What's the opposite of a lead screw?
July 5, 2021 5:33 AM   Subscribe

A lead screw is very efficient at pushing a nut when it turns. However, if you push on the nut it's not very good at turning the screw. I need the opposite of that: Something where if I push the nut, it's really efficient at turning the screw. What are some examples of things like that which I could buy? One example would be a "yankee" or spiral screwdriver. I'm particularly hoping for things with shaft sizes between 2mm and 6mm (5/64" to 1/4"). Thanks!
posted by clawsoon to Technology (9 answers total) 1 user marked this as a favorite
Search for "high helix" leadscrews and nuts, which are like regular leadscrews and nuts but with much steeper thread angle. They mostly get used for turning rotation into very fast linear motion. But where the thread angle is steep enough, you can also drive them the opposite way like those old screwdrivers.
posted by automatronic at 7:19 AM on July 5, 2021 [2 favorites]

If money is no object a ball screw can usually be back-driven... I've seen an expensive one broken when an engineer lifted it by the nut, the screw spun out and fell to the floor, balls everywhere. I've not seen them as small as you like but Misumi list some THK Ø4 & 6mm diameter.

A polymer nut with a multi start thread can usually be back-driven, this 3/16"-42.7 Thread looks promising (need to verify the spec, I don't think the 16:1 speed ratio agrees with the .375 travel per turn). Hayden Kerk Pittman would be another place to check.
posted by tinker at 7:22 AM on July 5, 2021 [1 favorite]

psst, I'm a bonehead on the speed ratio, its ok
posted by tinker at 7:42 AM on July 5, 2021

Response by poster: If money is no object

Money is an object, so the cheaper the better. :-)
posted by clawsoon at 7:51 AM on July 5, 2021 [1 favorite]

The reason why they are easier to drive from the screw is the high reduction ratio. The yankee screwdrivers move about one rotation per inch of travel; similar size screws are a dozen rotations per inch. So how much rotation do you need per inch?

Basically while back-driveable ball screws exist, it's not used for moving loads, just not breaking (so you could use it to move a light flag but not lift anything heavier). See 507 movements to find a different way to achieve your goal. Maybe one of the ratchet designs?
posted by flimflam at 9:25 AM on July 5, 2021

Response by poster: The goal is to have a screw represent a propeller in a model, so the screwy-ness of it is important. High load is not expected. I probably should've said that upfront.
posted by clawsoon at 10:28 AM on July 5, 2021

Best answer: If you just want a cheap and dirty demonstrator, look at the very steep leadscrew thread in this example from Bornemann's site, and note how similar it is to a drill bit.

Can you make a nut from a couple of ball bearings and a collar of some kind, that slides up and down a long drill bit of the diameter you want?
posted by automatronic at 10:37 AM on July 5, 2021 [1 favorite]

There are toys that are more or less a big top put in motion by vertical strokes on a mechanism like you describe. Example here.

Search on mechanical toy top.
posted by SemiSalt at 4:48 AM on July 6, 2021 [1 favorite]

Response by poster: Drill bits look like they'll work, and I can get sets of long bits with a collection of different forward motion per rotation, which will be perfect for me as I figure out what works best. Thanks!
posted by clawsoon at 8:46 AM on July 6, 2021

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