Which gear pitch to use

Does anyone know if 20 pitch gears will have sufficent strength for a drivetrain gearbox? Thanks in advance! :slight_smile:

You should be fine with 20; pretty much everyone uses 20 almost exclusively in their drivetrains.

20 pitch gears have good tooth strength, but you also have to remember that the width of the gear plays a big part in the gear’s strength. Generally teams use a 0.375" gear width, but you can go over or under, depending on the load the gear is supposed to take.

For example, you can safely go to a .25" gear width for a pinion gear on the CIM motor, because it takes little torque and high rpm. On the other hand, you may want to use a .5" gear width for the last gear in your transmission, the gear that spins the slowest and carries the combined power of all the motors in that transmission. This gear would have to transmit the most torque, so it needs a stronger gear tooth.

Thanks for the quick replies! I just wanted to make sure that I wouldn’t finish my design and then discover that it couldn’t take the power. :smiley:

Another question: would a 16 pitch worm be sufficently strong?

20 pitch, 14.5° pressure angle, 0.375 in face width, steel and iron gears are commonly and cheaply available from Boston and Martin; that’s probably why most teams use them. In general they’re strong enough for drivetrains, but might not be sufficient for an arm.

You don’t need to use 20 pitch on the high-speed stages, though; I’ve sucessfully used 32 pitch and 0.7 module, 20° pressure angle, 0.1875 in face width aluminum gears to gear motors together (linking the old Bosch drill motors, Fisher-Price motors and CIM motors). With that 1.4 HP combination, the gears ran safely (marginal wear after 2 regionals, championships and an off-season, including some impressive collisions and violent driving) at the equivalent of a 1.5:1 reduction from the CIM motors. Of course, I also tried to use those same gears at a total of 5:1 with respect to the CIM (i.e. 1.5:1 × 3.3:1 ≈ 5:1), and those broke in testing.

I’m also a fan of the 32 and 24 pitch hardened steel gears from Stock Drive Products. To replace the ones that broke, we used the 24 pitch, 20° pressure angle ones and never had an issue.

The final stages (parallel reductions, selected by dog shifter) of that gearbox used steel and iron 20 pitch gears, as described above. There was visible wear after the season, but it may have been due to the misalignment of the output gears due to somewhat inadequate support against the dog shifter. They weren’t ideal for that gearbox, but they didn’t fail.

Using 20 pitch gears can simplify things greatly, but just be aware that there are better solutions to keep weight and inertia down.

For what? I’d seriously advise against worms for the drivetrain, because of their efficiency characteristics (often between 50% and 80%) and (depending on the type) point-loaded teeth. Unless you’re investing in extremely expensive high-efficiency, high-strength worms and worm gears, avoid them like the plague for your main drive.

For drive gearboxes, 20 diametral pitch gears (made by Martin) are pretty much the norm for FRC. You can get them from Small Parts or Motion Industries. Small Parts usually always have them in stock but Motion is usually a bit cheaper, especially on the larger gears. 24 pitch would work too but they are less common. For low loads like the first stage of gearing off the fisher price motor, 32 pitch is good. 32 pitch would most likely hold up on the CIM too but why not standardize it all at 20?

For face width, we’ve always used 1/2 but 3/8 is fine.

Also usually these gears will have large hubs which you should/could turn down or face off to save a lot of weight. You can also take some material out of the gear too by making it thinner in the area between the teeth and the bore, or by drilling holes.

Let me know if you have any more questions.

a random question:

what is more readily available: 20 pitch 14.5 PA or 20 pitch 20 PA?
I have always used 20 degree pressure angles because of the wider tooth base, hence stronger gear. I know tons of suppliers of 20 PA, but what is more common in industry?


“14.5 degree pa gears were standardized by the ASA in 1927…officially this form is not recommened for new designs as it is considered obsolete. The 20pa was standardized in 1932”- Handbook for the metalworking industries. Now, idk how often gears get ‘redesigned’ lol but im guessing its saying 20pa is the new standard.

On that same note, by what factor is a 20pa stronger than a 14.5pa gear of the same material?

In 2004 we used 20DP, 20PA with a face width of 1/2’’ from Martin.

One CIM motor per transmission, three speeds.

In 2005 we used 20DP, 20PA with a face width of 3/8’’ from Browning.

One CIM motor per transmission, two speeds.

I think the 20DP, 20PA, 3/8’’ face width gears should hold up to any punishment you give them, no matter how many motors FIRST gives you at them.

By the way, does anyone have any stories of broken gears? If so please post what type of gears you used and why they broke!

2002, first time ever attempting a 4 motor drive. We used 3/16" face width 32P brass spur gears (yeah, oops). Geared the CIM and cluster gear to match the drill motor and gearbox. Very fast and very powerful. Worked great until we tried pulling two goals at once in a practice competition just a couple days before ship. Sheared off a few teeth and with that was the end of the four motor drive for 2002. Anyway, I guess you could say we’ve learned a thing or two about gearboxes since then.

We broke a tooth on a 32 dp stainless gear last year at nats. We should have increased the dp of the last stage of the gearbox. See attached pics.

It had a ratio of 200:1 if I remember right, most of the gears were 1/4" face and rode on a 1/4" keyed shaft. 1/4" keyed shaft? Is there such a thing? Yep, we made it using a 1/16" end mill. The transmission was used to operate our arm and worked well, even when the tooth broke on the one gear. However after some more use since then the bearings starting falling out of the transmission housing which was only 1/8" thick Al (that old 2x4 stuff they gave in 03 & 04).

Now we’re building a much stronger arm transmission using a worm gear (12 dp), 20 dp gears, 1/2" shafts, and two CIM’s housed in a 1/4" thick Al housing. Too bad it will weigh 10 x as much as the old one - but it’s just for fun.