The retarding moment (N·m) that the traction machine brake must generate to hold or decelerate the elevator car and rated load under the worst-case load and speed conditions specified by EN 81-20 or equivalent standard.
Full Definition
EN 81-20 Section 12.4 requires the brake to be capable of stopping and holding the car with a minimum braking torque equivalent to 125% of the rated load torque, applied at the machine sheave shaft. For a gearless machine with sheave radius R (m), rated load Q (kg), and rope suspension ratio i, the minimum brake torque is: T_brake ≥ 1.25 × (Q + car weight) × g × R / i. The brake torque is verified during commissioning by a no-load and full-load stopping test, measuring deceleration rate and final position. Two independent means of measuring brake torque exist: direct measurement via torque arm or deceleration measurement from an accelerometer. Where dual-brake circuits are fitted (as required by EN 81-20 for new installations), each brake circuit must independently achieve 100% of the required torque. Brake torque diminishes with lining wear and spring relaxation; periodic testing is mandatory.
