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This article takes an in depth look at Friction Materials.
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Friction materials are substances that produce friction between solid surfaces in order to control or stop forward or backward motion. The materials used to produce friction materials include organic and non-organic substances such as resin, ceramics, fibers, and metals. Friction materials have a limited lifespan due to the nature of their function. This factor requires that they be closely monitored and that plans be in place for their replacement.
The main use of friction materials is for safe and controlled braking or power transfer while minimizing wear. Some of the products associated with friction materials are brake pads and brake shoes, clutch plates, bonded assemblies, and friction bands, liners, and rolls. The forms of friction materials are bands, blocks, pads, and discs with rolls, sheets, and linings also being available.
Of the various available materials used to produce friction products, ceramics are the most durable and capable of withstanding the heaviest load. Beyond ceramics, there is a long list of types of friction materials that includes various metals, rubber, resins, aramid fibers, and graphite.
The main use for friction materials is in transportation as braking material, parts of clutching systems, and transmissions. Since their discovery, friction materials have become an essential part of a variety of product developments.
Friction is the resistance to motion of an object by an opposing object moving in the opposite direction or is static. This characteristic of friction is the reason that friction materials are used as braking, stopping, and direction changing devices. Though stopping is a common use of friction materials, making a connection between surfaces is another of their uses.
Since friction materials come in a variety of types and shapes, they are adaptable to any type of application. The uses of friction materials are determined by the form they take. Each of the shapes and configurations is designed to perform a particular function.
Friction discs, clutch discs, or brake discs are a major part of a disc brake system. They are designed to slow or stop motion of a vehicle such that it can turn or stop. The friction material is attached to a metal plate with an adhesive or rivets. Friction discs are parts of vehicles and industrial equipment.
The clutch discs, or linings, are the most important part of the clutch. Its hub is attached to the shaft of the transmission and engages the engines when the clutch is released. When the clutch is disengaged, the transmission is disengaged and allows for the changing of gears. Clutch discs are found on manufacturing equipment, large machines, and most vehicles. Regardless if the machine is electric, hydraulic, or pneumatic, clutch discs are used to connect and disconnect the transmission of the motor.
A slip clutch, known as a back torque limiter clutch, is a mechanism designed to protect the PTO and the gearbox of an engine. It reduces the impact of extreme force produced by a PTO and the potential damage from the PTO’s power. A slip clutch has a circular compact shape with multiple layers that include pressure plates, friction plates, and springs.
The designs of slip clutches include shaft to shaft and shaft to pulley, gear, or sprocket types. Its purpose is to allow a clutch to partially slip until the engine speed of a vehicle reaches the vehicle’s speed as well as control the torque. A slip clutch engages when the control torque level is higher than the low torque. It makes downshifts flawless and smoother during emergency braking.
Slip clutches are used on motorcycles when a rider has to quickly brake and downshift. It allows the rider to control traction on the rear wheel. With farm equipment, a slip clutch limits the torque being transferred from the tractor to the implement it is pulling. It makes it possible for both sides of a PTO shaft to spin at different speeds. On farms, slip clutches are used with augers, hay tenders, balers, and saws.
Clutch facings reduce the amount of noise produced by the clutch. It allows for smooth and consistent operation of the clutch, which results in a smoother engagement of the clutch. Like all friction materials, clutch facings are made from a variety of materials, including asbestos and non-asbestos ones. The kinetic energy from the engagement of the clutch creates a great deal of heat. Clutch facings are designed to endure and withstand the stressful conditions.
There are three types of friction materials that are used in the manufacture of brake pads, which are semi-metallic, non-asbestos organic, and ceramic. Each of the different materials have their advantages and disadvantages. The first brakes were drum brakes and had friction material applied to the brake shoe. When the brake on a device was applied, the shoe was activated and made contact with the braking surface or drum.
Later, disc brakes were developed with brake pads that use the same process to brake a device by applying friction to the rotor. A further improvement in friction materials was the development of semi-metallic materials that have metal fibers to provide friction.
The brake lining assists in the reduction of braking or stopping motion. They serve as a barrier between braking components and boost friction levels. Brake linings extend the life of braking components by keeping them from breaking down too quickly and are a buffer zone between components. In essence, the brake lining is another name for brake pads and serves the same function.
Brake shoes are widely used in an assortment of equipment as a method for stopping motion. Drum brakes use hydraulic pressure to move small pistons that push the shoes against the interior surface of the spinning portion of the mechanism. The friction material slows the motion of the device. Brake shoes are crescent shaped with the friction material applied on the rounded surface of the shoe. Though brake shoes serve the same function as brake pads, they apply force differently. While brake pads move inwardly toward the rotor, brake shoes move upward and outward to make contact with the drum.
A block brake is a hard material that is pressed against a wheel to stop motion. They can be made from a variety of materials as well as from chunks of rubber. Block brakes are a crude form of braking device that can be applied to a wide variety of applications. In their simplest form, they can be found on bicycles that have hand brakes.
Block brakes are a type of friction material that can be attached to the exterior of a brake shoe with rivets, screws, or by being bonded with a special adhesive. Molded brake blocks are used in slowing mining, engineering, and industrial machines. They are made of resin, wire, or viscose, glass, or yarn fibers.
The quality and endurance of friction materials depends on the process used to manufacture them. It is important to precisely blend the correct mixture of materials to produce the best performance. The process of producing friction materials is a complicated and intricate process that requires specialized knowledge and expertise.
With the development of recent technologies, exceptionally high performance friction materials have been developed that have a long lifespan and extraordinary capabilities. Over the last few years, new sturdier materials have been discovered that have enhanced the performance of friction materials.
The initial and important step in the production of friction materials begins with the selection of raw materials. The three types of friction materials are organic, metallic, and inorganic. Included in organic materials are asbestos and non-asbestos, which are semi-metallic, low steel, and non-steel. Metallic friction materials include sintered alloys such as copper, brass, and steel.
A key component in the manufacture of friction materials is phenolic resin, which is used as a binding agent. Phenolic resin is produced by combining phenol and formaldehyde. The properties of the resin vary depending on the catalyst used to produce it. The choice of using phenol resin as a binding agent is due to its exceptional resistance to heat and chemicals as well as its properties of non-conductivity and flame resistance.
In the manufacture of friction materials, phenol resin is ductile and easy to shape into complex contours and a wide variety of configurations. The positive qualities of phenol resin improve the lining and wear of friction materials.
Raw materials are over 60% of the final friction materials product. The blending process involves various grades of raw materials that are used at varying percentages. The grades of the materials are divided into tiers. Tier one consists of phenolic resin, fiberglass strands, and an abrasive agent. Tier two includes graphite, rubber, and particulate friction matter.
The mixing and blending process determines the grade of the friction material and its cost. Each mixture is designed to meet specific performance criteria and price points. The highest grades of friction materials use exceptionally pure raw materials and a lower percentage of fillers.
Blending and mixing vary depending on the type of friction material being produced. In the production of automobile friction material parts, a wet mixture is used, which consists of an interior mixer, mill rolling mixer, and pelletizer. For friction material for large equipment such as trains and industrial machinery, rubber is the primary friction material and is molded instead of being mixed. A third method uses pelletized mixtures for roll mill mixing and a calender machine made up of high pressure rollers to form the skin of the friction material.
The preforming process is designed to shape the friction material to meet the parameters of its design. During this stage of production, the final product is shaped prior to being subjected to various treatment processes, which improve the materials characteristics and properties.
The hot press procedure uses pressure to compact the raw material and induces sintering. It increases the density of the raw material and removes the need for reworking it.
In order to toughen and increase the strength of friction materials, they are put through a curing process. During the curing, the phenol resin develops a more adhesive bond with the components of the friction material. A chemical reaction occurs at the molecular level. The result of curing is the elimination of any openings or cracks in the material.
For the assembly of disc brakes and drum brake linings, curing is a common application for the production of automotive components. The process is completed using industrial ovens.
After the curing process, the friction material is allowed to cool prior to undergoing the final finishing process. The types of finishing processes vary according to the kind of friction material and its final use. A key factor in finishing is the ensuring of the resistance and friction coefficient of the material such that it meets the needs of the application.
There are a wide range of substances and metals that can be used to produce friction materials. Each of the various types generate friction in a different manner but produce the same effect. When manufacturers are producing friction material, they select materials in accordance with the characteristics of the material.
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Each type of friction material has special characteristics. Metal pads last the longest but are very loud. Semi-metallics are not as loud as metallic pads but do not last as long. The greatest advancement in friction materials has been the development of ceramics.
Ceramic friction materials are made of porcelain that is combined with sintered metals, such as copper, and are used for gentle stopping of motion. The main advantage of ceramic friction materials is how quiet and clean their operation is. They produce very little dust or noise when in use. Ceramic friction materials are not designed for heavy use and can easily overheat. The key to their lifespan is gently putting them to use.
Semi-metallic friction materials are made of sintered metals and synthetic fibers. They are considered to be the longest lasting type of friction material. The longevity of semi-metallic friction material is due to their heat tolerance and stopping power. These characteristics make them ideal for controlling and stopping heavy equipment. The rough finish of semi-metallic friction material is tough on rotors and other components with which it makes contact.
Kevlar, du Pont‘s name for aramid fibers, has a low abrasion rate, wear rate, and is resistant to oil. It does not contain metal particles and performs whether it is wet or dry. Kevlar is manufactured in rolls or sheets and lasts five times longer than asbestos and sintered metals. When used as a friction material, it does not abrade, damage, scratch, or score the opposing surface to which it makes contact. Kevlar makes noiseless smooth contact as a friction facing.
Rolls of Kevlar are flexible and used in tension devices, magnetic brakes, and clutches. Sheets of Kevlar are rigid and have a very slow wear rate with ground on both sides. They are used for tensioning units as well as overload and slip applications.
Feramic friction materials have a high friction coefficient and excellent static to dynamic ratio. They are quick to lock up, which makes them ideal for racing applications. Feramic friction materials are made of steel, silicon dioxide, bronze, and graphite. The composition of feramic friction material is similar to ceramic friction materials but produces more friction.
Carbotic friction material is a recent development that provides a smooth and even engagement that is similar to organic and Kevlar materials. It has a long lifespan and is able to handle high torque with extensive wear. The development of carbotic friction material was for use with heavy duty hauling equipment such as dump trucks that have to climb steep grades where slippage is dangerous.
Organic friction materials are made from a combination of materials that commonly include phenolic resin, metallic powder, and rubber. It is produced in woven and molded form. With the woven variety, fiberglass fibers are woven into the material to give it durability and a long lifespan. Woven organic friction fibers provide smooth engagement, wear resistance, and superior strength.
Molded organic friction materials are more affordable than woven ones but are not as durable. They are designed for use with light to medium applications due to their lack of strength. Molded organic friction materials do not contain any metal, which gives them a low abrasion quality.
Feramalloy friction material has the qualities of ceramic friction material with smoother engagement, wear, and temperature resistance as well as an excellent static to dynamic ratio. The main use of feramalloy friction materials is for heavy duty applications such as diesel trucks, commercial rigs, and heavy duty equipment. Feramalloy is a recent addition to friction materials due to its superiority over ceramic and feramic friction materials.
Wet friction is friction between surfaces with a different nature such as solid and liquid. It is a lower intensity friction compared to dry friction. Wet friction materials operate in fluid media and are compressible, porous, and permeable materials that are found in oil and transfer torque to and from metal plates in contact with wet friction plates. When a wet clutch engages, its kinetic energy converts into frictional heat that has to be endured by the wet friction material and absorbed by the metal or oil.
Paper wet friction materials are the most common and manufactured in paper making machines. Other types include air lay, woven, pultruded, sintered, and thermal spray. WFMs are classified by their function, which include holding, engaging, or slipping as well as high and low energy and high and low pressure.
WFMs must have smooth engagement, exceptional durability, thermal and pressure resistance, oil compatibility, glaze resistance, and high and consistent torque. There is very little wear with WFMs since there is very little contact between surfaces. The construction of WFMs includes bonding the material to a steel core plate. In the case of wet brakes, oil affects the friction properties and serves as a coolant agent. The friction coefficient for wet brakes is between 0.1 and 0.2, which is far lower than dry friction materials.
There are important properties and characteristics that are necessary for a friction material to offer high quality performance. The features that manufacturers include in their products are heat resistance, high friction coefficient, energy absorption, durability, and wear and tear resistance. The surface of friction materials are rough and textured to provide a sufficient amount of friction.
The key component of friction materials is the frictional force they create. The types of frictional force are dynamic, static, kinetic, fluid, sliding, and rolling. The materials used to produce friction materials depend on the type of friction required.
Friction is the force that opposes the motion of a solid object over another solid object. Friction and normal force are proportional to the contacting surfaces and do not depend on the hardness of the surfaces. As speed increases, sliding force reduces and fluid friction increases.
Static friction exists when surfaces are at rest to each other. The coefficient of static friction is how well objects move together. In the case of frictional materials, if the coefficient is high, there is little slippage between materials as they are in motion.
Sliding friction is the resistance between materials when they slide against each other. The force of sliding friction is the product of the coefficient of sliding friction and normal force.
Rolling friction is the resistance that a wheel or ball endures when in motion. It is the weakest form of friction. Friction materials are designed to catch the rolling object and stop it, which is found in the case of braking friction materials.
Kinetic friction is also known as dynamic friction and is the force that acts between moving surfaces. An object moving on a surface experiences force in opposition to the direction that it is moving. The magnitude of the opposing force depends on the coefficient of the kinetic friction between the object and the opposing object.
Fluid friction is the friction that exists between layers of fluids when they are moving relative to each other. The force of fluid friction restricts movement in it or the movement of something through it. The resistance to internal flow is referred to as viscosity. The denser a fluid is, the greater is its resistance to movement.
Dynamic friction is resistance that occurs when an object is in motion and is a critical aspect of friction materials. When too much force is applied by a frictional material when an object is in motion can result in slippage and poor performance. A friction material that has a high dynamic coefficient will grab, which leads to uncomfortable changes and transitions.
Clamping force is a measure of the amount of force necessary to be applied to an object to oppose the forces trying to move the object. In the case of friction material, the clamping force is the force the material is capable of applying when it is joined with an opposing surface.
As with any other type of force, clamping force is measured in pounds of Newtons. The higher the friction coefficient of a friction material enhances its clamping force and the retention of its grip on the opposing surface.
Fade is the reduction in the effect of a friction material after repeated applications and uses. The main reason for fade is a build up in the temperature on the surface of the friction material. Manufacturers of friction materials test and examine their friction materials to ensure a long use of the material and prevention of early fading. The most common causes for fade is misuse of materials or using the wrong material for the application.
The primary standards for friction materials have been established by the Friction Materials Standards Institute (FMSI), which was founded in 1948 as a trade association for aftermarket friction manufacturers. The FMSI has developed a standardized part numbering system for friction materials for brakes and clutch facings.
The FMSI system is available to producers and buyers as a means for identifying and categorizing friction materials. The scope of the FMSI has broadened from a national organization to an international one and includes scientific, engineering, technological, and statistical data on friction materials.
In the area of regulations for friction materials, the majority of states in the United States limit the amount of asbestos and copper that can be used in friction materials. In the case of asbestos, manufacturers limit the use of asbestos in friction materials with the majority having developed technologies that have replaced asbestos completely.
The Environmental Protection Agency (EPA) regulates the manufacture of friction materials and has standards regarding toxic substances produced by the production process. The concern of the EPA revolves around the solvents used in production. The first regulations were proposed in 2001 and have been steadily improved through 2019.
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Sliding friction must, logically always be less than static friction. This becomes obvious when you look at it in terms of forces rather than coefficients.
Consider a block resting on a flat surface. You apply a small force to one side of the block to try to slide it along. Initially it doesn't move as the reaction force provided by friction increases in line with the force you exert.
Say that applying 10N of force is just enough to overcome static friction and get the block moving. Now if the sliding friction becomes greater the 10N force is no longer enough to overcome it and so it stops and your back to a static condition.
Another way to look at this is that friction is a reaction force so it can never be greater than the force applied otherwise you would be effectively getting work from nowhere.
However it is certainly possible to have materials whose friction properties change as a result of friction, for example carbon-carbon brakes are most effective at quite high temperatures.
There is also viscous drag which is proportional some function of speed although it is not really very meaningful to talk about static friction in this context.
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