Crane braking: mechanical, electrical, and the sequence between them

A crane always brakes with two independent systems: a mechanical brake that holds, and electrical braking that controls the load in motion. The drive sequence between them decides whether the load ever drops. This lesson is course section 4.2.

Mechanical and electrical braking

Every motion carries a spring-applied electromagnetic brake, the same fail-safe principle as the lift brake of Chapter 3: released electrically while the drive is energised, applied by the springs whenever power is removed. It provides holding and the ultimate safety stop.

On top of it, electrical braking controls the load while it moves and absorbs the energy it returns:

1. dynamic (rheostatic) braking: a chopper diverts the DC-link energy into a braking resistor;
2. regenerative braking: an active front end returns it to the supply.

A hoist that lowers heavy loads for a large share of its duty generates a lot of energy, and returning it to the grid is then the better choice for efficiency and heat.

Sizing the holding brake

The holding brake is sized on the torque the load applies at the brake shaft, times a safety factor, not on the motor's rated torque:

$$T_\text{brake} \ge \beta\, \frac{m g R\, \eta}{i} \tag{4.4}$$

Look at where $\eta$ sits: it multiplies. When the brake holds a suspended load, the load drives the mechanism, so gearbox friction helps the brake. Putting $\eta$ in the denominator, the habit from motoring calculations, overstates the brake torque, and is one of the most common sizing errors on lifting equipment.

The braking factor $\beta$ is set by the standards and the duty group: typically 1.5 for general hoisting, 2.0 or more for hazardous loads (molten metal, nuclear material) or high duty groups. It covers lining wear, thermal fade and oil contamination.

Two brakes, apart. A single brake is a single point of failure above people and plant. Hoists in demanding service carry two independent brakes, each able to hold the rated load alone, on separate parts of the drive train: conventionally one on the motor shaft and one on the drum. Two brakes on the same shaft would share a failure mode, since a sheared coupling downstream would leave both holding nothing. Their separation matters more than their number. Each brake is tested alone at commissioning, and periodically in service, while the drive applies a proving torque.

Brake-lift sequencing

The brake must be released and applied in the right order relative to the drive. Releasing it before the drive holds the load lets the load drop briefly: the rollback of a badly commissioned hoist. The correct handshake, a standard function block in crane drive firmware:

• Starting: drive enabled and magnetising, torque proved against the closed brake, brake released, motion commanded.
• Stopping: motion ramped to zero, brake applied, and only then the drive disabled, after a short delay.

Click the steps in the order the crane drive should perform them, from start to stop, then run the sequence.

Release the brakeApply the brakeCommand the motionEnable the drive and magnetise the motorDisable the drive after a short delayRamp the motion down to zero speedProve torque against the closed brake

Run the sequenceUndoStart againShow the correct order

Predict first

An operator stops a hoist with the load at mid-height. Which should stop the load: the drive or the brake?

The brake, because it is the safety deviceThe drive, down to zero speed; the brake then only holdsEither, as long as it is fast

What the hoist drive must do

From all this, a hoist drive must provide full rated torque at zero speed (holding and starting without rollback), smooth and accurate low-speed control (spotting, gentle landing), and a wide constant-power range: fast hoisting of light loads and slow hoisting of heavy ones, by field weakening above base speed. This "light load fast, heavy load slow" behaviour gets the most productivity out of the installed power.

FoundationStart here if this is new to you

Think of carrying a heavy box down the stairs. Your arms hold it (the brake), your legs control how fast you go down (the drive). You never let go with your arms before your legs are ready, and you never stop with a sudden jerk of your arms. A crane does the same, in a fixed order, every single time.

ExplorerGo deeper: derivations and open questions

Brake for Tutorial 4.1. Using eq. 4.4 with $\beta = 1.5$, compute the holding torque for the Tutorial 4.1 hoist, on the motor shaft ($i = 243$) and on the drum ($i = 1$). Why is the drum brake so much larger, and why is it still worth fitting?

Rollback. If the brake opens 0.3 s before the drive holds the load, how far does the Tutorial 4.1 load fall, neglecting the rotor inertia? What does the rotor inertia, reflected through the gearbox, change?