Braking Point

Engineers are continually having to make decisions on how to stop machinery. John Dacombe of Twiflex describes the choices available

Over the past two decades a revolution has taken place in the industrial brake market with engineers world-wide having to make a choice between drum or disc brake technology.

Disc brakes were first introduced by the motor vehicle industry during the late 1950s and soon became the standard for modern passenger cars and trucks. The rail industry also adopted them, and with the advent of the high speed train, disc braking became the accepted standard.

In the industrial sector designers of safety critical installations such as mine winders also adopted disc brake technology and have now used it for over 25 years.

Many winches, however, particularly in the marine industry have stayed with the band brake technology, which is simple, rugged and inexpensive.

Wind turbines almost exclusively use disc brakes, while in sectors such as the crane, conveyor, lift, escalator, general mining and the steel industries you’ll find a mixture of both drum and disc brakes.

The disc brake revolution, then, is a very real one. If it’s not a technology you’ve given much thought to recently, there are compelling reasons to take a look.

SAFETY

One advantage of disc braking is that two or more callipers can be used to provide multi-line safety.

Braking also takes place on both sides of a disc which means that if one side becomes contaminated the other side will provide 50% braking.

On drum brakes, multi-line safety can only be achieved by the addition of another drum. Also, with only one braking path for each drum brake, contamination can cause a serious loss of braking power.

SPARES AND STANDARDISATION

One disc brake calliper can cover a wide range of braking torques and this means minimum spares and less stock holding.

With drum brakes, on the other hand, you’re going to be looking at a number of different sizes, and that equates to a large stock holding and a more complicated inventory system.

MAINTENANCE

Disc brake maintenance is very minimal and generally involves just replacing the pads. This operation usually takes no more than 5 to 10 minutes, with the minimum down time.

With drum brakes, spring inspection, pivot inspection and lubrication, frequent adjustment and a large degree of dismantling are required to change the shoes. This clearly leads to longer down times.

STABILITY

A disc brake has good stability when dealing with emergency stopping situations. A flat friction surface pressurised against a flat disc surface minimises the effect of friction losses due to heat, high speed or condensation.

Even if a disc suffers distortion from severe overheating, the friction properties will remain high because of the uniform pressure distribution over a relatively small area.

The performance of a drum brake, however, can be greatly reduced by the heat generated during an emergency stop. Braking torque, sometimes critically, depends on the pressure distribution along the arc of a brake shoe. This not only presents problems during `bedding-in’, but can be badly affected by the thermal expansion of the drum during braking.

Severe heating produces elliptical drum distortion which can also be accompanied by conical distortion resulting in poor pressure distribution and the serious risk of brake fade.

SHAFT END FLOAT

Disc brake callipers can be mounted on sliding pins to accommodate excessive shaft end float. Drum brakes can be used with shafts having excessive end float providing the axial movement does not occur during the braking cycle. End float during braking will impose excessive side loading on the pins and pivots.

SIZE, WEIGHT AND INERTIA

Discs have much less inertia than drums. That being the case, with constant stop/start applications significant power saving can be achieved when compared with a drum brake.

Drum inertia can be greater than the driving or driven machinery, particularly where two drum brakes have been paired for multi-line safety. Clearly this will result in higher operating costs.

Finally, the disc brake is smaller and more compact than its equivalent drum brake.

To summarise it would seem that for safety critical applications and where advanced controlled braking is required, the disc brake is essential.

The drum brake, however, still has a place in industry, but usually only where simple on/off stopping is needed and where size and performance are not operational criteria.