Specialty lubricants have particular properties and characteristics for special applications. They are used whenever stresses beyond the capacity of conventional lubricants occur, including high pressures, extreme high and low temperatures, vibration, shock loads, start-up and run-in phases, sliding surfaces heavily loaded during run-in, and aggressive environments. Specialty lubricants include greases, pastes, solid lubricants, and anti-friction (bonded) coatings.
Greases are thick or semi-fluid dispersions of thickening agents and other additives in lubricating oils. Often, mineral oil-based lubricating greases with conventional thickeners do not meet all technical requirements. Synthetic fluids—polyorganosiloxanes, poly-α-olefins, polyalkyleneglycols, perfluoroalkylethers, esters, etc.—and an appropriate thickener can help meet special technical requirements. Greases are used in several brake components.
Traditional pedal systems are made of metallic materials; new designs are utilizing polymeric composites. Glass-fiber-reinforced thermoplastics provide improvements in reliability, weight reduction, and cost-effective manufacturing. New material compositions in pedal systems led to different requirements for pedal joint lubrication. Lifetime lubrication; good low-temperature properties; and prevention of corrosion, noise, stick-slip, and wear are required by plastic-metal and metal-metal components. Plastic-metal components also require a lubricant with good plastic compatibility.
Vacuum brake boosters are used in most conventional passenger cars. Lubricant in the booster must provide good lubricity at temperatures as low as -40°C (-40°F) and be compatible with a wide range of plastics and rubbers. For these applications—especially the guide pin, sliding boot, reaction disc, plunger, and vacuum seal—lubricating greases are being used.
The outer body of a floating caliper slides on guide pins. The caliper holds the pads in place. Hydraulic pistons force the pads against the rotating surface of the disc when pressure is applied to the brake pedal. Generally, the lubricant should work between -40 and +180°C (-40 and +356°F). Requirements differ for rubber-bushing-mounted caliper pins or pins without sleeves. The bushing rubber—typically EPDM—shows very good damping and vibration absorbance, but the tendency for stiction (stick-slip) is very high. In this case, a lubricant with good low- and high-temperature behavior and good rubber compatibility, which influences stick-slip, is required.
Due to the caliper’s vibration, metal guiding pins without sleeves tend to develop fretting corrosion. Generally, a solid-containing lubricant with good lubricity and load-carrying capability over a wide temperature range is needed.
Electrical parking brakes (EPBs) are available in two types: motor-on-caliper and cable-puller systems. The more traditional cable type replaces the mechanical handle in the cabin with an actuator spindle system that needs to be lubricated by specialty grease. The grease needs to have a low friction coefficient to minimize starting torque of the electrical motor and ensure lubrication of the spindle at higher speeds; however, friction torque should be high enough to avoid self-release of the cable nut, which could lead to EPB malfunction. The grease also should work well at low and high temperatures.
Depending on the operation area, major requirements for grease in a brake component application are to ensure low-temperature lubrication, provide aligned lubricity/friction coefficient, protect from corrosion and fretting corrosion, and offer plastic/rubber compatibility.
Pastes contain high concentrations of solid lubricants dispersed in oil for ease of application. Where oils and greases are squeezed out of the lubricant contact area, solid lubricants form tenacious adherent films that prevent damage under extreme loads and low speeds. In brake applications, they are used for damping and for vibration and fretting corrosion prevention.
Brake pad back plates and shims are mainly lubricated with highly filled metallic copper, black or white solid lubricant pastes. Brake pad shims of different designs and types are often used as anti-squeal and anti-vibration shims. Major tasks of the plain steel or rubber-coated shims are to absorb brake pad or caliper vibrations, avoid noise, and dampen judder.
Anti-friction coatings are paint-like products containing submicron-sized particles of solid lubricants dispersed through carefully selected resin blends and solvents. They are applied by conventional painting techniques such as spraying, dip-spinning, screen printing, or brushing. After evaporating the solvent and/or curing the resin, the remaining coating exhibits good dry lubricating and corrosion protection properties.
The caliper’s sliding guide spring helps maintain proper brake pad movement. The major problem is to ensure the pad’s movement within the caliper guides while preventing corrosion during the pad’s lifetime. To avoid usage of “wet” greases, which could contaminate the rotor or pad surface, the brake pad’s sliding guide is equipped with a steel spring. Electrochemical potential between the spring and the zinc- or zinc-alloy-coated caliper means electrocorrosion can occur. A dry lubricant with insulating and corrosion-prevention properties—such as an anti-friction coating—is the proper solution.
Dry lubricant powders are solid substances applied between sliding surfaces to reduce friction and wear and prevent scoring. Solid lubricants can be inorganic or organic compounds. Typical inorganic solids include MoS2; graphite; and metal hydroxides, sulfides, halides, or phosphates. Typical organic solids include PTFE and PE waxes. Synergistic mixtures of finely distributed special graphite and metal compounds lead to remarkable lubricating properties. In brake systems, these powder additives can impart precise friction control properties to matrices for brake pads and linings.
A friction brake converts velocity into heat through friction. Brake components (pad/disc or lining/drum) are in sliding contact, which can be controlled by lubricants (friction modifiers) in the lining composition.
Friction control additives influence the performance of a brake system in several ways. For safety, the additives can help prevent fading, maintain a constant friction coefficient at all temperatures, and help ensure good friction recovery. For braking comfort, they can help reduce brake noise, prevent vibration (judder), and minimize material transfer to the disc. For wear/life of friction components, the additives can slow pad and disc wear, reduce corrosion of brake pads, and prevent spot formation (glazing) on discs.
These additives should not reduce friction but should stabilize frictional properties at a controlled level.
Selection of the right lubricant is one of the most challenging tasks for brake system designers and engineers. Brake component design should involve a lubricant specialist at a very early stage to help ensure the correct selection or initiate development of a suitable lubricant.
Detlev Hesse of Dow Corning GmbH, Wiesbaden, Germany, wrote this article for SAE Magazines.