How to Identify Proper Gear Oil Viscosity

"What should a synthetic gear oil with a viscosity of 8,200 centistokes at 40 degrees C (104 degrees F) feel like? Also, what would be some of its possible applications and recommended tests? Will it require any additional lab tests, or should a normal gear oil test slate be followed?"

As you are probably already aware, viscosity is the single most important physical property of a lubricant. It is a measure of an oil’s resistance to flow, or more simply, how thick the oil is. It is the viscosity of the oil that is critical in creating the oil film (hydrodynamic wedge) that keeps frictional moving surfaces separate.

Viscosity affects the frictional heat that is generated in all rotating equipment due to both metal-to-metal contact and the oil’s internal friction. It governs the sealing effect of oils and the rate of oil consumption. Viscosity also determines the ease at which machines may be started or operated under varying temperature conditions, particularly in cold climates. It is affected by contaminants in the oil, very high pressures, extreme heat, volatilization and shearing forces.

Distilled water has a viscosity of 1 centistoke. A gear oil with a viscosity of 8,200 centistokes would be 8,200 times as viscous as distilled water. Just imagine trying to run through that at the beach! For comparison, consider that honey has a viscosity of around 10,000 centistokes at room temperature. The gear oil in question would feel quite a bit like that. Obviously, the oil would have a higher level of lubricity, but the flow characteristics would be similar.

Typically, oils in this viscosity range are used for open gear applications and for other types of slow-moving gear trains. Currently, some grease formulations call for oils in this viscosity range. These are usually for small, low-duty gearboxes. The test slate for these types of oils would be the same as for any other type of gear oil.

The recommended tests would include viscosity (ASTM D445), acid number (ASTM D664 or ASTM D974), additive elements (ASTM D5185), oxidation (ASTM E2412 FTIR), nitration (ASTM E2412 FTIR), appearance (ASTM D4176), moisture level (ASTM E2412 FTIR), particle count (ISO 4406:99), contaminant elements (ASTM D5185) and wear debris elements (D5185).

Some of the non-routine or exception tests would be varnish potential, membrane patch colorimetry (ASTM D7843), RPVOT oxidation stability (ASTM D2272), Karl Fischer (ASTM D6304), ferrous density (proprietary methods) and PQ index (PQ90).