An emergency brake is a brake system that is generally only to be used in emergency situations to slow or stop a machine. The most well-known emergency brakes are those in trains and automobiles. Many people shorten emergency and call the devices e-brakes. Additionally, in the automobile context, they are also known as parking brakes and hand brakes.
In trains, control of emergency brakes is made available to the traveling passengers. Activating the brake will cause the train to automatically stop. Severe fines are often in place to dissuade people from activating the brake without good reason.
In cars, the emergency brake is a supplementary system that can be used if the vehicle's primary brake system (usually hydraulic brakes ) has a failure. Automobile e-brakes usually consist of a cable directly connected to the brake pads on one end and to some type of lever that can be actuated by the driver on the other end. The lever is most commonly a handle on the floor between the driver and front passenger (hence the hand brake name), or a pedal in the foot well in front of the driver. In the central handle configuration, the brake can be activated either by the driver or passenger (if the driver were to become unconscious, for instance).
However, the most common use for an automobile emergency brake is to keep the vehicle motionless when it is parked. Car emergency brakes have a ratchet locking mechanism that will keep them engaged. On vehicles with automatic transmissions, this is usually used in concert with a parking pawl in the transmission.
Historically, some cars with automatic transmissions were fitted with automatically releasing parking brakes. The parking brake would be released if the gear selector was placed in a forward or reverse gear. This automatic release system was eventually discontinued as a safety hazard, since there would be no protection against accidentally knocking the transmission into gear. Worse still, many North American-market Ford Motor Company cars from the late 1960s had a flaw in which, when the steering-column mounted shifter's bearings wore, the car could jump into reverse from park on its own. This and automatically releasing parking brakes were a deadly combination.
Electric parking brake
A popular new technology is the electric parking brake. First installed in the 2003 Lincoln LS, electric brakes have since appeared in a number of vehicles, including the Audi A6 and A8, BMW 7 Series and Jaguar S-Type and XJ. The 2006 Volkswagen Passat will also use this system.
Two variations are available: In the more-primitive "cable-pulling" type, an electrical motor simply pulls the emergency brake cable rather than a mechanical handle in the cabin. A more advanced unit uses a computer-controlled motor attached to the brake caliper to activate it.
It is expected that these systems will incorporate other features in the future, including "hill-holding" starts and automatic engagement.
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