Parking Brake

in Brake
Types of brakes



The hand brake lever in a Saab 9-5.



School buses which are equipped with a hydraulic brake system will have a hand brake lever to the left of the driver near the floor. It is operated by pushing the lever down with one's hand to apply the brake, and pulling it upwards to release it. However, this has been known to cause severe back problems in drivers who do this regularly[citation needed], and many choose to push it up with their feet.



Some cars with automatic transmissions are fitted with automatically releasing parking brakes. Later models require the foot brake to be depressed before the car's transmission can be moved from park. When reverse or drive is selected, the parking brake automatically releases. Earlier models would release the parking brake when the gear selector was placed in a forward or reverse gear without requiring any input on the brake pedal at all. These earlier automatic release systems were a safety hazard, since there would be no protection against accidentally knocking the transmission into gear.



In cars with rear drum brakes, the emergency brake cable usually actuates these drums mechanically with much less force than is available through the hydraulic system. In cars with rear disc brakes, the emergency brake either actuates the disc calipers (again, with much less force) or a small drum brake housed within the hub assembly.



A number of production vehicles have been made with a separate drum brake on the transmission tailshaft. This has an advantage of being completely independent of other braking systems. As long as the drive train is intact (propeller shaft, differential, and axle shafts) this is effective.



Large vehicles



Large vehicles are usually fitted with power operated or power assisted handbrakes. Power assisted handbrakes are usually found on large vans as well as some older heavy vehicles. These operate in the same way as a conventional handbrake, but pulling the lever will operate a valve that allows air or hydraulic pressure or vacuum into a cylinder which applies force to the brake shoes and makes applying the handbrake easier. When releasing the handbrake, the same mechanism also provides assistance to the driver in disengaging the ratchet. Particularly on commercial vehicles with air operated brakes, this has the added benefit of making it much harder or even impossible to release the parking brake when insufficient air pressure is available to operate the brakes. A reservoir or accumulator is usually provided so a limited amount of power assistance is available with the engine off. Power operated handbrakes are fitted to heavy commercial vehicles with air brakes, such as trucks and buses. These usually are 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 failsafe 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 can not 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.



New system: electric parking brake



A recent variation is the electric parking brake. First installed in the 2001 Renault Vel Satis, electric brakes have since appeared in a number of vehicles, including the Audi A4, A5, A6 and A8, the 2010 Subaru Legacy and Subaru Outback, the 2002 BMW 7 Series, Lincoln LS, Jaguar S-Type, XF and XJ, Renault Scnic, Espace , Laguna II.phase II , Vauxhall Insignia , Volkswagen Passat, Citroen C6 and the new Citroen C5.



Two variations are available: In the more-traditional "cable-pulling" type, an electric motor simply pulls the emergency brake cable rather than a mechanical handle in the cabin. A more complex unit uses two computer-controlled motors attached to the rear brake calipers to activate it.



It is expected that these systems will incorporate other features in the future. BMW, Renault and VW already have a system where the emergency brake initiates when the car stops and then goes off as soon as the gas pedal is pressed preventing the car from rolling. The vehicle operator can easily turn off the system.



There is also a single un-verified report of the brake disengaging randomly.



Jacking



It is important to know which wheels are providing the braking action when lifting the car with a jack. Typically the rear wheels are the ones that are stopped with parking brakes. The Saab 99s, Pre-Facelift 900's, the Citroen Xantia and most early Subarus applied the handbrake force to the front wheels, which makes them notable exceptions. If one lifts the braking wheels off the ground then the car can move and fall off the jack. This is why makers recommend that jacking be conducted on level ground and with chocks immobilizing the wheels that remain on the ground.



Railroad hand brakes



Virtually all railroad rolling stock is equipped with manually-operated mechanical hand brake devices that set and release the brakes. Most of these involve a chain linked to the brake rigging, most often at the brake cylinder, that when tightened pull the piston out against the releasing springs, thus applying the brakes on the car (if there is only one brake cylinder per car) or bogie (if there is more than once cylinder per car). Newer locomotives have electric systems that simply place an electric motor in place of the chain winding mechanism. This brake acts independent of the action of the automatic air brakes, which function collectively when coupled in a train and are under the control of the locomotive engineer.



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 and/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, designated railroad employees known as brakemen would move about the tops of cars, setting hand brakes 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.



See also



Handbrake turn



References



TRW's first-to-market EPB system



A Short Course on Brakes: Parking



Is a Parking Brake the Same as an Emergency Brake?



^ "Parking brake uncommanded release". http://legacygt.com/forums/showthread.php?t=131057. Retrieved 2010-02-15.



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Related brake articles



Air brake Disc brake Engine braking Pneumatics



Categories: Vehicle braking technologiesHidden categories: Articles lacking in-text citations from August 2009 | All articles lacking in-text citations | All articles with unsourced statements | Articles with unsourced statements from August 2008
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This article was published on 2010/10/09