Gear Experts Only
 

Last week I posted a question and got no response so I'm going to try one last time. So now that I've got you attention:

Does anybody know what the tooth profile is on the final gear in the Fisher-Price gearbox?. The best I can measure the pitch is 14. But what is the pressure angle? It looks to be more than 20deg.

The first person with a correct response will recieve, absolutely free, a custom encoder gear to use as you see fit. Correctness will be determined by the mesh of a fabricated gear with the FP gear. Partial credit if you can tell me how to figure out what the pressure angle is with a pair of calipers and not much else.

ChrisH

Re: Gear Experts Only
 

Chris,

I seriously doubt that it is a 14 DP. It is almost certainly 20 deg pressure angle.

What is the OD? To first order, the OD is (2+Nteeth)/DP Does this match up.

I confess that I don't have the gear in hand so I can only advise.

From talking to the folks at Fisher Price (several years ago, when I was explaining FIRST and begging them to donate motors/gearboxes) I am fairly sure it is an English gear, that it is not modified (enlarged) and that it is a standard 20 degree pressure angle gear. But... ...time passes, perhaps they have gone more exotic designs.

I wish I had more for you.

Joe J.

Re: Gear Experts Only
 

Dr Joe

I don't have one here with me either and I left my notes at home. The driving gear has 17 teeth and the pitch diameter is 1.2" + or - .030", with the tools at hand I couldn't get finer than that. I figured that out by sliding the gears around to where the mating surface was directly between the gear centers and measuring with a caliper to the estimated contact point. Since the gear has an odd number of teeth, measuring the OD is a bit of a challenge. I suppose our shop could figure it out with their CMM if I brought one over there by touching three points and letting the computer do the rest. But I will probably just check using the formula above and see which comes closest to, but slightly over the measured diameter

From what I remember, your formula did give a reasonable answer using 14 pitch, but the answer using 13 and 15 was obviously either too big or too small. Fortunately, pitches are most always integer numbers

For now I'll assume 14 pitch with a 20deg pressure angle. My attempted measurements came close to that number, but the teeth are so triangular looking I thought it must be more. My measurements also had a large variability so I didn't want to rely on them. I still need to talk to our controls guys and find out just what diameter we are going to use. There is some internal debate going on about whether it is better to have the encoder make multiple turns or something less than one. That is a subject out my area, so it will be a day or two before I can design the gear anyway.

Edit>> Upon reaching home, I checked my notes. The gear I measured was 1.25 max diameter with no allowance for the caliper jaw resting across two teeth. Plugging this into the formula above gives a DP of 15.2. Rearrainging and using 15 as the DP the formula gives an OD of 1.266. Pretty close to and slightly over what I measured. Too close for it to be anything else.

So Dr Joe, at the moment it looks like I owe you one BeachBot's special encoder gear. Should I send it when it's done or should I just hand it to you at Nationals? <<Edit