Hydraulic motors and winches: how flow makes speed and pressure makes torque

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.