Looking for Lead Screws

[Dominick Ferone]

Originally Posted by Jared

As others have said a 3/4" lead screw is more than likely overkill. We lifted our entire robot with a huge factor of safety with two 1/2" diameter lead screws.

The reason we were looking for 3/4 is for speed and torque purposes. With the current design we would go around 20 ft per second.

[electroken]

Haydon-Kerk has a large selection of lead screws. We used one last year with 8 starts and 2 inches of travel per revolution.

They're one of our sponsors. Maybe you can get them to sponsor you too?

[Joey Milia]

I'm not sure on the math behind the torque required for different diameters but I think the torque is dependent on the travel/rev not diameter. There are smaller diameters available in .5in/rev that would probably be a lot cheeper at lighter.

[JamesBrown]

Originally Posted by Dominick Ferone

The reason we were looking for 3/4 is for speed and torque purposes. With the current design we would go around 20 ft per second.

20 ft per second? Are we talking Vertical motion? that is very fast for our applications.

[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.

[Mike Marandola]

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.

Maybe you mean 20 in. per second?

[Steven Smith]

I'd recommend you post your application and math here. I just went through this last night for a different application, and I don't mind reviewing your numbers.

We're considering using them in part as a pincher closure. I'm going to run 3/8"-8, 2start rod at ~ 3 inch/sec closure speed on each of the two arms for 6 inch/sec closure speed total. It is more than adequate from a loading standpoint. The only reason we would want to consider bumping up to a larger diameter would be to more easily support it from a single end without too much whip in the free end... or I might just support both ends and go with 3/8".

Long story short, double check your numbers. These aren't 100% efficient devices, make sure you will have enough power to drive them in your application.

[Dominick Ferone]

Originally Posted by Mike Marandola

Maybe you mean 20 in. per second?

Yea guess i need some sleep, I think i got confused since we were looking at some that did feet per minute.

[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.

[Dominick Ferone]

Attached is a picture of our calculations. We are trying to optimize the Rpm, as to not draw to much power and have a good mix of speeed and torque.

[mastachyra]

Igus(FIRST sponsor) has some.

http://www.igus.eu/wpck/2371/drylin_trapezgewindemutter

[Dominick Ferone]

Do you guys have any recommendations for attaching a manipulator and slides to the lead screw from past experiences?

[Mike Marandola]

Originally Posted by Dominick Ferone

Do you guys have any recommendations for attaching a manipulator and slides to the lead screw from past experiences?

What kind mounting features are on the nut that you are using? Can you post a link or photo?

[asid61]

Originally Posted by Dominick Ferone

Attached is a picture of our calculations. We are trying to optimize the Rpm, as to not draw to much power and have a good mix of speeed and torque.

You are running the leadscrews far too fast. They literally cannot tolerate those kinds of rpms without failing.
There is another thread that I saw a few days ago that had a link to a chart of safe rpms for leacscrews.

[Steven Smith]

Not sure if it was the same chart, but here is a chart with some guidance on ballscrew/leadscrew design.

http://www.thomsonbsa.com/pdf/bsa_en...og_section.pdf