Finding the PERFECT MESH

Oct. 1, 2005
Gears are compact, motion-control components that determine a driven machine's speed, torque, and direction of rotation.

Gears are compact, motion-control components that determine a driven machine's speed, torque, and direction of rotation. To turn smoothly, they require lubrication — oil — which forms a film between tooth surfaces and cushions them from the pressures they produce.

Lubrication in any gear assembly serves two purposes: to reduce friction and wear at tooth surfaces, and to carry away heat generated at the contact areas. Extremely high temperatures occur instantaneously at the lines of contact, and continuous oil flow is required to quickly dissipate this potentially damaging energy. Otherwise, gear surfaces are compromised, leading to corrosion and wear.

Lubrication decisions

Many gear-set applications operate at high internal temperatures due to design, bearing type, loads, and materials. High-operating temperatures, however, tend to thin lubricants, resulting in an insufficient oil film between meshing gears. One way to combat this is to use a heavy-bodied synthetic lubricant, which also suits gears operating at high ambient temperatures. Those operating at low ambient temperatures require oils with low viscosity in order to flow more easily.

Gears operating at high speeds also need reduced-viscosity lubricants for adequate cooling and minimum tooth friction. By contrast, gear teeth have more contact time at low speeds, requiring heavier, more viscous lubricants.

The load or pressure on gear teeth also affects which viscosity is most appropriate. Heavily loaded gears require greater viscosity and more adhesive-type lubricants. These high-viscosity lubricants make a thick oil film that cushions tooth impact.

Because of the heavy sliding and rubbing that occurs in worm and hypoid gearing, lubrication containing extreme-pressure (EP) additives is suggested. EP additives modify rubbing surfaces to prevent welding of high spots or galling (destructive pitting) from inadequate oil film strength.

Systems exposed to water and other contaminants need lubricants that readily separate from the added material. Oxide formation on gears — due to heat, air, and moisture — is prevented and removed by sufficiently coating the metal with oil.

Method determines type

How gears are lubricated during assembly determines what type of lubrication to employ. Gears should have oil-tight housings that keep contaminants out and retain oil. Typical lubrication methods include circulating systems, bath/splash systems, idler/immersion systems, and intermittent lubrication systems.

Circulating systems are the most versatile, running well in adverse and continuous-operation environments. Here's how they work: A pump delivers the lubricant by spray or flood near or at the point of tooth mesh. The pump operates with a separate drive or connects directly (internally or externally) to the gears being lubricated. This system is fitting for gears turning at normal speed. However, because a comparatively low-viscosity lubricant is needed, an oil-tight housing is required to prevent leakage and increase pumping ease. Depending on the gear assembly's service and location, oil heaters or coolers may be utilized with these circulating systems.

In a sump or bath system, gears are mounted so they dip into a pool of oil that sits at the bottom of an enclosed gearbox. The bath/splash also requires an oil-tight gear housing. Oil is transferred from the dipping gear to contacting gears; excess oil returns to the sump or is thrown against the housing and guided by troughs into bearings.

Speed has a significant influence on this kind of system. At lower speeds, the entire tooth mesh is usually covered in oil. However, at speeds above 500 fpm much of the system's oil is splashed around, coating the gears, housing interior, and bearings. This means the sump's oil level drops. In these high-speed situations it's important that the oil level still reach between one-third and one-half of the largest gear's mesh (on helical and spur sets) or the underslung worm (in worm sets.) For speeds above 2,500 fpm, a lighter-grade oil reduces heat generation caused by lubricant churning.

Similar to bath/splash systems, idler systems pick up lubricant from the sump and deliver it to the gear teeth. In this system, one gear is included in the train solely to dip into the oil and distribute it to other gears transmitting power. This system is best for slow-turning gear assemblies.

In intermittent lubrication systems, a paddle, brush, drip, or hand pour applies heavy viscous lubricants to the gears. The method pertains to heavily loaded gears running at low speed as well as gears not enclosed in an oil-tight housing.

Lubrication viscosity: Critical to smooth operation

An oil's load-bearing and lubricating abilities depend on its viscosity. Abnormal viscosity indicates problems: increases signify oxidation or contamination with higher-grade oil, while decreases point to contamination with a solvent, fuel, or lower-grade oil.

The temperature at which oil is expected to operate influences lubricant selection. For instance, a high-operating temperature may lower the viscosity well below spec. Lubricants with high Vis are desirable for broad operating temperatures.

The viscosity of a liquid without a polymeric additive is independent of shear rate — the speed at which adjacent layers of fluid move with respect to each other, usually expressed as reciprocal seconds. However, oils with shear-rate additives, such as multi-grade engine oils, can temporarily lose viscosity. This is due to an increasing shear rate in lubricated machine elements where the large polymer molecules momentarily align in the direction of flow. These oils suffer permanent-viscosity loss in service when the large polymers split by shear stress into smaller molecules with less thickening power. In other words, oils with shear-rate additives break down during use, resulting in viscosity out of spec and increased friction, heat, and wear.

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