm (cubic feet per minute)

7.5HP Three phase Motor = 30cfm (cubic feet per minute)

10HP Three phase Motor = 40cfm (cubic feet per minute)