Parking brake
In cars, the parking brake, also called[1] hand brake, emergency brake, or e-brake, is a latching brake usually used to keep the vehicle stationary. It is sometimes also used to prevent a vehicle from rolling when the operator needs both feet to operate the clutch and throttle pedals. Automobile hand brakes usually consist of a cable directly connected to the brake mechanism on one end and to a lever or foot pedal at the driver's position. The mechanism is often a hand-operated lever (hence the hand brake name), on the floor on either side of the driver, or a pull handle located below and near the steering wheel column, or a (foot-operated) pedal located far apart from the other pedals.
Although sometimes known as an emergency brake, using it in any emergency where the footbrake is still operational is likely to badly upset the brake balance of the car and vastly increase the likelihood of loss of control of the vehicle, for example by initiating a rear-wheel skid. Additionally, the stopping force provided by using the handbrake is small and would not significantly aid in stopping the vehicle. The parking brake operates mostly on the rear wheels, which have reduced traction while braking but in some cases, parking brake operates on front wheel, as done in most Citroens manufactured since the end of World War II. The hand brake is instead intended for use in case of mechanical failure where the regular footbrake is inoperable or compromised. Modern brake systems are typically very reliable and equipped with dual-circuit hydraulics and low-brake-fluid sensor systems, meaning the handbrake is rarely used to stop a moving vehicle.
The most common use for a parking brake is to keep the vehicle motionless when it is parked. Parking brakes have a ratchet locking mechanism that will keep them engaged until a release button is pressed. On vehicles with automatic transmissions, this is usually used in concert with a parking pawl in the transmission.
Hand brakes are also used to assist in hill starts on vehicles with manual transmissions. Use of the handbrake frees both feet for use on the accelerator and clutch pedals, allowing the car to move off without rolling back at all.
Types of brakes
School buses which are equipped with a hydraulic brake system will have a hand brake lever to the left of the driver (in left hand drive buses) 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.
In cars with rear drum brakes, the parking 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 parking brake either actuates the disc calipers (again, with much less force) or a small drum brake housed within the hub assembly (the inner circumference of the disc is often used instead of a separate drum).
Hudson automobiles used an unusual hybrid hydraulic-mechanical dual-brake system which operated the rear brakes through the otherwise conventional mechanical emergency-brake system when a failure of the hydraulic system allowed the pedal to travel beyond its normal limit.[2]
A number of production vehicles, light and medium duty trucks, and motor homes have been made with a separate drum brake on the transmission output shaft; called a driveline parking brake. This has an advantage of being completely independent of other braking systems. This is effective as long as the drive train is intact — propeller shaft, differential, and axle shafts. In many vehicles, this type of parking brake is operated by either a foot pedal or a hydraulic cylinder controlled by the transmission gear selector, or by both.
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 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 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.
Electric parking brake
A recent variation is the electric parking brake. First installed in the 2001 BMW 7 Series (E65), electric park brakes have since appeared in a number of vehicles. Also note that the 1914 Norwalk Underslung Six, manufactured by the Norwalk Motor Car Company in Martinsburg, West Virginia-USA ( 1912-1922 ) utilized a Vulcan Electric Shift transmission, made by Vulcan Electric Shift of Philadelphia, Pennsylvania-USA, a division of the Cuttler-Hammer Corporation. This transmission had an electric parking brake in 1914 and can be seen on the only known surviving Norwalk Motor car located at www.norwalkmotorcar.com
Two variations are available: In the more-traditional "cable-pulling" type, an electric motor simply pulls the emergency brake cable on the push or pull of a button rather than a mechanical handle in the cabin. A more complex unit [3] uses a computer-controlled motor attached to each of the two rear brake calipers referred to as the Motor on Caliper(MoC) system.
It is expected that these systems will incorporate other features in the future. Jaguar, Landrover, BMW, Renault, Subaru and VW already have a system where the emergency brake engages when the engine is stopped and is released when the gas pedal is pressed. An extension of this system, called the hill-hold function, prevents roll-back when stopping and starting on a hill. The OEM can easily turn off the system.
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. Most Citroens, the Alfasud, Saab 99s, Pre-Facelift 900's, 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 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, 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.[4] In the U.S., an 1893 federal law, the Railroad Safety Appliance Act, required automatic brakes on all railroads, effective in 1900.[5]
See also
Notes
- ↑ http://www.tekstlab.uio.no/cambridge_survey/maps/81
- ↑ "Hudson Motor Car Company - Hudson Car Club - Hudson, Essex, Terraplane Cars". Hudsonclub.org. 2009-03-09. Retrieved 2011-04-24.
- ↑ http://www.trw.com/braking_systems/electric_park_brake
- ↑ McDonald, Charles (1993). The Federal Railroad Safety Program (PDF). Washington, DC: U.S. Federal Railroad Administration. pp. 2–5. Archived from the original (PDF) on 2009-07-31.
- ↑ United States. Act of Mar. 2, 1893, 27 Stat. 531, recodified, as amended, 49 U.S.C. § 20302.
External links
- Family Car Parts. "A Short Course on Brakes: Parking."
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