Hydraulic pumps
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Frequently asked questions
How do gear, vane and piston pumps differ?
Gear pumps are simple and tolerant of contamination, typically used at low to medium pressure. Vane pumps run quietly and suit machine tools and injection moulding at medium pressure. Axial piston pumps reach high pressure and come in variable displacement versions, but need cleaner oil.
How do I work out pump flow from cc/rev?
Theoretical flow in L/min equals displacement (cc/rev) times speed (rpm) divided by 1000. For example, a 20 cc/rev pump at 1450 rpm gives about 29 L/min before losses. Volumetric efficiency falls as the pump wears, so measured flow is lower than theoretical.
What must I check for a replacement pump to fit?
Flange type (SAE or metric), shaft type (keyed, splined, tapered), rotation viewed from the shaft end, and suction and pressure port positions and sizes. Running a pump in the wrong rotation can destroy it at start-up. For variable pumps, also identify the control type, such as pressure compensator or load sensing.
Why is a pump noisy and hot after replacement?
Usually cavitation from a restricted suction line, a blocked suction strainer, oil too thick when cold, or low oil level drawing in air. Piston pumps must have their case filled with oil before first start. Also check for air leaks at suction line joints.
How should a newly fitted pump be started?
Fill the pump casing with clean oil through the case drain port (for piston and vane pumps) before running, so bearings and sliding faces do not run dry. Open any suction line valve fully, set the relief valve low and jog the motor a few times. Then run unloaded for a few minutes to purge air before raising pressure gradually.
What is pump cavitation and how is it recognised?
Cavitation happens when suction pressure is too low, so vapour bubbles form and then collapse inside the pump, eroding metal surfaces. The signs are a gravel-like rattle, vibration and reduced flow. Common causes are a blocked suction strainer, a small or long suction line, oil too thick when cold and a tank set too far below the pump.
Does pump rotation direction matter?
Yes, most gear and vane pumps run in one direction only, and reversing them stops suction and may damage the shaft seal. Rotation is defined looking at the shaft end, usually marked R (clockwise) or L. When replacing a pump, match its rotation to the driving motor.
What does case drain flow on a piston pump reveal?
Case drain flow from a piston pump rises as internal wear increases, so it is a useful condition indicator. Measure it when the pump is new and track it periodically at the same pressure and temperature, and a marked increase means an overhaul is due. The drain line must run directly to the tank and must not be teed into a pressurised return line.
