Oil, grease, or dry: What is the best lubricant for my gear train?   

When designing any gear application, you must account for the effects of friction and take measures to minimize its impact on the performance of the gear system.

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Have you ever been driving down the road and the check engine light suddenly comes on? Sometimes the oil service light might come on too. With your automobile, it is tribal knowledge that 3,000 miles or three months is a good indicator that you should change the oil in your car. With today’s synthetic oils and computerized engines, you have more flexibility but the old adage holds true. If you change the oil on a regular basis, your vehicle will operate with fewer headaches. Part of this longevity is due to the physical changing of the oil; the other part is that while you are changing oil, you have time to inspect other parts of the vehicle. This inspection allows you to identify other issues with the vehicle before they become significant problems. Just like your vehicle, it is important to regularly check the lubricant in your gear mechanism.

Gears require different types of lubricant depending on the operating conditions. A simple hand-crank operation may not require any lubrication at all. However, applications that involve high speed or high torque must include a suitable lubrication method if the system is to run for any reasonable amount of time.

The reason lubrication is needed in a gear system is to counteract the effects of friction. As a gear interacts with another gear, the motion is transmitted via surface contact. The interaction of the surfaces on each gear creates friction. The friction in turn creates heat and it is the heat of the interaction that causes premature failure of the gears. When using metal gears, they can act as a heat sink and absorb some of the heat, pulling it away from the mesh. However, their ability to dissipate the heat is limited. A good lubricant will minimize the friction and will also cool the gear at the same time.

There are three gear lubrication methods in general use: grease lubrication, splash lubrication (also known as the oil bath method), and forced oil circulation lubrication.

There is no single best lubricant and method. Choice depends upon the tangential speed and rotating speed. At low speeds, grease lubrication is a good choice. For medium and high speeds, splash lubrication and forced oil circulation lubrication are more appropriate, however there are exceptions. For example, some systems have maintenance reasons for which a grease lubricant is used even with high speeds. Tables 1a and 1b present lubricants, methods, and their applicable ranges of speed.

Table 1a: Ranges of tangential speeds (m/s) for spur and bevel gears.
Table 1b: Ranges of sliding speeds (m/s) for worm gears.

Grease lubrication can be applied in low speed/low load applications; however, it is important to apply grease periodically, especially for gears used in open environments. Since lubricants diminish or become depleted in the long term, periodically changing the oil or refilling is necessary. Use of lubricants under improper conditions will cause damage to gear teeth. When using gears at high speed/heavy load, or when using easily worn gears such as worms or screw gears, care should be taken in selecting the right type of lubricant; quantity and methods. The proper selection of lubricant is especially important.

Grease lubrication is suitable for any gear system that is open or enclosed, so long as it runs at low speed. There are three major points regarding grease. The first is that you must choose a lubricant with suitable cone penetration. A lubricant with good fluidity is especially effective in an enclosed system. The second is that grease lubrication is not suitable for use under high load and continuous operation. This is due to the cooling effect of grease not being as good as lubricating oil. As such, using grease may become a problem as the temperature can rise under high load and continuous operating conditions. The third issue is the proper quantity of grease, as there must be sufficient grease to do the job. However, too much grease can be harmful, particularly in an enclosed system. Excess grease will cause agitation, viscous drag, and result in power loss.

Splash lubrication is used with an enclosed system. The rotating gears splash lubricant onto the gear system. Also known as oil bath lubrication, it requires a gear tangential speed of at least 3 meters/second to be effective. However, splash lubrication has several problems, two of them being oil level and temperature limitation. There will be excess agitation loss if the oil level is too high. On the other hand, there will not be effective lubrication or ability to cool the gears if the level is too low. Table 2 shows guidelines for proper oil level. Also, the oil level must be monitored during operation as the oil level will drop from when the gears are stationary and once the gears are in motion. This problem may be countered by raising the level of lubricant in an oil when the gears are at rest. The temperature of a gear system may rise because of friction losses due to gears, bearings, and lubricant agitation. Rising temperature may cause one or more of the following problems:

  • Lower viscosity of the lubricant.
  • Accelerated degradation of the lubricant.
  • Deformation of the housing, gears and shafts.
  • Decreased backlash.
Table 2: Adequate oil level.

New high-performance lubricants can withstand up to 80°C-90°C℃. These temperatures should be regarded as the limits of this method. If the lubricant’s temperature is expected to exceed these limits, cooling fins should be added to the gear box, or a cooling fan should be incorporated into the system.

Forced oil circulation lubrication applies lubricant to the point of contact of the tooth mesh by means of an oil pump. There are drop, spray, and oil mist methods of application. For the drop method, an oil pump is used to suck-up the lubricant and then directly drop it on the contact area of the gears. The spray method uses an oil pump to spray the lubricant directly on the contact area of the gears. For the oil mist method, a lubricant is mixed with compressed air to form an oil mist that is sprayed into the contact area of the gears. This method is particularly suitable for high-speed gearing. An oil tank, an oil pump, an oil filter, piping, and other devices are needed in order to use a forced oil lubrication system. As such, it is typically used only for specialty high-speed and large gear box applications. By filtering and cooling the circulating lubricant, the right viscosity and cleanliness can be maintained. This method is considered to be the best way to lubricate gears.

Gears can also be run dry. Typically gears that are run dry are plastic gears that have self-lubricating properties. Some plastics also have a lubricant such as molybdenum disulfide impregnated into them. Some surface finishes such as Polytetrafluoroethylene which allow them to run dry in many environments.

When designing any gear application, it is a requirement that you account for the effects of friction and take measures to minimize its impact on the performance of the gear system. Just like your vehicle, take the time to maintain the lubricant in your gear system and it will last a long time. 

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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.