window motor spec sheet confusion

[andy]

I'm designing the final stage of the articulation of our arm and we have choosen to use the window motor

<http://www2.usfirst.org/2004comp/Specs/Nippon-Denso_Window_Motor_specs.doc>

However, where the sheet mentions the specs for the output gear, they list 12-14 as the diamatral pitch. Why are there two pitches??? Which one should we use when we order gears to mesh with that gear? Have any teams used one/or the other with any success? Are we allowed to remove the output plate and press on our own gear?

Thank you in advance!

Good luck this year!
-Andy

[Joel Glidden]

I'd suggest using the coupler that came in the kit or making one of your own. Here's a drawing of the coupler we made last year. We also made a similar coupler in 2002. Let me know if you have any questions.

-Joel

[kiracofe8]

Originally Posted by andy

I'm designing the final stage of the articulation of our arm and we have choosen to use the window motor

<http://www2.usfirst.org/2004comp/Specs/Nippon-Denso_Window_Motor_specs.doc>

However, where the sheet mentions the specs for the output gear, they list 12-14 as the diamatral pitch. Why are there two pitches??? Which one should we use when we order gears to mesh with that gear? Have any teams used one/or the other with any success? Are we allowed to remove the output plate and press on our own gear?

Thank you in advance!

Good luck this year!
-Andy

This one confused me for a few minutes too, until I happened to come across a little paragraph in Machinery's Handbook. The drawings label these as "Stub tooth gears". There is an old standard for stub tooth gears called the "Fellows Stub Tooth" design which uses two pitches. The first is used for determining the pitch diameter, and the second is used for determining the addendum and dedendum. The result is that you have a 12 pitch gear, but the teeth are only as high as they would be for a 14 pitch gear. We have not tried it yet, but what we are planning on doing is using a standard 12 DP gear, but increasing the center distance a little as needed. I saw this idea posted awhile ago (like maybe years ago) by somebody (Joe J. I think). The gear was slightly different back then (different number of teeth?) but it was the same stub tooth, so I think it still applies. Do a search on "Window motor gear" and I think you'll turn it up...

As far as removing the plate and pressing on a new one, I'm pretty sure that that is explicity disallowed by rule R62.

[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?

[andy]

We, unfortunately do not have the capebilities to make our own gears...

However I found a supplier for 12 and 14 pitch gears, Ohio Gear, I think.
We were gonna try one of the 12 pitch gears and see if it works.

Since they are stub tooth gears, do you think we could turn a twelve pitch gear down so the teeth are shorter? Is that possible without distroying the teeth?

Thanks again!

-Andy

[Gary Dillard]

Years ago they supplied the whole window drive assembly in the kit; this motor attaches to a hard plastic coupling that drives a flexible plastic tape through a track. It doesn't have much horsepower so I wouldn't worry about bending stresses or contact stresses with a metal gear. You can turn down the teeth; you're changing the tooth profile but it wouldn't match this gear anyway. One suggestion - if you use a brass gear, the brass will "self-work" to the appropriate profile somewhat (we've used this method meshing to the small pinions on the other motors in our drive trains for years).

[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...