Friction Materials

Materials used in the production of friction are usually rough or textured in order to provide added friction and increase efficiency. Smooth surfaces are used to transport materials easily and with little friction, however adding texture to a surface increases the energy required to move over it, thus slowing a moving object more quickly. Heat-resistant materials are often utilized as the process of creating friction typically generates heat energy as a byproduct. While asbestos fibers were once the most popular choice for friction materials, due to health concerns about the use of asbestos, there has been a growth in manufacturing of alternatives such as ceramic brake pads. Ceramic is a durable and highly heat-resistant material which allows for its use in many high friction environments. As the process of friction occurs inevitably with wear and tear, the materials used need to be able to withstand a certain amount of physical stress. There are several applications for which friction materials are essential. Brake pads, clutch sets, brake bands, brake lining, clutch facing, disc brake pads, clutch discs and friction discs, brake shoes and brake blocks all demand friction materials in order to operate at optimum levels. These materials are utilized by a variety of industries such as mining, oil and gas, forestry and construction. The automotive industry relies heavily on these friction material components as well.

Friction is defined as the resistance to relative motion that opposes the travel direction of an object. At its most simple, it is used to slow an object down enough to stop it, or to bring it to a controllable speed. It is created by contact of one solid body with a divergent surface. The type of material suited to a given task is dependent upon the type of friction. Different types of friction include static, kinetic and rolling. Static friction is found between two solid objects that are not moving relative to each other, preventing movement entirely. Slope is an important consideration when utilizing this particular type of friction. Kinetic friction, also known as dynamic, occurs between two objects moving relative to each other. Both surfaces may be moving or one may be sliding along a stationary object. Rolling friction involves wheels or balls. The rough texture of friction materials catches the wheel and stops it from sliding or slipping. A car tire spinning on ice or snow, for instance, is an example of low rolling friction and thus the tire does not catch on the ground, but keeps moving around.

A high coefficient of friction and good energy absorption are the key requirements of all friction materials. It is necessary to know the system for which the material is needed in order to ensure efficiency and effectiveness. In instances where quick stopping is an issue, a material that creates a large amount of friction is needed. In addition to speed, heat generation is an important consideration. Resin bonded asbestos and carbon was once the most popular friction material, but recent technological advances have significantly expanded material possibilities. Ceramic, as aforementioned, has become a popular choice due to its desirable attributes. Semi-metallic brake elements are also available using copper, brass and steel wool bonded by resin as these compounds share many of the long-lasting characteristics of ceramic options.

Braking systems use friction materials to slow and stop wheels and other mechanisms from moving. When a brake is pressed, it activates a system that places friction material against the moving disc in order to slow the connected tire. Friction discs are most often constructed of durable metal. The drawback of metallic components is the inherent grinding noise created by their friction. For this reason several manufacturers coat them with rubber and other high friction materials. Rubber or other materials have the added benefit of increased surface friction. Brake pads, linings, blocks and shoes are different mechanisms engaged by the action of braking which presses them against the disc. Clutch facing and discs function in a similar manner to offer consistent clutch engagement and disengagement. Although it is an important aspect of many other industries, the production of friction materials has more recently become an industry in its own right. While generalizations can be made, manufacturers are often secretive of their specific friction material compositions. The major changes in the industry throughout the years have caused several companies to try and protect their own advancements in order to beat out competitors. The introduction of ceramics, for example, has provided several opportunities for efficient and reliable systems. Faster stopping and lower noise are common claims in brake and clutch related products. Friction materials continue to advance and developments are a consistent occurrence in the industry markets.

Friction Materials Types

• Asbestos materials used to be the most common type of friction materials, but due to connections to lung disease, they are being phased out of the market.
  
• Brake bands use friction material to slow vehicles by pressing against the brake disc.
  
• Brake blocks encompass both the brake shoe and brake pad or lining which apply pressure to the brake disc or spinning surface of a wheel to reduce speed as a result of increased friction.
• Brake lining is the heat-resistant material that creates friction in braking devices.
  
• Brake materials encompass a wide variety of durable and heat resistant substances used in the construction of devices which reduce the speed and stop spinning surfaces of vehicle wheels.
• Brake shoes house the brake pads or linings and press them into the brake disc or spinning surface of a wheel to reduce speed due to increased friction.
• Ceramic brake pads are implemented by brake shoes or calipers which press them against spinning wheels, rotors or discs to reduce speed as a result of increased friction.
• Ceramic friction materials are very popular because they eliminate squeal and audible vibrations. They are also less abrasive to rotors and their brake pads tend to have a longer life-span than those of other materials.
  
• Clutch discs are rounded plates used to connect the engine of a vehicle to the transmissions input shaft allowing the temporary separation needed to shift gears.
• Clutch facings are used to maintain lower coefficients of friction, which provide smooth and stable clutch engagement/disengagement. They help reduce clutch chatter, are available in molded and woven compositions and can be found with asbestos or asbestos-free materials.
  
• Clutch materials encompass a wide variety of durable and heat resistant substances used in the construction of devices used to connect the engine of a vehicle to the transmissions input shaft allowing the temporary separation needed to shift gears and coast.
• Clutch sets are pre-arranged assemblages of mechanical components used to enhance or replace existing clutch systems.
• Disc brake pads squeeze the rotor to slow the disc.
  
• Elastomeric materials are resilient and adaptable, and help maximize the consumption of power and energy. They were created especially for high speed and energy applications.
  
• Fiberglass materials are new to the market, and are currently being tested for durability, preservation of rotors and drums and noise level.
  
• Friction discs are metal plates bonded with friction materials that are used to brake.
  
• Graphitic materials are used in applications that have a higher temperature because of their good energy absorption. These kinds of materials help hold in heat for a long period of time.
  
• Non-asbestos lining includes all friction materials made without asbestos, due to concern over the affect of asbestos on health. These tend to be more abrasive and accelerate rotor wear.
  
• Paper materials have some of the best performance of the friction materials. They are made of fibers and fillers and, after the addition of water, are dried and compressed into a sheet.
  
• Semi-metallic friction materials are high performance and designed to prevent fade and squeal. They handle heat better than many others.
  
• Sintered metal materials are compacted friction modifiers and metallic powders, and work best with very little oil flow. They have the ability to operate in very high temperatures.

Friction Materials Terms

Balance - Refers to the stability between the front and rear brakes.
 
Bleeding - The process by which overheated or spoiled brake fluid and air bubbles are removed from the brake system.
 
Brake Drum - Metal housing bolted to an axle and vehicle wheel that looks like a large jar lid. Brake shoes are forced against the drum to stop rotating wheels.
 
Brake Lining - Heat-resistant friction material that presses against the disc/drum to create braking force.
 
Brake Pad - Made of friction materials and bonded to metal plates. Brake pads need to be replaced occasionally due to heavy wear.
 
Burnish - Process of wearing in brakes so that the contact between the friction material and the rotor or drum becomes stable.
 
Caliper - The assembly that houses the brake pad(s) and applies them to the rotor. This also houses the hydraulically operated pistons to which the pads are bonded.
 
Clutch - Any of various contraptions used to engage and disengage two moving parts of a shaft or shaft and driving mechanism. When changing gears, the clutch pedal is pressed, disengaging the clutch and allowing the gear change; when released, the clutch engages and transfers the rotating motion throughout the entire driveshaft.
 
Coefficient of Friction - The ratio of force necessary to move an object compared to the weight of the object itself.
 
Disc Brake - Consists of brake pads, caliper and rotor. This is the part of the brake system that actually stops the vehicle.
 
Drum - A large circular metal housing that looks like an oversized jar lid and is bolted to a vehicle's axle and wheel. Brake shoes are forced against the inner section of the drum, which then stops the rotating wheel.
 
Dynamometer - Apparatus in a laboratory used to test brake system performance.
 
Fade - Temporary reduced braking power. Fade results from overheating of the friction material.
 
Groove Patterns - Grooves on friction material that help dispel heat, get rid of fluid and eliminate noise.
 
Master Cylinder - The cylinder that contains hydraulic fluid and a piston. It is connected directly to the brake pedal and transmits pressure to the brake operating system.
 
Recovery - The return of braking performance to a normal level after fade has diminished. This measures friction materials' ability to perform after overheating.
 
Rotor - Also referred to as a disc or drum, this is the circular metal object to which the brake pads are applied, creating friction to slow and stop the vehicle.
 
Shoe - A steel semicircular form coated with a friction agent that presses against the inside of a drum when activated.
 
Squeal - High-pitched noise made when braking. Squeal indicates that brakes should be inspected for wear.