Determining Machining Center Distance (vs. theoretical CD)

[ewillia13]

I am designing a gear case to house two pinions mated in series with a bull gear (BG in the middle driving both). I've seen that the center distance that is machined into the gear case is usually a few mils larger than the theoretical center distance but is there an established method for calculating the minimum or nominal difference?

I've calculated the dynamic and thermal expansion of all three gears and calculated the amount of backlash I want. For simplicity I'm going to neglect the transition period when all components are heating up and expanding at different rates. I can formulate my own method I feel comfortable with but I wanted to check if there are any suggestions from gear designers with a little more experience then me. Thanks

 

[tbuelna]

ewillia13,

The nice thing about involute geometry gears is that they are somewhat insensitive to changes in PCD. And for most applications, a little bit of excessive backlash is not a problem.

If, for example, you have a set of steel external spur gears mounted in an aluminum housing, the PCD distance would increase with increasing temperature at a rate approx. equal to the difference in CTE between the steel and aluminum. And if you wished to compensate for this higher temp operating condition, you would machine the PCD slightly smaller at RT conditions. You would also need to be careful that your gears still have adequate backlash at your gearbox's low temp start-up condition.

Backlash is usually controlled by tooth thinning, and not by adjusting PCD. And unless your gears are very large diameter, it is probably not necessary to adjust your PCD dimensions from standard.

Hope that helps.
Terry