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Cooling and Lubrication of High-speed Helical GearsHelical gears are used to reduce the speed of centrifugal compressors and turbines to match nominal motor and generator speeds. Proper lubrication and cooling are critical to the successful operation of these gears. When gears mesh, the tooth surfaces roll and slide against each other (Figures 1 and 2).
This meshing creates enormous contact and shear stresses. Oil is used to lubricate the mating gear teeth and prevent scuffing, wear and pitting damage to gear tooth surfaces due to metal-to-metal contact. Oil viscosity increases with pressure; therefore oil can separate the mating gear teeth despite enormous stress. Oil is also used to cool the gear teeth and prevent excessive tooth temperatures and overheating of the oil. This article presents various issues associated with proper lubrication and cooling requirements of high-speed helical gears. High-speed helical gears are defined as those gears that operate pitch line velocities greater than 120 m/s. Gear Surface Distress Gear tooth distress includes scuffing, wear and surface fatigue (micropitting and macropitting). ANSI/AGMA 1010-E95 (1995) provides descriptions and pictures of these types of damage. Scuffing
According to AGMA 925-A03 (2003), “The basic mechanism of scuffing is caused by intense frictional heat generated by a combination of high sliding velocity and high contact stress.” Scuffing will normally not start at the pitch line because the sliding velocity is zero. Thus, scuffing generally starts in either the top or bottom half, or root, of the teeth. The contact temperature is equal to the flash temperature plus the tooth temperature before entering the mesh. AGMA 925-A03 (2003) states: “The flash temperature is the calculated increase in gear tooth surface temperature at a given point along the line of action resulting from the combined effects of gear tooth geometry, load, friction, velocity and material properties during operation.” Several sources can cause higher tooth temperatures at high pitch line speed. According to AGMA 925-A03 (2003), “For pitch line velocities above 80 m/s, churning loss, expulsion of oil between meshing teeth and windage loss become important heat sources that must be considered.” The other factor is how effectively the oil spray cools the teeth. When spraying the gear mesh, oil must cover the tooth surfaces for a slight time interval before being flung off. At high pitch line speeds, the teeth may be moving so fast relative to the oil velocity that not all the teeth get covered with oil. Wear Micropitting
Micropitting can lead to a reduction in gear tooth accuracy, which increases gear tooth loading, vibration and noise. Micropitting can lead to macropitting and gear tooth breakage. There is a basic lack of understanding of the mechanism of micropitting. One theory claims that micropitting starts when the asperities on gear tooth surfaces carry a significant portion of the load. These asperities then deform, which produces local residual tensile stresses. The cyclical loading is then high enough to cause local fatigue cracks that take on the form of small pits. Thus, surface roughness is a big factor in the risk of micropitting. According to AGMA 925-A03 (2003), micropitting was eliminated in some cases when the gear tooth surfaces were finished to a mirror-like finish. Micropitting can occur anywhere on the gear tooth surface. However, according to researchers Cardis and Webster, it generally starts in areas associated with high sliding velocity, which is in the bottom or top of the tooth profile, not at the pitch line where the sliding velocity is zero. Lubricants also play a key aspect in the risk of micropitting. Studies completed by Cardis and Webster have shown that micropitting is more apt to occur in gears that use oils with antiscuffing additives. Also, micropitting resistance tends to decrease with higher gear tooth temperatures, but it has been reported that other additives actually improve micropitting resistance at higher temperatures. Macropitting
Macropits typically occur if there are high asperities and metal-to-metal contact occurs between meshing teeth. However, in high-speed gears, the surface finish is typically very smooth and the oil film is thick enough to prevent metal-to-metal contact. In these cases, the cause of the macropits is generally an inclusion or small void in the material that provides the initiation point for the crack, and the subsurface shear stresses propagate the crack. Thermal Problems Gear Spray Arrangements
Some gearbox manufacturers use special baffles to reduce windage losses inside gearboxes. Air inside the gearbox is accelerated because of the high rotational speed of the gears. The energy that it takes to accelerate the air is a loss. By fitting plates close to the sides of the gears, the amount of air that is accelerated is limited and this loss is reduced. These plates can also be used as shields to prevent oil that is squeezed out of bearings from hitting the gears, causing acceleration, which would result in additional loss. Finally, other gear manufacturers use a false bottom in the gearbox. This is typically a perforated plate that is fitted between the gears and the bottom of the gearbox. It is used to help the oil drain by preventing the oil in the bottom of the gearbox from being lifted back up and reaccelerated. Special Designs One company has a patented a gearbox that operates in a vacuum and virtually eliminates windage losses. This invention has reportedly lowered the gearbox power loss by up to 50 percent in comparison to conventional gearboxes. One documented case was a comparison of two 90 MW gearboxes: one with conventional gearing and the other with the vacuum gearbox. The calculated losses in the conventional gearbox at full load were 1,407 kw, while the losses in the vacuum gearbox were measured at 628 kw. This is a difference of 779 kw, or about one percent of the rated power. Several companies also manufacture high-speed helical gears with special axial grooves (Figure 6).
These grooves provide a path for the hot air/oil mixture to escape before overheating and allow fresh, cool oil to be supplied to the gear mesh at an intermediate point in the mesh. Typically, these grooves are added on the bull gear and sometimes on the bull gear and the pinion. Although Figure 6 shows only a single groove, other designs may have more. Various high-speed gearboxes are also equipped with a special high-pressure/high-velocity-type spray arrangement that is fitted close to the gear mesh (Figure 7).
As stated above, it is important for proper cooling for the oil to cover the tooth surfaces for a slight time interval before being flung off. At high pitch line speeds, higher velocity oil may be needed to get sufficient cooling oil into the tooth spaces before being stuck by the next tooth and flung out of the gear mesh. Generally, more oil is required for cooling than for lubrication. So while there may be enough oil on the gear tooth surfaces for lubrication, there might not be enough for cooling. There are many good sources available to end users to assist in the evaluation of high-speed helical gear designs. Although lubrication and cooling problems are rare, they can be difficult to solve. When evaluating high-speed helical gears, it is important to understand the manufacturer’s experience. Lubrication and cooling are complex issues and a number of parameters must be considered. Editor’s Note: References
(BY Patrick J. Smith)
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