This article takes an in depth look at Friction Materials.
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Friction materials are engineered substances crafted to generate friction between surfaces, enabling the control or stoppage of motion. These substances, encompassing both organic and inorganic elements such as resins, ceramics, fibers, and metals, are vital across numerous applications. Due to the nature of their function, friction materials possess a limited lifespan, necessitating routine checks and timely replacements.
Friction materials are chiefly employed for efficient braking and power transfer, all while minimizing wear. Some familiar products incorporating friction materials include brake pads, brake shoes, clutch plates, as well as bonded assemblies, friction bands, liners, and rolls. These materials are available in various forms, including bands, blocks, pads, and discs, while also being offered in rolls, sheets, and linings.
In the broad array of materials for friction products, ceramics are notable for their robustness and capacity to endure heavy loads. Besides ceramics, other friction-inducing materials include metals, rubber, resins, aramid fibers, and graphite, each bringing distinct qualities to various specialized uses.
The primary application of friction materials is within the transportation industry, particularly for braking systems, clutch mechanisms, and vehicle transmissions. From their advent, these materials have played an essential role in a multitude of product innovations.
Friction refers to the resistance encountered when one object moves against another or remains stationary while an opposing force acts upon it. This property makes friction materials ideal for applications involving braking, halting, and altering direction. In addition to stopping motion, these materials are also used to establish connections between surfaces.
Friction materials are available in various types and shapes, making them versatile for different applications. The specific functions of these materials are dictated by their form, with each design tailored to perform a distinct role.
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, also referred to as a back torque limiter, is a device designed to safeguard the PTO and gearbox of an engine. It mitigates the effects of excessive force generated by a PTO and prevents potential damage to the system. The slip clutch typically features a compact circular design with several components, including pressure plates, friction plates, and springs.
Slip clutches come in various designs, such as shaft-to-shaft or shaft-to-pulley, gear, or sprocket configurations. Their primary function is to allow controlled slippage of the clutch until the engine speed aligns with the vehicle's speed, thereby managing the torque. The clutch engages when the applied torque exceeds a preset threshold, ensuring smooth and precise downshifts, especially during emergency braking.
In motorcycles, slip clutches are used to enable riders to brake and downshift quickly while maintaining rear wheel traction. For agricultural machinery, slip clutches control the torque transmitted from the tractor to the attached implement, allowing for differential rotation of PTO shafts. They are commonly utilized with equipment such as augers, hay tenders, balers, and saws on farms.
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.
Subsequently, disc brakes were introduced, utilizing brake pads that operate by applying friction to the rotor to stop the device. An advancement in friction materials came with the creation of semi-metallic materials, which incorporate metal fibers to enhance braking performance.
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 consists of a rigid material pressed against a wheel to halt movement. These brakes can be crafted from various materials, including rubber chunks. Block brakes represent a basic form of braking device suitable for numerous applications, and they are commonly seen on bicycles equipped with hand brakes.
Block brakes, a type of friction material, can be affixed to the outer surface of a brake shoe using rivets, screws, or specialized adhesives. Molded brake blocks are utilized in industries such as mining, engineering, and manufacturing for deceleration purposes. They are composed of materials like resin, wire, viscose, glass, or yarn fibers.
The durability and effectiveness of friction materials are heavily influenced by their manufacturing process. Achieving optimal performance involves carefully blending the right combination of materials. The production of friction materials is a complex and detailed procedure requiring specialized expertise.
Advancements in technology have led to the creation of high-performance friction materials with extended lifespans and remarkable capabilities. Recent developments have introduced more robust materials, significantly improving the performance and longevity of friction products.
Starting the production of friction materials requires a crucial first step: choosing the right raw materials. Friction materials generally fall into three categories: organic, metallic, and inorganic. Organic materials encompass both asbestos-based and non-asbestos variants, including semi-metallic options, low steel, and non-steel types. On the other hand, metallic friction materials are typically made from sintered alloys like copper, brass, and steel.
Phenolic resin plays a vital role in creating friction materials, serving as a crucial binding agent. This resin is synthesized through the reaction of phenol with formaldehyde. Its characteristics can differ based on the specific catalyst used during production. Phenolic resin is chosen for its remarkable heat and chemical resistance, along with its non-conductive and flame-retardant properties.
When producing friction materials, phenolic resin’s flexibility allows it to be molded into intricate shapes and various configurations. Its advantageous properties enhance the durability and performance of friction linings.
Raw materials constitute over 60% of the final friction material product. The blending process involves mixing different grades of raw materials in varying proportions. These materials are categorized into tiers: Tier one includes phenolic resin, fiberglass strands, and abrasive agents, while Tier two consists of graphite, rubber, and particulate friction components.
The grade and cost of friction materials are influenced by the mixing and blending processes. Each blend is tailored to achieve specific performance requirements and price points. Higher-grade friction materials are produced using highly pure raw materials with fewer fillers.
The methods of blending and mixing depend on the type of friction material being manufactured. For automotive friction components, a wet mixing process is employed, involving equipment such as interior mixers, mill rolling mixers, and pelletizers. Conversely, for large machinery and train friction materials, rubber is used as the primary friction material and is molded rather than mixed. Another approach involves using pelletized mixtures combined with roll mill mixing and calender machines equipped with high-pressure rollers to form the friction material's surface.
The preforming phase focuses on molding the friction material to conform to its design specifications. At this stage, the product is shaped before undergoing further treatment processes that enhance its characteristics and properties.
The hot pressing method applies pressure to compress the raw material and promotes sintering. This process enhances the material's density and eliminates the need for additional reworking.
To enhance the toughness and strength of friction materials, they undergo a curing process. This stage allows the phenolic resin to form a stronger adhesive bond with the material components through a molecular-level chemical reaction. Curing also helps to eliminate any voids or cracks in the material.
Curing is frequently used in the production of automotive parts such as disc brakes and drum brake linings. This process is typically carried out in industrial ovens.
Following the curing process, the friction material is allowed to cool before proceeding to the final finishing stage. The specific finishing techniques used depend on the type of friction material and its intended application. A crucial aspect of this finishing process is ensuring that the material's resistance and friction coefficient meet the required performance standards for its use.
Friction materials can be made from a diverse array of substances and metals. Although different materials generate friction in distinct ways, they ultimately achieve the same effect. Manufacturers choose materials based on their specific properties to meet the requirements of the friction material being produced.
Different types of friction materials have unique attributes. Metal pads are known for their durability but tend to be quite noisy. Semi-metallic pads offer quieter operation compared to metal pads but have a shorter lifespan. The most significant innovation in friction materials has been the introduction of ceramic materials.
Ceramic friction materials are composed of porcelain blended with sintered metals like copper, designed for smooth and quiet braking. Their primary benefit is their ability to operate with minimal dust and noise. However, ceramic friction materials are not suited for heavy-duty applications as they are prone to overheating. To maximize their lifespan, they should be used gently and not subjected to excessive stress.
Semi-metallic friction materials consist of sintered metals combined with synthetic fibers. They are known for their durability and are the longest-lasting type of friction material. This durability is attributed to their high heat tolerance and effective stopping power, making them well-suited for managing and halting heavy machinery. However, the coarse texture of semi-metallic materials can be abrasive on rotors and other components they interact with.
Kevlar, a type of aramid fiber developed by DuPont, boasts low abrasion and wear rates and is resistant to oil. Free from metal particles, Kevlar performs reliably in both wet and dry conditions. It is produced in rolls or sheets and has a lifespan that is up to five times longer than asbestos and sintered metals. As a friction material, Kevlar does not cause abrasion, damage, or scratching to the surfaces it contacts, providing a smooth and silent friction experience.
Kevlar rolls are flexible and are commonly used in tensioning devices, magnetic brakes, and clutches. In contrast, Kevlar sheets are rigid, have a very slow wear rate, and are ground on both sides. These sheets are utilized in tensioning units as well as for managing overload and slip situations.
Feramic friction materials offer a high friction coefficient and a superior static-to-dynamic ratio. Their quick engagement makes them well-suited for racing applications. Made from a blend of steel, silicon dioxide, bronze, and graphite, Feramic friction materials share similarities with ceramic types but deliver increased friction performance.
Carbotic friction material is a newer innovation designed to offer smooth and consistent engagement, comparable to both organic and Kevlar materials. It boasts a long lifespan and can withstand high torque and significant wear. Carbotic materials were specifically developed for heavy-duty hauling equipment, such as dump trucks, which operate on steep gradients where slippage poses a safety risk.
Organic friction materials are crafted from a blend of components, typically including phenolic resin, metallic powders, and rubber. They are available in both woven and molded forms. Woven organic friction materials incorporate fiberglass fibers, enhancing durability and extending their lifespan. This type offers smooth engagement, wear resistance, and high strength.
Molded organic friction materials, while more cost-effective than their woven counterparts, generally have lower durability. They are suited for light to medium-duty applications due to their reduced strength. Unlike woven types, molded organic materials are metal-free, resulting in lower abrasion properties.
Feramalloy friction material combines the advantageous properties of ceramic materials, including smooth engagement, durability, and temperature resistance, while also offering an excellent static-to-dynamic ratio. It is primarily used in heavy-duty applications such as diesel trucks, commercial vehicles, and robust machinery. As a newer development in friction materials, Feramalloy surpasses both ceramic and feramic materials in performance.
Wet friction occurs between surfaces with differing properties, such as solid and liquid interfaces. It involves lower friction intensity compared to dry friction. Wet friction materials function within fluid environments and are characterized by their compressibility, porosity, and permeability. These materials are found in oil and facilitate torque transfer between metal plates and wet friction plates. When a wet clutch engages, kinetic energy is converted into frictional heat, which must be managed by the wet friction material and absorbed by the surrounding metal or oil.
Paper-based wet friction materials are among the most common and are produced using paper-making machines. Other types include air-laid, woven, pultruded, sintered, and thermal spray variants. Wet friction materials (WFMs) are categorized based on their functions—such as holding, engaging, or slipping—as well as their energy and pressure levels.
For WFMs to be effective, they must provide smooth engagement, excellent durability, resistance to thermal and pressure extremes, oil compatibility, glaze resistance, and consistent torque. WFMs experience minimal wear due to reduced surface contact. They are typically constructed by bonding the material to a steel core plate. In wet brakes, oil not only influences the friction properties but also acts as a coolant. The friction coefficient for wet brakes ranges from 0.1 to 0.2, which is significantly lower than that of dry friction materials.
To ensure high-quality performance, friction materials must possess several essential properties and characteristics. Key features included by manufacturers are heat resistance, a high friction coefficient, energy absorption, durability, and resistance to wear and tear. The surfaces of these materials are intentionally rough and textured to provide adequate friction.
The primary factor of friction materials is the frictional force they generate. Frictional forces can be categorized as dynamic, static, kinetic, fluid, sliding, and rolling. The choice of materials used in friction products is influenced by the specific type of friction required.
Friction is the force that resists the motion of one solid object against another. The magnitude of friction and the normal force are proportional to the area of contact and are independent of the surface hardness. As speed increases, sliding friction decreases while fluid friction rises.
Static friction occurs when surfaces are stationary relative to each other. The coefficient of static friction indicates how well objects resist motion when at rest. A higher coefficient means there is less slippage between the materials when they start to move.
Sliding friction refers to the resistance encountered when two materials slide against each other. This frictional force is determined by the product of the coefficient of sliding friction and the normal force.
Rolling friction is the resistance experienced by a wheel or ball as it moves. It is the least intense form of friction. Friction materials are designed to engage with rolling objects to stop them, which is crucial in braking systems.
Kinetic friction, also known as dynamic friction, is the force opposing the motion of moving surfaces. The force acting against the direction of movement depends on the coefficient of kinetic friction between the object and the surface.
Fluid friction occurs between layers of fluid as they move relative to one another. This type of friction restricts movement within the fluid or the motion of objects through it. The resistance to flow within the fluid is known as viscosity, and denser fluids exhibit greater resistance to movement.
Dynamic friction arises when an object is in motion and is crucial in the performance of friction materials. Excessive force from the friction material while in motion can lead to slippage and decreased effectiveness. A friction material with a high dynamic coefficient may result in abrupt changes and transitions.
Clamping force measures the amount of force needed to counteract forces attempting to move an object. For friction materials, clamping force refers to the force the material can exert when in contact with an opposing surface.
Clamping force, like other types of force, is measured in pounds or Newtons. A higher friction coefficient enhances the clamping force and improves the material’s grip on the opposing surface.
Fade refers to the decrease in effectiveness of a friction material after repeated use. This reduction is primarily caused by a buildup of heat on the friction material's surface. Manufacturers test and assess their materials to ensure prolonged performance and prevent premature fading. Common causes of fade include improper use or selecting the wrong material for the application.
The Friction Materials Standards Institute (FMSI), established in 1948, sets the primary standards for friction materials as a trade association representing aftermarket friction manufacturers. The FMSI has created a standardized part numbering system for identifying brake and clutch facings.
The FMSI system provides a framework for producers and buyers to classify and identify friction materials. Over time, the FMSI has expanded its reach from a national to an international organization, offering a wide range of scientific, engineering, technological, and statistical data on friction materials.
Regarding regulations, most U.S. states impose restrictions on the amount of asbestos and copper that can be used in friction materials. Many manufacturers have developed alternative technologies to eliminate asbestos entirely from their products.
The Environmental Protection Agency (EPA) oversees the production of friction materials, with standards focused on controlling toxic substances generated during manufacturing. The EPA's regulations, first proposed in 2001 and updated through 2019, primarily address concerns related to the solvents used in the production process.
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