The purpose of lubrication is is to minimize friction and wear to extend equipment service life.. To provide extreme temperature and pressure protection in order to prevent wear, pitting, spalling, scoring, scuffing and other types of damage that result in equipment failure. Other important purpose for lubrication is to protect against thermal degradation, rust and corrosion to equipment.
Methods of Lubrication
There are generally three type of gear lubrication.
- Grease lubrication
- Splash lubrication
- Spray lubrication
Grease Lubrication (0 to 6 m/s tangential gear speed )
Grease lubrication is suitable for any gear system that is open or enclosed, so long as it runs at low speed. The grease should have a suitable viscosity with good fluidity especially in a enclosed gear unit. Grease is not suitable for high loads and continuous operation and there is virtually not cooling effect. The must be sufficient grease to ensure the gear teeth are lubricated but an excess can result in viscous drag and power losses.
Splash Lubrication(4 to 15 m/s tangential gear speed )
Splash lubrication is the normal method for lubricating spur, helical, bevel and worm gears. The gears simply dip into a bath of oil as the rotate. Splash lubrication needs at least 3 m/s tangential speed gear speed to be effective. It is important that provisions are made to ensure the teeth are not immersed in the bath such that excessive losses result from the oil being churned up. The oil level should be monitored under static and dynamic conditions to ensure it is correct for the application
Spray Lubrication (above 12 m/s tangential gear speed )
For the higher speed units (10 to 20 m/s peripheral speed) engineered spray lubrication is genally provided using shaped nozzles with oil at a circulated pressure of about 0.7 barg. At higher speeds the system for directing the oil at the teeth needs to be carefully engineering to ensure the oil actually reaches the contacting surfaces as centrifugal forces and escaping air flow will tend to deflect the oil jet.
Types of Gears
Gear Design Dictates Lube Design
Gear designs vary depending on the requirements for rotation speed, degree of gear reduction and torque loading. Transmissions commonly use spur gears, while hypoid gear designs are usually employed as the main gearing in differentials. Common gear types include:
Spur (straight cut) gears are widely used in parallel shaft applications, such as transmissions, due to their low cost and high efficiency. The design allows for the entire gear tooth to make contact with the tooth face at the same instant. As a result, this type of gearing tends to be subjected to high shock loading and uneven motion. Design limitations include excessive noise and a significant amount of backlash during high-speed operation.
Bevel gears (straight and spiral cut) transmit motion between shafts that are at an angle to each other. Primarily found in various types of industrial equipment as well as some automotive applications (differentials), they offer efficient operation and are easy to manufacture. As with spur gears, they are limited due to their noisy operation at high speeds, and are not the top choice where load carrying capacity is a requirement.
Worm gear sets employ a specially-machined “worm” that conforms to the arc of the driven gear. This type of design increases torque throughput, improves accuracy and extends operating life. Primarily used to transmit power through non-intersecting shafts, this style of gear is frequently found in gear reduction boxes as they offer quiet operation and high ratios (as high as 100:1). Downfalls with this type of gear set are its efficiency, high price per HP and low ratios (5:1 minimum).
Hypoid gear sets are a form of bevel gears, but offer improved efficiency and higher ratios over traditional straight bevel gears. Commonly found in axle differentials, hypoid gears are used to transmit power from the driveline to the axle shafts.
Planetary gear sets, such as those found in automatic transmissions, provide the different gear ratios needed to propel a vehicle in the desired direction at the correct speed. Gear teeth remain in constant mesh, which allows for gear changes to be made without engaging or disengaging the gears, as is required in a manual transmission. Instead, clutches and bands are used to either hold or release different members of the gear set to get the proper direction of rotation and/or gear ratio.
Helical gears differ from spur gears in that their teeth are not parallel to the shaft axis; they are cut in a helix or angle around the gear axis. During rotation, parts of several teeth may be in mesh at the same time, which reduces some of the loading characteristics of the standard spur gear. However, this style of gearing can produce thrust forces parallel to the axis of the gear shaft. To minimize the effects, two helical gears with teeth opposite each other are utilized, which helps to cancel the thrust out during operation.
Herringbone gears are an improvement over the double helical gear design. Both right and left hand cuts are used on the same gear blank, which cancels out any thrust forces. Herringbone gears are capable of transmitting large amounts of horsepower and are frequently used in power transmission systems.
The differences in gear design create the need for significantly different lubrication designs. For instance, gears normally seen in automotive differentials are hypoid gears and require GL-5 concentration and performance of extreme pressure additives.
“This is because of the spiral sliding action that hypoid gears have,” said Dinwiddie.
Most manual transmissions have helical gears and do not require GL-5 performance.
“The helical gear is almost a straight cut gear, but on an angle,” said Dinwiddie. “There is spiral action and very little sliding action, hence there is less need for extreme pressure additives.”
Gl-4 gear lubes have half of the extreme pressure additives of GL-5 lubes.
Gear oil ,a fluid lubricant made specifically for transmissions, transfer cases, and differentials in automobiles. It is formulated with specially processed base oil and sulphur phosphorus additives to ensure exceptional chemical and thermal stability. It is used in gears (gearboxes) for reduction of friction and wear of the gear tooth surfaces, removal of the heat generated by the operating gear and corrosion protection of the gear parts. The high viscosity ensures transfer of lubricant throughout the gear train.
Most lubricants for manual gearboxes and differentials are hypoid gear oils. These contain extreme pressure (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears.
American Petroleum Institute (API) established a performance grading system for gear oils (mostly automotive gear oils). According to the system gear oils are designated by the letters GL (Gear Lubricant) followed by a number 1,2,3,4 or 5:
- GL-1-GL-1 gear oil has rust and oxidation protection effect but it does not contain extra pressure (EP) additives. the oil is used in low load applications only.
- GL-2-GL-2 gear oil contain more additives than GL-1, but without EP effect. It is used in medium loaded worm gears.
- GL-3-GL-3 gear oil possesses light EP effect. It is used in non-hypoid gears.
- GL-4-GL-4 gear oil possesses moderate EP effect. It is most widely used oil.
- GL-5-GL-5 gear oil possesses high EP effect. It is used in hypoid and other highly loaded gears.