Brake
A brake is a mechanical device used for retarding or stopping motion by friction or power means, essential for controlling the speed and direction of vehicles, machinery, and other moving systems. In automotive applications, brakes function by converting the kinetic energy of a moving vehicle into heat energy through friction between components, allowing drivers to slow down or halt safely. This process is typically initiated by pressing a brake pedal, which activates hydraulic or pneumatic systems to apply force to friction materials against rotating parts.[1][2][3] The two primary types of brakes in modern passenger vehicles are disc brakes and drum brakes, with disc systems predominant due to their superior heat dissipation and performance. Disc brakes employ a caliper assembly that squeezes brake pads against a rotating disc (rotor) attached to the wheel, generating friction to slow the vehicle; this design is efficient for frequent or heavy braking and is often used on all four wheels in contemporary cars. Drum brakes, conversely, feature curved brake shoes that expand outward to press against the inner surface of a rotating drum, a simpler and more cost-effective option commonly found on rear axles or in older vehicles, though they are prone to overheating during prolonged use. Hydraulic fluid transmits the pedal force in most systems, ensuring even distribution of braking power across wheels.[2][3] Braking technology has evolved significantly since early mechanical systems in the late 19th century, with hydraulic brakes introduced in the 1920s for automobiles and air brakes developed in 1872 for railways and heavy vehicles. Modern advancements include anti-lock braking systems (ABS), first commercialized in the late 1970s, which modulate brake pressure to prevent wheel lockup on slippery surfaces, enhancing steering control and reducing stopping distances. Electronic stability control and regenerative braking in electric vehicles further improve safety and efficiency by recovering energy during deceleration. These innovations reflect ongoing engineering efforts to balance performance, durability, and environmental impact in braking design.[4][5][6]Overview
Definition and Purpose
A brake is a mechanical device that inhibits motion by absorbing energy from a moving system, typically converting kinetic energy into heat or other forms such as potential energy or electrical energy.[2] This process applies resistance to rotating or linear components, enabling controlled deceleration, complete stops, or stationary holding of loads.[7] The primary purposes of brakes encompass deceleration to ensure safety during operation, maintaining stationary positions for loads in various systems, and regulating speed in dynamic environments like machinery, vehicles, and elevators.[8] In essence, brakes counteract the natural tendency of objects in motion to continue moving, as described by Newton's first law of motion, which states that an object remains in uniform motion unless acted upon by an external force.[9] During braking, this opposing force—often generated through friction between brake components and the moving parts—produces deceleration in accordance with Newton's second law, where the net force equals mass times acceleration (F = ma), allowing the system to slow or halt predictably.[10] Brakes find essential applications across diverse sectors, including automotive vehicles for routine stopping and emergency maneuvers, rail systems for managing train speeds on tracks, aviation for aircraft landing and taxiing, and industrial settings for controlling heavy machinery and conveyor operations.[2][11][12][8] Effective braking plays a critical role in safety, with advanced systems like automatic emergency braking (AEB) projected to prevent at least 360 fatalities and 24,000 injuries annually in the United States by reducing rear-end and pedestrian crashes.[13] Studies indicate that such technologies can lower rear-end crash rates by 46-52% in passenger vehicles, contributing to overall collision reductions of up to 50% in equipped vehicles.[14]Historical Context
Early wheeled vehicles in ancient civilizations relied primarily on controlling draft animals to stop, with mechanical braking mechanisms emerging much later. Simple friction brakes, such as wooden blocks or spoons pressed against wheels, appeared in horse-drawn carriages by the 17th century.[15] The Industrial Revolution brought significant progress, particularly in rail transport, where iron-shod wheels and basic lever systems were developed for locomotives. Early locomotives like George Stephenson's Rocket, introduced in 1829, lacked dedicated brakes, relying on engine reversal; manual wheel brakes were soon applied to tenders and cars in the 1830s to manage speeds on early railways.[16] In the early 20th century, automobiles drove further innovation, with cable-operated drum brakes adopted by 1900 in vehicles like the Maybach, enclosing brake shoes within a drum for enhanced durability and weather resistance. Hydraulic systems followed, patented by Malcolm Loughead in 1918 for a fluid-actuated design that transmitted pressure evenly to all wheels.[17] This hydraulic evolution addressed limitations of mechanical linkages, enabling safer and more consistent braking as vehicle speeds increased.Types
Friction Brakes
Friction brakes are the most prevalent braking systems in automotive applications, relying on the direct contact between frictional surfaces to decelerate vehicles by converting kinetic energy into thermal energy.[2] These systems generate stopping force through the rubbing action of brake pads or shoes against rotating components, dissipating heat to slow or halt motion.[18] The core mechanism involves applying normal force to press friction materials against a rotating surface, producing a tangential frictional force that opposes wheel motion. This process follows Coulomb's law of dry friction, where the maximum frictional force F is expressed asF = \mu N
with \mu as the coefficient of friction and N as the normal force perpendicular to the contact surfaces.[19] The resulting heat from friction must be effectively dissipated to maintain performance, as inadequate cooling can lead to thermal issues.[20] Key subtypes include disc brakes and drum brakes. Disc brakes feature a rotating rotor (disc) attached to the wheel, clamped by brake pads housed in a caliper that applies hydraulic pressure for even force distribution on both sides.[21] This design, patented by Frederick William Lanchester in 1902, provides superior heat dissipation due to exposed surfaces.[22] Drum brakes, conversely, use internal expanding shoes pressed against the inner surface of a rotating drum, a configuration often employed on rear wheels for its self-energizing effect and integration with parking mechanisms.[18][23] Friction materials have evolved significantly for safety and performance. Historically, asbestos-based composites dominated due to their high thermal stability and durability, but health risks from inhalation prompted a phase-out beginning in the 1980s, with most manufacturers ceasing production by the 1990s. More recently, the U.S. Environmental Protection Agency finalized a ban on chrysotile asbestos in 2024, effectively eliminating its remaining use in automotive brakes.[24][25] Modern alternatives include semi-metallic pads, incorporating steel or copper fibers for enhanced heat resistance and torque, and ceramic composites, which use carbon or aramid fibers for low noise, minimal dust, and operation at temperatures up to 800°C.[20] These materials balance friction coefficients typically between 0.3 and 0.5 while reducing wear on mating surfaces.[26] Friction brakes offer advantages such as high braking torque for rapid deceleration, mechanical simplicity in design, and cost-effectiveness compared to advanced alternatives.[27] However, wear factors like heat dissipation are critical; poor ventilation can cause brake fade, a progressive loss of effectiveness when interface temperatures exceed 500°C, reducing the friction coefficient by up to 50% due to material degradation or fluid vaporization.[28] Effective cooling, often via ventilated rotors or airflow, mitigates this, ensuring sustained performance under repeated loading.[29]