Content area
Full Text
Most commonly found in mining, mineral-processing and milling applications, these unique lubricants come in a variety of types.
The May and lune Best Practices Notebook introduced information about the composition of fluid and grease lubricants, the nature of lubricant raw materials used in those products and how the materials protect machine surfaces from wear and degradation. This month we'll look at another lubricant category that exists between these two well-recognized lubricant products-semi-fluid lubricants.
In this article, semi-fluid lubricants (i.e., semi-fluid grease) will be addressed as a prelude to addressing the topic of lubricant selection for open gears, low-speed enclosed gears and bushings and machines with cases experiencing severe leakage.
Types and applications
Semi-fluid lubricants exist in a variety of configurations, each having a designed function and end-use. By their very nature these lubricants are designed to serve the reliability and machine protection requirements of heavily loaded and low-speed machines. The dominant applications driving technological development for this class of lubricants has been, and will continue to be, large mining and mineral processing/milling applications, including gears, bushings and element bearings.
Semi-fluid lubricants could fall into one of six different categories:
1. Asphaltlc oils or black oils.
2. Asphaltic cutbacks.
3. Semi-fluid greases.
4. Semi-fluid grease cutbacks.
5. Polymerized lubricant gels.
6. High viscosity synthetics.
An argument could be made to characterize the first, second and last options as fluid lubricants, given that no thickener is used to stiffen the material for application, but they remain in the semi-fluid category because these are often barely fluid at room temperature and because of the common types of applications for which these lubricants are constructed.
Asphaltics & Asphaltic cutbacks
Asphaltic lubricants are also referred to as black oils or bituminous oils. They are made from the tacky tar-like substances left over from the vacuum disdilation portion of the solvent refining of petroleum. Asphaltic materials also occur naturally in the form of asphalt lakes (sometimes incorrectly called tar pits such as the La Brea Tar Pits in Los Angeles) or oil sands. These non-volatile oil stocks can be characterized by their high molecular weight structures (hydrocarbons in excess of 25 carbon atoms and a high ratio of hydrogen atoms) and slight amounts of sulfur, nitrogen and trace metals. They...