Q & A

Lock-Step Trends

“I am responsible for oil analysis on our fleet of ore haul trucks. We are taking regular samples from the wheel motors, which always come back with high iron (550 ppm), high copper (165 ppm) and high aluminum (115 ppm). I believe the wear is caused by dirt because we routinely find high silicon levels (400 ppm), but neither our oil supplier or wheel motor OEM can explain the high aluminum wear since there are no aluminum components in the wheel motor. Can you explain what is going on?”

In oil analysis, one of the keys to diagnosing the root problems is to look at “lock-step trending” of different parameters. For example, in your case, it appears that every time silicon increases, iron also increases indicating that the iron wear is caused by the presence of silicon, likely from dirt ingression. This direct correlation between silicon and iron is called a lock step trend.

Often a more complete root cause diagnosis can be made by trending not just major elements, but minor elements, which may be present; this is the key to understanding your problem. Dirt is predominantly comprised of two minerals, silica and alumina. These two minerals are typically present in dirt and dust in the approximate ratio 3.4:1, with silica being the major component and alumina the minor component. Spectrometric analysis of oil samples that contain high levels of dirt and dust typically reflect this ratio of silica to alumina in the ppm of silicon and aluminum measured.

In your case, the ratio of silicon to aluminum is 3.5:1, indicating that the aluminum is not in this instance a wear metal, but simply a minor, trace component of dirt.

See an example of Lock-Step Trends.

Today's Tips

• Here are some steps to consider when flushing a new hydraulic system:

  • Flush with light rust and oxidation inhibited oil, operate system at maximum flow-rate to generate turbulent flow, check filters, refill with operating charge, reach operating temperature, exercise activators, drain and flush non-circulating zones.

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Book Bits

"Most mineral-oil-based greases (of adequate dropping point) will operate successfully to about 250 degrees F (121 degrees C). A smaller number can handle 300 degrees F (149 degrees C). A few mineral-oil-based greases can operate to about 350 degrees F (177 degrees C). Around this temperature, synthetic fluids are preferred or required. As service temperature rises, frequency of lubricant addition and relubrication must increase."

For more information about "The Lubricating Grease Guide" go to:

http://www.noria.com/secure/product_detail.asp?catalogid=14

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