Worm Gears

Technical Technical Discussion
#1

Team 1089 is planning to use a gearbox that uses worm gears to acheive a sufficient reduction. Here is an inventor design of the gears we are planning to use…

http://img.photobucket.com/albums/v156/caboose_nullee/ubergearbox.jpg

We plan to mount the motor to the worm, and a wheel to the gear.
Has any team here attempted to use a worm gear system for their drive train before ? How well did the gearbox work?

#2

Worm gears were in the kit in 2004 with the drill motors. They had a few problems of binding and overheating the motors. I think that is why FIRST got away from them. They can be very good if there is very little friction.

I also recommend putting a fan on top of the motors to cool them down when running worm gears.

#3

The gears that were in the kit were not exactly worm gears, they were 45° gears.

Worm gears have a serious drawback for drivetrains: you cannot back drive them. Your robot will not coast, you will not be able to pull it across the floor when its turned off

and worst of all, if you get slammed by another robot the worm gear (or mating gear) will get its teeth stripped off.

#4

My old high school Team 195 used the a worm gear setup in 2002 with much success. I will try to swing by the high school today and snap some pics of it for you. Also in 2002 we used the no backdrive feature to our advantage. We had grippy tank treads, when we shut down out motors the only way to move us was to drag the robot across the carpet, no small feat with ~120 square inches of gum rubber on the floor with 130lbs pinning it down. Anyway if I grab the pics i’ll toss them up for you.

#5

We used two worms for our arms last gear with great success.
Biggest drawback to a worm is efficiency - about 65% at best - so you lose a lot of your available power. I would look at using say the Dewalts.
With that being said I found some nice worms at http://www.qtcgears.com/
You might also search ebay.

#6

Here are the pictures I promised, all the gusseting was very necessary due to the massive torque of the drill + CIM motor. It had a tendency to pull the worm away from the gear and de-mesh the two. This had the cute effect of turning the worm into a downright end mill and shaving the teeth off the brass gear quite nicely. Also the center gear was made out of soft brass to be a sacrificial gear of sorts. This allowed the worm to mate itself very nicely with the gear, albeit, with a few chips in the bottom of the chassis after the first few runs with a fresh gear. With this setup we had to change the center gear about every competition and a half.

#7

:slight_smile: Worm gears are great…but there are a few things to consider:

  1. Because the gears are “sliding” by each other, there is considerable friction between the two. This requires more torque than using spur or spiral cut gears and substantial lubricant (grease) to make work.
  2. If made of metal, this may be heavy.
  3. On the other hand, this set-up can create excellent ratios in one reduction, not two or three.

Just food for thought :yikes:

Phil Paspalas
Head Coach - Team 1676
The Pascack Pioneers
2005 NJ Rookie All-Stars

#8

Owch!

#9

Worm gears have 2 great advantages: High ratio and (possible) not backdriveablity

They have many great disadvantages: tricky centerline distance sensitivity and extremely low effeciencies (compared to spur gears, for example).

In general, in a FIRST application, the disadvantages almost always outweigh the advantages. IMHO.

your mileage may vary…
Joe J.

#10

I agree with Joe. Just say no to worm gears. The only application I would use one again on a FIRST robot would be for a light load situation.

ymmv,
Andy B

#11

Have you seen Metal Gear Solid 3 ?

I try to convince my team to use Worm gears like this last year, but the mechanical guys think I’m crazy: :yikes:

http://img.gamespot.com/gamespot/images/2004/reviews/914828_20041116_screen014.jpg

#12

Non-backdriveability is the greatest reason that we wanted to use them in this first place.

#13

Not being backdriveable means that as soon as another robot rams into you (which most assuredly will happen), you’re going to shear half the teeth off your gears.

Not to mention the fact that they’re horribly inefficient, something that isn’t such a large issue in an arm (where no backdrive is also generally helpful), but is definitely an issue in the drivetrain.

#14

I’m going to have to disagree with that on account of the fact that in 2002, which by all means was a push and shove game, we dealt and received many hard blows and never sheared one tooth. FWIW and as always your mileage may vary.