By Leslie Langnau, Managing Editor
Friction is an important force in mechanical designs. But it can be a challenge to find the right friction material for industrial applications. A relatively new material may make your selection easier, and help you spend less money replacing or resurfacing rotors, drums or flywheels.
Facings lined with 100% Kevlar fiber composite lining for installation in truck and other vehicle clutches.
When asbestos became an undesirable material for industrial brake linings, engineers and researchers began to develop application specific friction lining formulations. Many turned to organic alternatives, such as natural amorphous and fine flake graphite. In these formulations, other organic materials such as rubber, cashews, glass, mineral wool, ceramic, Kevlar®, and various fibers are added to provide application specific features. Assorted polymer-based resins not only bind these materials, they also boost strength, enhance heat absorption properties, improve manufacturability, and provide other desirable features. In some formulations, metallic and carbon materials, such as iron and steel fiber and copper or iron based materials sintered with inorganic components are included. Common examples include solid-state sintered bronze with mullite and sintered iron with graphite.
Carbon-carbon friction material, which is carbon fiber bonded with amorphous carbon, can be a lightweight material that suits applications with high operating temperatures, such as 2000°C or more.
However, for friction systems on the rotors, drums or flywheels of trucks, paper mills, cranes, stamping presses and other industrial equipment, many of these alternative friction lining formulations are not quite as good as asbestos-based linings in terms of life and wear. In addition, many lining materials are dusty, noisy, or wear down the mating part too quickly. Thus, the search continues for a suitable replacement for industrial friction applications.
A little known material formed almost completely from Kevlar offers a wear life that is up to five times longer than other materials, does not wear down the mating surface, and its close static-to-dynamic ratio provides smooth engagement. Plus, there is no dust to clean.
Kevlar was developed in 1965 by Stephanie Kwolek while she worked at DuPont as a research chemist. Through experimentation and original thinking in the field of liquid crystalline polymers, she created long-chain molecule fibers of poly-paraphenylene terephthalamide. Because the chains are oriented with strong interchain bonding along the fiber axis, they have a unique combination of properties.
These properties include high load at specified elongation, high tensile strength at low weight, structural rigidity, low electrical conductivity, and high chemical resistance to name a few.
The Kevlar fibers are considered aramids, a class of heat resistant and strong synthetic fibers. Other related aramids include Nomex and Technora. When Kevlar material is stacked in layers (up to 20), it can stop a bullet traveling at more than 1000 fps.
Developers of friction lining materials have long wanted to use this man-made fiber because of its rare combination of high strength, nonabrasiveness, and resistance to heat and tearing.
Various clutch and brake plates for off-road and industrial applications lined with 100% Kevlar fiber composite lining.
However, limitations on manufacturing processes and the cost of Kevlar have forced developers to include this material as ground up pulp, which reduces its special features.
One manufacturer, however, has discovered a different manufacturing technique for Kevlar. It is proprietary, but this technique uses a textile sheet-making process that takes advantage of Kevlar’s long fibers. Somewhat like the woven looping of carpet manufacturing, this developer’s production technique intertwines Kevlar fibers that are up to a foot long into an intricate matrix to create a textile like cloth. The long fibers in this cloth will not pull out under use, as can happen in linings with ground up bits of Kevlar or components of other materials.
In addition, the company adds a resin for binding that was developed by NASA for use on reentry vehicles. This polyimide resin has much higher heat resistance than the phenolic resins used in standard friction facings.
This friction material’s matrix configuration and ‘space age’ binder enables it to last up to five times longer than asbestos, sintered bronze, molded graphitics and other original equipment facings. It is not affected by heat or cold temperatures, or by ethylene glycol, a solvent that tends to eat away at many friction linings. Thus, it is well suited to applications that require longer lasting, predictable, and reliable operation; applications with scheduled downtime for example. It handles both wet and dry construction friction applications such as clutches, power transmissions, PTOs, torque converters, synchronizers and off-road brakes.
Because this friction lining is more like a cloth, it doesn’t really have a flatness dimension. Therefore, it can conform more cleanly to mating surfaces, even those with irregularities. Plus, it will not abrade, scratch or score the opposing surface.
The lining takes heavy loads and can withstand high temperatures and pressures to deliver more torque. There is virtually no dust from this material, rather, “it oxidizes, somewhat like a person breathing,” noted Nick Bale, sales manager at Tribco Inc., the manufacturer of this material.
Currently, this material is used on certain parts of the new Airbus in Europe, friction discs in cameras placed up in satellites, and on 6000-ton forging presses. It can also be found on truck clutches, and in paper mills, cranes, and stamping presses. UPS has about 50,000 trucks using this material on the flywheels.
This material’s coefficient of friction is at a good mid range. Dry, it is at about 0.36. After burnishing, when the high and low spots from the interweaving process are broken down, the range moves to about 0.45. Until that happens, the first few uses will show only about 40% contact. Once broken it, the mating percentage goes up to 70% and higher.
But because this material can last three to five times longer than most other friction materials, it can take three to five times longer to break in.
Various clutch and brake plates used in oil-immersed applications (such as automatic transmissions) lined with 100% Kevlar fiber composite lining.
While suitable for off-road vehicles, it is not recommended for highway vehicles. That area is outside of Tribco’s business focus, plus the material has not been tested for these applications, so the degree of safety is unknown. In addition, this material tests poorly using traditional highway vehicle friction testing methods. For example, new outputs and new test parameters must be developed in order to test it with a dynamometer as this material goes outside usual friction parameters.
:: Design World ::
Friction is necessary to start, stop, and keep objects in motion. The force of friction can describe several actions. Static friction describes the force between two objects that do not move relative to each other. It is the force that must be overcome in order to move an object. Its coefficient is typically denoted as µs, and is usually higher than the coefficient of kinetic friction.
Kinetic (or dynamic) friction occurs when two objects move relative to each other and rub together. The coefficient of kinetic friction is typically denoted as µk, and is usually less than the coefficient of static friction.
Since friction is exerted in a direction that opposes movement, kinetic friction usually does negative work, typically slowing something down. There are exceptions, for instance if the surface itself is under acceleration.
Rolling friction is associated with the rotational movement of a wheel or other circular objects along a surface. Generally it is less than that associated with kinetic friction. Typical values for the coefficient of rolling friction are 0.001.
The standard equation to determine the resistive force of friction of two solid objects sliding together states that the force of friction equals the coefficient of friction times the normal force
pushing the two objects together. This equation is written as:
Fr = µ x N
Fr = the resistive force of friction
µ = the coefficient of friction for the two surfaces
N = the normal or perpendicular force pushing the two objects together
Fr and N are measured in pounds or newtons. µ is a dimensionless number between 0 (zero) and infinity.
The details of the Tribo material
The Tribo friction lining does not include metal, abrasives, cotton or fillers. Here’s a quick look at its specifications:
Density: 0.91 g/cu cm (0.033 lb/cu in.)
Thermal conductivity: Extremely low
Shock resistance: Excellent (does not crack, or break)
Lubricant contaminant resistance: Does not degrade
Abrasiveness: Non-abrasive to opposing iron, steel, and copper surfaces
Static pressure: Up to 6900 kPa (1,000 psi), or as limited by substrate
Dynamic pressure: 140 – 3100 kPa (20-450 psi)
Temperature: Ambient to 315° C (600° F)
Surface speed: Static to 40 m/s (8,000 fpm)
Opposing surface: Machined and unfractured surface required, no fine finishing necessary. Surface speed, temperature, and pressure are interdependent energy parameters. Values represent typical conditions and are not the ultimate limits of the material. Burnish time to achieve full mating surface contact can be three to five times that of conventional materials.
Wear rate: 1/5 to 1/10 that of asbestos materials, 1/2 to 1/3 that of sintered bronze materials
Dynamic coefficient of friction: 0.36 µ ± 0.1 in the 95 – 345° C range (200° F – 650° F). Approximately 25% higher than molded asbestos, glass-fibered, and graphitic materials
Static to dynamic ratio: 1.05
Fade: Significant fade at 260° C (500° F), accelerating at 370° C (700° F)
Filed Under: Brakes • clutches, Materials • advanced, Mechanical