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Choosing a low-speed hydraulic motor: displacement, duty and the interface

Orbital and radial-piston motors compared, how continuous and intermittent ratings work, and why the flange, shaft and port code must be fixed before ordering.

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Two families of low-speed, high-torque motor

Winches, travel drives, augers and slewing drives usually use a low-speed, high-torque motor rather than a fast motor with a big gearbox. Two families dominate. Orbital (gerotor or roller) motors are compact and economical for moderate torque. Radial-piston motors, including the ball-piston QJM type widely made in China, deliver more torque at lower speed and tolerate heavier duty.

Both families appear here. Hewell catalogues orbital motors such as item 001 alongside geared drives; Horey lists nine BM1/BYM orbital sizes. Radial-piston QJM families come from LSWR as a QJM motor family and from Ninggong as ball-piston motors with brake variants. Haiyu offers its LHM series for marine duty.

Size from torque and speed, then check the duty class

Displacement sets torque per unit of pressure; flow sets speed. Work out the torque the load needs, including start-up, and the speed range, then pick a displacement that reaches the torque below the motor's continuous pressure. Makers publish separate continuous, intermittent and peak values. Danfoss, for its TMVW orbital motors, defines intermittent operation as values that may occur for at most 10% of every minute and peak load as at most 1% of every minute. Other makers use their own definitions, so ask for them in writing.

Return pressure matters too. Many orbital motors limit pressure in the return line or need a drain line to protect the shaft seal. If the motor sits in a closed-loop drive fed by a piston pump, the charge circuit matters as much as the motor; a replacement A4VG56 charge pump shows how many coupler variants one pump can have. Protect the circuit with a correctly set relief valve such as the AOER RP-14W-20A.

Fix the mounting flange, shaft and ports

Two motors with the same displacement are not interchangeable unless the flange, shaft and ports match. ISO 3019-2 sets dimensions and an identification code for metric mounting flanges and shaft ends of displacement pumps and motors, including keyed and splined shafts; many Chinese motors follow it, others copy a specific foreign maker. Ask for a dimensioned drawing with flange, pilot diameter, shaft type and length, port threads and drain port, and quote the full order code, not the family name.

When the motor drives a gearbox, the gearbox input fixes the shaft. Unimotics builds custom input interfaces for exactly this reason. Tracked chassis integrate motors into travel drives; the Xumai profile asks who is responsible for the hydraulic station and controls.

Ask for test data, not only a catalogue

ISO 4409:2019 describes how to test and present the steady-state performance of pumps and motors, including efficiency. A supplier that has run such tests can give you torque, speed and volumetric efficiency at stated pressures and oil viscosity; a catalogue table without test conditions is a starting point only. For repaired or used units, ask what was replaced and how the unit was tested after repair.

Electric drives are an alternative on some machines; the Hixee MCU controller page lists the motor and VCU interface questions for that route. For valves, fittings and seals around the motor, see hydraulic components.

Sources

  1. ISO 3019-2:2001 Mounting flanges and shaft ends — metric series
    Dimensions and identification code for metric flanges and shaft ends of displacement pumps and motors; confirmed 2025.
  2. ISO 4409:2019 Methods of testing pumps, motors and integral transmissions
    Test installations, procedures and presentation of steady-state performance and efficiency.
  3. Danfoss Technical Information, TMVW Orbital Motors (520L0728)
    Defines intermittent operation as max. 10% and peak load as max. 1% of every minute; lists separate continuous, intermittent and peak pressures.

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