Insight from Millers Oils: why do different gear boxes require different oils?

Different types of gears are used for different applications, and selection depends on several variables including gear ratio, load, and noise tolerance. The key types of industrial gears include spur, helical, bevel and worm, and in each type the gear teeth mesh in a different way, meaning the metal-to-metal contact and therefore lubrication requirements vary:

What is micropitting? Micropitting, also called grey staining, is a type of wear caused by metal-to-metal contact at microscopic level and leaves the metal surfaces with a ‘dull’ appearance. Although the surfaces of gear teeth may look smooth to the naked eye, microscopic inspection reveals roughness and asperities that break through the oil film – especially at high temperature and high load, and high speed and low torque. Micropitting can be avoided by selecting the correct viscosity of oil (i.e. high enough to provide a good film thickness), and by using oil with extreme pressure additives.

Figure 1: Types of gears

Selecting the right gear oil Often there will be several products that could be suitable for your application and it’s important that the gear oil should match the recommendations of the original equipment manufacturer (OEM). Key properties of gear oil include:

• Viscosity: gear oil must be thick enough to maintain the lubricating film, but not to cause drag and energy wastage.

• Oxidation stability: anti-oxidant additives boost oxidation resistance so the oil lasts longer – this is particularly important at higher operating temperatures.

• Thermal stability: synthetic oils offer superior thermal stability so the oil lasts longer, especially at higher temperatures.

• Extreme pressure (EP) performance: high loads result in high pressures that squeeze out the oil between the gear teeth - EP additives prevent wear and damage in these conditions.

• Demulsibility: gear oil should separate quickly from water, with the exception of Polyalkylene glycol (PAG) based oil.


What are the benefits of synthetic gear oil? Although modern gear oil additive technology is extremely advanced, using base oils that offer inherently better properties can reduce the additive level required, and/or further improve the performance of the gear oil.

In general, synthetic oils offer: • Better lubricity for better gear teeth protection and reduced wear, extending the life of the gears.

• Improved viscosity index, meaning better and more consistent performance across a wider range of operating temperatures.

• Higher thermal and oxidative resistance, so the oil is less likely to degrade due to high temperature or contamination, meaning fewer deposits and less sludge, as well as extended drain intervals.

• Reduced volatility, evaporation and flammability, making them safer to work with.

What’s the difference between mineral, Polyalphaolefin (PAO) and Polyalkylene glycol (PAG) based gear oils?

Mineral oil is the most commonly used base oil in gear oil. It offers good lubricity and corrosion protection to protect gear teeth against wear, and is often formulated with EP additives for extra protection at high loads.

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