# How to make gears mesh properly.

Let’s say you have a gear with a pitch diameter of 1" and then you have another one with a pitch diameter of 2". Would you place these gears exactly 1.5" apart? Or would you leave a few thousandths for backlash? Is a little backlash already accounted for in gear sizes? If you must account for it yourself, how do you know how much more to put them apart? Thanks

You should generally mesh the gears using the pitch diameter, so 1.5" would be correct. You do not want to go any closer than this for sure, so if you are worried about tolerancing you could make them a little bit further apart. Also, if it is a large gear ratio, then the tips of the gears will interfere, and two teeth or more will touch at a given time. This causes the gears to be loud and wear each other down. So you might want to add a couple thousandths. I suspect the real engineers will be able to provide better advice, which I am curious to hear since I’m designing a gearbox for RoboCup as well. I’ll post how to actually calculate what distance to use if I can successfully interpret my “Machine Component Design” textbook.

• Patrick

I think for calculating the distance, there are three #'s that should be summed to get your center distance. The outer radius of the first gear, the inner radius of the second gear, and the working depth of the gears that are being used. The working depth takes into account that gears do not mesh all the way to the bottom, adding an additional thousandth or two will give you a good center distance #. I am not positive on this though…

-Rc

Good question. Keeping tight tolerances on center-distance spacing when designing gearboxes is a good thing to do. In your case, it is OK to specify 1.500" distance if the fabricator keeps a tolerance of +/- 0.001".

Another way to dimension it would be to call out 4 places, at 1.5000". Traditionally, the tolerance for this dimension would be +/- 0.0005".

I know that there are other engineers who say to add 0.002-0.003" to the center distance. That will work also, as it is a more conservative way to design. It will add a bit of backlash (not sure how much).

It really depends on the level of quality and tolerance your fabricator can handle. If they are very good, go with a four-place tolerance. If they are just OK, then add a few thousanths for backlash.

Good luck,
Andy B.

The see Andy’s anwser – I have not yet read it, but I know Andy and I suppose you will be fine with what he recommends.

Joe J.

P.S. The addendum and dedendum of standard (i.e. off the shelf) gears is typically 1/DP and 1.2-1.25/DP This means the standard pitch dia is NOT the average of the Root Dia and the O.D. For the record, In FIRST, I almost always add a few thousandths to the theoretical center distance. extra backlash will almost never kill you in FIRST. Jamming the gears together because the gears are a bit out of round will eat up your power like opening the frig on a hot summer day.

One of our machinists showed me a trick. You can put a piece of ordinary notebook paper between the two gears and then mash them together as tightly as you can. You can then measure the center distace with your calipers. In my experience it is 0.002 to 0.003 inches more than the pitch diameters would indicate. The result are gears that spin freely with almost no backlash. Basically what Joe J. said.

Some teams I’ve talked to use the pitch circle spacing and then wear them in with a few hours of operation. I think I remember someone saying they let them run over night.

Does anyone have any direction as to where i can find some decent 39 tooth gears, and about how long they will take to deliver. Thank you all for your help and enjoy the 6 weeks this season brings. I look forward to seeing all who plan on attending the UTC New England Regional!!!

What pitch are you looking for?