A bump test, unlike most other CM methods, requires that the machine be switched off. An accelerometer is placed on the part of the structure suspected of causing significant resonant frequencies. The most likely sources of these will involve parts (such as fan guards, thin panels and pipe work) that “ring” for a long time after being hit.
The structure is repeatedly but gently hit; during these impacts a measurement from the accelerometer records the responding ring. Its frequency content then is compared with norms.
By identifying a shift in the natural frequency, bump tests can help detect mechanical faults such as cracking in metallic components. (Cracked or poorly bonded structures will exhibit less stiffness, resulting in a change in natural frequency.) The test also can identify weak or unstable structures.
Lubricants represent vital sources of information ready to be unlocked and evaluated as part of a CM program. Results enable operators to confirm use of the proper lubricant, prevent potential over- or under-lubrication, track lubricant use and waste, raise flags about quality (including inorganic contamination, debris from wear or lubricant degradation), and contribute to the desired cleanliness and optimized performance of machines and systems.
Lubricant analysis can satisfy two primary objectives: detecting a problem and diagnosing its source. Many lubricant suppliers often provide basic lubricant analysis as an added-value service for using their lubricants. However, the analysis only may confirm that the lubricant meets specifications and offer little information regarding machinery health. For this reason, one of the first steps in establishing an analysis program for lubricants is to identify the lubricant testing technology employed to make analytical assessments.
While laboratory analysis of lubricants can play an important role in managing machinery assets more effectively, the good news is that not all testing has to be performed in a laboratory. Many of the important characteristics of working lubricants can be examined visually or with the aid of very simple tools.
For example, you can check clarity and water contamination with a standing sample. A magnet drawn up the side of a glass jar containing lubricant diluted with a solvent can detect ferrous materials (filings and metal dust). A bull’s eye sight glass can show flow and discoloration. Simple in-plant tools enable viscosity monitoring. These are good day-to-day observations.
On a broader and more in-depth scale, you should routinely evaluate several critical machine and lubricant parameters including machinery wear particles, contamination, and lubricant or additive degradation.
Truly meaningful lubricant analysis programs encompass testing a wide range of parameters using a variety of methods. Some of the more common test areas are:
- Color and appearance. Regularly check these characteristics. For oils too dark for effective appraisal, reduce the volume of oil to a constant depth for proper observation.
- Viscosity. Oils found to be outside specification always are considered abnormal. However, a change within a grade also can be a sign of trouble. Watch for changes of 10% from new oil.
- Base number. Compare the alkalinity values (base number) of the used diesel engine oil to new oil. As a general rule, change oil when the alkalinity value of the used oil is 50% of the new oil.
- Acid number. Acidity varies in new unused lubricating oils based on the concentration of antiwear (AW), antiscuff (EP) or rust additives. Increases above the new oil reference indicate oil degradation. Lubricants having additives such as zinc dithiophosphate and EP generally exhibit higher acidity than those containing only rust and oxidation additives.
- Emulsion. Water separability testing is primarily used to evaluate steam turbine, hydraulic and circulating oils susceptible to high water contamination.
- Foam. In systems where foam is perceived to be a problem, perform a foam test to confirm whether the lube oil is the source. If the oil isn’t the problem, turn your attention to other influencing parameters (mechanical or operational) to resolve the issue.
Tools aren’t enough