Understanding pneumatic pump ratios & associated performance — Advance Fluid Control

A piston pump is just that – a piston which is connected to the air motor, and reciprocates up & down. Fluid is drawn into the piston, through a series of check valves, and then expelled from the outlet.

Air operated piston pumps are where ratios are important, and very high fluid pressures can be generated.

The very simplistic piston pump diagram on the right displays the basic idea of a piston pump, which is that the ratio relates to the effective surface area of the air piston relative to the effective surface area of the fluid piston.

So with a 3:1 oil pump, this means that the effective area of the air side of the piston is 3 times greater than the effective area of the fluid side of the piston.

This also means that if the air supply to the pump is 100psi, then the fluid pressure can reach 300psi. At 300psi, the forces on the air side and the fluid side are equal, and therefore the pump will stall and maintain this pressure in the system until such time as either the air or fluid pressure changes (i.e. somebody dispensing from the pressurized system).

With high pressure pumps such as grease pumps, the ratios are usually anywhere between 50:1 and 100:1, so how is this achieved while keeping the air motor down to a respectable size?

There are two points to remember here:

1. It is the effective difference in area between the two ends of the piston which matters.

2. There is a relationship between pressure and flow.

Effective Area difference:

This is usually impacted by the size of the rod used to connect the two piston ends, and this usually varies in size in the air chamber compared to the fluid chamber.

On some very high pressure pumps, the piston rod on the fluid side is only minimally smaller than the actual fluid piston, therefore reducing the effective area of the piston. Meanwhile the air side has a rod which is big enough to provide the strength needed but no more, therefore maximizing the effective area.

Pressure vs Flow relationship:

When the fluid piston has a very small effective area relative to the air piston, the pressure is high, but the flow is very low.

Conversely, when the fluid piston has a very large effective area relative to the air piston, the pressure is low, but the flow is very high.

In practice with oil and grease pumps the fluid piston never has a larger effective area than the air piston, as there is not enough pressure to push the fluid through hose or piping. Equal effective area is the lowest these go (1:1 pumps).

Leaving aside different pump technologies for a minute and assuming we had 2 pumps with the same operating principle, same ratio (same effective area differential), and same operating pressure range, but one with a higher rated flow than the other, the only way this higher flow can be achieved is by making both pistons larger than the lower flow model while still retaining the same ratio of area difference on the pistons.

There are numerous other methods which improve the performance of piston pumps which are outside this topic, however the piston size difference is still what determines the ratio, and therefore the potential pressure output.

So just to recap; for a 1:1 pump, the theoretical maximum output pressure is equal to the air inlet pressure, a 3:1 pump is 3 times the air inlet pressure, 5:1 is 5 times air inlet pressure, 50:1 is 50 times etc.