Gears are used in all aspects of life, from bicycles to tiny watch gears, car transmissions, and even highly specialized surgical equipment. Gears keep the world moving. However, when they move, they often rub against each other, and if this friction is not managed, it can cause wear and eventually lead to significant damage or failure. This is where gear oil makes a difference.
In this article, we will explore the various types of gear lubricants, their composition, how they degrade, some storage and handling tips, and what the future holds for these types of oils.

If you’re familiar with gears, you know that despite the standard emoji keyboard, more than one type of gear exists. There are several types of gears, each suited for various applications. As such, each application will have varying environmental conditions, which will require specialized lubricants to reduce friction and wear.
One of the main operational conditions for gears is the transfer of torque. Even when torque is transferred, gears will have sliding and rolling contact, leading to frictional losses and heat generation. Therefore, the lubricants selected for these applications must be able to significantly reduce these frictional losses and cool the gears.
As per (Pirro, Webster, & Daschner, 2016), several types of gears can be classed into three groups based on the interaction of the teeth of these gears and the types of fluid films formed between the areas of contact:
- Spur, Bevel, Helical, Herringbone, and spiral bevel
- Worm gears and
- Hypoid gears
Figure 1 shows some of the types of gears which exist.
It must be noted that hypoid gears transmit motion between nonintersecting shafts at a right angle. Additionally, there is a difference between rolling and sliding.
Rolling indicates continuous movement, whereas sliding varies from a maximum velocity in one direction at the start of the mesh through zero velocity at the pitch line and then back to maximum velocity in the opposite direction at the end of the mesh, as seen in Figure 2.
According to Mang, Bobzin, and Bartels (Industrial Tribology—Tribosystems, Friction, Wear and Surface Engineering, Lubrication, 2011), hypoid gears require heavily loaded lubricants. These should have high oxidation stability, good scuffing, scoring, and wear capacity, as the tooth contacts have a high load.
The lubricant must also have a high viscosity at operating temperature such that the formed film can sufficiently support the load while cooling the gears.
Conversely, hydrodynamic gears such as torque converters, hydrodynamic wet clutches, or retarders require high oxidation stability characteristics but do not need good scuffing or scoring load capacity characteristics. Unlike hypoid gears, hydrodynamic gears experience viscosity-dependent losses, so they must have a lower viscosity at operating temperature.

According to (Mang & Dresel, Lubricants and Lubrication – Second Edition, 2007), there are some frequent failure criteria for gears and transmissions, including:
- Extreme abrasive wear
- Early endurance failure, fatigue of components in the form of micropitting and pitting
- Scuffing and scoring of the friction contact areas
Continuous abrasive wear is usually observed at low circumferential speeds and during mixed and boundary lubrication. Typically, continued wear can cause damage that extends to the middle sector of the tooth flank. Understandably, lubricants with a high viscosity and a balanced quantity of antiwear additives promote a higher tolerance to wear.
Micropitting can be observed on tooth flanks at all speed ranges. Those with rough surfaces are prime candidates for micropitting. Typically, this develops in negative sliding velocities or the slip area below the pitch circle.
Usually, microscopic, minor fatigue fractures occur first, which can lead to further follow-up damage such as pitting, wear, or even tooth fractures. A lubricant with a sufficiently high viscosity and a suitable additive system can help reduce this type of fatigue.
At predominantly high or medium circumferential speeds, scuffing and scoring of the tooth flanks occur, and the contacting surfaces can weld together for a short time. Due to the high sliding velocity, this weld usually breaks, causing scuffing and scoring.
Typically, this damage is seen on the corresponding flank areas at the tooth tip and root, which experience high sliding velocity. In this case, lubricants with higher EP (Extreme Pressure) additives can help reduce this damage.
According to (Ludwig Jr & McGuire, March 2019), the type of gear can aid in determining the most appropriate industrial gear oil. The following table is an adaptation from the article:

As per (Mang & Dresel, Lubricants and Lubrication – Second Edition, 2007), transmission gears can be broken down into two main types: those with a constant gear ratio and those with a variable gear ratio. These can be seen in Figures 3 and 4 below.

