Lubrication oils - Gear Lubrication

                                              Gear Lubrication

 

        Gear lubricant must possess the following  characteristics:

               

The following characteristics are applicable to all gear lubricants. The lubrication requirements for specific gears are over and above this general requirements:

(a)Viscosity and Pour point

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.

 Gear oils should have a minimum pour point of 5 0C lower than the lowest expected temperature. The pour point for mineral gear oil is typically -7 0C  When lower pour points are required, synthetic gear oils with pour points of -40 0C may be necessary.

 (b) Film strength. Good film strength helps prevent metal contact and scoring between the gear teeth.

(c) Lubricity (oiliness). Lubricity is necessary to reduce friction.

(d) Adhesion. Helps prevent loss of lubrication due to throw-off associated with gravity or centrifugal force especially at high speeds.

(e) Gear speed. 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.

(f) Temperature. Ambient and operating temperatures also determine the selection of gear lubricants.

Normal gear oil operating temperature ranges from 50 to 55 0C 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.

           In addition to the general requirements, lubrication for open gears must meet the following requirements:

(g) Drip resistance. Prevents loss of lubricant, especially at high temperatures which reduce viscosity.

(h) Brittle resistance. Lubricant must be capable of resisting embrittlement, especially at very low temperatures.

   In addition to the general requirements, lubrication for enclosed gears must meet the following requirements:

(i) 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.

(j) 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.

                                      Types of gear lubricants

(1)       Oil.

(a) Rust and oxidation oils. These petroleum-based oils are frequently referred to as RO gear oils.

RO oils are the most common gear lubricants and have been formulated to include chemical additives that enhance their performance qualities. RO lubricating oils have easy application properties for gear and bearings, good lubrication qualities, and adequate cooling qualities and they are economical to use.

Disadvantages include restriction to enclosed gear applications to prevent contamination.

(b) Compounded gear lubricants. These oils are a blend of petroleum-based oils with 3 to 10 percent fatty or synthetic fatty oils. They are particularly useful in worm gear drives. Except as noted in the AGMA applicable specifications, compounded oils should comply with the same specifications as RO oils.

(c) Extreme pressure lubricants. These gear lubricants, commonly referred to as EP lubricants, are petroleum-based and specially formulated to include chemical additives such as sulfur-phosphorus or other similar materials capable of producing a film that provides extreme pressure and anti-scuffing protection.

(d) Synthetic oils. Synthetic oils have the advantage of stable application over wide temperature range, good oxidation stability at high temperatures, high viscosity indices, and low volatility. Because gear oils must be changed periodically, the main disadvantage of synthetics is high cost, which can only be justified for applications at high temperature extremes where other lubricants fail. Another disadvantage of synthetics is possible incompatibility with seals and other lubricants.. Gear units should be flushed of all mineral oils before the filling with the final synthetic oil.

(e) Residual compounds. These are higher-viscosity straight mineral or EP lubricants that are mixed with a diluent to facilitate application. Viscosities range from 400 to 2000 mm2 /s at 100˚C without diluent. Once applied, the diluent evaporates and leaves a heavy residual lubricant coating on the treated surface.

                   (2) Special compounds and greases.

These lubricants include special greases formulated for boundary lubricating conditions such as low-speed, low-load applications where high film strength is required. These lubricants usually contain a base oil, a thickener, and a solid lubricant such as molybdenum disulfide (MoS2) or graphite. The gear manufacturer should be consulted before using grease. The primary disadvantage of using grease is that it accumulates foreign particles such as metal, dirt, and other loose materials that can cause significant damage if adequate maintenance is not provided. Grease also has a tendency to be squeezed out of the gear tooth meshing zone, and it does not provide satisfactory cooling.

                   (3) Open-gear lubricants.

Open-gear lubricants are generally reserved for slow-speed low-load boundary lubricating conditions. Due to the open configuration, the lubricants must be viscous and adhesive to resist being thrown off the gear teeth surfaces. The disadvantages of these lubricants are similar to those noted above for grease.

                   (4) Solid lubricants.

The solid lubricants most commonly used in gear trains are molybdenum disulfide, graphite, polytetrafluoroethylene (PTFE), and tungsten disulfide (WoS2). Because they are expensive to apply, use of these lubricants is reserved for special applications such as high and low temperature extremes where other lubricants fail to perform adequately.

                             Applications in various gearing systems

(a) Spur and helical gears. Spur and helical gears usually require mineral oils with RO inhibitors.

high-speed, low-load gear units : Low-viscosity RO oils, such as turbine oils, mineral oils without ant scuff/extreme pressure agents can be used successfully, provided the oil viscosity is capable of maintaining the required film thickness. However, low speed gears are usually heavily loaded so ant scuff/extreme pressure agents are necessary to ensure adequate protection..

(b) Hypoid gears. Hypoid gears combine the rolling action and high tooth pressure of spiral bevel gears with the sliding action of worm gears. These severe operating conditions result in high load, high sliding speeds, and high friction. Therefore, hypoid gears are very susceptible to scuffing. Mineral oils for this application must have high lubricity and high concentrations of anti-scuffing/extreme pressure additives.

(c) Worm gears. Worm gears operate under high sliding velocity and moderate loads. The sliding action produces friction that produces higher operating temperatures than those that occur in other gear sets. Normal operating temperature for worm gears may rise to 93˚C. Lubricants for worm gears must resist the thinning due to high temperatures and the wiping effect of sliding action, and they must provide adequate cooling. Mineral oils compounded with lubricity additives are recommended.

Extreme pressure additives are usually not required for worm gears. When EP protection is required, the additive should be selected with a caution to prevent damaging the bronze worm wheel.

(d) Gear combinations. Many applications use different gears in the same gear housing. For these applications the lubricant must be suitable for the gears with the most demanding requirements. The other gears will operate satisfactorily with such high-performance lubricants.

(e) Gear shaft bearings. Gear shaft bearings are frequently lubricated by gear oil. In most instances this condition is acceptable. Bearings in high-speed, low-load applications may operate satisfactorily with the gear oil. However, low-speed, heavily loaded gears usually require a heavy oil. For these conditions a low-viscosity EP oil may provide adequate lubrication for the gears and bearings. The low-viscosity oil will adequately lubricate the bearings while the EP additive will protect the gear teeth from the effects of using low-viscosity oil.