Screws for Larger Machines ?

[Clockwork]

Just aquired some parts to build a 4' by 7' machine.

The 7' axis mechanism will be moving a 110 pound gantry. This axis will be composed of 4 Thomson linear slides and 2 screws (2 motors). I have the slides but need to decide on the screws to use. Need something in the range of 90 inches in length.

I'm thinking that smaller diameter screws - say 1/2 inch - will be subject to vibration under load. Perhaps a 1 inch screw will be more appropriate.

Will appreciate comments from anyone who has built a machine in this range to learn from your experience.

Thanks

[Rodm1954]

You really are past the limit of screw at 90 inches unless you go rotating nut. A 90 inch screw will whip.
Rack and pinion is a popular option for this size axis and some have use the belt drives as well.

[Clockwork]

Rod

Thanks for the response. It seems that a rack and pinion setup will require a substantial gear reduction on the motors to get a decent resolution. It opens up a whole new set of problems. I'm thinking about it - seems like a good solution.

Belts? Yuk!

[georgeburns]

I'm going to be using a 1/2inch acme which will be 55 inches long. I'm worried about whipping as well. I devised an idea to stop the whipping, not sure if someone has used it before.

Basically... the screw will have several spring loaded doors on each side.

The hinges will be double action such as this... they bring the door to the "closed" position, but let it swing both directions.

http://www.hardwaresource.com/Store_...ts.asp?Cat=118

Planning to use a 1/2ID teflon washer of some sort in between and cut in half.

Not sure if it will work, and is just a contingency plan, so I may never know.

[markus_detroy]

Any screw over 5/8 will cost you and arm and legs....go rack and pinion
its simpler and less issues ,,,can seam no problem long lenghts, most professional machine are rack and pinion