Question about worm gears

[kajeevan]

I've seen a bunch of swerve drive this season from 118 to 1625 and I planning on CADing up my own but was wondering would worm gear be a good idea. Is it efficient and durably enough for drive train usage. Also wondering from teams that have already done swerve what is a weight i should be aiming for and what motor should I be using to turn the modules so I have easily enough torque but have optimum turning speed.

[Joe G.]

I have no firsthand experience with crab or swerve drive, but I suggest you contact team 269, about their "sworm" drive from 2007.

http://www.chiefdelphi.com/media/photos/28618

[Aren_Hill]

For drivetrain usage, i'd suggest staying as far away as possible from worm gears, the main reason being efficiency. A swerve done with bevel gears has enough stages to make it less efficient than other drivetrains, but putting worm gears into that will make it even worse.

Worm gears are quiet though which can make for a stealthy robot lol.

As far as turning goes, we used a fischer price motor and geared the modules at 60rpm free speed, so there is no question it has the power to move the wheels.
I've also seen teams use dual globe motors or window motors to steer.

[dtengineering]

A team we saw in Portland in 2007 (I believe) had done up a standard 6wd system using worm gears and had calculated the relative efficiency of using a single worm gear vs. multiple spur gears. Their calculations showed that the two systems were comparable... if not equal, at least close enough that they chose to build the worm gearbox in hopes of achieving some anti-backdrive benefits to make them more resistant to pushing.

Now I know it would be really helpful if I could remember what team it was or where to find a picture of their robot... but perhaps someone will recognize the team I am talking about and post that info here.

I don't remember their drive system as being either devastatingly effective, or terribly problematic... I just remember it being cool because it was something different, and quite elegantly designed.

In many cases a 4cim drive has more power than a team actually needs most of the time and some teams have been quite effective with 2cims driving a 4wd or 6wd drive train. This suggests to me that if you try building a worm swerve you might come up with a cool system... perhaps not the ultimate in mechanical efficiency, but perhaps lighter and simpler than using bevel gears... and ultimately something a bit different, unique and kind of cool. So not to disagree with the concept that you lose some efficiency with a worm drive, but perhaps it is not prohibitive if you see the potential for benefits in other ways.

Jason

[Al Skierkiewicz]

Most teams shy away from worm gears in the drive train since they rely so heavily on mechanical alignment. Without the strong bearings and mounts needed for this application the gears tend to climb on each other under heavy loads which just increase the loads on the bearings and drives the efficiency down. Generally, the support structure for the bearings becomes unwieldy in these drive systems. Worm gears work well in other applications and for those that need backdrive compensation. Arms and manipulators are both good applications.
The Globe motors are great fro steering, they high torque and good control at low speeds needed for steering. We have used them for all of our crab drive systems.

[AndyB]

I am a former member of 269, and yes, we have done "sworm" drive. Even though its really cool, and nice and quiet, I really can't recommend it over more traditional swerve concepts. (As traditional as swerve drives can be at least)

Efficiency is obviously a major factor, but another thing to take into account is how the whole thing fits together on the robot. With a worm reduction, it is impossible to center a motor ontop of the turning radius (unless your wheels are offset from the turning radius, like a caster, which is not recommended). Our sworm drive turns only 180 degrees, which also makes programming a little tricky. Again, I don't know if I'd recommend that method either.

If you could come up with a design that had 360 degree rotation, using worm gears, I don't know if it would be any more advantageous than just mounting the motor sideways ala 111 or 71.

[Joe Ross]

A team we saw in Portland in 2007 (I believe) had done up a standard 6wd system using worm gears and had calculated the relative efficiency of using a single worm gear vs. multiple spur gears. Their calculations showed that the two systems were comparable... if not equal, at least close enough that they chose to build the worm gearbox in hopes of achieving some anti-backdrive benefits to make them more resistant to pushing.

Now I know it would be really helpful if I could remember what team it was or where to find a picture of their robot... but perhaps someone will recognize the team I am talking about and post that info here.

I don't remember their drive system as being either devastatingly effective, or terribly problematic... I just remember it being cool because it was something different, and quite elegantly designed.

The team was 1425. I didn't see any info on their website about their worm gears, but they do have a forum. There are also a few people that read chiefdelphi, but haven't been active in a month or so.

Here's a picture of their worm gear (http://www.chiefdelphi.com/media/photos/26656) and of their whole robot (http://www.chiefdelphi.com/media/photos/28171).

[joshsmithers]

This is a tough trade-off situation, and I can honestly say I would never come to a definite conclusion on this. After pondering 269s sworm drive and Andy B's post, I would suggest a more traditional swerve. Like Al said, the drivetrain, where you should be maximizing your efficiency, just isn't the right place for worms. Also, worm gearing needs to be aligned to perform at its best. However, it may save a pound or two if you aren't using a bunch of spur gears. I have no idea what swerves typically weigh, but it's always a good idea to design with weight in mind.

I have no experience with swerves, but I would asume that if you don't want your wheels to backdrive when being pushed, then you could just turn them perpendicular to the pushing force.

[Pat Roche]

In the past, teams I have been involved with have used worm gears. We've used them both in drive train and manipulator situations. I would recommend staying away from the drive train application only in the sense of using it to power your wheels. In the case of rotating your drive modules a worm gear should be feasable due to the loses in efficiency being less important in that part of the drive train compared to the actual powering of the wheels. One of the big trade-offs with worm gears however is the weight. You have to weigh the value of the worm gear vs. other gear reducing systems to determine if its necessary to put such a heavy system on the robot.

Best of luck,

Pat

[AndyB]

Now on an opposite note of my last post, 269 has been using worm gears for the past 4 or 5 seasons, and has never once had a single problem with a gearbox. Worm drives are pretty simple to build but do require lots of care as far as lubrication goes.

Worm drives are also very nice for the no-backdrive feature many find, but not all worms will provide this effect, so be careful in those regards.

In an ordinary drive scenario, (6wd, 4wd, front-wheel steering), I would say worm gears provide enough benefit to at least not rule them out right away. But on a more complex system like crab, the only real advantage for the worm gears are the noise, and there is an obvious coolness factor.

I've heard stories from one of the Hill's that 269's robot can really sneak up on you on the practice field if you aren't watching it. :)

[Aren_Hill]

its creepy quiet.....