42:1 3DP gearbox that does not backdrive

This is probably the last I get to do before I move to my new place. Cant test it as I packed up my 2mm key stock and dont remember what box

2 viewws from the bottom the center gear atm is suspended in mid air as its missing a CIM with the aforementioned 2mm key

A view from the top

back view with holders for the hex head and holes for limit switches this is a test rig so it will be tested under different scenarios

I prior iteration of the worm gear x2 / HTD pulley (I decided to change the spacing to make mounting the CIM easier

What you see is about 350 g of filament (HIPS) 2 608 bearings and 4 688 bearings on the Worm The 608 take the load of the lifting. A 608 is rated at a static load of 1.4 KN (308 lb) static load so strong enough dynamic is more that 2x that.

The pinion on the motor is 19 Tooth the 2 gears mounted to te worms are 42 tooth the worm/worm gear is a 19:1 giving an overall 42_1 reduction.

Figuring running the CIM at optimal 27 Amp - 64 oz/in - 4 inlb * 42 = 168 in/lb The output pulley is 28 tooth - 0.88 in pitch radius so theoretically if nothing breaks at optimal it should lift 184 lb. I guess at about 70 % efficiency on this thereabouts due to the 2 stages and the Worm setup. So in the neighborhood of 130 lb

The output should turn at about 100 rpm at full speed So lift at about 9 in/sec

We can gain about up to 35 % more torque running it up to 40 Amps which at 70 % efficiency and a slightly reduced speed should lift up to 180 lb so point is a robot is quite feasable with them topping out at about 150 including battery and bumpers.

The pitch of the Wormgear assy is 2.5 mm puts enough plastic on them to give me reason to believe the teeth will hold. The gears teeth on the pinion and gears only deal with about 4 in/lb torque That is close to 8 lb distributed over at least 3 of the pinion gears teeth as its double helical involute setup.

Well in a few weeks when I am moved I can test - unless someone wants to know sooner then I can make the files available so you can print it yourself and try it.

Oh the axles are 5/16 bolts which are 8mm and work well for 8mm bearings and I got a ton of them


Excellent use of 3D printing for customizing!
I’m pretty sure 19:1 is enough ratio to get your friction up.
I’ve always wished there was a solid FRC worm gear solution that was high enough ratio to be non-backdriveable!
You may want to think a little bit about how to get load sharing between the worms. Angular error on the herringbone gears or axial error on the the worms will shift load between the parts. If you has the worms set up where they were keyed or hexed onto their shafts, you could have rubber washers between the worm and the big herringbone gear. You would need to orient it so that the thrust load is squeezing the washer.
Another thought would be to arrange things where you have worms on each side of a worm wheel; this would balance the separating force, but then you need the two shafts running in opposite directions (IIRC).
I’ve seen a limited slip differential that used helical gears in it; the torque between the two output shafts would drive the helical gear one way or the other, and it had friction surfaces on each end to limit the slip.

If you could arrange things where the worms were pushed against something high friction when the bot was hanging you could get nice positive locking. Ideally it would be set up so that when you were lifting that would automatically lift the worm off of the friction surface. I’m not sure if its possible to get that to work out right, though :wink:

Test will show but for now I think we might be ok

1.) during assy you can turn the worms so they touch in the same direction on the gear. So they touch on the power side of the tooth
2.) I am using 5/16 bolts instead of M8 so there is a tiny bit of slop so there is a tiny bit of wiggle room
3.) Its plastic so it will deflect a tiny bit

So the theory is that if one side has more torque it will push within the limits of the "wiggle"on its side and with that transfer some torque to the other tooth. The wiggle is about .1 to .2 mm so not that much but that makes for quite a turn on the worm. So there is no guarantee that both worms have exactly the same torque all the time but they do have some - at least hand tested. Also with the X and Y on the inexpensive printer a round gear is not absolutely round. I know on the predator I am about .04mm oval so a 40 mm in the Y equals a 40.04 mm in the x. Hence going to a 5/16 instead of a M8 allows everything to be quite smooth. That also allows the gear and pinnion to slightly shift back and forth (much less than .5mm I currently cant measure any closer) and you got a little play in the bearings always under torque. The way I print it and designed it the worm/wormgear has a backlash of .2mm thereabouts. Plus after a while there will be some material creep or wear that will even things out.

Now I doubt that setup - if it survives the torque tests - will run for 10s of thousands of hours year after year. But I am pretty confident we get a season at least out of it. So 12-20 climbs or 5ish seconds - so less than 2 minutes use/day. For 60 days (competitions, practice max) so 120 minutes use = 2 hours. Now That is much more likely. I intend to test it (after I mover) for a day or so under load and if it survives I call it a success. You are talking 1/3 kg of filament - HIPS in this case including the body (about 1/2 of that for the gears) So if I can stay with HIPS $ 1.50. Assy time is minutes - so replacing a gear/worm peace could be done while queuing at a competition.

Take a toughbox mini - 4 on a bot for 4 wheels we have at least 3-5 needs for repair a season, Usually the Eclip came off or something like that They are hardly automotive or military grade gear assemblies.

The goal is to design something for FRC that will last reliably a season - why a season? As you build it yourself its not KOP so you got to chuck it at the end of the season.

If its promising I might do it in Taulman 910 Nylon as then you probably can forgo lubrication and the only question is do you want to spend about $3 in filament or $25. ( probably $60 if you go onyx) .

So I target the use of the device and test for it. Hence I will not do Stall tests as stalling a CIM - you wont as that will blow the fuse rather quickly and render the device inoperable as it looses power even if it mechanically survives. So I target for (like we do) limiting the current by design and program to <40 Amps on a CIM and then test at those current/torque loads for at least 2x the time the max use is expected even if we go all the way to the finals in every competition including Nationals.

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Looks like a great design. I would be interested in printing one!

Uploading it to grab cad. Please be aware there are some prior iterations in that directory too. The worm/Gear Assy is G42T1125W2500… Files the Pulley/Gear is WG2500_HTD, the gear that goes on the CIM is P19T1125HH, The enclosure is testholder. Grabcad does not let me upload the ipj(project file) The other files are either components or prior iterations so for example the G42T1125W2500… is made up out of the G42T… gear and the W… worm. The naming is
G for Gear
P for Pinnion
WG for WormGear
Ww for WormgearWorm
xxT number of teeth followed by 4 character or the modulus then either nothing for spur gear, H for Helical spur gear HH for double helical spur gear. Combined parts have combined numbers usually shortened so they fit on the display of the printer which makes print selection easier

The gears need to be printed with a .6 nozzle or smaller. The Test holder with any nozzle. Unless you print for show only its designed to be printed with enough perimeters to make a 5mm wall or with 100% infill and at least 5 perimeters. The rectangular holes are for 1/2 in 16 gauge square tubing to mount it we either use aluminum or steel. The bolts for the CIM are M4 12mm or 15mm long then you need 4 688 bearings and 2 608 bearings. The axles are 5/16 bolts I currently only have 4.5 in and 5.5 in in stock and they are a bit long so IDK for sure if a 4 and 5 in bolt will fit. The Test holder prints upside down (the motor and worms are on the bottom when running it otherwise the motor is in the way of the 1/2 in tubing. And everything is designed to print without supports. Due to that you might have to clean up the 21mm hole for the Cim a little. A Neo will fit too. The key way in the 19T gear is a little taller than it needs be. Reason is for safety we usually us a 3.2x2mm key instead of a 2x2mm. IOW we take an 1/8 in key and grind or file one side down to 2mm

If you print in PLA use ample lube as PLA usually grinds down to nothing at a gear running > 200rpm and the CIM does almost 5k under load. We use either HIPS or Nylon. CF Nylon or GF Nylon should work great too. All overhangs are 45 deg max so that means keep the layer height < 50% nozzle diameter. Make sure your extrusion is dialed in to make the dimension between 98-100% nominal otherwise you might have to not only drill the holes to size besides cleaning up things like brim or elephants foot but you also might get some Center distance problems on the gears.

The url is here : https://grabcad.com/library/worm-climber-42-1-1


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