My team is thinking about using a 2-stage cascade elevator for this year’s game but we are stuck between using either a belt and pulley system or a lead screw system.
I’m aware the belt and pulley would have more backlash but also more efficiency. It could also reach faster speeds and have more torque. On the other hand the lead screw is stronger/sturdier and has more precision. Am I missing any cons or pros and which do you think is best for this game? (Also we are thinking of using the elevator to climb)
The biggest difference is going to depend greatly on your resources and intended game strategy. Whichever one you can build better to more completely fill your needs is the one for you.
This may be a mistake. A cube and its delivery system will probably weigh under 25 lb (in some cases way under), where a climber will have to support a much greater weight (100-400 lb depending on whether you’re planning to lift another robot or two as well). This means that you will have to build your lift several times as strong as it would have needed to be to do the cube delivery task. We are intending to use our lift to set a hook on the rung, but actually climb using a cable/winch system.
How much accuracy do you think you really need to be effective? How much speed do you think you will really need to be effective? Watching the 4 videos of the Ri3D scrimmage held yesterday (linked in post 20 of this thread) might help you make your decision.
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.
I would consider it a gross generalization giving one team as an example.
But I would agree with that generalization. A well designed lead screw can keep up with a belt, however a typical rookie built lead screw would tend to be much slower.
You simply HAVE to do the math to have a good climber. Download the JVN calculator spreadsheet from the Vex website or here and plug in some numbers. It only takes about 30 minutes starting from zero and will give you a much better chance than guessing.
You can also do chain or a winch. We are doing chain. We did think about belt or lead screw. A long and strong lead screw was considered too expensive and heavy. My team was scared of belting after some bad experiences in previous years with tension problems.
I gave examples of times we used them in previous competitions for them to see. I never said that they shouldn’t use them either. If you have a different setiment to share compared to our experiences, feel free to share them for OP to look into.