Anchor windlasses, mooring winches, towing winches, crane hoists, slewing drives, capstans and conveyors all turn hydraulic flow into rotation with a hydraulic motor. The relationships are the same as for a cylinder, just round instead of straight: flow decides how fast it turns, pressure decides how hard. Haul the load in the animation, then make it heavier, change the motor and the drum, and break something.
In simple terms
A hydraulic motor looks and works much like a pump, but the other way round. Oil is pushed into it, and the oil pushing on its pistons or gears turns the shaft. Every turn swallows a fixed amount of oil, so more oil per minute means more turns per minute. A motor that takes 400 cm³ per turn, fed 60 litres a minute, turns 150 times a minute.
How hard it turns depends on the pressure. The heavier the load on the rope, the more pressure the motor needs. A bigger motor (more cm³ per turn) gives more turning force from the same pressure, but turns slower on the same flow. It’s the same trade as the jack in lesson 1: force against speed.
The drum turns that twisting force into pull on the rope. A small drum gives more pull and less rope speed. A big drum gives more speed and less pull. Most winches also have a gearbox between motor and drum to multiply the torque.
The brake is the safety part. It’s held on by strong springs and only released by oil pressure. When the lever is in the middle, or the power pack stops, or a hose bursts, the pressure drops and the springs clamp the brake on. The brake holds the load, not the oil.
When you pay out, the load wants to fall and drives the motor round. A brake valve (a kind of counterbalance valve, lesson 11) holds it back so the load can only come down as fast as the pump fills the motor. The energy of the falling load turns into heat in that valve.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free account.
For engineers
| Quantity | Formula | Example (3 t, 500 mm drum, 10:1 gearbox, 400 cm³/rev) |
|---|---|---|
| Line pull | F = m × g | 3,000 × 9.81 = 29.4 kN |
| Drum torque | T = F × D / 2 | 29.4 kN × 0.25 m = 7,358 Nm |
| Motor torque | T(drum) / i / η(gb) | 7,358 / 10 / 0.95 = 774 Nm |
| Pressure needed | Δp = T × 20π / (V × η(m)) | 774 × 62.8 / (400 × 0.9) = 135 bar |
| Motor speed | n = Q × 1000 / V × η(v) | 60 × 1000 / 400 = 150 rpm |
| Line speed | v = π × D × n(drum) | π × 0.5 × 15 = 23.6 m/min |
Two consequences come straight out of the table:
- Maximum line pull is set by the relief valve, the motor size and the drum. With a 210 bar relief, this winch can pull about 4.7 t. A heavier load stalls it and dumps the flow over the relief.
- Line pull falls as rope builds up on the drum. Each layer of rope increases the effective diameter, so the same pressure gives less pull on the top layer. Winches are rated at a stated layer, usually the first.
Motor types. Gear and orbital (gerotor) motors are compact and cheap but leak more. Axial piston motors are efficient at high speed and are used with planetary gearboxes. Radial piston motors give very high torque at low speed and often drive a drum directly. On deck machinery, low-speed high-torque radial piston motors and piston motors with planetary gearboxes are the most common.
Internal leakage in a motor behaves like a pump’s: roughly proportional to pressure. At low speed and high load it becomes a large share of the flow, so a worn motor loses speed exactly when the load is heaviest. If the leakage exceeds the flow being fed, the motor turns backwards under the load. With the brake released, that means the load creeps down while the lever says haul.
Deep dive
Brake release pressure and sequencing. Multi-disc brakes in winch gearboxes typically release fully at around 20 to 35 bar and are rated to hold well above the motor’s maximum torque. The release pressure is taken from whichever motor line is pressurised, through a shuttle valve, often with a small orifice so the brake releases a moment after the motor has built torque. If the brake releases before the motor has torque, the load drops a little at the start of every haul. If it releases late or only partly, the motor drives against the brake and the linings overheat.
Dragging brakes. The usual causes are a release pressure lower than the brake needs (a worn pump, a relief or reducing valve set wrong), a restricted pilot line, a brake piston seal leaking, or contaminated oil in an oil-immersed brake. The symptoms are a hot gearbox or brake housing, burnt smell, more pressure needed for the same load, and a winch that won’t free-spool when it should.
Paying out and heat. When lowering, the load’s potential energy has to go somewhere. Lowering 3 tonnes at 24 m/min is about 11.8 kW, all turned into heat in the brake valve and oil. Long lowering operations, such as paying out a heavy anchor chain, are a common cause of high oil temperatures in UAE summers. The power pack’s cooler has to be sized for lowering as well as hauling.
Testing a winch motor. On a bench, a motor is tested by driving it with a known flow against a load and measuring speed, torque and case drain flow. On board, a practical check is a stall test: apply a measured pull with a load cell and compare the pressure needed with the calculated figure, then hold the load with the brake released and the motor ports blocked and watch for creep. Any test with a suspended load needs the brake able to hold and the area cleared.
Key numbers
| What | Typical value |
|---|---|
| Torque per 100 bar | V × 1.59 Nm (V in cm³/rev) |
| Low-speed high-torque motor sizes | 200 to 10,000 cm³/rev |
| Brake full-release pressure | about 20 to 35 bar |
| Brake holding torque | about 1.5 to 2 × motor maximum torque |
| Heat when lowering | F × v (3 t at 24 m/min ≈ 11.8 kW) |
