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In road vehicles, the parking brake, also known as a handbrake is a mechanism used to keep the vehicle securely motionless when parked. Although it is also called the emergency brake (e-brake), that is an incorrect term as it will not stop a car in an emergency. The mechanical leverage, the size of the brake shoes inside the rotor "hat" on many vehicles with rear disc brakes, are insufficient to effectively stop the vehicle. Parking often consist of a pulling mechanism attached to a Wire rope which is connected to two wheel brakes. In most vehicles, the parking brake operates only on the rear wheels, which have reduced traction while braking. The mechanism may be a hand-operated lever, a straight pull handle located near the steering column, or a foot-operated Car controls located with the other pedals.
While automatic transmission vehicles have a "Park" gear with a parking pawl that immobilizes the transmission, it is still recommended to use the parking brake, as the pawl in the gearbox could fail due to stress or another vehicle striking the car, causing it to roll.
When parking on an uphill gradient, it is recommended that the front wheels face away from the curb. This would prevent the car from rolling into the roadway by using the curb to block the front passenger tire in the event of a parking brake failure. Similarly, on a downhill gradient, the front wheels should face the curb for the same reason, and the wheels should face to the side of the road on which vehicles are driven on an uncurbed road regardless of orientation. In a manual transmission, leaving the car in first gear (or in reverse if pointing downhill) is also advised, as the Engine braking will prevent the car from rolling if the parking brake fails.
The parking brake in most vehicles is still completely mechanical. Traditionally engaged by pulling a lever, the cables manually engage part of the car's braking system, usually the rear disk or drum brakes. The mechanical nature allows the driver to apply the brake even if the main hydraulic brake system fails.
Pictograph and/or lights may be used to indicate the location of a parking brake, its application or release. Parking brake indicator
In the event of hydraulic brake failure, the parking brake can be used to slow a vehicle. In such cases, the lever should be slowly engaged to prevent locking up the wheels.
Many car manufacturers such as Jaguar, Land Rover, BMW, Renault, Subaru and Volkswagen sell models whereby the parking brake automatically engages when the vehicle is stopped and is released when the gas pedal is pressed, eliminating the need for the driver to operate a button. An extension of this system, known as hill-hold or hill-assist, prevents the vehicle from rolling back when moving-off on an uphill gradient.
Another less common setup for rear discs is the use of a separate, smaller, cable actuated caliper (such as the Wilwood MC4), which is used exclusively for the parking brake. This is sometimes used as an alternative to a separate drum parking brake when multi-piston main calipers are used.
Hudson automobiles used an unusual hybrid hydraulic-mechanical dual-brake system which operated the rear brakes through the otherwise conventional mechanical parking brake system when a failure of the hydraulic system allowed the pedal to travel beyond its normal limit.
A number of production vehicles, light and medium duty trucks, and motor homes have been made with a separate drum brake on the driveline; called a transmission brake. This has an advantage of being completely independent of other braking systems. This is effective when there are multiple driving axles: all driven wheels are braked at once.
A line lock is a temporary parking brake that makes use of the vehicles standard hydraulic brakes. They are often used for off-road conditions or when stopping on steep grades is required. By trapping hydraulic pressure in the brake lines, all four wheels can be locked.
Power operated parking brakes are fitted to heavy commercial vehicles with air brakes, such as trucks and buses. These are usually spring applied, with air pressure being used to hold the brake off and powerful springs holding the brakes on. In most cases, a small lever in the cab is connected to a valve which can admit air to the parking brake cylinders to release the parking brake, or release the air to apply the brake. On some modern vehicles the valve is operated electrically from a lever or button in the cab. The system is relatively safe since if air pressure is lost the springs will apply the brakes. Also, the system prevents the parking brake being released if there is insufficient air pressure to apply the foot brake. A disadvantage to this system is that if a vehicle requires towing and cannot provide its own air supply, an external supply must be provided to allow the parking brake to be released, or the brake shoes must be manually wound off against the springs.
Manual hand brakes serve to keep a piece of rolling stock stationary after it has been spotted in a rail yard or at a customer for unloading or loading. They are also used to secure a parked train from inadvertent movement, especially while unmanned.
Before the development of locomotive-actuated train braking systems in the late 19th century, American railroads employed brakeman to move about the tops of cars, setting hand brakes on the engineer's signal in an effort to stop the train in a timely manner. This process was imprecise and extremely dangerous. Many brakemen lost life and limb as a result of falling from a moving train, icy and wet conditions often adding to the hazards involved in negotiating the top of a swaying boxcar. In the U.S., an 1893 federal law, the Railroad Safety Appliance Act, required automatic brakes on all railroads, effective in 1900.United States. Act of Mar. 2, 1893, , recodified, as amended, .
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