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Gears Lubrication

Energy is transmitted from a power source to a terminal point, through gears that change speeds, directions, and torque. Gear lubricants are formulated and applied to prevent premature component failure, assure reliable operation, reduce operating cost, and increase service life. The important objectives accomplished by these lubricants include: reduction of friction and wear, corrosion prevention, reduction of operating noise, improvement in heat transfer, and removal of foreign or wear particles from the critical contact areas of the gear tooth surfaces.

Gears vary greatly in their design and in their lubrication requirements. Proper lubrication is important to prevent premature wear of gear tooth surfaces. When selecting a lubricant for any gear application the following issues must be considered: type and materials of gear, operating conditions, including rolling or sliding speed, type of steady load and temperature, method of lubricant application, environment and type of service. Enclosed gears -- those encased in an oil-tight housing -- usually require an oil with various additives, depending on the operating conditions. Rust, oxidation, and foam inhibitors are common. Extreme pressure (EP) additives are also used when loads are severe.

Worm gears are special because the action between the worm and the mating bull gear is sliding rather than the rolling action common in most gears. The sliding action allows fluid film lubrication to take place. Another significant difference is that worm gears are usually made of dissimilar materials, which reduces the chance of galling and reduces friction. EP additives usually are not required for worm gears and may actually be detrimental to a bronze worm gear. Lubrication can be improved by oiliness additives.

In open gear applications, the lubricant must resist being thrown off by centrifugal force or being scraped off by the action of the gear teeth. A highly adhesive lubricant is required for most open gear applications. Most open gear lubricants are heavy oils, asphalt-based compounds, or soft greases. Depending on the service conditions, oxidation inhibitors or EP additives may be added. Caution must be exercised when using adhesive lubricants because they may attract and retain dust and dirt, which can act as abrasives. To minimize damage, gears should be periodically cleaned.

Gear Wear, Failure and Lubrication

The most critical function provided by lubricants is to minimize friction and wear to extend equipment service life. Gear failures can be traced to mechanical problems or lubricant failure. Lubricant-related failures are usually traced to contamination, oil film collapse, additive depletion and use of improper lubricant for the application. The most common failures are due to particle contamination of the lubricant. Dust particles are highly abrasive and can penetrate through the oil film, causing “plowing” wear or ridging on metal surfaces. Water contamination can cause rust on working surfaces of the gears and eventually destroy metal integrity. To prevent premature failure, gear selection requires careful consideration of the following: gear tooth geometry, tooth action, tooth pressures, construction materials and surface characteristics, lubricant characteristics and operating environment.

Gear Pitting and Lubrication

Pitting occurs when fatigue cracks are initiated on the tooth surface or just below the surface. Usually pits are the result of surface cracks caused by metal-to-metal contact of asperities or defects due to low lubricant film thickness. High-speed gears with smooth surfaces and good film thickness may experience pitting due to subsurface cracks. These cracks may start at inclusions in the gear materials, which act as stress concentrators, and propagate below and parallel to the tooth surface. Pits are formed when these cracks break through the tooth surface and cause material separation. When several pits join, a larger pit (or spall) is formed. Another suspected cause of pitting is hydrogen embrittlement of metal due to water contamination of the lubricant. Pitting can also be caused by foreign particle contamination of lubricant. These particles create surface stress concentration points that reduce lubricant film thickness and promote pitting. The following guidelines should be observed to minimize the onset of pitting in gear units:

  • Reduce contact stresses through load reduction or by optimizing gear geometry.
  • Steel should be properly heat-treated to high hardness. Carburizing is preferable.
  • Gear teeth should have smooth surfaces produced by grinding or honing.
  • Use proper quantities of cool, clean and dry lubricant with the required viscosity.

Gear Lubricant Characteristics and Viscosity
Gear lubricant must possess the following characteristics:

Good viscosity is essential to ensure cushioning and quiet operation. An oil viscosity that is too high will result in excess friction and degradation of oil properties associated with high oil operating temperature. In cold climates gear lubricants should flow easily at low temperature. Gear oils should have a minimum pour point of 5° C (9° F) lower than the lowest expected temperature. The pour point for mineral gear oil is typically -7° C (20° F). When lower pour points are required synthetic gear oils with pour points of -40° C (-40° F) may be necessary. The following equation from the provides a method for verifying the required viscosity for a specific gear based on the operating velocity:

V40 =

7000

V0.5

Where:

V40 = lubricant kinematic viscosity at 40° C (105° F) (cSt)
V = pitch line velocity (ft/min) given by :
V = 0.262nd

n is the pinion speed in rev/min and d is the pitch diameter (inches).
Film strength: Good film strength helps prevent metal contact and scoring between the gear teeth.
Lubricity (oiliness): Lubricity is necessary to reduce friction.
Adhesion. Helps prevent loss of lubrication due to throw-off associated with gravity or centrifugal force especially at high speeds.

Gear speed: The now superseded Industrial Gear Lubrication Standards, AGMA 250.04, used center distance as the primary criterion for gear lubricant selection. The new version of this standard, designated AGMA 9005-D94 Industrial Gear Lubrication, has adopted pitch line velocity as the primary selection criterion. As noted above, gear speed is a factor in the selection of proper oil viscosity. The pitch line velocity determines the contact time between gear teeth. High velocities are generally associated with light loads and very short contact times. For these applications, low-viscosity oils are usually adequate. In contrast, low speeds are associated with high loads and long contact times. These conditions require higher-viscosity oils. EP additives may be required if the loads are very high.

Temperature: Ambient and operating temperatures also determine the selection of gear lubricants. Normal gear oil operating temperature ranges from 50 to 55° C (90 to 100 °F) above ambient. Oils operating at high temperature require good viscosity and high resistance to oxidation and foaming. Caution should be exercised whenever abnormally high temperatures are experienced. High operating temperatures are indicative of oils that are too viscous for the application, excess oil in the housing, or an overloaded condition. All of these conditions should be investigated to determine the cause and correct the condition. Oil for gears operating at low ambient temperatures must be able to flow easily and provide adequate viscosity. Therefore these gear oils must possess high viscosity indices and low pour points.

Open gears: In addition to the general requirements, lubrication for open gears must meet the following requirements:
Drip resistance: Prevents loss of lubricant especially at high temperatures which reduce viscosity.
Brittle resistance: Lubricant must be capable of resisting embrittlement especially at very low temperatures.

Enclosed gears: In addition to the general requirements, lubrication for enclosed gears must meet the following requirements:
Chemical stability and oxidation resistance: Prevents thickening and formation of varnish or sludge. This requirement is especially significant in high-speed gears because the oil is subjected to high operating oil and air temperatures.
Extreme pressure protection: Provides additional galling and welding protection for heavily loaded gears when the lubricant film thickness fails. Extreme pressure lubricants are available for mild and severe (hypoid) lubricant applications.

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