hi guys i have been given an assignment at uni to make a vice

i am fine with everything for the part building but when it comes to assembly im struggling,

[IMG]http://i43.tinypic.com/20rmxxg.png[/IMG]

this is a test screw i am using to try and learn from but i run into a problem:

at the moment the screw and threaded cylinder are mated by a screw mate and that seems ok, the thing is when i put a motor to the end of the screw it turns it but does not make it go up and down the cylinder like it should.

its probably a really easy thing that im missing but i cant work it out! no help on youtube either

any help to get my screw moving up and down would be much appreciated (i can upload more photos if needed)

cheers