window motor spec sheet confusion

[kiracofe8]

Originally Posted by Raul

Here is something that adds to the confusion: The spec says it has a pitch diameter of 20.2184mm. This equates to .796 in. So if you do the standard calc for gears (NT/ PD = DP) you get: 9 / .796 = 11.3 Diametral Pitch! Huh!

So, what is going on here. I guess we should ignore the 20.2184 number?

I cannot confirm this without looking at the prints of the gear that this pinion is supposed to mate with, but I'm guessing that this pitch diameter is the operating pitch diameter, instead of the generating pitch diameter. The operating pitch diameter (and diametral pitch, and pressure angle) are equal to the generating pitch diameter (and diametral pitch, and pressure angle) if and only if the two gears operate at the standard center distance. If they operate at a different center distance, the effective pitch diameter changes. You can find an explanation of this in any standard machine design textbook.

Usually, gear prints show the generating specifications, but I have know several companies to put the operating specifications on their prints (helicopter transmissions, for example, are sometimes specified this way).

So, unless anyone can come up with further evidence, I would ignore that pitch diameter...