Lubrication of Clarifier Gear Drives - Why Oil is Preferred Over Grease

Daniel Harker, McNish Corporation
Tags: gear lubrication

Clarifier drives, used in the treatment of wastewater, are expected to provide uninterrupted service for decades. Commercial drives for the water and wastewater industry generally pass the minimum test of proper design by demonstrating that they function reliably and, if properly lubricated, do not fail prematurely. Gear manufacturers or assemblers must reduce the risk of pitting and tooth breakage by proper material selection, but can easily pass along future gear and bearing problems by improper design of a maintainable lubrication system.

The consulting engineer responsible for specifying a clarifier drive’s minimum design features can provide added assurance of a long life by specifying a well-designed lubrication system. A drive unit must pass all criteria for sound design, and the designer must also take into account the realities of 20 or more years of exposure to weather and temperature alterations when selecting the type of lubrication system for the gears and bearings. The choices involve systems that use either oil or grease as the means for lubrication.

Lubrication Methods
Oil is the preferred lubricant for gearing and is universally recognized as the most reliable lubricant type by gearing and bearing experts.1 Lubrication is a friction-reducing film, and liquid lubricants in the form of oil provide the optimum film formation.


Figure 1

Oil, either petroleum-based or synthetic, because of its fluid properties, circulates through the gearing components. The meshing action of the gear teeth forces lubricant up the face of the gear teeth and over components, providing the boundary layer film necessary for proper lubrication.

Grease formulations are usually petroleum oils thickened by soap dispersions1 and are used in situations where fluid lubricants are difficult to retain. The thickener matrix (soap) in a grease formulation immobilizes the base oil and acts as a storehouse from which the oil bleeds at a slow rate. The semifluid nature of grease solves the problem of containment for commercially available bearings and for the suppliers of gear housings that may leak, but introduces other difficult properties to the user.

Grease formulations solidify at a higher temperature than oil of the same viscosity, so at low temperatures most grease formulations exhibit a low base oil separation rate that increases the risk of starvation. When base oil viscosity is high, for example during low temperature conditions, then the base oil cannot separate out of the thickener.2 Without proper additives, grease can become virtually solid at low temperature, causing bearings operating in this medium to simply cut a channel and run without any lubrication.

Contaminant Removal
Contaminants are inevitable in any nonsealed lubrication system. Any void area in the gearing or bearing is subject to water condensation caused by daily temperature fluctuations. In reality, it is not possible to keep large turntable-type bearings such as those applied to clarifier drives 100 percent filled with grease. As a result, condensate and other contaminants become trapped in the matrix. Complete purging of grease from a large gear and bearing area is just not possible without disassembly and hand cleaning.

Contaminants such as condensation, dust or metallic particles - either airborne or from other sources - will settle in an oil bath. However, these may be easily removed by draining and flushing without disassembly of the unit. The daily accumulation of condensation is easily drained from housings that are designed as an oil bath, or such draining may even be accomplished with an automatic condensate removal system called a drain trap, or water trap. Also, maintaining the proper lubricant level is possible and made easier in gear assemblies that use oil.

 

Grease must be periodically purged or replaced. Purging requires a low-volume housing with no dead zones so new grease can displace old grease through the seals or through purge points when plugs are removed. Replacing the grease that cannot be purged demands time-consuming disassembly, cleaning and repacking by hand.

Removing debris and condensation is a concern in grease-lubricated spur gear housings because these contaminants cannot be readily flushed out of the contact zone of gear teeth or bearing elements. Oil, on the other hand, flushes particles and debris from the contact zone and allows the daily accumulation of condensate to flow to take off points.


Figure 2

Useful Life Considerations
It is not enough to reference American Gear Manufacturers Association(AGMA) and American National Standards Institute(ANSI) standards to assure a long useful life for a clarifier drive. Assuming a drive is initially designed, assembled and installed correctly, gear maintenance will ultimately determine overall life. Oil is the best lubrication for slow-moving elements of a clarifier drive. The drive manufacturer who is not confident that the gear housing will retain oil will use grease and pass a maintenance problem on to the user.

References

  1. Oberg, E., Jones, F., Horton, H. and Ryffel, H. Machinery’s Handbook, (24th ed). Industrial Press Inc. p. 2218, Lubricating Oils.
  2. “Bearing the Cold.” Motion System Design, June 2001.
  3. Drago, R. Fundamentals of Gear Design. Drive Systems Technology, Inc. Butterworths. p. 426, Lubrication.