Pick a hydraulic motor by working the four input numbers first: required shaft speed (rpm), required shaft torque (Nm), available pressure drop (bar), and volumetric displacement (cm3/rev). Sizing is the inverse of pump sizing: torque equals displacement times pressure drop divided by 2π, and flow equals displacement times rpm, so a 100 cm3/rev motor at 200 bar produces about 318 Nm of theoretical torque and needs 60 L/min to reach 600 rpm [S1][S3].
Hydraulic motors convert pressurised fluid into rotary mechanical work, and the same unit can run as a pump when driven mechanically, which is why sizing math is symmetric across the two directions [S1]. Fluid cleanliness, viscosity, and temperature band are non-negotiable inputs, not afterthoughts: 42-74 mm2/s at 40 °C, 20-60 °C normal oil temperature, 90 °C upper limit for no more than one hour, return filtration at 10-30 µm, and solid contamination not worse than ISO 4406 19/16 [S4].
Family Gate 1: Gear vs Piston vs Vane, by Displacement Band
Gear motors (external and internal) cover the small-displacement end, typically 0.45-50 cm3/rev, and accept pressure peaks up to about 250 bar with simple construction and low cost [S3]. Piston motors (axial and radial) start around 5 cm3/rev and scale past 500 cm3/rev, hitting 350-450 bar continuous pressure and volumetric efficiencies above 95%, which is why they dominate high-torque mobile and injection-moulding duty [S1]. Vane motors sit in the middle band, roughly 10-300 cm3/rev, with smooth low-speed running but a more limited pressure ceiling near 175 bar, and they are sensitive to contamination above ISO 4406 19/16 [S1][S4].
For the same shaft speed, a piston motor will give roughly 1.5-2.0 times the torque density of a gear motor at the same displacement, which is the first number to weigh when frame size is constrained. The selection criteria on GlobalSpec rank operating specifications, features, and type ahead of brand, and the catalogue search is built around displacement, speed, torque, pressure, and mounting [S1].
Family Gate 2: Unidirectional vs Bi-directional, and Why It Kills a Sizing
Unidirectional motors are set at the factory to rotate only clockwise or counter-clockwise, and a few types with asymmetrical shaft-flange ports require a port swap to reverse, which is fine for conveyor and fan duty where direction never changes [S3]. Bi-directional (reversible) motors swap rotation under live load using a standard directional valve, and the load can reverse without draining the case, which is the only option for swing drives, winch free-fall, and reel take-up [S3].
A unidirectional motor specified on a reversing duty will either stall at zero shaft speed or spike the case-drain pressure past the shaft seal limit, and either failure shows up as a leak at the output shaft within the first 200 hours. The Vivoil selection guide explicitly lists "repeated direction change during use" as the trigger to upgrade from unidirectional to reversible, and the same rule applies across all four dimensional groups (Group 0 at 0.45-2.28 cm3/rev up to Group 3 in the 60+ cm3/rev range) [S3].
Sizing Math: Torque, Flow, Pressure Drop, and Service Factor
Use the three equations: Torque (Nm) = Displacement (cm3/rev) × ΔP (bar) / (2π × 100), Speed (rpm) = Flow (L/min) × 1000 / Displacement (cm3/rev), and Power (kW) = Torque × Speed / 9550. A 50 cm3/rev piston motor on a 200 bar ΔP delivers about 159 Nm theoretical, dropping to roughly 145 Nm at 90% mechanical efficiency, which is the figure to enter into the driven-machine calculation [S1][S3].
Apply a 1.25-1.5 service factor for continuous duty, 1.0-1.25 for intermittent, and add 10-15% to flow for volumetric inefficiency and heat soak. For a load-sensing circuit, the load-sensing regulator forces pump pressure to equal ΔP across the flow control plus load-induced pressure, so the motor sees only the differential it needs and the rest of the pump pressure is not burned as heat across a meter-in valve [S2]. On conventional open-centre circuits, the meter-in drop is a pure loss, and switching to load-sensing commonly cuts motor-circuit heat generation by 30-50% on the same duty cycle [S2].
Hydraulic Circuit Context: Pumps, Valves, and Actuators
A motor does not size in isolation; the upstream hydraulic pump sets the flow ceiling and the relief valve sets the maximum ΔP the motor will ever see. A hydraulic cylinder on the same machine runs from the same pump but consumes flow only on stroke extension or retraction, so the motor branch needs its own flow divider or priority valve if both are active at once, otherwise the motor will starve and the cylinder will creep [S1].
For swing, winch, and track-drive duty the motor is usually fed through a counterbalance valve to absorb overrunning load, and the case drain must be piped back to tank at less than 2 bar, never to the return line at the valve, or the shaft seal fails inside 1000 hours. On machines that mix linear and rotary hydraulic actuators, the linear-motion spec on a reference like the hydraulic cylinder selection: bore, stroke, duty, and mounting rules should be read alongside this guide so cylinder speed and motor speed are sized off the same pump curve, not separately.
Who Should NOT Pick the Mainstream Gear Motor
Gear motors are the wrong pick for high-cycle reversing duty, for continuous service above about 200 bar, and for any application requiring low-speed high-torque output below 50 rpm with held load, where a radial piston motor with integrated planetary gearbox is the standard. They are also the wrong pick for high-back-pressure circuits where case pressure can exceed 5-7 bar, because external gear motors depend on internal balance and a separate case-drain port is not always provided [S1][S4].
Piston motors are the wrong pick when the budget is tight and the duty is below 100 bar, where the efficiency gain does not pay back the cost premium, and when the system filtration cannot be held at 10 µm absolute or better, because the close clearances score fast. Vane motors are the wrong pick on contaminated systems above ISO 4406 20/18, on cold-start duty below -10 °C where the vanes stick, and on continuous high-pressure circuits above about 150 bar [S4].
Selection Shortlist: 4 Profiles and Their Sizing Logic
For conveyor and low-pressure fan duty, 5-25 cm3/rev external gear at 100-160 bar, unidirectional, 1.0 service factor, ISO VG 46 oil, 25 µm return filter. For injection-moulding clamp and die-cast ejector, 50-160 cm3/rev axial piston at 280-350 bar, bi-directional, 1.5 service factor, load-sensing pump upstream, 10 µm return filter, case drain direct to tank at less than 2 bar [S2][S3].
For mobile swing and track-drive, 80-400 cm3/rev radial or axial piston at 350-420 bar peak, bi-directional, 1.5 service factor, counterbalance valve on each motor port, planetary gearbox at 15-40:1 to match load rpm. For low-speed high-torque winch, 250-800 cm3/rev radial piston at 300-350 bar with integral two-speed or planetary reduction, bi-directional with overcentre valve, and dedicated case-cooler if continuous running exceeds 30 minutes [S1][S3].
Cross-check the shortlist against the operating spec search on GlobalSpec (displacement, max continuous pressure, peak pressure, speed range, torque, port type, mounting flange) before locking the part number, and re-check the contamination code and oil temperature band against the published limits [S1][S4].