Quick questions on Gear trains and torque in lifting devices: HSC Engineering Studies Lifting Devices

For gear stages in series, the overall ratio is the product:

Given a load $F$ to be lifted at speed $v$ on a drum of radius $r$:

A worm gear pair has a screw (worm) meshing with a gear wheel. The lead angle is typically small (under 10 degrees), which gives high gear ratios (40:1 to 100:1 in a single stage) but limits efficiency to about 40 to 70 percent.

Ship-to-shore container cranes at Port Botany use multi-stage helical-spur gearboxes between the 200 kW lifting motor and the wire-rope drums to lift 50-tonne containers at 1 to 2 m/s. Tower cranes on Sydney CBD building sites use planetary gearboxes for the hoist and worm gearboxes for the slewing drive. Construction site hoists for personnel use worm gearboxes for self-locking safety; they are also fitted with mechanical brakes as a backup.

Real gear stages lose 1 to 5 percent per stage; worm stages lose 30 to 60 percent. Always include efficiency before sizing the motor.

Self-locking is a property of the gear geometry. A separate friction brake is still required on hoists for safety; relying on worm-gear self-locking alone is not compliant with Australian Standard AS1418.

When accelerating heavy loads, the motor must also accelerate the gear train. For high-acceleration lifts (lifts in commercial buildings), include rotational inertia in the calculation. :::