Gears 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.

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.

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.