Mechanical brakes arrest the energy of a machine or object via force, most commonly friction. Most individuals are familiar with automotive brakes, but mechanical brakes are also essential in material handling, manufacturing, and other power transmission applications.
Mechanical brakes function via force delivered to a body in rotary or linear motion, such as an axle, shaft, or wheel, to slow or stop motion. Mechanical brakes are often in an assembly with a mechanical clutch for engaging and disengaging shafts.
Friction-based brakes utilize a coarse and rugged material (i.e. brake liner) that is tightened or pressed against a body in motion to decelerate. Friction-based braking generates immense heat and some noise, degrading all of the engaged surface areas. Brake capacity decreases with every cycle and requires inspection and replacement. Friction-type brakes are heavily used in automotive applications.
Toothed brakes have tooth-shaped contact surfaces that transmit power without slippage or heat generation. Teeth are engaged only when stopped or running at slow speeds.
Non-contact brakes use a technology such as a magnetic field or eddy currents to provide the braking action. Braking force is generated proportionately to velocity. An advantage of eddy current and magnetic brakes is that there is no friction, so there are no parts that wear out. A disadvantage of these brakes is that they do not have any holding force for stationary objects.
A wrap-spring clutch and brake works by connecting an input shaft and output shaft with an interference fit helical clutch spring. When the input shaft rotates in the direction of the wind of the clutch spring, torque is transmitted to the output shaft. A separate brake spring spins until the brake spring control tang is locked, which tightens the brake spring around the output shaft in position to a stationary brake hub that is bolted to a plate. Simultaneously the clutch spring releases. Wrap-spring brakes provide precise stopping (± 0.5°) in industrial machinery, but fully-mechanical wrap-spring clutches and drives are limited to smaller sizes; larger sizes are electromagnetic.
The following brakes are types of friction brakes.
In drum brakes the brake lining is adhered to the external surface of a curved bracket, called a shoe. The most common configuration includes two shoes mounted inside of a drum on a plate. A cylinder presses the shoes onto the insides of the drum to initiate deceleration. A drum brake that presses on the outside of the drum is called a clasp brake; a double clasp brake applies braking pressure to both the inside and outside of the drum.
Cone brakes are a type of drum brake where the drum and shoe are mating sections of conical frustums. The shoe (i.e. cone) is outfitted with brake lining and pressed into the drum (i.e. cup) to apply friction. The advantage is increased surface area and quicker deceleration.
Disc brakes utilize a metal disc, also called a rotor, that is connected to the axle. The rotor spins between a caliper that contains between one and 12 cylinders, which pushes a lining material outfitted on a brake pad against the rotor surface.
An Ausco-Lambert disc brake utilizes two discs along with two 360° brake pads positioned inside the discs; one of the pads is stationary. Instead of pinching, the brake pads expand and initial contact between brake lining and disc is provided by actuators. Ball bearings are positioned between the brake pads in tapered recesses. When the pads expand the bearings ride the incline and jam the pads against each other, thereby providing additional, self-sustaining friction to the discs. When stopped the brakes auto-release. The Ausco-Lambert provides space savings, last longer, and run cooler, but are considered oversensitive. They have not been used on autos in 60 years.
Band brakes tighten a ribbon of high-friction material around a pulley attached to the rotating axle; they are often employed on bicycles. If the pull on the band is in the direction of axle rotation the brake is self-energizing. Differential band brakes attach both ends of the brake ribbon to the lever to supply braking power for bi-directional shafts.
Common parts of applicable mechanical brakes are as illustrated.
Brake lining materials vary considerably by application. Materials must be soft, tough, heat resistant, and possess a high coefficient of dynamic friction. They are applied to pads or shoes made of welded or riveted sheet metal.
- Non-metallic - composite organic or synthetic materials, including cellulose, aramids, polyacrylnitrile, and sintered glass. Non-metallic linings minimize rotor wear, but have short service lives.
- Semi-metallic - composite materials filled with metallic flakes to improve wear resistance and service life with increased wear on the rotor or drum; requires higher braking torque.
- Metallic - often reserved for performance or high energy applications; composed of sintered steel that wears quickly on rotors, requires higher braking torque, and generates noise.
- Ceramic - clay and porcelain mixed with copper flakes and filaments. Moderate durability, lifespan, and torque requirements with zero perceived sound. However high operating temperatures can warp pads and other components.
Calipers are typically metal plates with cylinders made of plastic, aluminum, or chrome-plated steel. Disc materials include:
- Ceramic composite - carbon fiber-reinforced ceramic provides stable friction at high speed and all temperatures, is 50% lighter than grey iron, but is also quite expensive.
- Grey iron - cast iron with graphitic microstructure that has high heat and damping capacities with wear resistance and machinability. It is the most common material for automobile rotors.
- Steel - stainless steel is commonly integrated into bicycle and motorcycle brakes, however has poor thermal conductivity. Steel may be acceptable for applications with higher thermal requirements, but will also corrode.
- Aluminum - a lightweight metal with excellent thermal characteristics; only suitable for low RPM applications because of decreased wear resistance and strength; sometimes found on bicycles.
- Titanium - only compatible with organic or resin-based liners; lightweight with good strength and corrosion resistance, but has low surface friction and lifespan.
Some common drum materials are:
- Cast iron - the most common brake drum material
- Aluminum - lighter than iron with improved heat dissipation and brake fade; needs iron or steel liner to retain structural integrity
- Steel - steel shells are a common means to reduce weight and improve cast iron drum performance
- Composite - hybrid materials, such as cast-aluminum filled with silicon carbide, provides weight savings with adequate friction and wear resistance