Gear Center to Center question (and discrepancy)

Technical Technical Discussion
1

Hello.

I was looking at designing a custom gearbox and came across the WCP center to center distance calculator. After playing with a couple values, I wanted to check if they were accurate. I downloaded the cad model for the VexPro 2 CIM ball shifter and discovered a problem; the distances calculated by the WCP calculator were significantly larger than those on the shifter. Namely, the distance between 40-44 gears should be 2.1020 inches. The shifter, upon inspection in solidworks, shows only a distance of 1.9882 inches center to center.

Could someone please explain this discrepancy? I fear that it may be me simply not using one of the two tools incorrectly, so I’d love if someone could clear up the issue.

Thanks,
Hades

http://www.wcproducts.net/how-to-gears/

2

Could you be measuring point to point or shortest distance instead of center to center in SW? Sometimes SW decides to switch the measuring tool to closest or point to point measurements randomly, when what us FRC folk want 99% of the time is center to center (for holes, etc).

The nominal value for the 40-44 20 DP gearset is 2.1000". Whether or not you add in the .002" adder depends where you’re using the gears in your design, and the quality of the gears.

3

I heard 0.001" is fine for clearance, but it really depends on who’s machining it and how good you tolerance it.
The formula I use for gearboxes is (T1 + T2)/(P*2),
where T1 is the first gear teeth, T2 is the second gear teeth, and P is the pitch (usually 20 for FRC). Then I add 0.002" because I manual mill gearbox plates. Next year we’re sending them out though.
Make sure that your cim mounting is really good. I add 0.002" for the cim gear to the first gear, because the cim does not press fit into the plate like a bearing. Rather, it is a very close fit with about 0.002" of clearance (diameter).

4

Totally agree with Roger, Solidworks sometimes gets funky on the measure tool. But just verified real quick:

http://puu.sh/90hqe/ecce375103.jpg

and

Let me know if you have any other questions

5

Thanks everyone for the speedy replies. I must have just been doing something wrong with SolidWorks then.

Now that I know that the WCP calculator is correct (not that there should’ve been much doubt, I suppose), I’ve created this spreadsheet with values for the center to center distances with various gears sold by WCP/VexPro: http://www.chiefdelphi.com/media/papers/3036

Thanks again!