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 ASM Handbook provides a method for verifying the required viscosity for a specific gear based on the operating velocity:
where 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 The now superseded Industrial Gear Lubrication Standards, AGMA 250.04, used center distance as the primary criterion for gear lubricant selection.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.