Looking for Lead Screws

[Jared]

Originally Posted by Dominick Ferone

20 ft per second is what we had calculated. We have a mentor who works with them often and he helped us figure it out.

20 feet per second is WAY too fast. It's far beyond the critical velocity for any lead screw that an FRC team could ever have access to. I'd be willing to bet that you meant 20 inches per second.

You may be right that you actually need such a crazy thick lead screw in this case. For high speed stuff, I recommend a McMaster (or amazon equivalent) 1/2"-8 lead screw with 8 starts, resulting in a 1" of travel per rotation. These are the fastest commonly available lead screws you can buy.

However, the concern your mentor has is likely due to critical speed. A lead screw can only spin so fast before it starts to vibrate like crazy and destroy itself. When we lifted our robot's weight, we drove the nut instead of the screw, so the screw moved relative to the robot. With this configuration, critical speed isn't a concern. This configuration doesn't work so well for elevators, where you want to drive the screw and let the nut move up and down. I don't remember what the exact critical speed for a 1/2" lead screw is that's well supported at both ends and is around 70" tall, but IIRC, it was in the ballpark of 600 rpm. That's 10 inches per second, which is slower than what you wanted.

My recommendation is not to use a lead screw if you plan to have a 70" tall elevator, but if you plan to have a shorter elevator (3 feet or so), go with a 1/2" lead screw. The critical speed increases quickly (meaning you can go faster) as you decrease the unsupported lenght of the lead screw.