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"Oil Viscosity"

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    • Oil Viscosity

    Which really has the lower viscosity: SAE 50wt motor oil or SAE 90wt gear oil?
    In the process of switching from grease to oil in the blade grips of my Safari, the question got raised because of some guidance provided in a Bearing Handbook. The manual stated that for thrust bearings under high thrust loads (sounded like my pitch bearings to me) use a HIGH viscosity oil or grease. Then later when they were talking about thrust bearings that did not rotate but just oscillated back and forth and had a tendency to fret*. (sounds like my pitch bearings again). A LOW viscosity oil was recommended. AAH. Now unless there was an oil out there that was both low and high viscosity, I was stumped.

    Looking further I discovered the attached chart. Voila!! My education continues. I had always thought (and based on their answers when I posed the above question, so did all my friends) that SAE 90wt gear oil was higher in viscosity than the SAE 50wt engine oil. Look at the chart. SAE viscosity numbers for engine oils and gear oils are obviously on different scales. Evidently, from the chart, it is possible to have a 90wt gear oil with a lower real viscosity, cSt (Centistoke) number, than a 50wt engine oil at the same temperature.

    After much head scratching, a friend called it to my attention that Frank Robinson's R-22 used ATF (Automatic Transmission Fluid) in their grips. If it was good enough for Frank, it is good enough for me. It has another benefit: It's color. If it leaks you will surely see it. I now have 50hours on the ATF filled grips and no leaks and cool grips.

    * An engineer-proof reader asked: What is Fretting? Fretting, or false Brinelling, can occur in a bearing that doesn't move but is in a vibrating environment. Unused machines, in an area where other machines are in use, can suffer bearing damage from fretting. The results look like little dents in the races, similar to those incurred thru the use of a Brinell hardness tester, hence the term "False Brinelling". Thanks to Mark Swett for the questions, as well as the proofing.

    Stu
    Oil Viscosity Cha
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    Experimental Helicopter magazine - On Line
    2011
    Last Update 8/28/2011