Types Of Wear
Abrasive wear particles are most commonly the result of dust or dirt in the oil. The dirt particles become wedged between two moving parts, embed in the softer surface, and cut into the harder one. The wear debris from this process appears to be miniature shavings from a machining operation.
Abrasive wear particles can be several hundred microns long. Hard metals tend to form smaller abrasive particles that may have a needle-like appearance.
The primary corrective action for abrasion is to filter the oil to remove the contaminants. It is also important to minimise the ingress of contaminants, especially dust.
Fatigue wear is the result of repeated cyclic loading of surfaces with compression and shear or compression and tension. This is most common in industrial bearings and gears.
The repeated loading of the same point on a gear or bearing causes micro-cracks to form and become interconnected. When the cracks intersect surfaces, spall occurs and flakes or chunks are released into the oil. These particles are commonly 10 to 30 microns at first and later grow to be 100 microns or more. Fatigue is often from one of the following root causes:
- Improper assembly
- Other conditions which concentrate loading in a non-uniform distribution.
Corrosion and Lubrication Degradation
Corrosive problems are caused by water or other corrosive process media in oil, such as natural gas or sulphur.
Corrosion is especially a problem in refineries and crude oil processing facilities. Corrosive wear in industrial machinery is normally caused by contamination of the oil by water or other corrosive fluid.
Corrosive wear in engines can also be caused by degraded oil. Oxidation is a common way that oil gets degraded. Oxidation is caused when hydrocarbon oil molecules chemically react with oxygen from combustion gases, the atmosphere or moisture.
Long term high temperatures cause rapid oxidation. Measure the change in the dielectric constant, Total Acid Number or Fourier Transform Infra-red (FT-IR) to give an indication of when to change the oil.
Also, look at the colour of the oil. If it is degraded, then it will be very dark in colour (brown to black). Keep in mind that it may be dark and still be perfectly good, but if it is bad due to oxidation or other chemical deterioration it should also be very dark. Dielectric increase of 0.1 usually means its time to change the oil.
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