Brake

Brakes serve to reduce or limit the speed of moving machine parts or vehicles. They usually function by converting the supplied kinetic energy through friction into thermal energy, which is lost (heating of the surrounding air).^[1] In the less frequently used regenerative brake, the kinetic energy is converted into another usable form of energy, usually electrical energy.^[2] The brake can be regarded as a force-locking clutch (friction clutch), in which one of the two components coupled together does not move. Vehicles within the meaning of the StVZO must have a dual-circuit braking system in Germany, that is, two independently functioning brakes.^[3]

The types of brakes used by far most frequently in vehicles are the drum brake and the disc brake.^[4] The oldest type, which is no longer often used, is the block brake. A brake is usually used to reduce the rotational speed of rotating parts, and less often for linearly moving parts.

Brakes are also classified according to the type of their actuation (force transmission): lever brake, cable brake, compressed-air brake, and hydraulic brake.^[5]

A special brake is the parking brake, with the help of which the movement of machine parts or vehicles is temporarily blocked.^[6]

All mechanical brakes are friction brakes and are based on reducing motion through friction between a stationary body and a moving body.^[7]

The principle of the scraping brake has been known since the earliest times. A lever is attached or clamped to a vehicle in such a way that the (as short as possible) shorter part points toward the ground and the longer part toward the operator.^[8] By pulling the brake lever, the short lower end is pressed into the ground through leverage, thereby braking the vehicle. This technique was widespread for a long time and is still used today, e.g. on sleds, sports equipment, or children’s vehicles.

The drag shoe represents a primitive form of the block brake.^[9]

The overwhelming majority of all brakes used on land vehicles in the 19th century can be assigned to the principle of the block brake.^[10] Spindle brakes on historic carriages, for example, have brake blocks made of linden wood.

The shoe brake is a mechanical brake in which a rotating cylinder is braked from the outside by pressed-on brake linings.^[11]

The drum brake has a cylindrical rotating housing (drum), against which stationary brake shoes located on the inside or outside are pressed during braking.^[12] The brake shoes are usually actuated by a hydraulic cylinder inside the drum or by rotating eccentric bolts from the outside. Depending on the design, further types are distinguished.

The disc brake has a brake disc rotating together with the shaft, against which the brake pads are pressed from both sides. Such brakes are found today on all common vehicles such as passenger cars, trucks, motorcycles, bicycles, and also trains.^[13]

Among other things, in brake motors, electromagnets that attract a spring-loaded brake disc made of soft magnetic iron ensure that the brake is released. In some brake motors, the magnetic field of the motor itself is used to release the brake, or the direct voltage for releasing the brake is obtained from the operating voltage by means of a rectifier.

A subcategory of the disc brake is the oil-bath brake (often also referred to as a “wet brake”).^[14] Here, one brake disc (or several brake discs separated by intermediate discs) rotates in an oil bath and is braked through friction with the pressure plate, friction ring (outside), and the intermediate discs. The pressing is carried out by a pressure plate consisting of two discs. Balls are installed between the plates in elongated recesses that become shallower. By rotating the two discs relative to each other, causing them to move toward or away from each other, the pressing force on the brake lining discs and intermediate discs is adjusted.

The oil serves to remove thermal energy. The advantage of this system is that it is temperature-stable (no fading) and has very low wear and therefore requires little maintenance.^[15] In addition, no environmentally harmful brake dust is produced. A disadvantage is the usually high cost in the event of a repair. This type of brake is found in some tractors and quads. The oil-bath brake is related to the oil-bath clutch, which is frequently used in motorcycles.

In the electronically controlled wedge brake (a type of disc brake), a small electric motor pushes a brake pad with a wedge-shaped rear profile between the brake shoe and the brake disc.^[16]

In the conventional wedge brake (used on horse-drawn carriages), the coachman drives a wedge between the wheel and the wheel arch.

A magnetic rail brake (abbreviated Mg) is a brake for rail vehicles. Here, a brake block is pressed by magnetic force onto the rail on which the vehicle is moving.^[17] The brake consists of iron friction shoes with built-in electromagnets. When current flows through the electromagnet, the friction shoe is pulled toward the rail. Friction occurs between the rail and the friction shoe pressed against it and moving forward with the vehicle, converting the kinetic energy of the motion into heat (dissipation) until the kinetic energy is consumed or the brake is deactivated.^[18] In addition, eddy-current induction occurs in the rail, producing a force that acts against the motion. Since the frictional forces increase as speed decreases and the eddy-current forces decrease, the brake acts relatively uniformly over the entire range compared with a wheel brake with metallic brake blocks.^[19]

The band brake is also a mechanical brake, but in contrast to the shoe brake, a band is wrapped around a drum.^[20]

Centrifugal brakes are generally not used directly for a strong reduction in rotational speed, but rather for limiting it.^[21] They operate according to the same principle as centrifugal clutches. A common application was limiting the return rotation speed of the dial of telephones.

Track brakes are shunting technology in tracks at marshalling yards, i.e. installed shunting-technical equipment (RTE).^[22] They reduce the kinetic energy of the wagon rolling down the hump. Energy conversions occur through impacts, friction, and electrodynamic operating principles at the wheelsets or buffers. The types are distinguished according to function and operating principle.

• Eddy Current Brake – It uses the eddy current effect. In this type of brake, an electrically conductive material (usually a metal disc) is moved through a magnetic field.^[23] Electrical eddy currents are induced in the material. These in turn generate a magnetic field that opposes the torque producing them. The disc is thereby braked.
• Electromotive Brake – The drive motor is used as a generator during braking.^[24] In modern generator brakes, the energy obtained is fed back into the power grid (rail vehicles and trolleybuses) or into an energy storage system (electric cars). This process is also called recuperation.^[25]

• The magnetic retarder or eddy current brake operates according to the eddy current principle – when permanent magnets are used, without additional auxiliary energy.^[26]
• The hysteresis brake uses the effect of a magnet or electromagnet on a moving, ferromagnetic, hard-magnetic material.^[27] The energy loss results from the repeated reversal of the magnetization of the material. In contrast to the eddy current brake, the force/torque generated by the hysteresis brake is not dependent on speed or rotational speed.
• Magnetic Powder Brake – A magnetic field generated with the aid of a coil causes a ferromagnetic powder to become linked or rigid, thereby producing braking friction.
• Asynchronous motors can be used as motor brakes by passing direct current through the windings.
• The magnetorheological brake or magnetorheological clutch operates with a magnetorheological fluid.^[28]

• A retarder uses the viscosity of a liquid (oil) to slow down the rotational movement of a shaft.^[29] It operates without wear and is therefore often used as a continuous brake in trucks or buses. In the latter, it is also used because its deceleration performance can be regulated almost continuously, and therefore without jerking.
• In high-speed vehicles, particularly in aviation and aerospace, braking parachutes and air brakes are used to increase air resistance and reduce speed.^[30] In the Mercedes-Benz SLR McLaren or Bugatti Veyron 16.4, for example, during strong deceleration the rear wing is positioned at an angle of 65 degrees in order to increase air resistance through a vortex and thereby achieve better deceleration and greater rear downforce (and thus increased braking performance of the rear wheels) (see also air brake).
• Another example of an air brake is the fly in the striking mechanisms of wheel clocks.
• Water vortex brakes are among the power brakes used for stationary test facilities, for example engine test benches.^[31] They are used to brake a test specimen (internal combustion engine, electric motor, or other drives).
• Sea anchors or drag lines reduce the speed of boats or ships in rough seas or emergencies.

In certain railway vehicles (for example steam locomotives with the Riggenbach counter-pressure brake), aircraft, and ships, the drive is switched or redirected in the opposite direction for braking.^[32] In aircraft, this is referred to as thrust reversal. The principle of thrust reversal is also used for braking boats and ships.