Time for an oil change

There are specific requirements and lots of choices – what is the right lubricant for your gear train?

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When you have to change the oil in your automobile, the specification for the type of oil is clearly detailed in the service manual and, in many cases, it is also stamped on the oil fill cap. For gear trains, the selection of viscosity, additives, and other properties is set by the operating conditions.

Gears require different types of lubricant depending on their operating conditions. A simple hand crack operation may not require any lubrication at all. Other applications that involve high speed, or high torque, might require an oil or grease lubricant. 

In order for any lubricant to work properly in a gear system, an oil film must be formed at the contact surface of the teeth in order to minimize friction and to prevent dry metal-to-metal contact. When deciding on a lubricant you need to consider a solution that has the properties listed in Table 1.

Table 1: The properties that a lubricant should possess.

A key factor in selecting the best lubricant for your application is to determine the optimum viscosity of the lubricant. As detailed in Table 2, the viscosity grades of industrial lubricants are regulated in the standard JIS K 2001.

Table 2: ISO viscosity grade of industrial lubricant (per JIS K 2001).

There are some gear oils that are designed for industrial applications and others that are designed for automotive applications. Table 3 is created from the data in standard JIS K 2219-1993: Gear Oils Standards. It shows the various viscosity grades and their suitable industrial applications.

Table 3: Types of gear oils and their usage.

As there are many options available, it is most practical to select a lubricant by following the information in a catalog, a technical manual, or from the web site of a gear oil manufacturer, as well as following the JIS, JGMA, and AGMA standards. Table 4 details the proper viscosity for enclosed gears, as recommended by several gear oil manufacturers.

able 4: Recommended viscosity for enclosed gears.

After making a choice about which grade of viscosity to select, taking into consideration the usage (for spur gear, worm gear pair, etc.) and usage conditions (dimensions of mechanical equipment, ambient temperature, etc.), then you can choose the appropriate lubricant. Table 5 details several options.

Table 5: List of several industrial oils from representative oil manufacturers.

Worm gears require a different lubricant than other gearing as the material properties of worm gear pairs typically differ from that of other gearing. In most cases, the worm wheel is made from bronze or brass. If an oil with an EP additive is used, the active sulfur in some EP additives can react with the copper within the brass or bronze. Under the presence of heat, the sulfur, when in contact with copper, forms copper sulfide. This chemical reaction can have devastating effects on the gearing. When an EP oil is present in extreme pressure situations, copper disulfide can be formed. Both of these crystalline forms of copper are very hard and will cause abrasive damage to soft tooth surfaces of brass and bronze worm wheels. Table 6 details the reference values for proper viscosity gear oil that are recommended in accordance with the strength calculations per JGMA405-01(1976). Table 7 details some of the representative lubricants suitable for use in  worm gear pair applications.

Table 6: Reference viscosity for worm gear lubrication.
Table 7: Example of worm gear oils.

In addition to gear oils, there are also gear greases. Dependent on the application, a grease may be better suited than an oil. Greases are sorted into seven categories that are further segmented by components and properties, and by consistency numbers. Table 8 details the four categories of grease that are suitable for gearing. These details are drawn from the standard JIS K 2220:2003 Lubricating greases. Table 9 lists grease products from representative manufacturers.

Table 8: Grease types.
Table 9: List of representative greases from various manufacturers.

When designing any gear application, it is imperative to account for the effects of friction and take measures to use an appropriate lubricant in the gear system. As one gear interacts with another gear, the motion is transmitted via surface contact. The interaction of these surfaces on each gear creates friction. The friction in turn creates heat and it is the heat of their interaction that causes premature failure of the gears. A good lubricant will minimize the friction and will allow for the gear to perform as designed.

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is general manager of KHK USA Inc, a subsidiary of Kohara Gear Industry with a 24-year history of working in the industrial automation industry. He is skilled in assisting engineers with the selection of power-transmission components for use in industrial equipment and automation. Dengel is a member of PTDA and designated as an intern engineer by the state of New York. He is a graduate of Hofstra University with a Bachelor’s of Science in Structural Engineering.