You know that two surfaces mate, but what is a friction clutch or brake? Using friction has been the most popular way to engage and stop loads since the 1800s. Surfaces interact with counter-frictional surfaces (and in wet systems, oil) to control performance during engagement and disengagement, thermal loading limits, and speed of wear. Methods of actuation may vary, but the mode of physical contact does not. Whether mechanically, centrifugally, magnetically, or hydraulically actuated, plates, shoes, or pads pivot or clamp to engage larger disc or drum surfaces. The best materials combination for plates, pads, and the larger surfaces they engage depends on the application.
This is the material of a brake or clutch surface that gets the job done, as its coefficient of friction is a measure of braking effectiveness. Though more often than not the objective is to increase the coefficient of friction, there are also times when a friction material is selected for its ability to lubricate surfaces.
Changes in the coefficient of friction during engagement or disengagement are determined by the static coefficient of friction, which in turn is determined by sliding speed, ambient and friction surface temperatures, friction surface materials, design, and structure. Because many brakes and clutches are used for safety stops, it’s important that this value be well tested. Otherwise a developing misalignment, a misplaced workpiece, or a shifted load could threaten to damage machinery - or people.
If the coefficient of friction is too high, the brake will be overly grabby. If the coefficient of friction is too low, the brake will not provide enough stopping power. Several forms of carbon are used to adjust friction and recovery; though inexpensive and convenient, they do burn at about 700°C and are affected by moisture. Semi-metallic friction surfaces containing brass chips, another common additive, are generally more grabby. The hotter a semi-metallic surface gets, the more the brass chips stick. (This does wear drums and discs out faster than carbon-based materials.) Ceramics exhibit the same benefits as carbon-based surfaces with increased temperature resistance and are growing in use.
These materials hold the brake or clutch friction surfaces together. Common binders today include copper, iron, and nickel alloys, as well as phenolic and modified resins. If too much resin is included in a brake, the material becomes weak. When too little is used, the coefficient of friction drops off at higher temperatures. With very severe use, the resins can even seep to the outer friction surfaces and glaze them. To combat this problem, cresol, epoxy, rubber, boron, and even linseed oil is sometimes added to alter bonding characteristics.
Fillers aren’t just fillers; when added to friction pads and shoes, they can decrease cost, increase temperature thresholds, help maintain material stability, and increase the coefficient of friction. The original and best-known filler is asbestos; however, other fillers are in wider use now. Potassium titanate is sometimes an acceptable replacement for asbestos because it is inert, has insulation capabilities, and adds structure to friction surfaces. Other replacements include polyacrylonitrile polyester, mica, and blast furnace slag.
Quartz, silica, and aluminum oxide are a few examples of abrasives; they help maintain the cleanliness of mating surfaces and control the buildup of friction films. They also increase stopping friction, or bite. To further increase braking effectiveness and control fade, mineral wool fibers made of magnesia, silica, and other inorganics are used. Nitrile and diene rubbers are used as stabilizers to promote cross-linking within shoes and pads. They also alter compressibility of friction surfaces for varied gripping.
When free of road dirt, recycled tire peels lower brake and clutch cost. Finally, anti-oxidants aren’t just good for humans; in friction surfaces, they help optimize oxide film thickness. Graphite is a common anti-oxidant that also ensures cooler operation, reducing heat transfer to the surrounding system and brake or clutch actuator.
An oiled system - or wet system - is based on the paradox that some oils actually increase internal friction. Normally brakes and clutches must be kept completely free of oil to ensure optimum operation. Wet brakes and clutches have a sealed housing filled with oils containing long molecule strands. When compressed, these molecules create high internal friction. Wear on oil brakes and clutches is reduced to a fraction of a dry system’s normal wear; lubrication between the mixed or boundary friction areas smoothes over surface roughness with a layer of oil molecules. The oil absorbs most of the wear, reducing maintenance to a simple oil change. Another benefit is that sealed housings impede corrosion and provide fire and environmental protection.
A wet system’s coefficient of friction (and torque) can be more accurately maintained than that of a dry system. Though wet coefficients of friction are somewhat lower, rapid engagement with low dynamic excitement (for more strength and lower noise emissions) can be achieved. Friction spikes do occur, but only during coasting. Interfacial moisture suppresses the formation of thin transfer layers on friction surfaces, increasing grabbing just before stopping. Binding forces that adhere oil to surfaces are stronger than shearing forces caused by sliding, allowing coatings to stay in place. These binding forces, boosted by lubricant additives, combat the dispersal effects of pressure and temperature.
Adequate lubrication must be guaranteed when a clutch or brake is set up for wet operation. Though the component might be able to perform as usual for a time, clutches and brakes will fail prematurely. If oil continuously circulates and cools the friction surfaces, brakes and clutches can operate for very long periods, such as in continuous slip applications. Systems can also be designed with external cooling for near-continuous operation. Sometimes notches on clutch and brake plates promote flow, splash, or spray lubrication.