Looking for Lead Screws
 

Hi! We are looking for a lead screw for our robot. With the math we did we would need a 3/4" diameter screw with 2 threads per inch to reach our optimal speed. We are having a lot of trouble finding a lead screw that we could use and were wondering if anyone had any suggestions! Thank you, and we will see you at the competition!

Re: Looking for Lead Screws
 

3/4"-2 is a non-standard thread, so I don't think you'll find what you're looking for.

Take a look at McMaster-Carr:
http://www.mcmaster.com/#acme-precis...screws/=vgs0pf

Re: Looking for Lead Screws
 

3/4 sounds like a massive lead screw to me but IDK your application. For that kind of speed you're most likely going to need a multi threaded rod. If I'm understanding you correctly you need 1/2in of travel per rotation. McMaster has threaded rods with that spec :
http://www.mcmaster.com/#99030a500/=vgs23q

That is also available in a 6ft length.

Re: Looking for Lead Screws
 

You must learn the ways of the Master, if you're to come with me to Eliminations.

Re: Looking for Lead Screws
 

Thank You for the help!

Re: Looking for Lead Screws
 

For the best price, buy from amazon.com. If you have prime, you get free shipping too!

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.

Re: Looking for 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.

Re: Looking for Lead Screws
 

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?

Re: Looking for Lead Screws
 

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.

Re: Looking for Lead Screws
 

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

Re: Looking for Lead Screws
 

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.

Re: Looking for Lead Screws
 

Maybe you mean 20 in. per second?

Re: Looking for Lead Screws
 

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.

Re: Looking for Lead Screws
 

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

Re: Looking for Lead Screws
 

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.

Re: Looking for Lead Screws
 

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.

Re: Looking for Lead Screws
 

Igus(FIRST sponsor) has some.

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

Re: Looking for Lead Screws
 

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

Re: Looking for Lead Screws
 

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

Re: Looking for Lead Screws
 

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.

Re: Looking for Lead Screws
 

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

Re: Looking for Lead Screws
 

That's the one. It does depend on what diameter leadscrew you use, but running so fast with a 3/4" will not work. You are exceeding the absolute maximum with a fixed-fixed configuration even at the maximum speed of 2000rpm.

Re: Looking for Lead Screws
 

This is the info from the other thread: http://www.chiefdelphi.com/forums/sh...69&postcount=7

Depending upon design, you might have to consider buckling issues too. The first pdf has a graph to help with that calculation.

Re: Looking for Lead Screws
 

We ordered yesterday from www.Roton.com We used them because I wanted to make sure all parts "mated" the first time rather than having to modify/send it back/wait for right part. We prototype d with smaller lead screws from Igus that we had hoarded from past years. We used Igus' sleeve nut and flange to test. It worked great! We're having a build off between a cable/Modulox modified transmission/CIM lift system and the screw next week. I'll post a video of the build off unless it really sucks badly for both.

Here's what we ordered:

59284 Acme Lead Screw, 1/2 - 10, RH, Steel

89403 Acme Sleeve Nut, 1/2 - 10, RH, Bronze

89838 Flange, Steel

19660 Acme Anti-Backlash Flange Nut, 1/2 - 10, RH, Plastic

Re: Looking for Lead Screws
 

I'll second what other people are saying, this is not going to yield good results. My team used a large lead screw attached directly to a CIM last year to lift our ball holder into the air, and it performed very poorly. If you absolutely have to do this I recommend using more motors geared down some, and making absolutely sure that you leadscrew is fully supported on either end. There is limit for the length of unsupported rod you can have in a system that is based on the size of the rod itself.

I don't think that a leadscrew powered elevator (or whatever you are planning) is a bad idea, just that there are a lot of concerns when using leadscrews that are easy to overlook.

Re: Looking for Lead Screws
 

I second Roton.com as a lead screw source. I've used them for years and had good results. We're using the 3/8" x 1.000 torquesplines this year.

Re: Looking for Lead Screws
 

Whew...I feel better now. I pretty much just fished for the screw system that we needed. It truly stinks that I'm just a teacher and not a seasoned mechanical engineer. We're a bit short on mechanical engineer mentors this season. That forces me to expand my scope...well as much as I can...LOL

Re: Looking for Lead Screws
 

Hello,

I too looked at lead screws for 70", but the critical speed for a long unsupported shaft I calculated to be under 400 RPM which with 1/2" lead which makes it under 3ips. We felt it was not fast enough. Lots for force, but not enough speed.

This is a good reference for calculating force, and critical speed
http://pic-designcatalog.com/images/.../section_3.pdf