Hydraulic Pumps (Gear, Vane, Piston)

What it is

A hydraulic pump moves a set volume of fluid each revolution, called its displacement. Pressure comes from resistance in the system, not from the pump. Gear pumps use meshing gears; they are simple, low cost and fixed displacement. Vane pumps use sliding vanes in a cam ring; they are quiet and come in fixed and variable versions. Piston pumps use pistons in a rotating block; they suit high pressure and come in fixed and variable displacement, including pressure-compensated and load-sensing versions.

How to identify

Read the tag for model number, displacement, rotation and maker. Note the mounting flange (SAE two-bolt and four-bolt mounts are common), the shaft (keyed, splined or tapered and the spline count), the port type and size and the rotation viewed from the shaft end. Variable pumps have a compensator or control mounted on the housing.

Pump type comparison

Flow formula

Flow (gpm) = displacement (in³/rev) x shaft speed (rpm) / 231

Flow (L/min) = displacement (cc/rev) x shaft speed (rpm) / 1000

These give theoretical flow. Actual flow is lower because of internal leakage, shown as volumetric efficiency in the maker's data. More formulas are on the fluid power formulas chart.

Key points

• Rotation must match the drive. Many pumps are built for one rotation only.
• Inlet conditions matter. A restricted or long suction line causes cavitation and pump damage.
• Fill the pump case and inlet before startup as the maker instructs. Piston pump case drains must go straight to tank.
• Fluid cleanliness and viscosity limits are in the maker's data. See ISO 4406 cleanliness codes explained.
• Pressure ratings and compensator settings come from the maker and the system design.

Common mistakes

• Ordering by flange and shaft without checking rotation.
• Plumbing a case drain into a return line with back pressure.
• Running a new pump dry at startup.
• Replacing a failed pump without finding the contamination that killed it.

Governing standards: ISO 4409 (pump performance test methods); ISO 4413 (hydraulic system safety requirements); SAE J744 (pump mounting flanges and shafts).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.