Worm drive gearbox

I’m looking for a worm drive gearbox that can’t be back driven for a climber or a worm and worm gear that aren’t as expensive as these and would be strong enough:

These are what 2910 used so I know they’re strong enough but they’re really expensive and still require designing a custom gearbox.

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One thing to consider is whether you actually need a worm gearbox. An inexpensive way to prevent backdriving is a ratchet; a very inexpensive way is a ratcheting 1/2” socket wrench on a 1/2” hex shaft. The only downside of a ratchet versus a worm gearbox is it only allows driving in one direction, but that is often not a problem for climber mechanisms. Climbing in 2017 was worth so many points that teams would give away climber assemblies to their partners at events that consisted of just a medium sized motor, a planetary gearbox, a ratcheting socket wrench, and a winch assembly.

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The downside of a ratchet is that if the driver makes a mistake and misses the chain they don’t have a second shot.

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You can find winches at hardware stores that use worms, but those can be very inefficient if they’re poorly made. But they do work for the purpose.

Finding gears of similar size (around 1.5 module) might be cheaper at other places like KHK, McMaster is pricey.

Doesn’t really solve the cost issue but…

You may be able to find gearsets on KhK gears for cheaper than McMaster

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Maybe look into this. Solves the issue of only being able to do it once with a wrench as a ratchet and worm gears can be back driven.

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It’s also fairly inexpensive, as the unit cost of the maxplanetary mod is ~$25.
Obviously you do need a maxplanetary, but they can be easily reused season to season.

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I believe the RAW box can back drive, which doesn’t solve the OPs issue.

It’s overkill for a robot, but here’s an inexpensive solution. You could ditch the wire rope and instead use Dyneema rope to save some weight.

https://www.harborfreight.com/automotive/winches/2000-lb-capacity-geared-winch-5798.html

Word of caution - we had a lot of challenges with our custom worm gearbox. We blew up a few motors and it was extremely power hungry. That non-backdriveability comes at a cost of efficiency. It was a late design add and we were able to make it work, but it’s far from an optimal design here.

I’m not aware of any COTS worm gear solutions that are non-backdriveable and accept FRC motor shaft inputs natively. Worm gears have a lot of weird forces which make custom designs hard.

IMO, unless you’re dead set on learning how to design/use a worm gear drive just for that knowledge building, a ratchet solution is a much better choice. If you have pneumatics or an available motor slot you can actuate the ratchet so you can have more than one climb attempt, but even if you only have the one shot, 90% of the time you’re not going to have enough time left on the clock to actually reset and retry even if the robot is capable of doing so.

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Would you be willing to share the CAD for your gearbox? It would also be helpful if you could explain what were the challenges you encountered and how you solved them

We’re in the process of cleaning up the full robot CAD and tech docs for release hopefully in the next week or so. We’ll have more details then.

We never really “solved” any of the problems with it, or even really have solid ideas of how we would change the design to solve the issues, we just have workarounds. Like “hit the E-stop as soon as the robot is climbed so we don’t burn out the motor because it wasn’t getting to the setpoint” workarounds. We only had it on for our last 2 events and at that stage in the season it was just about finding a way to make the system work through an event.

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Even simpler is just gearing using a higher ratio. At the end, climbing at like 1ft/s vs 1.5 ft/s is not a big deal, but sometimes raising the gear ratio makes climbs hard enough to backdrive that they stay up for long enough.

That’s what my team did. We had two Climber in a Boxes with a 16:1 vp and a neo on either one. The friction in the gearboxes combined with the neos being in brake held it up for more than long enough at the end of a match (105 lbs robot).

It’s important to remember that, as a general rule, worm gears can be back driven, they just shouldn’t be back driven.

Think of a worm gear as a very shallow ramp. When driven normally, the applied force is nearly in line with the ramp and the normal force and thus the friction is fairly low. When back driving the worm gear, you’re pushing from the other direction so the normal force and and friction are very high. But the friction is the only thing keeping a worm gear from back driving, so a well lubricated worm gear might be fairly easy to back drive.

A higher ratio is going to be both more efficient and harder to back drive, so its probably a good idea to use as high a ratio as possible. Increasing friction would reduce the likelihood of back driving, but is probably a very bad idea as worm gears are already pretty inefficient. Getting the relative position of the gears just right is very important for many worm gears (depending on the cut), so you’re probably going to want to be able to shim the positions.

You can find cheap worm gears on amazon, something like this might work if you really want to play with worm gears: Amazon.com

You might highkey be on to something with that >.>

I would start with a window motor as a proof of concept then consider if it is worthwhile to build custom.

My team used this gearbox for our climber. I don’t remember the exact supplier but the NMRV030 is the model number. You can get up to a 100:1 gear reduction. They are heavy but they get the job done.

https://www.amazon.com/NMRV030-Worm-Reducer-Speed-Ratio/dp/B09HGH4S3F?th=1

Lele

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One solution that 343 used many many years ago (2010), but would still work, if a little unconventional, is to integrate a window motor into the drive you are trying to make non-backdriveable. Window motors are by nature non-backdriveable worm gearboxes and are FRC legal. You would need another motor (or 2) in a more standard gearbox configuration to actually power the winch, but the window motor would prevent undesired backdrive while allowing bi-directional drive. Keep in mind that window motor casings and gears are all plastic so you can’t go putting shock forces on them but they are strong enough to hold a steady braking force on a winch to keep an FRC robot suspended, especially if your drive motors are also set to brake mode so they take some force too.

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