eSource 101 Various Sources of a Popular Wear Metal
Copper is used extensively in many bearing configurations. Copper alloys used in bearing materials are grouped into several classes: copper lead, copper tin (tin bronze), leaded bronze, aluminum bronze, and beryllium copper. Different metals are alloyed with copper for requirements based on load capacity, bearing strength, hardness, wear resistance, and fatigue strength. A common bearing that utilizes copper is a Babbitt bearing. Babbitt bearings typically use a softer external layer comprised of a soft metal such as lead. Babbitt bearings form many advantages including increased cyclic loads. The copper layer is not intended to come into contact with the rotating component; it is simply an intermediate layer, normally around 0.30-0.38 mm (0.012-0.015 in) thick between the backing and the soft metal alloy (often called white metal or 'Babbitt') that forms the actual bearing surface.
The process to make a Babbitt bearing (main bearing) utilizes a process called electroplating. Copper is used as a conductive media layer during the production process and is most often used over aluminum alloys to ensure full adhesion of the plated lead alloy layer. Electroplating a thin precision Babbitt layer (lead or tin) on an accurately machined bimetal shell is more economical then a casting process and has also proven to improve fatigue resistance of the bearing. Copper that is alloyed in small concentrations to the Babbitt based bearing material exhibit better fatigue strength compared to lead or tin based Babbitt but lacks other essential properties. Copper-alloy bearings provide greater load capacity, better high-temperature operation, greater wear resistance, but poorer scoring resistance. Each manufacturer of the bearings has their own recipe and design for the bearing. The design provides excellent seizure resistance provided by the lead based overlay. Seizure resistance drops sharply when the overlay is removed in direct metal-to-metal contact, embed ability of small dirt particles, and conformability for small misalignments.
Copper is present in main bearings as well as rod bearings, oil cooler core, clutch plates, brass and bronze bushings and the roller bearing outer cage. When copper is discovered in engine oil samples, it may be due to a water pump leak or coolant core breach. If this is the case, potassium, sodium and glycol may also be present. The tubing in coolers and heat exchangers is often made of Copper. New components will leach metals while they build up a protective oxidative layer which is seen by elevated Copper levels during first few thousand hours of service before a varnish layer is formed that reduces the copper leaching. Some anti-seize compounds contain Copper. If work is done on a component and a copper based anti-seize is used, copper may show up in a sample.
There has been instances when a Sulphur-Phosphorus EP additive in a gear oil is used in a Copper or brass system will lead to chemical wear. This occurs when there is an increase in Copper in conjunction with either sudden appearance of Phosphorus not previously detected when R&O oil is normally used, or stable Phosphorus and decrease in Zinc when Anti-Wear oil is normally used.
ALS is the global leader in oil analysis and can help in establishing a reliability centered maintenance practice at your facility. Contact your local ALS lab for more details.
Michael D. Holloway MLT1, OMA1, MLA1, CLS
Manager – Strategic Accounts and External Training, Tribology