Low Oil Delivery to the Bearings

The oil delivery (flow rate) to the bearings was observed through Plexiglas windows mounted in the side and end of the bearing housing. From the test observations, the hypothesis was that oil delivery was sufficient to ensure adequate lubrication.

To verify this hypothesis, an additional oil feed gallery was added to increase oil delivery (Figure 6). Tests were conducted using this arrangement and when the results were compared with previous tests, there was no improvement in these oil samples’ color. The only observation was that increased oil delivery resulted in increased bearing temperature due to more oil churning within the bearings.

It was therefore concluded that oil delivery was not a contributing factor to black oil formation. The actual oil requirements for lubrication of a rolling element bearing are only a few drops per minute. Additional oil only serves to help remove the heat generated by the bearing.

Oil Viscosity and Additives

Because oil viscosity affects the oil film thickness at the ball/raceway/cage interface, oil viscosity was identified as a factor important in black oil formation.

Therefore, a test to determine the significance of an increase in oil film thickness on black oil formation was conducted. Under ball skidding and other less-than-ideal conditions, the oil film acts as a barrier separating the components from metal-to-metal contact. Thus, generally speaking, the thicker the oil film, the better its separation potential. It was concluded that higher viscosity oil reduces discoloration.

Oil additives were evaluated to determine whether antiwear additives had any effect on the level of black oil. In addition to the standard VG32 viscosity turbine oil used by Sulzer Pumps for many years, testing was performed using VG46 and VG68 turbine oils (from the same manufacturer) and antiwear VG32, VG46 and VG68 oil.

The antiwear VG32 and VG46 oils were found to give similar results to a standard turbine VG32 and VG46 oils. The conclusion was that antiwear additives alone do not reduce the level of blackening.

Test Conclusions

Based on the tests described, the following conclusions were reached:

The bearing arrangements with the lowest wear levels and the least oil discoloration on long-term test were:

Nonmetallic oil ring, VG46 oil and a medium preloaded bearing.

Nonmetallic oil ring, VG46 oil and a medium-clearance bearing (with controlled assembly).

The bearing arrangement with the worst wear levels and the most oil discoloration on tests was the bronze oil ring, VG32 oil, medium-clearance bearing and an inter-bearing shim.

The primary source of bronze debris in the oil was due to the oil ring wearing against the bearing housing and oil ring carrier. A nonmetallic ring successfully eliminated this wear.

The viscosity of the oil affects the level of discoloration of the oil. Higher viscosity oil reduces the discoloration, but it does not eliminate it if the other design problems are not addressed.

The use of antiwear oils alone does not prevent the discoloration of the oil.

Varying the amount of oil delivery to the bearing did not affect the discoloration of the oil. Oil delivery rate, beyond that required for good lubrication, is not a factor in black oil formation.

The use of thrust bearings without an inter-bearing shim will reduce the blackening of the oil by eliminating ball skidding within the bearing.

The use of preloaded thrust bearings reduces the probability of ball skidding, which is a factor in black oil formation. This type of bearing will have a tendency to run hotter (comparative tests show a 0°F to 10°F increase in sump oil temperature compared to non-preloaded bearings).

Clearance thrust bearings give comparable oil color to preloaded bearings provided nonmetallic oil rings, VG46 oil and controlled assembly techniques are used.

In the cases studied, black oil formation was not caused directly by cleanliness- or chemistry-related issues.

Implementation of Design Changes

Following the testing and based on the conclusions reached, the following design changes were implemented on all Sulzer Pumps’ ring oil-lubricated pumps:

Bronze oil rings were replaced with nonmetallic oil rings.

The standard viscosity of the oil used in the bearings was changed from VG32 to VG46.

Bearing temperature alarm/trip settings were raised by approximately 5°F to allow for a slightly higher operating temperature (due to the higher viscosity oil).

Medium clearance thrust bearings without an inter-bearing shim became the standard thrust bearing choice.

Assembly procedures and the installation and operating manuals for the pumps were revised to reflect the best practice identified during the tests.