Hydraulic Motor Symbol

Definition: The Hydraulic Motor symbol represents the rotary actuator that converts hydraulic fluid power back into mechanical shaft power, drawn per ISO 1219-1 as a circle with a filled (solid black) triangle whose apex points inward — energy flowing from the fluid into the unit — with Pressure In and Return ports.

Also known as: fluid power motor, gerotor motor, orbital motor, gear motor (hydraulic), piston motor, wheel motor, hydrostatic motor.

What the Hydraulic Motor symbol means

The Hydraulic Motor symbol denotes the mirror image of the pump — mechanically similar hardware run in reverse. Pressurised fluid enters at the Pressure In port, forces the internal elements (gears, gerotor star, vanes, or pistons) to rotate, and exits at the Return port back to tank; the shaft delivers torque to the driven machine. The ISO 1219-1 triangle convention is the entire identification: the filled triangle's apex points inward, toward the centre of the circle, meaning hydraulic energy flows into the component and out as mechanical work — exactly opposite to the pump's outward triangle.

Motors give hydraulics its signature advantage: enormous torque from a small envelope, stall-tolerant and continuously reversible. Torque is proportional to displacement times pressure drop (T = Δp × D ÷ 20π in N·m with bar and cm³/rev), while speed is flow divided by displacement — so the schematic reader can size the whole drive from the motor symbol's displacement annotation and the line pressures. Two opposed triangles mark a bidirectional motor (the normal case for winches and track drives, fed from a directional valve or closed-loop pump); a diagonal arrow marks variable displacement, used to trade torque for speed on the fly.

How to identify the Hydraulic Motor symbol

The symbol is a circle with a solid black triangle whose apex points inward from the port toward the circle's centre — read the triangle as an arrow showing fluid energy being consumed. Bidirectional motors show two opposed inward-pointing triangles; a struck-through diagonal arrow adds variable displacement; a dashed line leaving the case marks the external case drain that piston and high-speed motors require. Port labels follow A/B for the work lines with T (or L) for the drain.

The pump/motor distinction trips up every newcomer precisely because the hardware looks identical on paper: same circle, same triangle geometry — only the direction differs. A memory hook: pump pushes out (apex out), motor absorbs (apex in). Open triangles instead of filled ones would make it a pneumatic air motor per ISO 1219-1's medium convention. As with pumps, there is no separate ANSI shape — legacy ANSI Y32.10 matched the ISO grammar and US practice now cites ISO 1219-1.

Function in a circuit

Fluid entering at Pressure In acts on the motor's internal displacement elements: in a gerotor/orbital motor the pressurised zones walk an inner star around a fixed ring, producing high torque at low speed; in axial-piston motors fluid drives pistons against a swashplate for high speed and efficiency; gear and vane motors mirror their pump counterparts. The pressure drop across the motor (Pressure In minus Return backpressure) times displacement sets the torque; the flow through it sets the speed — making the motor a transparent flow-to-speed, pressure-to-torque converter.

Circuit context matters for safe reading: an overrunning load (a winch lowering, a vehicle descending) turns the motor into a pump, so real circuits add counterbalance valves or brake valves to prevent runaway, and crossover relief valves to absorb shock when the directional valve closes on a spinning load. Many motors also need their case drain routed directly to tank — blocking it blows the shaft seal — which is why that little dashed line on the symbol deserves attention during troubleshooting.

Standards: IEC vs ANSI

Terminals / pins

Typical values

Displacements span ~8 cm³/rev (high-speed gear motors) to 250–800 cm³/rev (orbital/LSHT motors) and beyond for radial-piston cam motors (multi-litre class). Speed ranges: LSHT orbital motors 10–800 RPM; axial-piston and gear motors 500–5,000+ RPM. Continuous pressure drops: gerotor ~140–175 bar, geroler disc-valve ~210–240 bar, piston motors 350–450 bar. Torque follows T(N·m) = Δp(bar) × D(cm³/rev) ÷ 62.8 × ηmech — e.g. a 200 cm³/rev orbital motor at 175 bar delivers roughly 500 N·m. Overall efficiencies run 80–92%; case-drain backpressure limits are typically 2–5 bar.

Where the Hydraulic Motor symbol is used

• Excavator and skid-steer track/wheel drives, often with integral planetary gearboxes and brakes
• Winches on cranes, recovery trucks, and marine deck machinery (with counterbalance valves against overrunning loads)
• Auger, trencher, and drilling-rig rotary heads needing high stall-tolerant torque
• Conveyor, feeder, and mixer drives in agriculture and food processing (orbital motors on drum mixers, augers, bale spinners)
• Cooling-fan drives on heavy equipment, where a small motor replaces long mechanical fan shafts
• Hydrostatic transmissions, closed-looped with a variable pump for seamless speed and direction control

Example

In a winch circuit drawn to ISO 1219-1, the Hydraulic Motor symbol's Pressure In pin is fed from the directional valve's A line and its Return pin goes back through a counterbalance valve to tank. The bidirectional 315 cm³/rev orbital motor at a 160 bar pressure drop produces about 760 N·m at the drum; at 60 L/min it turns roughly 190 RPM. The counterbalance valve on the Return side prevents the descending load from overspeeding the motor — the classic protection the inward-pointing-triangle symbol should prompt you to check for.

Key facts

• ISO 1219-1 identification: a filled triangle pointing INWARD (apex toward the circle centre) marks a hydraulic motor — fluid energy flows into the unit and out as shaft power; outward means pump.
• Torque comes from pressure drop times displacement (T ≈ Δp × D ÷ 62.8 in N·m/bar/cm³-rev terms); speed comes from flow divided by displacement — the two are independently controllable.
• Two opposed triangles mark a bidirectional motor, the normal configuration for winches, track drives, and any reversing load.
• A diagonal arrow through the symbol means variable displacement — the motor can trade torque for speed while running (two-speed travel motors).
• LSHT (low-speed high-torque) orbital/gerotor motors deliver hundreds of N·m at 10–800 RPM without a gearbox; piston motors cover high-speed high-pressure duty to 350–450 bar.
• Overrunning loads turn a motor into a pump — counterbalance or brake valves are required to prevent runaway on winches and slopes.
• The dashed case-drain line on the symbol is functional: block a required case drain and internal leakage pressurises the housing and blows the shaft seal.
• Crossover (cross-port) relief valves protect a spinning motor from pressure spikes when the directional valve suddenly blocks both work ports.

Frequently asked questions

What is the difference between the hydraulic pump and hydraulic motor symbols?

Only the triangle direction. Both are circles with a filled triangle, but the pump's apex points outward (energy leaving the unit into the fluid) while the motor's apex points inward (fluid energy entering the unit, leaving as shaft power). ISO 1219-1 defines both; on a busy schematic the triangle direction is the single detail to check.

How do I calculate hydraulic motor torque and speed?

Torque (N·m) ≈ pressure drop (bar) × displacement (cm³/rev) ÷ 62.8, times mechanical efficiency (~0.85–0.95). Speed (RPM) = flow (L/min) × 1000 ÷ displacement (cm³/rev), times volumetric efficiency. Example: 200 cm³/rev at 175 bar gives about 500 N·m; feed it 40 L/min and it turns roughly 190 RPM. Torque is set by pressure, speed by flow — independently.

What is an LSHT or orbital motor?

A low-speed high-torque motor using a gerotor/geroler element: an inner star orbits inside a fixed ring, so each shaft revolution displaces many chamber volumes, producing large torque at 10–800 RPM without a gearbox. They are the workhorse motors on augers, conveyors, mixers, and wheel drives. On the schematic they carry the same inward-triangle symbol — the displacement annotation (large cm³/rev) hints at the type.

Why does a hydraulic motor need a case drain?

Internal leakage past pistons or vanes collects in the motor housing. Piston motors and many high-speed motors route this leakage out through a dedicated case-drain port (the dashed line on the symbol) directly to tank at low backpressure (typically under 2–5 bar). If the drain is blocked, plumbed into a pressurised return, or forgotten, case pressure rises and destroys the shaft seal — a classic commissioning failure.

What happens when the load overruns a hydraulic motor?

The load drives the shaft faster than the supply flow supports, the motor starts acting as a pump, inlet pressure collapses, and the load can run away — a lowering winch or descending vehicle is the textbook case. Circuits prevent this with counterbalance valves (pilot-operated restriction on the return side that meters the load down) or brake valves, plus crossover reliefs to absorb the shock of stopping. Seeing a motor with a hanging or rolling load on the schematic should trigger a search for these valves.

Related symbols

• Check Valve symbol
• Flow Control Valve symbol
• 3-Phase Motor symbol
• Pump (Motor-driven) symbol
• Solenoid Valve symbol
• Pneumatic 5/2 Valve symbol

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