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Copper Pollution in South San Francisco Bay: A Brake Researcher's Perspective. Part 2 of 4

By: Arnold Anderson

Heavy Duty Drum Brake Linings

Drum brakes are used on most heavy trucks and their trailers. Similarly, most transit buses also use drum brakes.These use two brake blocks on each brake shoe. Brake blocks may be attached by rivets or bolts. Line-haul trucks, the 18-wheelers that are commonly seen on our highways, generally neither require nor use high copper content brake blocks. The line-haul trucks, and others that use low copper conte t brake blocks, make up most of the heavy trucks and the most truck brake block wear in the Bay area.

High copper and high brass content truck brake blocks are used in some severe service heavy truck brake applications.Their copper or brass content may range from around 8% to 30% by weight. These heavy-duty truck brake blocks aresimilar in formulation to those of high-performance passenger car disc brake linings.

Medium duty trucks and school buses use both drum and disc brakes. A survey of around twenty recent heavy dutydrum brake lining formulations indicate that these typically are under 4% copper or brass content. Some use zinc instead of copper or brass.

Except for severe service truck applications, it appears that drum brake linings average about 2.5% copper content

Brake Linings for Aircraft

Early commercial aircraft used drum brakes, with brake lining formulations similar to those for race cars. Withthe advent of jet engines, brake loadings increased dramatically, and multi-plate full disc brakes weredeveloped. Originally, these also used high copper content organic brake linings. However, sintered bronzefriction materials soon became common. These used a solid-state-sintered bronze

matrix, often combined with a ceramic abrasive particle. The rotors were made of an alloy steel. These multiple disc aircraft wheel brakes are still in common usage on many commercial aircraft, and even some military aircraft.

Most new commercial and military aircraft are now being designed with carbon-carbon based disc brakes. Theseuse carbon (often called graphite) fibers that are bonded with an amorphous carbon phase. The same carbon-carbonmaterial is used for both the stators and rotors. High costs, $250 to $500 per pound, limited their introduction to commercial aircraft. Weight savings (to 1,000 pounds or more) remains an incentive for their usage. No copper orother metal is used as a wearable material on these brakes.

No current reference document was found listing the numbers of aircraft using sintered bronze, carbon-carbon, orother brake materials. It is known that the Boeing 747 aircraft originally used bronze, but newer versions now usecarbon-carbon. The Boeing 757 was reported also to use bronze-based brakes.

Most of the wear from sintered copper-based aircraft disc brake linings occurs during landing brake applications.Taxiing wear tends to be minimal. Aircraft brake wear particulate should mostly be beside and downwind of the landing strips.

Aircraft brakes are designed for a wear life of 1,000 landings. With several large airports in the Bay area, aircraft are a major source of friction material copper wear debris. Not all aircraft use copper-based brake linings, but in the past, and for the next decade or more, aircraft copper brake usage should not be ignored.

Most wear come from the San Francisco International Airport (430,000 flights/year), but Oakland International Airport (63,000 flights/year) and San Jose International Airport (82,000 flights/year) are significant. San Jose's Airport abuts the Guadalupe River and could affect the South Bay if copper brake

wear debris is transportable by stormwater and if copper brake wear debris is soluble. These two 'ifs' are critical and will be discussed later.

Why Copper is Used in Friction Materials

Most people are aware that brakes and clutches get hot when used. Frictional processes involve intimate contact atvery small contact sites, called contact asperities. Contact pressures are very high at these asperities. When rubbing speeds exceed one meter per second (a slow walking speed) the asperity contact temperatures may reach l000C.Higher speed produces more of these hot spots. Surface melting limits hot spot temperatures to around 1250C.Frictional performance remains good, as long as these contact asperities are well behaved. That is, as long as they keep moving around the rubbing surface and stay only briefly at one point. With high-speed braking, there is atendency for the heating to take place along thin bands around the brake lining rubbing surface. These can form aneffective seal that prevents gases from escaping the contact zone. If so, the braking surface pressurizes, and friction drops dramatically. This is called brake 'fade.' When a brake fades, the driver must push disproportionately hard on the brake pedal. Clearly this is not desirable.

Many years ago, brake compounders discovered that copper and brass, when added to an otherwise reasonable formulation, made the brake less prone to fading. We now know why. Copper and brass particles melt and smear atthe surface under high speed and high temperature usage. The smearing action does many things, including controlling the surface heating and its resultant distortion. Thus, it prevents gas pressure buildup and associatedbrake fade. Surface oxidation of the copper provides a mild abrasive action that temporarily increases friction. Intime, compounders discovered which alloys and particle sizes work best, and how much was needed for a particular usage.

High speed, high temperature friction materials use copper, iron/graphite, or carbon fiber/carbon for stability.When an organic binder resin is used, as with car and truck brake linings, copper or some copper alloy is best.When braking speeds do not exceed about 80 mph, copper may not be necessary, unless brake bulk temperatures and pressures are high. In Europe, most high-speed touring cars use copper in their disc brake linings. US cars withnon-copper original equipment brake linings can safely make a high-speed brake stop, but often at a price. Brakeeffectiveness is reduced somewhat, requiring greater pedal efforts, and lining wear rates may be temporarily high.

Although few car owners in the US really need performance brake linings for motoring safety, their usage appears to be on the rise. The reason is simple. Vehicle power-to-weight ratios are rising, and test drivers from car enthusiast magazines give new cars hard workouts. This requires brake systems that provide a good 'pedal feel,' even during severe service. Vehicle manufacturers are very sensitive to criticisms of their new cars, especially safety items like brakes. The pressure for higher copper content brake linings will decrease when lower performance, higher fuel economy cars return to fashion. Until then, vehicle manufacturers may use copper in brake linings to:

• reduce brake lining wear rates for high speed, high temperature braking.

• reduce brake pedal travel during hard braking.

• get higher friction/reduced brake fade tendency, with resultant lower brake pedal force requirements.

• reduce thermal rupture stresses for lessen risk of rotor cracking, distortion, and rupture.

• reduce brake noise and vibration problem occurrence.

A somewhat different situation exists for heavy duty drum brakes. It has been traditional to add brass chips to their brake lining formulations. Brass serves both to control brake fade and hasten recovery from fade. Copper, even atlower concentrations helps to control fade in drum brakes. Zinc, and its smeared zinc oxide transfer layer, speeds recovery from brake fade. Brass chips help both fade and recovery properties in drum brakes. Brass also aids removal of oil, grease, and other organic contamination, by providing local 'hot spots' and accelerating the breakdown of the organic contaminant.

Arnold Anderson, known as Arne in the brake technology world, has devoted most of his professional life to studies in tribology, the science of friction and wear. Much of this was done as a researcher at the Scientific Laboratory of Ford Motor Company. After his retirement in 1987, Arne was a consultant to vehicle manufacturers, brake manufacturers, and regulatory groups. He recently retired after having accumulated 12 patents, 42 technical papers, as he performed diagnostic studies of car, truck, aircraft, and aerospace brakes and clutches.

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