CAB AS 2500961 - Caterpillar

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.
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.
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.) 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 overlay wear. Oil dilution can be determined by regular oil analysis.3. Rapid Engine Starts - Rapid engine starts, following an oil and filter change, momentarily starve the bearings of oil and can wear the LTO. After oil changes, always crank the engine with the fuel shut off before starting. This procedure will fill all filters and adequately prime the system.4. Dry Starts - Dry starts can occur after an engine has been inoperative for over 30 days. The lubricating system should be primed with a pressure lubricator prior to starting the engine. Priming can also be accomplished by cranking the engine with the fuel shut off until system pressure is obtained.5. Extended Oil Drain Intervals - Extended oil drain intervals raise the contaminants level of oil and increase oil viscosity. Contaminants wear the LTO. Follow manufacturer's recommendations for oil change periods for your application.6. Plugged Oil Filters - Plugged oil filters can be caused by extended filter change intervals or sooting of the oil because of incorrect fuel ratio control adjustment. Plugged filters allow unfiltered oil to circulate through the engine resulting in bearing