Reliability Engineering Snapshot TM

Lubrication - Case Study No. 45: Open Gear Oil Contamination

When an oil vendor promises that their open gear lubricant can handle any environment, they mean it; but can the gear handle it too? Beware of contamination. The 10 ft. diameter spur ring gear to the left turns a large rotary dryer. The protective casing surrounding the gear was destroyed years ago. In the world of outdoor rotating equipment contaminants can get around any casing in short order and build up inside the casing. The build up continues until the gear actually picks it up and grinds it between its teeth. In the picture to the right, there is clear evidence of abrasive wear at the very tip of the gear tooth. Gear teeth should have very smooth surfaces and scratch marks only lead to further deterioration of the tooth profile. The white line indicates the pitch line. This line indicates where the relative velocity between the two teeth, as they mate, is equal to zero. Zero velocity means that for a very brief moment the load is transferred statically, not dynamically; this is known as "point contact". Any grit in this region is crushed between the two teeth.

As the two mating teeth travel further away from their pitch line, the relative velocity between the two teeth increases and the grit gets dragged across the mating surfaces; that's because the two surfaces are actually sliding across each other in this region. The greatest relative velocity between the two mating teeth is at the root and the tip of each tooth. Notice that there are scratch marks at these locations, and none as one gets closer to the pitch line (white line). The pitch line is marked on every gear, as can be seen in the top left picture. There is a scribe mark about half way down each tooth from the tip. When setting open gears in the field, the pitch line on each gear has to be as close to each other as possible. Otherwise, there will be a terrible increase in the sliding motion portion of the gear contact and you will wear out the gear in short order. The lubricant will be fine and will provide uniform film thickness, it's just that the film is full of grit, and that is what kills the gear. There is another problem going on with this gear, and this will be discussed in the "Machine Design - Case Study No. 46: Matched Gear Set Hardness" article.

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