# gearbox design question

Never designed a gear box before. My question is (assuming andymark gears) how do you determine the correct inter-axle distance for a pair of gears?
Bruce

Try this,

http://www.team1323.com/resources/manafacturing/team1323/all_in_one_gear_calculator.html

Btw, Andy uses 20 DP gears, just enter tooth count after that.

The functional diameter for center-to-center calculations is the pitch diameter. This diameter is located in roughly the middle of the height of the gear teeth. It is often given as a dimensional property of the gear, or can be calculated from other properties such as the pitch, no. of teeth, diametral pitch, etc.

For a gear pair, add the pitch radii (half the pitch dia.) of both gears to find the center-to-center distance.

Andy-Mark supplies the diametral pitch (they call it the “diametric” pitch) for their gears, which is the number of teeth per inch of pitch diameter. Divide the number of teeth by the diametral pitch to find the pitch diameter:

(Number of teeth)/(Diametral Pitch) = (50 teeth)/20 = 2.5 inches

Pitch radius = 2.5/2 = 1.25 inches

A summary of spur gear properties here: http://shopswarf.orconhosting.net.nz/spur.html

All gears (and sprockets) have a pitch diameter, and all gears should mesh at this pitch diameter.

http://www.fi.edu/time/Journey/Time/Lessons/pitchgear.gif

To find the pitch diameter for gears, take the total number of teeth on the gear and divide by the diametrical pitch (DP). For example, the 50t gear in an AndyMark Toughbox gearbox has a pitch diameter of 2.5". (50 teeth / 20 DP) = 2.5 inch

To find the theoretical center-to-center (CTC) distance between two gears, divide the two pitch diameters by half (to get the radius) and add together. For an AndyMark 50:14 reduction in a Toughbox, this is ((14/20) / 2) + ((50/20) / 2) = 1.6".

But in reality, you should add a small value onto this theoretical CTC distance; I usually add 0.003" to all calculated CTC values. This serves two purposes: 1) it adds in a small buffer to account for manufacturing tolerances, and 2) it’s better for gears to “run loose” than it is to “run tight”.

Ditto the .003" number. On carefully machined gearboxes where I’ve used this tolerance, I’ve never had an issue with binding or wear.

-Brando