In this section, the hydraulic piston/plunger is attached to the air piston and its bottom section is housed inside the hydraulic pump head. Its size determines the pressure ratio of the pump, which in turn designates output flow and maximum pressure capability. Its purpose is to pull liquid into the hydraulic body through the inlet check valve and push it out through the outlet check valve at an elevated pressure.

These check valves are spring loaded and direct the passage of liquid through the pump. During the suction stroke of the hydraulic piston/plunger, the inlet check valve opens to its maximum. The liquid is pulled into the pump while the outlet check valve is held shut by a spring and differential pressure. On the pressure stroke, the inlet check valve is closed as the hydraulic piston/plunger moves the liquid out through the outlet check valve.

A seal is positioned around the hydraulic piston/plunger and is one of a few parts that may wear. The seal’s purpose is to hold the liquid under pressure during cycling and to prevent both external leakage and leakage into the air drive. Various seal materials and designs are utilized based on the liquid to be pumped, operating temperature and pressure rating.

NOTE: With most pumps, a separation or distance piece may be utilized between the air drive section and the hydraulic section. This allows for total separation and contaminant-free operation.

MAXPRO air driven liquid pumps work on a standard reciprocating differential area principle utilizing a large air drive piston connected to a smaller hydraulic piston/plunger to convert compressed air power into hydraulic power.

The nominal ratio between the area of the hydraulic plunger and the air drive piston is shown by the dash number in the model description and estimates the maximum pressure the pump is able to generate. The actual ratio can be higher than the nominal so the pump will still cycle when the ratio of the output hydraulic pressure to the air drive pressure equals the nominal ratio. Consult technical information chart in the catalog for actual pressure ratios. Example: an S35 has an actual ratio of 1:39.

This means that the actual ratio of the area of the air piston is 39 times the area of the plunger.

Example:

If the air drive pressure is raised to 100 psi then the outlet pressure will be near 3,900 psi at stall. The maximum air drive pressure rating on all pumps is 160 psi.

When drive air is initially introduced to the pump, the pump will cycle at maximum speed, providing maximum flow and also functioning as a transfer pump by filling the test piece or actuator with liquid. The pump will then begin to cycle at a slower rate as the outlet pressure rises and offers more resistance to the reciprocating differential piston assembly. The piston assembly then stalls when the forces balance, i.e. when air drive pressure x air drive piston area = outlet (stall) pressure x driven hydraulic plunger area.

The hydraulic pressure drop (hysteresis) needed to cause the air driven pump to restart is very low due to little frictional resistance from the large diameter air drive piston seal and hydraulic seal. This can be as low as two times the pump’s ratio under certain conditions.

The minimum air drive pressure to operate a pump is 15 psi and the maximum is 145 psi depending on pump used.