The Right Lubricant Will Lengthen the Life of Your Drive
By Jorge Iduate, Director of Engineering
Gear and Bearing Assemblies
Many people wonder whether oil or grease lubrication is best for main gear and bearing assemblies. Jack L. Dillon Jr., assistant director of utilities for the City of Fort Smith, AR, recently faced this decision in specifying a replacement drive.
"I do not have preference as to whether the ball bearings are being lubricated with grease or oil lubricated," Dillon said. "However, our maintenance personnel have expressed displeasure with oil lubrication because they have to drain the condensation and change the oil."
Principles of Lubrication, which is published by Exxon, defines grease as "petroleum oils thickened with special soaps that give them an unusual ability to stay in place. It is often used in applications for which it is not practical to provide a continuous supply of oil." But for longer bearing and gear life, DBS believes that a continuous supply of oil is needed.
For one thing, bearings and gears require regular re-lubrication because of high exposure to contaminants in the air. However, grease cannot be changed easily, and simply adding more grease will not necessarily purge debris and deteriorated grease from the bearing and/or gear system. The presence of such debris and the displacement of grease between the gear mesh can cause excessive gear wear or failures in drive units, which are normally installed in fairly hostile environments. So if, for example, maintenance personnel performed sandblasting operations near a drive unit with grease lubrication, the unit could end up needing a complete overhaul of the bearings and gears.
Grease doesn't entirely eliminate problems with water, either. Condensation still occurs in the housing -- the water simply gets trapped between the steel surfaces and the grease. Oil, on the other hand, is fluid in nature and since drive units operate very slowly, at clarifier speeds, water and oil will separate. The water will then collect in the bottom of the housing where it can easily be removed.
The bottom line? Since debris and water can be easily removed using oil bath lubrication for the main gear and bearing, and since oil provides continuous lubrication, DBS believes this system is the better alternative.
Using proper lubricants is an important factor in maximizing the life of the components that comprise a DBS drive unit. While this article provides general information and guidelines about lubrication, recommendations from the operation and maintenance manual provided with each drive should be followed. You can contact DBS directly for answers to questions about specific lubricants or applications. The majority of DBS drive units consist of three separate speed reducers that must be lubricated: a primary speed reducer, planetary gearbox, and main gear housing. There are four subsets primary speed reducers: types E, F, H, and W.
The E and W type primary speed reducers both consist of a parallel helical gearbox; some E and W type drives require double parallel helical gearboxes. These gearboxes are factory-filled with an NLGI grade 00 semi-fluid grease formulated with a synthetic base fluid and extreme pressure additives. When replacing this lubricant, the NLGI grade must be matched; grades 1 and 2 do not possess the semi-fluid property that is necessary. The ISO viscosity of the base oil in the grease is also important, as higher viscosity will provide more film thickness between meshing gear surfaces. An ISO viscosity of 460 is ideal for the base oil. Due to the long drain intervals for these gearboxes, greases with synthetic base oils such as Mobilith SHC 007 and Castrol SHL 00 are worthwhile investments due to their excellent extended performance over wide temperature ranges.
The other two primary speed reducer types, F and H, are hydraulic systems. The selected hydraulic oil should have rust and oxidation inhibitors as well as anti-wear additives. For most ambient temperatures, a hydraulic oil with an ISO viscosity of 32 is appropriate. For startup temperatures below 20 degrees Fahrenheit, lower viscosity oil or oil heaters may be required. There are many oils meeting the ISO 32 viscosity that should be avoided because they do not contain anti-wear additives that protect the hydraulic pump and motor in the high pressure F and H type systems.
The planetary gearbox is factory-filled with an NLGI grade 00 semi-fluid grease with extreme pressure additives. Lubricant changes are not required on a regular basis for the planetary gearbox; it is essentially lubricated for life.
The main gear housing is the most expensive and difficult to access component in the drive unit, so proper lubrication is a concern in spite of the fact that the integrated gear bearing is typically oversized and able to handle years of abuse. The recommended lubricant for the common temperature range of 30 to 125 degrees Fahrenheit is an ISO 680 gear oil with extreme pressure (EP) additives. The difficulty in properly lubricating the main gear arises from its extremely slow pitch velocity. In most applications, the pitch velocity is too slow for hydrodynamic lubrication to be possible; boundary lubrication is inevitable because of limits on the available viscosity of gear oils. Because of this, the EP additives in the gear oil play a key role in preventing metal to metal contact between meshing gear teeth.
One problem associated with the high viscosity ISO 680 gear oil is its lower temperature limit. Gear oils should not be used at less than 10 degrees above their pour points; this limits ISO 680 gear oils made from mineral oil to about 30 degrees Fahrenheit. The temperature limit can be overcome by using a synthetic ISO 680 gear oil or switching to a lower viscosity mineral based gear oil during winter months. Oil changes should be performed on a regular basis to replenish the much needed EP additives in the gear oil. Oil changes will also remove any water that has condensed into the gear housing. Note that automotive gear oils should be avoided because some do not have the ability to separate condensation from the oil.
Special care must be taken when selecting lubricants used in water treatment plants or other applications requiring food grade approval. Though options are somewhat limited, there are available oils and greases with NSF H1 approval for all speed reducers.
To summarize, remember the following two key points. (1) Always choose the proper grease grade and/or oil viscosity for the gearbox and temperature. (2) Always verify the relevant features of the lubricant such as extreme pressure, anti wear, or minimum pour point.