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Best Practices for Analyzing Helical Gear Failures(2014-03-25)

Appearance Test
The simplest test is visual appearance. Often this test will disclose problems such as gross contamination or oxidation. Look at the lubricant in a clean, clear bottle. A tall, narrow vessel is best. Compare the sample to a sample of new, unused lubricant. The oil should look clear and bright. If the sample looks hazy or cloudy, or has a milky appearance, there might be water present. The color should be similar to the new oil sample. A darkened color might indicate oxidation or contamination with fine wear particles. Tilt the bottle and observe whether the used oil appears more or less viscous than the new oil. A change in viscosity might indicate oxidation or contamination. Look for sediment at the bottom of the bottle. If any is present, run a sedimentation test.

Sedimentation Test

If any sediment is visible during the appearance test, a simple test for contamination can be performed on-site. Place a sample of oil in a clean, white cup made from a non-porous material that is compatible with the lubricant. Cover and allow it to stand for two days. Carefully pour off all but a few milliliters of oil. If any particles are visible at the bottom of the cup, contaminants are present. Resolution of the unaided eye is about 40 microns. If the particles respond to a magnet under the cup, iron or magnetite wear fragments are present. If they don’t respond to the magnet and feel gritty between the fingers, they are probably sand. If another liquid phase is visible or the oil appears milky, water is likely present.

Odor Test
Carefully sniff the oil sample. Compare the smell of the used oil sample with that of new oil. The used sample should smell the same as new oil. Oils that have oxidized have a “burnt” odor or smell acrid, sour or pungent.

Crackle Test
If the presence of water is suspected in an oil sample, a simple on-site test can be performed. Place a small drop of oil onto a hot plate at 135 degrees C. If the sample bubbles, water is above 0.05 percent. If the sample bubbles and crackles, water is above 0.1 percent. When carrying out the crackle test, the inspector’s health and safety must be taken into consideration by wearing eye protection, for example.