Proper lubrication is critical for achieving satisfactory performance of gear reducers. Mineral and synthetic are two basic types of lubricants used in these products. Synthetic lubricants fall into two categories: hydrocarbon (PAO) and polyglycol (PAG). Lubricants can also be classified as biodegradable or food grade. Each type of lubricant has its own specific characteristics, including viscosity, viscosity index, pour point and additives. Improper lubrication is among the leading causes of gearbox failure. This includes choosing the incorrect viscosity, not maintaining the correct oil-fill level, improper type of lubricant and operating the gearbox with dirty or contaminated oil.
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, by providing a thin layer of oil between the moving components. This film of oil, called the elastohydrodynamic film, actually separates the mating surfaces of components. There’s no metal-to-metal contact and this minimizes wear and prevents corrosion. The most important properties to consider are the viscosity, viscosity index and pour point.
The viscosity of a lubricant, which is its ability to resist flowing when subjected to a force, helps determine the thickness of the oil film. The viscosity index indicates the ability of the lubricant to resist viscosity change as the temperature of the lubricant changes. The higher the viscosity index, the wider the operating temperature range of the oil, while still maintaining its rated viscosity.
Another important property of a lubricant is its pour point. The 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 degrees Fahrenheit lower than the coldest expected ambient temperature.
The lubricant chosen for a specific application should have an ISO viscosity that will match the ambient temperature in which the speed reducer is expected to operate. The lubricant must be able to maintain the elastohydrodynamic oil 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. The higher the operating temperature of the speed reducer, the greater the viscosity requirement will be.
Speed reducer maintenance
The correct amount of oil must be maintained in a speed 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 unit has had sufficient time to cool.
On new applications, the reducer should be operated under normal loads for approximately two weeks. During this “break-in" time, fine particles of metal will be removed from the mating surfaces of the internal components and gears. After the two-week, break-in period, the oil should be drained and the reducer flushed to remove all metal particles. The drain plug must also be cleaned prior to re-installation.
The story of oil life
As a speed reducer operates, the lubricant begins to break down and oxidize. This process continues and the oil begins to form sludge and varnish deposits along with acids. The acids can attack yellow metals, such as bronze gears or bearing cages. Sludge formation prevents effective lubrication and interferes with the elastohydrodynamic oil film and can cause metal-to-metal contact between moving components.
That’s why speed reducers, like other types of equipment, should have a preventive maintenance (PM) schedule for changing the lubricant. The schedule is dependant on the local conditions in which the speed reducer operates and on the type of oil selected. The lubricant should be changed every 2,500 hours under normal operating conditions. In extremely harsh environments, the lubricant must be changed more frequently.
In helical or spur-gear speed reducers, the operating hours between oil changes can be extended with the use of synthetic lubricants. Typically, when very cold or hot ambient conditions exist, synthetic oil should be selected. An added benefit of synthetic oil is that it tends to reduce the overall operating temperature of the reducer. For worm-type speed reducers, polyglycol synthetics are extremely effective in reducing sliding friction and extending the operating hours before a lubricant change is required.
Extreme-pressure (EP) additives, such as graphite or sulfur-phosphorus, are highly effective in reducing friction. These types of lubricants, however, shouldn’t generally be used with internal backstops or brakes that rely on friction to operate correctly. The manufacturer of the speed reducer should be contacted if there’s any doubt about the suitability of EP lubricants.
In addition to decreased oil life, excessive operating temperatures can lead to increased lubricant contamination. The surface temperature of a heavily-loaded speed reducer can reach over 200 degrees Fahrenheit. Once the reducer is shut off, it starts to cool down and condensation begins to form on the inside of the gear case. Over a period of time, depending on the ambient conditions, the amount of water forming on the inside of the housing can be substantial.
This type of contamination can lead to bearing and gearing failure. The condensation displaces the lubricant and results in a thin 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 filters and breathers will help minimize condensation damage.
Selecting the proper lubricant and maintaining and implementing a good oil sampling/analysis program will help to determine the proper servicing intervals. Changing the oil more frequently than necessary will result in wasted resources and drive up maintenance costs. Not changing the oil when needed will lead to premature reducer damage and possible catastrophic failure.
Always remember that the proper selection of a lubricant and establishment of a routine PM program will result in less equipment downtime and higher productivity.
Don Miller is application engineering manager with Dodge Gearing, Rockwell Automation. For more information call (905) 792-1739 or visit www.dodge-pt.com