Improper lubrication is among the leading causes of gearbox failure. Thus, it makes sense that proper lubrication plays a key role in achieving satisfactory performance and extending the life of gear (or speed) reducers. Let’s review some important points.
A big step toward success is choosing the proper lubricant from one of two basic types: mineral-based or synthetic (with the most common synthetic types being hydrocarbon polyalphaolefins [PAOs] and polyalkylene glycols [PAGs]). That choice should be predicated on a product’s characteristics—i.e., viscosity, viscosity index, pour point and additive package.
Conversely, choosing and/or using the wrong product, wrong viscosity, wrong additives, etc., are steps toward improper lubrication. Likewise, not maintaining the correct oil-fill level, operating the gearbox with dirty or contaminated oil and other poor lube practices also can be the kiss of death for your equipment.
Lubricant function in gear reducers
In all speed reducers or gear drives, friction is created between internal moving components. The primary function of the lubricant is to minimize the friction caused by the sliding and rolling action of the gears and bearings and to dissipate heat by providing a thin layer of oil between moving components.
With a typical thickness of just 0.00005 of an inch, this layer of oil, known as elastohydrodynamic (EHD) film, separates the mating surfaces of components, preventing metal-to-metal contact and minimizing wear. If the EHD film is insufficient for the transmitted load, metal-to-metal contact of the mating surfaces occurs and causes pitting of gear teeth. No EHD film—or an insufficient amount of it—can also cause scuffing of the gear teeth and leads to bearing and gear failure.
The most important properties to consider when choosing the correct lubricant for a gear reducer are viscosity, viscosity index and pour point.
Viscosity, a lubricant’s ability to resist free flowing when subjected to a force, helps determine the thickness of the oil film throughout the expected operating temperature. This resistance to free flowing is what produces the elastohydrodynamic film between the moving components.
The International Standards Organization (ISO) viscosity classification system and the Society of Automotive Engineers (SAE) grading system specify the viscosities of industrial lubricants. The ISO and SAE viscosity of a given lubricant is measured at both 40 C (100 F) and 100 C (210 F). The viscosity range of a lubricant, by ISO grading standards, is usually narrower than that used by SAE. As shown in the “Oil Viscosity Equivalency” chart above, the viscosity of SAE 90 gear oil overlaps the ISO 150 and the ISO 220 viscosity grades and partially overlaps the ISO 320 viscosity grade.
Viscosity index (VI) indicates the ability of the lubricant to resist viscosity change as the temperature of the lubricant changes. The higher the VI, the wider the operating temperature range of the oil while still being able to maintain its rated viscosity. Synthetic lubricants have a much higher VI than mineral-based products and can be used in very cold or very hot ambient conditions.
Pour point is the lowest temperature at which a lubricant will pour. Temperatures lower than the pour point will cause the lubricant to solidify. For proper lubrication, the pour point of a lubricant should be 10-15 degrees F lower than the coldest expected ambient temperature.
The lubricant chosen for a specific application should have an ISO viscosity suitable for the ambient temperature in which the reducer is expected to operate. The lubricant must be able to maintain the EHD film when subjected to the forces created by the application. Insufficient viscosity will cause metal-to-metal contact and premature wear—and even possible catastrophic failure. As the operating loads on a gear reducer increases, the reducer operating temperature and required viscosity increases.
Gear reducer maintenance
The correct amount of oil must be maintained in a gear reducer to ensure long and satisfactory performance. If the reducer operates with an insufficient amount of oil, premature gear or bearing failure can occur due to oil starvation. Over-filling the reducer, however, will create excessive churning that leads to excessive air entrapment. If this occurs, overheating will most likely result due to the reduced ability of the air and oil mixture to dissipate heat. To get the most accurate reading, the oil level should be checked with the reducer not operating and after the reducer has had sufficient time to cool.
Once the reducer is shut off, it starts to cool, and condensation begins to form on the inside of the gear case. Over time, depending on ambient conditions, the amount of water forming on the inside of the gear reducer housing can be substantial. This type of contamination can lead to bearing and gearing failure. The condensation displaces the lubricant and results in an insufficient oil film between mating components. Depending on the severity of the water contamination, the frequency of servicing the lubricant may need to be as short as 250 hours. Proper selection of desiccant filters and breathers will help minimize condensation damage.
Gear reducers operating under heavily loaded conditions at slow speeds will generate high operating temperatures. This high operating temperature decreases the useful life of the lubricant and results in more frequent lube changes. This is true for both mineral-based and synthetic lubricants. In fact, the useful life of a typical mineral-based product at 200 F can be as short as 2000 hours.
Selecting the proper lubricant and maintaining and implementing a good oil sampling and analysis program will help to determine the proper servicing intervals of your gear reducers. Changing out a lubricant more frequently than necessary will result in wasted resources and drive up maintenance cost. Not changing the oil when needed, however, will lead to premature reducer damage and possible catastrophic failure. Keep in mind that the proper selection of a lubricant and establishment of a routine PM program will result in less equipment downtime and higher productivity. LMT