In previous articles we’ve spoken about the benefits of selecting the correct synthetic gear oil for the application; however this would be meaningless if we didn’t explain the importance of selecting the correct synthetic gear oil viscosity.

Selecting the Gear Oil Type
In order to select which type of gear oil to use in a gearbox, you must understand the gearbox’s mode of operation. The
application factor identifies the type and magnitude of load the gears will experience.

Steps to selecting the correct synthetic gear oil viscosity

Correct synthetic gear oil viscosity is the most important physical property of a lubricating oil. Because the viscosity changes with temperature, the rate of change is an important property identified by the Viscosity Index (VI). Most mineral-based gear oils will have a VI of 95. A lower VI indicates that the oil’s viscosity changes to a greater extent with change in temperature.

Conversely, a higher viscosity index indicates a much lower rate of change in viscosity with respect to change in temperatures. The advantage of a high VI is that in lower temperatures, the oil will tend not to increase viscosity as much as a lower VI product.

The ability of an oil to maintain a small viscosity differential over the operating range of the gearbox provides a more consistent lubricating film to the gears and more predictable wear performance.

Establishing the correct synthetic gear oil viscosity

As stated earlier, the correct synthetic gear oil viscosity is an important parameter in proper selection of a gear oil. The manufacturer of the gearbox or gear system generally offers a viscosity recommendation and these recommendations should be followed in most cases. If the OEM of the gear unit has not provided a recommendation and the viscosity has not been calculated based on lubrication theory, it can be selected in accordance with various worksheets and graphs. The differing viscosity-temperature and viscosity-pressure behaviour of synthetics as compared to mineral oils should also be taken into account.

The correct synthetic gear oil viscosity must be selected independently of any specific gear stage, realizing that a compromise is required for multistage gears. The selection of the correct viscosity is based on the oil’s expected operating temperature, such as sump temperature or the temperature of the injected oil. This temperature is calculated by determining the gear’s thermal economy, taking into account the frictional losses; or in the case of gears already installed, by measuring the temperature of the sump.

It might be required to select a lower viscosity to ensure lubricant is supplied during a cold start or at lower ambient temperatures. In each individual case, it is necessary to check the viscosity at the existing starting temperature, especially in the case of oil circulation systems.

A typical worksheet method for determining the correct synthetic gear oil viscosity required for a spur gear drive and a worm gear drive starts with the calculation of the force-speed factor. Because of different viscosity-temperature behavior of different oils, different ISO viscosity grades are selected for the same Kluber Viscosity Number.

This article presents only a few of the important factors in gear lubricant selection. Technical and performance specifics about lubricants lead to better, more precise decisions in making lubricant selection. It is also helpful to use a reputable lubricant supplier, such as Habot synthetic lubricants, who are knowledgeable in selection options that affect energy consumption, machine life, lubricant consumption and waste oil generation.

If you’re still unsure about selecting the correct synthetic gear oil viscosity, give Habot Synthetic Lubricants a call, we’re here to assist with professional service and advice.