I’d like to see a ball screw and nut assembly that could be driven either through a geared assembly or directly via a Vex stepper or motor.
I’m thinking a standard vex shaft machined with threads, end bearings and mounts, and enough shaft left stock to attach a gear or clutch.
I realize the load capacity of such a part wouldn’t be huge, but coupled with the linear actuator rails, it would make possible some interesting designs.
Joe
Ball screws would be a bad idea. Just use normal threads, maybe acme. Ball screws would have such little friction that loads could actually turn the actuator backwards. Regular threads will give enough ‘stick’ to keep the load where it goes.
Can anyone point me to a diagram so I can better understand what you guys are talking about?
My FVC team (FVC-5) from last year had an early design that utilized this very same concept that was put together using only vex kit parts. Ours was basically a lead screw with a threaded beam used as the nut. I will have to post a photo of it later on but for right now, all we did was:
Take a length of square shaft stock and thread one end using an 8-32 die from a tap and die set. We threaded about 10" of the 12" length. You won’t end up with deep threads but step 4 will help overcome that.
Drill an appropriate larger size hole in a pillow block bearing to allow a threaded beam to be forced into it and be held with a good bit of friction. We undersized ours a good bit and used a mallet to force it in.
Drill in a pilot hole in the top of the pillow block to allow a setscrew to ride on one of the flat sides of the threaded beam. We used the long motor mounting screws as the set screw. This was used to additionally secure the beam to insure it did not move when the threaded shaft rotated.
Insert a 1/2" long threaded beam through the hole drilled out in step 2. The 1/2" long threaded beam is a good choice because it is threaded all the way through, and the length and number of threads provides extra bearing surface for the shaft threads to push against without striping out.
Now we had a threaded shaft that could drive a pillow block mount. Thereby converting rotating to linear movement. It was pretty slow moving though. With 32tpi and a 100 rpm vex motor bumped up to 300 rpm through gears, we could only manage 1.5" of movement in 10 seconds. For our application this was just not fast enough, but it worked beautifully.
Ball nuts are like regular nuts but with ball bearings on the inside, so instead of the threads slipping on metal, they roll on ball bearings. I saw a good diagram once but I can’t remember where 
Ok I think I understand what you guys are talking about. It seems to me though that there must be easier ways of achieving the same goal.
I’m lost i dont get what you guys are doing:confused:
a diagram would be very helpful:)
My thoughts have been to use the ball screw to drive the linear actuator.
I’m trying to prototype a small cnc router type setup. It’s more for the programming excercise than anything else, but it’s been something I thought would be neat to build for awhile.
Joe
Well for official vex competitions, you can’t use any parts other than those that come in the kit, available at vexlabs or those few parts that you can obtain on your own like rubber bands, etc.
Ball nuts and screws are the definate choice when you are talking about doing precision cnc work. Other methods (lead screws) have too much backlash to be accurate enough for my taste or purposes.
You can eliminate most backlash in the programming.
I know it wont work for competition but I got a piece of 12 inch long threaded shaft the same size and thread as the vex screws. I then went to work with a dremel on one end to square it so a gear would fit. I used one of the + plates from the vex kit and bent 2 sides of it around a nut, putting a short screw and nut on each side to make it captive. It has a little play in it but remains captive. This then can run up and down the shaft and with the 2 wings still straight out it can be attached to things to make a linear actuator.
Enjoy,
Ray Moore
Get Moose and Squirrel!