Fundamentals of Wear Debris Analysis
When a machine wears, material from the contacting surfaces enters the lubricant. The forces, material and geometry of the wear site all influence the size, shape and quantity of the final wear product. The wear debris carried away in the lubricant can be considered to be the ‘signature of the wear process. This signature can be read by wear debris analysis techniques to identify the mode of wear, its severity, assess the machines health and help form a prognosis.
There are some fundamental features of wear and wear debris that must be considered when using wear debris analysis techniques:
The size distribution of wear particles generated by different wear mechanism and available for detection/analysis, varies with the wear mechanism (Figure 1).
Figure 1. (Click to enlarge)
As the wear particles are carried by the lubricant, other mechanisms, settling, grinding, filtration, etc, influence the final quantity and size distribution of the wear particle population remaining for analysis.
Due to settling effects, oil usually contains only recently generated wear products. Consequently, elemental analysis (SOA) of an oil sample generally only provides a ‘snapshot in time of the wear process. However, for some machinery problems the multi element SOA analysis can provide excellent trends.
In contrast, very little particle settling if any occurs with grease; it accumulates wear particles and so can provides a wear history for the assembly. Grease analysis is excellent for very slow speed machinery that would demand complex vibration analysis techniques. For over ten years grease analysis has provided 100% fault targeting of defective bearings in the low speed gearing of Sugar Cane Crushing Mills.
Wear mechanisms that produce small wear particles (scuffing, micropitting, etc.), generally progress steadily, whereas the fatigue wear mechanisms, which produce larger and fewer particles, are often only release particles in random bursts.
Once a defect has developed, the wear particles generated can act as an abrasive or become broken up in other contacts. This secondary wear usually aids fault detection by introducing a broader range of particle sizes (Figure 1). It has also been shown that the dynamic energy (vibration) generated by a developing fault can increase the wear rate of other wear processes elsewhere in a machine.
Figure 2.(Click to enlarge)
The wear debris monitoring technique chosen will have its own detection size range limitations (Figure 2).
As a general rule, the capture efficiency of the wear debris analysis technique employed should increase with the size of wear particle it is being used to monitor (see Figure 3).
Figure 3. (Click to enlarge)