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.
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.
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.
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.
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.
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.
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.
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.