Equipment designers/manufacturers (OEM) and operating manuals reflect the minimum lubricant requirements for gearbox applications and this very complex equipment/application specific selection is not the within the scope of this article. These requirements however, include the type of oil, the oil’s specifications and typical operating temperatures. These requirements vary based on the three major classes of gears:
- Spur, bevel, helical, herringbone and spiral
The differences in gear teeth actions and materials influence the formulation and properties of the lubricants required but the size, speed, load, and temperature of the bearings typically determine the viscosity requirements. During operation, the heat generated by metallic friction between the tooth surfaces and by fluid friction of the oil, will cause the temperature of the oil to rise. The final operating temperature is a function of both this temperature rise in the oil and the ambient temperature surrounding the gear case. Thus, a temperature rise of 90’F (50’C) and an ambient temperature of 60’F (15.6’C) will produce an operating temperature of 150’F (66’C). The same temperature rise at an ambient of 100’F (38’C) will produce an operating temperature of 198’F (88’c). For gear sets equipped with heat exchangers in the oil system, both the ambient temperature and the temperature rise are less important since the operating temperature of the oil can be adjusted by varying the amount of heating or cooling.
The lubricant is thrown from the gear teeth and shafts in the form of a mist or spray. In this atomized condition, it is exposed to the oxidizing effect of air. Bearing operating temperatures may be increased above normal by heat conducted to the bearing from a hot shaft, or spindle, or by heat radiated to the housing from a hot surrounding atmosphere. This high temperature will also increase the rate of deterioration due to oxidation. Sludge or deposits formed as a result of oil oxidation, can restrict oil flow or result in an insulating deposit on the internal surface of the gear case to decrease cooling and cause further increases in the rate of oxidation. Allowing the gear case to become covered in cement dust, wood chips, debris, or even multiple layers of paint will create an insulating effect allowing the internal temperatures to rise and potentially above the OEM planned range.
The viscosity of the lubricant decreases drastically with this increasing temperature. Even though the lower viscosity reduces the churning losses, it also decreases the ability of the lubricant to fulfill its main function, namely separate the components in contact by creating an Elasto-Hydrodynamic (EDH) film. Without this separating film the components would score, wear out, pit and fail within a short period of time. Manufacturers typically provide a range of viscosity requirements for the equipment, which may or may not be able to be improved upon in practice.