STARTING MOTOR ASSEMBLY 3A1872 - Caterpillar

New copper bonded connecting rod and main bearings are used in production for the 3200, 3300. and 3400 Series Truck Engines. This new bearing design uses a copper bonding agent (material) instead of a nickel bonding agent still being used by other manufacturers.
Caterpillar's research indicates the new copper bonding agent is superior to nickel-type bonding agents used in aluminum bearings, because the copper reduces contact friction by 50% over the nickel. Test results show copper bonded bearings are more resistant to scuffing than nickel bonded bearings.
Up until the mid-1970's Caterpillar and many other manufacturers used copper as a bonding agent. Environmental problems with the cyanide process, required to plate the copper, caused manufacturers to change to a nickel bonding agent. Caterpillar has again developed bearings with a copper bond.
The following discussion of bearing design gives the advantages of the copper bond. The first layer of the bearing is the lead-tin overlay (LTO) which is sometimes referred to as babbit. See Illustration 1. The LTO is a soft, low friction material and provides protection under conditions of marginal oil lubrication. The LTO is bonded to the aluminum with a bonding agent.
Illustration 1. Bearing construction.Nickel bonded bearings are sensitive to conditions of marginal lubrication once the LTO has become worn. Once the LTO has worn away, the nickel is exposed and approximately twice as much contact friction can be produced. This friction during marginal lube conditions may result in scuffing and subsequent failure.
Based on these findings, it is recommended that nickel bonded bearings be replaced with copper bonded bearings once the lead-tin overlay has become worn. Illustration 2 shows that as the LTO is worn away, the nickel and possibly even the aluminum bearing layers are exposed. The nickel mixes with the other two layers and can appear dark, while the LTO remains gray and the aluminum is silver. Additional photographs of worn bearings can be found in "Engine Bearings and Crankshafts", Form No. SEBD0531.
Illustration 2. These bearings show how, over a long period of time, wear In the load area has slowly removed the lead-tin overlay from a large area. (The aluminum and bonding agent can be seen.)The chart has recommended inspection intervals for nickel bonded bearings. The roll-in interval is determined by the results of those inspections, oil analysis results, and the customer's past experience with that engine model.
Bearings can last the life of the engine, till overhaul, if bearing wear and marginal lubrication are avoided. Conditions of marginal lubrication follow. If any of these conditions occur, bearings can be damaged and should be inspected.
Conditions Of Marginal Lubrication
1. Engine Overheating - Engine overheating raises the lubrication oil temperature and reduces the film strength of the oil. The oil can no longer support the bearing loads, and contact occurs between the bearings and the crankshaft journal which prematurely wears the lead-tin overlay (LTO).2. Oil Dilution - Oil dilution (either by fuel or coolant) reduces the film strength of the oil resulting in crankshaft-to-bearing contact with subsequent lead-tin