The bearing overlay can play a critical role in determining bearing life. The authors analyzed two different mechanisms of overlay removal from production engines operating in major fleets. The first involves the loss of tin in the overlay and the second was due to a small coolant leak. Both were returned at low mileage.

Overlay Loss to Tin Migration
Bearings were returned from the field due to a camshaft bushing spinning after short mileage: 280,000 miles (448,520 km). An investigation into the problem revealed that two different bearing suppliers provided bearings for this application and one supplier’s bearing had failed more frequently in both engine dynamometer and field tests. Both exhibited overlay removal in the field.

Neither of these bushings had a nickel dam between the overlay and the lining. In examining new bearings from both suppliers, it was found that the tin in the overlay had migrated during manufacture and reacted with the copper of the lining. Based on X-ray diffraction analysis, the bushing with the shortest life had formed Cu3Sn at the overlay-lining interface. Its thickness was approximately 77 micron inches. The overlay contained seven percent tin. In contrast, the other bearing formed Cu6Sn5 with a thickness of 48 micron inches. Its overlay content was 19 percent tin.

The migration of tin from the overlay resulted in loss of the overlay due to corrosion. When the camshaft contacted the hard and brittle intermetallic compound of Cu3Sn, it caused the bearing to spin, resulting in an engine failure. The solution to the problem is the use of a nickel dam between the overlay and copper lining, in addition to careful control of temperatures during bearing manufacture to prevent tin migration.


Figure 1. Mechanism of oil ball formation
in crankcase oil with coolant leak.

Overlay Loss from Coolant Leakage
In 1978, bearings with only 200,000 miles (320,100 km) were returned for analysis to determine the cause of overlay removal. Examination of the failed bearings in the SEM showed that there were spherical particles embedded in the surface. Electron beam microprobe analysis (EBMPA) of the particles revealed three kinds of particles:

  1. White spherical particles containing the additive elements: 50 percent calcium, 15 percent phosphorous, 6 percent sulfur and 20 percent oxygen.

  2. Black irregularly shaped particles, mostly carbonaceous with small amounts of calcium, phosphorous and sulfur present.
  3. Small numbers of sand (SiO2) particles embedded in the exposed copper-lead alloy areas.

The spherical particles plowed into and embedded in the overlay. These spherical particles were also present in the engine sludge. The average particle sizes in the overlay were 15 microns: in the sludge the particle sizes ranged from 4 to 12 microns.

Shortly after this work, Magan Patel (Cummins Engine Company) reported on spherical particles abrading connecting rod bearings.1 Because the elemental composition of these particles was similar to the elemental composition of the additive package, the term “oil balls” was applied to these spherical particles. In Patel’s work, the particle size range was found to be 5 to 40 microns. He also observed that coolant or water contamination accelerated, connecting rod bearing overlay removal by oil balls.


Oils balls found in bearing
overlay (1,000x magnification)

Laboratory experiments by Godfrey showed that oil balls could be produced by mixing two percent glycol with a typical fully formulated engine oil in a beaker at 150°C for two hours. Hydrolysis of overbased calcium sulfonates and zinc dithiophosphates will lead to inorganic salts that, in principle, may react to form very hard precipitates in calcium-containing oils (Table 1).


Table 1. Formation of oil balls. Hydrolysis reaction
for calcium-containing oil.

Additionally, both H2S and H3PO4 may react with either calcium carbonate (from the overbased detergent) or with calcium hydroxide, forming either CaS or Ca3(PO4)2 (calcium phosphate). These reactions will preferentially occur in the water phase. In the crankcase, the oil is vigorously agitated by the actions of the crankshaft and the connecting rods. Any aqueous or glycol-containing phase will be well dispersed in the form of small droplets. Consequently, precipitates that form in these droplets will also have a spherical shape, hence the formation of ball-like (spherical) inorganic concretions (Figure 1). Calcium phosphate is very hard (RC ~48). In fact, it is a major constituent of bones and teeth. Hence, those oil balls that survive being embedded in overlay are likely to be those concretions that are rich in this very hard material.

Reference

1. M. Patel. “Influence of Oil Balls on Premature Overlay Removal of Diesel Engine Connecting Rod Bearings.” SAE Paper 810501.