In specialized operations such as nuclear reactors, it takes the careful application of lubrication theory to choose the right lubricant for the job. In situations with less extreme temperature and pressure conditions, making a selection based on trial and error is possible. When it comes to lubricants for nuclear reactors, there is not much room for experimentation, which is where the precise science of tribology comes into play.
Tribology – the science of lubrication
Tribology is the study of friction, lubrication and wear. It provides an accurate method for the selection of lubricants. Tribological calculations take factors such as speed, temperature, load, type of motion and operating environment into account. These parameters make up the application’s specific tribological system. Once the nature of the tribological system has been identified, the appropriate lubrication theory is selected, enabling the precise choice of lubricant. The factors are analyzed as follows:
- Type of motion: the motion may be sliding, which requires the hydrodynamic lubrication theory for analysis, or rolling, which is analyzed using the elastohydrodynamic (EHD) theory. A combination of the two is also possible and would require a specific lubricant chemistry to optimize.
- Speed: The speed of components is broken up into three broad ranges – fast, moderate and slow. Within each of these categories, the optimum attributes of the required lubricant can be calculated using the equation ndm=(ID+OD)/2, where ndm= operating speed multiplied by mean bearing diameter, ID = inside diameter and OD= outside diameter. The Stribeck curve can also be used to find the appropriate lubricant for the system’s speed.
- Temperature: Every lubricant has a specific optimum temperature range. Taking the system’s temperature into account enables the engineer to eliminate all lubricant options that do not function well within range. In the conditions of nuclear reactors, this eliminates many options, leaving only a few specially formulated lubricants for use in extreme heat.
- Load: Light loads may indicate systems that are sensitive to frictional torque, requiring a lubricant that minimizes fluid friction while protecting metal components in direct contact. Heavy loads may require lubricants with additives that protect against wear, pitting and galling.
- Operating environment: The various conditions of the operating system, such as the presence of moisture, or certain chemicals, or working in a vacuum, would all dictate the selection of specific lubricants suited to those conditions.
Tribological analysis accounts for all five of these variables to pinpoint the lubricant that will enable optimal functioning across all of them.