Hydraulic Motors

What it is

A hydraulic motor works like a pump in reverse. Oil under pressure pushes on gears, vanes or pistons and turns the output shaft. Speed comes from flow. Torque comes from pressure. Displacement, the volume of oil per revolution, links them. Motors are used on wheel drives, conveyors, augers, winches, sweepers and fans.

How to identify

Read the nameplate or the part number for displacement (cubic inches or cubic centimeters per revolution), mounting flange, shaft type and port type. The body shape gives clues:

• Gear motors: compact, rectangular body with two meshing gears inside. Look like gear pumps.
• Vane motors: round cartridge body, quiet, smooth at medium speed.
• Piston motors: axial (bent-axis or swashplate) or radial piston. Large and heavy for their size class.
• Orbital motors: short, round motors with a gerotor or geroler set, often with a wheel or flange mount. Built for low speed and high torque.

Motor types compared

Basic formulas (theoretical)

• Speed (rpm) = flow (gpm) x 231 / displacement (cubic inches per rev)
• Torque (lb-in) = pressure (psi) x displacement (cubic inches per rev) / 6.28

Real speed and torque are lower because of leakage and friction. Use the maker's efficiency data or curves. See the fluid power formulas chart.

Key points

• Most motors need a case drain line back to tank. Blocking it can blow the shaft seal.
• Bi-directional motors run either way. Uni-directional motors must be plumbed for the correct rotation.
• Starting torque is lower than running torque on many motors. Check startup torque for loaded starts.
• Shaft side loads must stay within the maker's limits. Wheel motors have bearings sized for it; others may not.
• Low-speed performance is tested to ISO 4392 methods. Smoothness at very low speed varies widely between types.

Common mistakes

• Leaving the case drain plugged or tied into a pressurized return line.
• Replacing a motor by flange alone without checking displacement.
• Ignoring shaft type: keyed, splined and tapered shafts are not interchangeable.
• Using a motor with no brake to hold a load. Motors leak and will creep.

Governing standards: ISO 4392 (low-speed motor characteristics); SAE J744 (mounting flanges and shafts); maker's performance data.

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.