Purchasing gear lubricants: be careful when playing the numbers game
Written by John Sander
7. Identify Viscosity Recommendation
The viscosity of a gear lubricant is a measurement of its ability to flow in an application. This is a very important consideration in selecting gear oil. If the oil is too thick, it will not flow into the gear contact zones. If the oil is too thin, it will be compressed out of the contact zones or fling off the gears while they are in motion. In either case, lubricant starvation will occur, which can result in premature wear-related failures. The primary means of gear lubricant selection, with regard to viscosity, is to use the OEM requirement. If an OEM recommendation is not available, there are two other methods to obtain viscosity recommendations. The first is to use the viscosity ranges recommended by the American Gear Manufacturer's Association, per its 9005-E02 standard (3), illustrated in Table 3.
The second method is attributed to renowned gear expert Robert Erricello (4), and is based upon a calculation method that employs the following equation:
V40 = 7000 / √V1
Where V40 is the viscosity at 40°C, in cSt
V1 = pitchline velocity of the lowest speed gear in the gearbox in feet per minute = 0.262 X speed (pinion rpm) X pinion diameter (inches)
If there is no oil cooler on the industrial gear drive, it is best to determine the maximum expected ambient temperature during operation and:
a. Increase one ISO viscosity grade if the ambient temperature exceeds 35°C (95°F).
b. Increase two ISO viscosity grades if the ambient temperature exceeds 50°C (122°F).
If there is an oil cooler, the maximum ambient temperature is less important because the oil's temperature can be controlled.
Therefore, the oil's temperature should determine the viscosity.
c. Increase one ISO viscosity grade if the oil temperature exceeds 65°C (150°F).
d. Increase two ISO viscosity grades if the oil temperature exceeds 85°C (185°F).
If the oil temperature exceeds 90°C (194°F), use a cooler such as a fan or a heat exchanger.
