Because the disc is exposed to the air on both faces and around its full circumference, so it sheds heat continuously while it works.

An enclosed brake - a drum brake or an electromagnetic unit - has its friction surfaces inside a housing. Heat generated there must conduct through the structure before it can reach the air, which is slow, so the friction faces run hot and the sustainable energy rate is limited.

A brake disc is out in the open. It is a large exposed mass of metal with a great deal of surface area, spinning in air, so it both absorbs a large amount of energy without a big temperature rise and rejects that heat quickly afterwards.

Ventilated discs, with radial passages between two friction faces, pump air through themselves as they rotate and improve this further.

The practical result is that a caliper installation handles energies and cycle rates that would rapidly destroy an enclosed brake of comparable torque rating.

By a larger disc, more calipers, higher pressure, or better pads - usually in that order of preference.

Torque is the clamping force multiplied by the friction coefficient and by the effective radius at which the pads act. Increasing the disc diameter increases that radius, so it raises torque directly and also adds thermal mass - which is why it is the first option where space allows.

Adding calipers around the same disc multiplies the clamping force. Because the mounting is modular, two, four or more calipers on one disc is a standard arrangement, and it also spreads the heat input around the disc rather than concentrating it.

Raising the supply pressure increases force per caliper, within the component ratings.

Changing pad material to a higher friction coefficient works but usually trades against wear rate and disc wear.

The modularity is a real advantage: an installation can be uprated without replacing the whole brake.

The braking torque follows the applied pressure closely and repeatably, so it can be modulated continuously rather than switched on and off.

In web handling - paper, film, foil, textile - an unwinding reel must be held back with a controlled torque so the web stays at constant tension. As the reel diameter falls during the run, the torque required falls with it, so the brake must track a continuously changing demand.

A caliper brake does that naturally: a proportional valve or a pressure regulator driven by the tension control system sets the pressure, and the resisting torque follows. Response is fast enough to correct disturbances.

The complication is that this is continuous slipping duty rather than intermittent stopping, so all the energy of the tension times the web speed goes into the brake for the whole run. Thermal capacity is therefore the governing selection criterion, and forced cooling or a generously sized disc is often required.

Yes - spring-applied hydraulic and pneumatic calipers are standard, and they are widely used on hoisting and winding duty.

In a spring-applied caliper the springs clamp the pads onto the disc, and pressure is applied to hold them off. Loss of pressure - a burst hose, a pump failure, a power cut - lets the springs close the brake.

Heavy installations frequently combine both types on the same disc: power-applied calipers for modulated service braking during normal operation, and spring-applied calipers for emergency and holding duty. That gives fine control in normal running and an independent fail-safe function, which is what winder and hoist standards generally require.

When specifying a spring-applied caliper, check the release pressure required as well as the clamping torque - a hydraulic system that cannot reach the release pressure leaves the brake dragging, which is a common commissioning problem.

Cast iron or steel for most industrial duty, sized for thermal mass as much as for torque, and mounted so it can grow when hot.

Cast iron has excellent thermal capacity and damping and is forgiving of pad materials. Steel discs are used where higher strength or lower mass matters, and specialist materials appear in high-energy applications.

The mounting detail matters more than it seems. A disc heats substantially during braking and expands radially; if it is rigidly clamped at its bore it cannot grow freely, and the resulting stresses cause coning, distortion and heat cracking. Discs are therefore commonly mounted on a hub arrangement that allows radial growth while transmitting torque.

Runout matters too. A disc running out of true knocks the pads back on each revolution, which increases the pedal or pressure travel needed and can cause vibration and uneven wear. Check runout at installation and after any disc replacement.

By pad thickness and by caliper piston travel, and increasing travel is the practical indicator.

As pads wear, the pistons must extend further to close the gap to the disc. That increases the volume of fluid or air needed for each application and lengthens the response time, so the brake applies later - which on a stopping duty directly increases stopping distance.

Many calipers include a wear indicator or a visible scale on the piston position. Some installations use a proximity sensor to signal wear or a fully retracted position electronically, which suits unattended and safety-critical plant.

On spring-applied calipers, wear reduces the spring extension and therefore increases the clamping force slightly - the opposite of the electromagnetic case - but it also increases the release pressure required, so a worn brake may begin to drag.

Inspect the disc at the same time. Heat checking, scoring and thickness loss all limit life independently of the pads.

On high-energy, high-cycle or modulated braking duty - broadly, anywhere an enclosed brake would cook.

Typical applications are mine and shaft winders, cranes and hoists, large belt conveyors and their emergency braking, wind turbine rotor and yaw braking, unwind and rewind tension control in converting and paper, mill and press drives, marine deck machinery, and test rigs where a load must be absorbed continuously.

What those share is either a large inertia stopped from significant speed, a high number of stops per hour, or continuous slipping under control - all thermal problems.

They are less appropriate for small machines with modest energies, where an electromagnetic brake is far more compact and needs no fluid supply, and for pure holding duty with no stopping requirement, where a simpler spring-applied unit will do the job for less.