Hydraulic Brake
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A hydraulic brake is an arrangement of
braking A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. Backgroun ...
mechanism which uses
brake fluid Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works ...
, typically containing
glycol ether Glycol ethers are a class of chemical compounds consisting of alkyl ethers that are based on glycols such as ethylene glycol or propylene glycol. They are commonly used as solvents in paints and cleaners. They have good solvent properties while h ...
s or
diethylene glycol Diethylene glycol (DEG) is an organic compound with the formula (HOCH2CH2)2O. It is a colorless, practically odorless, and hygroscopic liquid with a sweetish taste. It is a four carbon dimer of ethylene glycol. It is miscible in water, alcohol, ...
, to transfer pressure from the controlling mechanism to the braking mechanism.


History

During 1904, Frederick George Heath (Heath Hydraulic Brake Co., Ltd.), Redditch, England devised and fitted a hydraulic (water/glycerine) brake system to a cycle using a handlebar lever and piston. He obtained patent GB190403651A for “Improvements in hydraulic actuated brakes for cycles and motors”, as well as subsequently for improved flexible rubber hydraulic pipes. In 1908, Ernest Walter Weight of Bristol, England devised and fitted a four-wheel hydraulic (oil) braking system to a motor car. He patented it in Great Britain (GB190800241A) in December 1908, later in Europe and the USA and then exhibited it at the 1909 London Motor Show. His brother, William Herbert Weight improved the patent (GB190921122A) and both were assigned to the Weight Patent Automobile Brake Ltd. of 23 Bridge Street, Bristol when it was established in 1909/10. The company, which had a factory at Luckwell Lane, Bristol, installed a four-wheel hydraulic braking system on a Metallurgique chassis, fitted with a Hill and Boll body, which was exhibited at the November 1910 London Motor Show. Although more cars had the brake system installed and the company advertised heavily, it disappeared without achieving the success it deserved. Malcolm Loughead (who later changed the spelling of his name to Lockheed) invented hydraulic brakes, which he patented in 1917. "Lockheed" is a common term for brake fluid in France.
Fred Duesenberg Frederick Samuel Duesenberg (December 6, 1876 – July 26, 1932) was a German-born American automobile and engine designer, manufacturer and sportsman who was internationally known as a designer of racecars and racing engines. Duesenberg's eng ...
used
Lockheed Corporation The Lockheed Corporation was an American aerospace manufacturer. Lockheed was founded in 1926 and later merged with Martin Marietta to form Lockheed Martin in 1995. Its founder, Allan Lockheed, had earlier founded the similarly named but ot ...
hydraulic brakes on his 1914 racing cars and his car company,
Duesenberg Duesenberg Automobile and Motors Company, Inc. was an American race car, racing and luxury car, luxury automobile manufacturer founded in Indianapolis, Indiana, by brothers Fred Duesenberg, Fred and August Duesenberg in 1920. The company is ...
, was the first to use the technology on the Duesenberg Model A in 1921. Knox Motors Company of Springfield, MA was equipping its
tractor A tractor is an engineering vehicle specifically designed to deliver a high tractive effort (or torque) at slow speeds, for the purposes of hauling a trailer or machinery such as that used in agriculture, mining or construction. Most common ...
s with hydraulic brakes, beginning in 1915. The technology was carried forward in automotive use and eventually led to the introduction of the self-energizing hydraulic drum brake system (Edward Bishop Boughton, London England, June 28, 1927) which is still in use today.


Construction

The most common arrangement of hydraulic brakes for passenger vehicles, motorcycles, scooters, and mopeds, consists of the following: * Brake pedal or lever * A pushrod (also called an ''actuating rod'') * A master cylinder assembly containing a
piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...
assembly (made up of either one or two pistons, a return spring, a series of
gaskets image:gaskets.jpg, Some seals and gaskets A gasket is a Seal (mechanical), mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression (physical) ...
/
O-rings An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more part ...
and a fluid reservoir) * Reinforced hydraulic lines * Brake caliper assembly usually consisting of one or two hollow aluminum or chrome-plated steel pistons (called ''caliper pistons''), a set of thermally conductive
brake pad Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are composed of steel backing plates with friction material bound to the surface that faces the disc brake rotors. Function Brake pads convert the kin ...
s and a
rotor Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator * Helicopter rotor, the rotary wing(s) of a rotorcraft ...
(also called a ''brake disc'') or
drum The drum is a member of the percussion group of musical instruments. In the Hornbostel-Sachs classification system, it is a membranophone. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a she ...
attached to an axle. The system is usually filled with a glycol-ether based brake fluid (other fluids may also be used). At one time, passenger vehicles commonly employed drum brakes on all four wheels. Later, disc brakes were used for the front and drum brakes for the rear. However disc brakes have shown better heat dissipation and greater resistance to 'fading' and are therefore generally safer than drum brakes. So four-wheel disc brakes have become increasingly popular, replacing drums on all but the most basic vehicles. Many two-wheel vehicle designs, however, continue to employ a drum brake for the rear wheel. The following description uses the terminology for / and configuration of a simple disc brake.


System operation

In a hydraulic brake system, when the brake pedal is pressed, a pushrod exerts force on the piston(s) in the master cylinder, causing fluid from the brake fluid reservoir to flow into a pressure chamber through a compensating port. This results in an increase in the pressure of the entire hydraulic system, forcing fluid through the hydraulic lines toward one or more calipers where it acts upon one or more caliper pistons sealed by one or more seated O-rings (which prevent leakage of the fluid). The brake caliper pistons then apply force to the brake pads, pushing them against the spinning rotor, and the friction between the pads and the rotor causes a braking
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
to be generated, slowing the vehicle. Heat generated by this friction is either dissipated through vents and channels in the rotor or is conducted through the pads, which are made of specialized heat-tolerant materials such as
kevlar Kevlar (para-aramid) is a strong, heat-resistant synthetic fiber, related to other aramids such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, the high-strength material was first used commercially in the early 1970s a ...
or
sintered glass Fritted glass is finely porous glass through which gas or liquid may pass. It is made by sintering together glass particles into a solid but porous body. This porous glass body can also be called a frit. Applications in laboratory glasswar ...
. Alternatively, in a
drum brake A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum. The term ''drum brake'' usually means a brake in which shoes press on the inner surfac ...
, the fluid enters a
wheel cylinder A wheel cylinder is a component of a hydraulic drum brake system. It is located in each wheel and is usually positioned at the top of the wheel, above the shoes. Its function is to exert force onto the shoes so as to bring them into contact wit ...
and presses one or two
brake shoes A brake shoe is the part of a braking system which carries the brake lining in the drum brakes used on automobiles, or the brake block in train brakes and bicycle brakes. A device that is put on a track to slow down railroad cars is also called ...
against the inside of the spinning drum. The brake shoes use a similar heat-tolerant friction material to the pads used in disc brakes. Subsequent release of the brake pedal/lever allows the spring(s) in the master cylinder assembly to return the master piston(s) back into position. This action first relieves the hydraulic pressure on the caliper, then applies suction to the brake piston in the caliper assembly, moving it back into its housing and allowing the brake pads to release the rotor. The hydraulic braking system is designed as a closed system: unless there is a leak in the system, none of the brake fluid enters or leaves it, nor does the fluid get consumed through use. Leakage may happen, however, from cracks in the O-rings or from a puncture in the brake line. Cracks can form if two types of brake fluid are mixed or if the brake fluid becomes contaminated with water, alcohol, antifreeze, or any number of other liquids.


An example of a hydraulic brake system

Hydraulic brakes transfer energy to stop an object, normally a rotating axle. In a very simple brake system, with just two cylinders and a
disc brake A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc or a "rotor" to create friction. This action slows the rotation of a shaft, such as a vehicle axle, either to reduce its rotational speed or to hol ...
, the cylinders could be connected via tubes, with a piston inside the cylinders. The cylinders and tubes are filled with incompressible oil. The two cylinders have the same volume, but different diameters, and thus different cross-section areas. The cylinder that the operator uses is called the
master cylinder In automotive engineering, the master cylinder is a control device that converts force (commonly from a driver's foot) into hydraulic pressure. This device controls slave cylinders located at the other end of the hydraulic brake system. A ...
. The spinning disc brake will be adjacent to the piston with the larger cross-section. Suppose the diameter of the master cylinder is half the diameter of the slave cylinder, so the master cylinder has a cross-section four times smaller. Now, if the piston in the master cylinder is pushed down 40 mm, the slave piston will move 10 mm. If 10 newtons (N) of force are applied to the master piston, the slave piston will press with a force of 40 N. This force can be further increased by inserting a
lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or ''fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is div ...
connected between the master piston, a pedal, and a pivot point. If the distance from the pedal to the pivot is three times the distance from the pivot to the connected piston, then it multiplies the pedal force by a factor of 3, when pushing down on the pedal, so that 10 N becomes 30 N on the master piston and 120 N on the brake pad. Conversely, the pedal must move three times as far as the master piston. If we push the pedal 120 mm down, the master piston will move 40 mm and the slave piston will move the brake pad by 10 mm.


Component specifics

''(For typical light duty automotive braking systems)'' In a four-wheel car, the
FMVSS The Federal Motor Vehicle Safety Standards (FMVSS) are U.S. federal vehicle regulations specifying design, construction, performance, and durability requirements for motor vehicles and regulated automobile safety-related components, systems, and ...
Standard 105, 1976; requires that the master cylinder be divided internally into two sections, each of which pressurizes a separate hydraulic circuit. Each section supplies pressure to one circuit. The combination is known as a tandem master cylinder. Passenger vehicles typically have either a front/rear split brake system or a diagonal split brake system (the master cylinder in a motorcycle or scooter may only pressurize a single unit, which will be the front brake). A front/rear split system uses one master cylinder section to pressurize the front caliper pistons and the other section to pressurize the rear caliper pistons. A split circuit braking system is now required by law in most countries for safety reasons; if one circuit fails, the other circuit can still stop the vehicle. Diagonal split systems were used initially on
American Motors American Motors Corporation (AMC; commonly referred to as American Motors) was an American automobile manufacturing company formed by the merger of Nash-Kelvinator Corporation and Hudson Motor Car Company on May 1, 1954. At the time, it was the ...
automobiles in the 1967 production year. The right front and left rear are served by one actuating piston while the left front and the right rear are served, exclusively, by a second actuating piston (both pistons pressurize their respective coupled lines from a single foot pedal). If either circuit fails, the other, with at least one front wheel braking (the front brakes provide most of the braking force, due to
weight transfer Weight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects: *the change in load borne by different wheels of even perfectly rigid vehicles during acceleration *the change in center of mas ...
), remains intact to stop the mechanically damaged vehicle. By the 1970s, diagonally split systems had become common among automobiles sold in the United States. This system was developed with front-wheel-drive cars' suspension design to maintain better control and stability during a system failure. A Triangular split system was introduced on the
Volvo The Volvo Group ( sv, Volvokoncernen; legally Aktiebolaget Volvo, shortened to AB Volvo, stylized as VOLVO) is a Swedish multinational manufacturing corporation headquartered in Gothenburg. While its core activity is the production, distributio ...
140 series from MY 1967, where the front disc brakes have a four-cylinder arrangement, and both circuits act on each front wheel and on one of the rear wheels. The arrangement was kept through subsequent model series 200 and 700. The diameter and length of the master cylinder has a significant effect on the performance of the brake system. A larger diameter master cylinder delivers more hydraulic fluid to the caliper pistons, yet requires more brake pedal force and less brake pedal stroke to achieve a given deceleration. A smaller diameter master cylinder has the opposite effect. A master cylinder may also use differing diameters between the two sections to allow for increased fluid volume to one set of caliper pistons or the other and is called a "quick take-up" M/C. These are used with "low drag" front calipers to increase fuel economy. A proportioning valve may be used to reduce the pressure to the rear brakes under heavy braking. This limits the rear braking to reduce the chances of locking up the rear brakes, and greatly lessens the chances of a spin.


Power brakes

The vacuum booster or
vacuum servo A vacuum servo is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort. In the US it is commonly called a brake booster. A vacuum servo also known as a power booster o ...
is used in most modern hydraulic brake systems which contain four wheels. The vacuum booster is attached between the master cylinder and the brake pedal and multiplies the braking force applied by the driver. These units consist of a hollow housing with a movable rubber diaphragm across the center, creating two chambers. When attached to the low-pressure portion of the throttle body or intake manifold of the engine, the pressure in both chambers of the unit is lowered. The equilibrium created by the low pressure in both chambers keeps the diaphragm from moving until the brake pedal is depressed. A return spring keeps the diaphragm in the starting position until the brake pedal is applied. When the brake pedal is applied, the movement opens an air valve which lets in atmospheric pressure air to one chamber of the booster. Since the pressure becomes higher in one chamber, the diaphragm moves toward the lower pressure chamber with a force created by the area of the diaphragm and the differential pressure. This force, in addition to the driver's foot force, pushes on the master cylinder piston. A relatively small diameter booster unit is required; for a very conservative 50% manifold vacuum, an assisting force of about 1500 N (200n) is produced by a 20 cm diaphragm with an area of 0.03 square meters. The diaphragm will stop moving when the forces on both sides of the chamber reach equilibrium. This can be caused by either the air valve closing (due to the pedal apply stopping) or if "run out" is reached. Run out occurs when the pressure in one chamber reaches atmospheric pressure and no additional force can be generated by the now stagnant differential pressure. After the run out point is reached, only the driver's foot force can be used to further apply the master cylinder piston. The fluid pressure from the master cylinder travels through a pair of steel brake tubes to a pressure differential valve, sometimes referred to as a "brake failure valve", which performs two functions: it equalizes pressure between the two systems, and it provides a warning if one system loses pressure. The pressure differential valve has two chambers (to which the hydraulic lines attach) with a piston between them. When the pressure in either line is balanced, the piston does not move. If the pressure on one side is lost, the pressure from the other side moves the piston. When the piston makes contact with a simple electrical probe in the center of the unit, a circuit is completed, and the operator is warned of a failure in the brake system. From the pressure differential valve, brake tubing carries the pressure to the brake units at the wheels. Since the wheels do not maintain a fixed relation to the automobile, it is necessary to use hydraulic brake hose from the end of the steel line at the vehicle frame to the caliper at the wheel. Allowing steel brake tubing to flex invites
metal fatigue In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts o ...
and, ultimately, brake failure. A common upgrade is to replace the standard rubber hoses with a set which are externally reinforced with braided stainless-steel wires. The braided wires have negligible expansion under pressure and can give a firmer feel to the brake pedal with less pedal travel for a given braking effort. The term 'power hydraulic brakes' can also refer to systems operating on very different principles where an engine-driven pump maintains continual hydraulic pressure in a central accumulator. The driver's brake pedal simply controls a valve to bleed pressure into the brake units at the wheels, rather than actually creating the pressure in a master cylinder by depressing a piston. This form of brake is analogous to an air brake system but with hydraulic fluid as the working medium rather than air. However, on an air brake air is vented from the system when the brakes are released and the reserve of compressed air must be replenished. On a power hydraulic brake system fluid at low pressure is returned from the brake units at the wheels to the engine-driven pump as the brakes are released, so the central pressure accumulator is almost instantly re-pressurised. This makes the power hydraulic system highly suitable for vehicles that must frequently stop and start (such as
bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
es in cities). The continually circulating fluid also removes problems with freezing parts and collected water vapour that can afflict air systems in cold climates. The
AEC Routemaster The AEC Routemaster is a Front-engine, rear-wheel-drive layout, front-engined double-decker bus that was designed by London Transport Executive, London Transport and built by the Associated Equipment Company (AEC) and Park Royal Vehicles. The ...
bus is a well-known application of power hydraulic brakes and the successive generations of Citroen cars with
hydropneumatic suspension Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Sha ...
also used fully powered hydraulic brakes rather than conventional automotive brake systems. Most large aircraft also use power hydraulic wheel brakes, due to the immense amounts of braking force they can provide; the wheel brakes are linked to one or more of the aircraft's main hydraulic systems, with the addition of an accumulator to allow the aircraft to be braked even in the event of a hydraulic failure.


Special considerations

Air brake systems are bulky, and require
air compressor An air compressor is a pneumatic device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces m ...
s and reservoir tanks. Hydraulic systems are smaller and less expensive.
Hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backhoe ...
must be non-compressible. Unlike air brakes, where a valve is opened and air flows into the lines and brake chambers until the pressure rises sufficiently, hydraulic systems rely on a single stroke of a piston to force fluid through the system. If any vapor is introduced into the system it will compress, and the pressure may not rise sufficiently to actuate the brakes. Hydraulic braking systems are sometimes subjected to high temperatures during operation, such as when descending steep grades. For this reason, hydraulic fluid must resist vaporization at high temperatures. Water vaporizes easily with heat and can corrode the metal parts of the system. Water which enters brake lines, even in small amounts, will react with most common brake fluids (i.e., those which are
hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substan ...
) causing the formation of deposits which can clog the brake lines and reservoir. It is almost impossible to completely seal any brake system from exposure to water, which means that regular changing out of brake fluid is necessary to ensure that the system is not becoming overfilled with the deposits caused by reactions with water. Light oils are sometimes used as hydraulic fluids specifically because they do not react with water: oil displaces water, protects plastic parts against corrosion, and can tolerate much higher temperatures before vaporizing, but has other drawbacks vs. traditional hydraulic fluids. Silicone fluids are a more expensive option. "
Brake fade Vehicle braking system fade, or brake fade, is the reduction in stopping power that can occur after repeated or sustained application of the brakes, especially in high load or high speed conditions. Brake fade can be a factor in any vehicle that ...
" is a condition caused by overheating in which braking effectiveness reduces, and may be lost. It may occur for many reasons. The pads which engage the rotating part may become overheated and "glaze over", becoming so smooth and hard that they cannot grip sufficiently to slow the vehicle. Also, vaporization of the hydraulic fluid under temperature extremes or thermal distortion may cause the linings to change their shape and engage less surface area of the rotating part. Thermal distortion may also cause permanent changes in the shape of the metal components, resulting in a reduction in braking capability that requires replacement of the affected parts.


See also

*
Air brake (road vehicle) An air brake or, more formally, a compressed-air-brake system, is a type of friction brake for vehicles in which compressed air pressing on a piston is used to apply the pressure to the brake pad or brake shoe needed to stop the vehicle. Air bra ...
* Anti-lock braking system *
Bicycle brake systems A bicycle brake reduces the speed of a bicycle or prevents it from moving. The three main types are: rim brakes, disc brakes, and drum brakes. Most bicycle brake systems consist of three main components: a mechanism for the rider to apply the ...
* Brake bleeding *
Brake-by-wire In the automotive industry, brake-by-wire technology is the ability to control brakes through electrical means. It can be designed to supplement ordinary service brakes or it can be a standalone brake system. This technology is widely used on al ...
*
Fuse (hydraulic) In hydraulic systems, a fuse (or velocity fuse) is a component which prevents the sudden loss of hydraulic fluid pressure. It is a safety feature, designed to allow systems to continue operating, or at least to not fail catastrophically, in the ev ...
*
Hydraulics Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...
*
Hydraulic circuit Hydraulic machines use liquid fluid power to perform work. Heavy construction vehicles are a common example. In this type of machine, hydraulic fluid is pumped to various hydraulic motors and hydraulic cylinders throughout the machine and b ...
*
Railway air brake A railway air brake is a railway brake power braking system with compressed air as the operating medium. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on April 13, 1869. The W ...
*
Torque converter A torque converter is a type of fluid coupling that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the power ...
*
Vehicle brake A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. Background ...


References


External links

* * * Erjavec, Jack (2004). ''Automotive Technology: A Systems Approach'', Delmar Cengage Learning.
Allan and Malcolm Loughead (Lockheed) Their Early Lives in the Santa Cruz Mountains including the invention of the hydraulic brake.


Patents

* . Kinchin 1956-05-22 * . Dubois 1952-04-08 * . Martin 1951-03-13 * . Bryant 1949-10-08 * . Johnson Wade C, Trishman Harry A, Stratton Edgar H. 1949-04-12 * . Fitch 1947-02-12 * . Lambert Homer T. 1946-08-06 * . Lambert Homer T. 1945-05-15 * . Forbes Joseph A. 1944-12-26 * . La Brie 1938-12-20 * . Poage Robert A. and Poage Marlin Z. 1937-06-15 * . Avery William Leicester 1936-02-21 * . Buus Niels Peter Valdemar 1934-05-15 * . Norton Raymond J 1934-04-10 * . Boughton Edward Bishop 1929-07-16 * . Borgwar Carl Friedrich Wilhelm 1940-09-06 * . Hall Frederick Harold 1932-07-28 * . Rubury John Meredith 1932-01-06 {{DEFAULTSORT:Hydraulic Brake Vehicle braking technologies American inventions