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We have been asked to galvanize a large shaft which is part of a drive mechanism. Now the design engineer has stated that in order to properly size the motors, he needs to know the coefficient of friction of the zinc coating. Where can I find this information?

The coefficient of friction? You got that question out of your kids high school physics book just to try and fool Dr. Galv, didnt you? Nice Try! I cant give you information on the coefficient of friction, but I can give a value for the slip factor, which should satisfy the engineers need. The information is from W.H. Munse paper on High Strength Bolting of Galvanized Connections; which states the slip factor for a conventional galvanized coating is about 0.14, as compared to 0.35 for clean as-rolled steel. This means that the galvanized surface is more slippery than bare steel. However, as the galvanized surface weathers, or is roughened, the slip factor increases:

 Average Slip Factor
Weathered Galvanized0.20
Galvanized Wire Brushed0.31
Galvanized  Grit Blasted0.31
Bare Steel, As Rolled0.35

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I'm not sure how slip factor is helpful when you're trying to work a simple force equation that calls for the coefficient of friction. In my case I'd need the coefficient of friction to determine what minimal angle to store my zinc plated carbon steel plate to allow water droplets to flow off to prevent white rust from forming during shipping and subsequent storage. My assumptions include the water is condensate and not due to water ingress, so per the USGS these droplets range in size between 0.0001 and 0.005 centimeter in diameter. I can derive the weight of the droplet based on this assumption. Additionally, assume the zinc plating is applied per ASTM B633, SC3, Type V. To determine the minimal angle needed I'd need the coefficient of friction between the water droplet and the zinc plating. How would slip factor help me, is there some other way to get to the angle I need using slip factor? Coefficient of friction has a formula. I'm not finding how slip factor is defined or derived so I can't currently draw any relations to the two so that I can again, calculate minimal angle needed. I'm still looking...


Hi Cynthia, The slip factor, along with the pretension force, allows friction to be quantified in bolted connections. Therefore, it does not represent the static or kinetic coefficient of friction. Here at the AGA, we deal only in batch hot-dip galvanized coatings but according to Zinc Handbook by Frank C Porter, the coefficient of friction of zinc (a rolled zinc surface to a rolled zinc surface) is 0.21. We do not have values for plated zinc with or without a passivation coating.

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