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NEW YORK — The following is the second of four articles by Arnold Anderson posted on the Brake Academy’s website concerning the interaction between copper pollution in South San Francisco Bay and emissions from brake components.
Copper Pollution in South San Francisco Bay: A Brake Researcher’s Perspective
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 content 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 are similar 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 duty drum 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. With the advent of jet engines, brake loadings increased dramatically, and multi-plate full disc brakes were developed. Originally, these also used high copper content organic brake linings. However, sintered bronze friction 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. These use carbon (often called graphite) fibers that are bonded with an amorphous carbon phase. The same carbon-carbon material 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 or other 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, or other brake materials. It is known that the Boeing 747 aircraft originally used bronze, but newer versions now use carbon-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 at very 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 a tendency for the heating to take place along thin bands around the brake lining rubbing surface. These can form an effective 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.
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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 at the 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 associated brake fade. Surface oxidation of the copper provides a mild abrasive action that temporarily increases friction. Intime, compounders discovered which alloys
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