First of all, theoretically there is no speed difference between a frictionless leadscrew and a cable/belt/chain setup. Your final lift speed is a function of the motor’s output power, which defies E = mgh to lift your cube. 2 775s can easily raise a 30lb lift in under a second. Our lift uses 4 775s running at under a 10 amp draw each to raise our lift in under a second, and if we want to risk it we can actually use the same gearbox to climb with.
The difference between the two options is that leadscrews have crazy frictional losses, especially when you start going fast. Multi-lead leadscrews let you go a bit faster but there’s always going to be more friction than a belt/chain system. In return, backdriving is impossible or just very difficult when using a leadscrew. Ballscrews are another option but all fast-spinning screws suffer from the possibility of “whipping” at high speeds.
A leadscrew and a chain setup of the same weight will actually favor the chain in terms of pure strength. Your accuracy is slightly reduced but who cares? You’ll easily be able to hit 1/16" positional accuracy with either.
If you use a belt/chain/cable there’s far fewer frictional losses, mainly just those in the bearing assembly on the carriage. But you can backdrive your gearbox so the lift will sink unless you are careful about holding it up. IMO it’s also easier to execute than a screw-based alternative, but either can be made to work fine.
EDIT: some back-of-the-napkin calculations (using this table) say that a 48" 1/2"-10 leadscrew with 5 leads can spin at around 1000rpm if you support both ends with bearings. 1000rpm * 0.5ipr = 500 inches per minute, which is only 8 inches per second. I’m certain that you could go faster than this if you wanted, probably up to around 3000rpm depending on how good your carriage is, but that’s still only 2ft/s. Robot in 3 Days robots were able to lift much faster than that using pulleys, up to 3ft/s. We’re gunning for a 6ft/s lift speed ourselves. A ballscrew might end up faster but that’s going to cost a lot of money unless you want to wait a week for China. linearmotionbearings2008 from ebay is a good seller if you go that route.
