Gearbox lubrication: What are the best methods?

Like other types of rotating equipment, gearboxes rely on lubrication to reduce friction and provide cooling for optimal operation and life. Gearbox manufacturers provide recommendations for the type of lubrication to be used and the typical lubrication intervals, but the actual gearbox lubrication requirements depend on the environmental conditions the gearbox is exposed to, whether it is properly maintained, and whether it experiences overloading.

There are several methods for lubricating a gearbox, with the most common being grease lubrication, oil splash, and forced oil.

Grease lubrication is suitable for low-speed operation, but it provides less cooling than oil, and is not recommended for continuous-duty or heavily loaded applications, even at low speeds. With any lubrication, using the proper quantity of lubricant is important, and this is especially critical with grease. Using too little lubrication prevents an adequate lubrication film from forming, but too much lubrication – especially grease – adds viscous drag and results in power loss.

Oil splash lubrication is often used for helical, spur, and bevel gearboxes. This method is also referred to as an oil bath, because it uses a reservoir is filled (or partially filled) with oil. As the gears rotate, they dip into this oil bath and splash the oil onto the other gears and bearings. But if the gear teeth are fully submerged, a condition known as “churning” occurs.

Essentially, churning is when the gears or bearings must work harder to “push through” the lubricant. A good analogy is walking along the edge of the water at the beach. Walking through ankle-deep water is relatively easy, but if you move to knee-deep water, walking requires much more effort.

The effectiveness of oil splash lubrication is heavily dependent on the speed of the gears. A common rule of thumb is that a tangential speed of at least 3 m/s is required in order for splash lubrication to be effective.

Forced oil lubrication is preferred for high-speed applications, and includes methods such as oil mist, oil spray, and oil drop. In the oil mist method, oil is atomized so that it saturates all areas of the gears and other internal components. In contrast, the oil spray method applies oil lubricant directly to the gears and other components, but this method is not always effective, as high centrifugal forces affect the direction of the oil spray.

The oil drop method pumps, or “drops,” oil directly onto the surfaces where it’s needed. A pump takes lubrication from a reservoir and delivers it to the gears and/or bearings via an internal piping system. This method can often be found in conjunction with the splash (aka oil bath) method, where some components may be difficult to reach via oil splash.

Regardless of the lubrication method used, the right type of lubrication is critical to gearbox performance. And of the parameters involved in selecting a lubricant, viscosity is the most important.

A lubricant with a viscosity that is too high for the application (i.e. the lubricant is too “thick”) won’t flow sufficiently as the gear teeth engage to protect mating surfaces and provide cooling. But lubrication with a viscosity that’s too low won’t provide a sufficient film thickness to prevent metal-to-metal contact between the mating surfaces.