It’s a fact of life that bearings wear out and will eventually fail at some point. One factor impacting on bearing life is external environmental conditions. Even with good maintenance strategies put into place, premature bearing failure will happen when excessive environmental contamination occurs.
It’s well known in the industry that over-greasing can cause more bearing failures than not having enough grease. However, when you have limited control over external environmental conditions, such as in a mine, a common maintenance strategy is to pack bearings with as much grease as possible (upwards of 95% of the bearing cavity). The grease acts as a wall preventing excessive contamination from entering the bearing. Nevertheless, this doesn’t work 100% of the time and eventually the contamination does interact with the grease around the seals. As the bearing rotates, some of the contamination is carried into the bearing cavity where it collects.
Real World Example
Let’s look at the experience of our customer, a large potash mining company. The amount of contamination (i.e. salt / potash) their equipment experiences is very high. Aside from the usual problems with hard, discreet contaminants causing abrasion (which happens everywhere), salt is extremely corrosive, so it destroys bearings much faster than normal.
The mine’s solution to this known failure mode is to fill up the bearing with grease as much as possible, and then keep flushing out the old grease with new grease to keep the overall potash contamination levels reasonably low. If new grease is added too quickly, or excessive pressures are used when adding the grease (i.e. in order to push out the old grease), the result is blown seals which then allows even more contamination to enter the system. The opposite problem is that if new grease is added too slowly, or lower pressures are used to do this, then the old grease stays in the system too long and causes corrosion and abrasion. Neither situation is good, but getting the right balance is tricky.
Bearing Condition Monitoring
Most mines that operate in severe environmental conditions (like this customer) are used to dealing with these issues. They will do condition-monitoring on their bearings, including such things as thermography and vibration analysis. Thermography allows them to know if the bearing temperatures are changing. If the bearing is getting too hot or cold, the bearing may have too little or too much grease, preventing easy rotation. Vibration analysis tells them similar information, but with more precision around the alignment, balance, and rotation of the bearing. If a bearing is making excessive noise, it is more likely to fail prematurely unless the problem is corrected or lubricated properly.
Bearing Grease Analysis
Which brings us to grease analysis. Relatively few companies are doing grease analysis on either a problem basis or a routine basis. However, this tool adds some relevant information to the discovery process. For starters, this allows users to compare the relative amounts of contamination and wear in multiple bearings that are all subjected to the same environmental conditions. If you use the failed bearing as a baseline, and compare the other bearings against that, then it becomes easier to understand which other bearings are at high risk of failure.
Going back to our mining customer, they experienced a bearing failure which triggered an investigation into other bearings on the same unit as well as some other similar bearings nearby to inspect their condition. The customer had Fluid Life conduct Advanced Grease Analysis which includes Analytical Ferrography. The failed bearing (Figure 1) showed the highest level of wear materials/iron out of the 5 total bearings sampled with lots of red flags and unusual additive concentrations. The bearing on the opposite side of the shaft (Figure 2) showed much lower levels of wear and a fairly healthy condition compared to its counterpart, indicating that this bearing was fine to keep running and not worry about a replacement at this time.
Another bearing (Figure 3) also showed very high wear particles but had not yet failed. They used this result to flag this bearing for a flush to see if they can keep it going as well as order a spare for future replacement.
All 16 bearings from this location in the plant will receive regular quarterly grease analysis to start making changeout and flush decisions based on hard data instead of waiting for them to fail. The Ferrography graphs were a great way to visually compare the different bearings and set a baseline for future comparison.
Additional benefits of grease analysis
Aside from the benchmarking benefits, grease analysis provides insights into the condition of the grease itself. If customers are filling up the bearing and pushing new grease through all the time, then they should expect the grease condition to always be in relatively good shape, except for the build up of contaminants. However, if the grease is not being pushed through sufficiently well, then the grease may be degrading, and this may require a change to the greasing procedures. For example, if the consistency of the grease became thinner, it may indicate some shearing of the product. If the grease consistency became thicker over time, it may indicate some separation of the grease and thickener, leading to grease bleed out. Either one of these conditions will accelerate wear and is easily correlated with other condition-based technologies, such as thermography or vibration analysis.
Performing grease analysis on a routine or as-needed basis provides useful information about the condition of the grease and bearing, while gauging the effectiveness of your maintenance strategy. If ever in doubt about the type of grease intended for your application, contact your OEM and lubricant supplier. Contact us if you’d like to set up a routine or triggered grease analysis program.