Gear
lubrication Moving synfully forward!
During the NLGI golden anniversary meeting in 1983, a paper entitled “Gear Lubrication: Fifty Years of Progress” [1] was presented which looked at the development of extreme pressure (EP) additives and multi grade lubricants during the preceding 50 years. The authors also tried to predict advances for the next 50 years: “Lubricants designed to save energy in the form of petroleum or electricity will become necessary. These energy-efficient oils will be based on new chemistry, use of synthetics, and optimization of viscosity to provide optimum boundary lubrication in complex gearing systems.”
Since the predictions were made, most, if not all of them, have come true, especially the use of synthetic lubricants. Due to higher initial cost, synthetic lubricants traditionally tended to be used where there was a technical need or they provided an economic benefit. Today, we see global trends are continuing to drive the need for synthetics. Conventional mineral oils cannot meet higher performance requirements, and the economic benefits of energy savings, extended drain intervals, lower maintenance and disposal costs are becoming much more attractive. The benefits also help deliver improved environmental performance in a world where this is becoming an essential part of business strategy. Numerous market studies confirm there will be steady growth in the use of synthetic lubricants across almost all applications [2].
For gear applications, the use of synthetic lubricants makes a lot of sense, so let‘s look at some of the key characteristics which make them so attractive.
As with most applications, viscosity is the key parameter of focus with respect to lubricant selection based on operating conditions. Selecting the proper lubricant helps minimize power losses and heat build-up associated with using thick oil, while helping to minimize the wear associated with thin oil. In equipment that runs at steady load and speed, we can optimize the viscosity grade relatively easily. However, for many gear boxes the speeds and loads can vary considerably, especially those in power transmission applications, which makes optimization difficult. Consequently, the viscosity grade chosen should provide sufficient protection under all expected working conditions. Although gear oils contain additives to provide wear protection under potential boundary lubrication conditions,
understanding and optimizing base oil viscosity changes relative to fluctuations in temperature and pressure remain key to help ensure the effective lubrication of a gear set.
Viscosity Index (VI) defines the relationship between viscosity and temperature and provides an indication of an oil’s ability to maintain its viscosity over a wide range of temperatures; the higher the VI, the smaller the change of viscosity with temperature. An oil can thus be chosen to provide the required protective film at normal operating conditions, while providing improved fluidity at low ambient temperatures to reduce starting torque and save energy. Synthetic lubricants can offer benefits in these conditions as they typically have natural VIs of over 130 or more, while traditional mineral gear oils will have VIs of 90-110. Figure 1 shows the viscosity at the operating conditions can be maintained with lower ISO viscosity grades when using high VI polyalphaolefin (PAO) based oils. At the same time, PAO-based oils can decrease the viscosity of the oil considerably at low temperatures (in this example from around 8000cSt@20°C to around 800cSt@ 20°C), providing faster circulation at startup and increased energy savings at low temperature. Figure 2 shows the magnitude of savings that could be achieved when switching from mineral gear oil to fully synthetic gear oil with the same viscosity grade at low startup temperatures.
Figure 1. The benefits of high VI (source: ExxonMobil data).
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LUBE MAGAZINE NO.129 OCTOBER 2015
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