Drill motor gear measurement

Does anyone know what the size (pitch, pressure angle) of the gear on the drill motor is? We want to get a gear that will match up to it correctly. Thanks for your help.

.7 module (metric pitch) with 20 degree pressure angle. the only place i’ve found with that pitch is PIC.

Try this. (From FIRST’s 2003 spec sheets.)
But watch out for this. (From FIRST’s 2004 spec sheets.)

Notice, that the documents are not the same! The 2004 describes a 13-tooth gear, and the 2003 describes a 15-tooth gear. The 15-tooth gear was used in 2004 and 2003–the 2004 spec sheet is in error. (I’ve linked to it, because it may be easier to read–but use the numerical values from the 2003 sheet.)

Also, PIC Design is just about the only source that anyone’s been able to find for 0.7 module gears (high-precision metal ones, anyway). They’ve been very slow with orders (think 1-2 months, when they promised a couple of weeks, even with nagging phone calls, etc.), so it might be wise to order ahead. If you think you can make them work, Stock Drive Products has plastic moulded spur gears in 0.7 module, but they obviously aren’t as strong or as precise.

Also, I’ve found that the drill motor pinion seems to like a little bit of extra centre distance, in order to mesh correctly (due to a slightly enlarged, non-standard tooth form). I’d suggest using about 0.007" to 0.010" extra centre distance, to allow for a better mesh. If you use the standard theoretical centre distance (based on pitch diameters), the gears will mesh too tightly.

Mark Koors on the TechnoKats made a replacement gear for this pinion last year, and instead of using a 0.7 module gear, he made a 36dp gear. They are almost the same.

Andy B.

In fact, 32 dp is so close to .7 mod that we just used a 32 dp gear with the drill motor.

The pinion on the drill motor is hardened steel. You can take a 32 dp gear of a softer metal (we used brass) and run them in dry. Lube them up and you’re good to go.

SPAM used this method on their 2000 and 2001 bots and we used it on our 2004 bot. Works great.

One word of advice: The number of teeth probably won’t work right for calculating this ratio.

FYI, 0.7 module is exactly 25.4 * 1 / 0.7 = 36.2857 pitch. The 32 = 36.2857 thing has been used successfully, but it really isn’t optimal, and will likely yield unpredictable wear on the large (softer) gear. Instead, I would try to predict an appropriate size of 0.7 module gear, and order it before the season starts. Once the kit of parts arrives, make any design decisions that need to be made, bearing in mind that you already have a proper gear, and won’t have to wait 5 weeks for it. Only if you can’t use what you preordered, should you consider using some sort of crude alternative. (And if your design can tolerate plastic 0.7 module gears, they can be had within a week from SDP/SI–even more reason not to go with 32 pitch, even when pressed for time.)

I too cringe when I hear folks using 32 DP gears with .7 module gears, but it is hard to argue with success. Apparently many teams do it (can the TechnoKats, WildStang and SPAM all be wrong? – well, yes, but… …how wrong can they be?).

Here is what I recommend, get the gears from PIC or have them burned from a wire EDM source (Lou Odin at Capital Tool & Die is a Detroit area source we use a lot – he takes credit cards and will ship anywhere – PM me if you want contact info). Failing that, press the gear off and press on whatever gear you can get easily. We’ve found the 5mm shaft is pretty easy to work with.

Joe J.

Notice that I said we used a 36dp gear, NOT a 32 dp gear.

Andy B.

Sorry, it was just a typo.

Joe J.

We had slow turn-around for our first order with PIC (over the summer). However, when ordering a spare gear after ship date, they supplied us in a fairly reasonable time frame. I guess it depends on whether they have stock on hand or not. Probably something that can be verified with a phone call if time is tight.

In 2001, 2002, and 2003, we used the first stage of the drill motor gear box “as is” (we made a custom planetary shroud).

In 2004, we went with a straight up spur gear stage using PIC design gears (48 teeth, I think). We used the theoretical center-to-center distance and did not experience any significant losses or loads. We did a no load current measurement with and without the gear box on both drill motors and did not see a rise in current.

We’ve also inspected to see if there is excessive wear, and things look fine.

I suppose, in the off-season, we can make some test fixtures with different center distances and measure load currents to see if this is an issue. However, I don’t see the need to use extra center distance.

This was discussed before - pay attention to the advice on running in the brass gears under no load: