Re: How do you build telescopes/forklifts?
 

Don't even think about trying to telescope aluminum tubing without some sort of non-metalic "bearings" between the tubes. Just grease alone will not work. Trust me on this one....

Re: How do you build telescopes/forklifts?
 

From personal experience, don't try extending a telescoping fishing pole (or anything really) by pushing bicycle break cable sheathing or similar cable into it between two wheels (either both driven or with one idling).

I've seen a cool rolamite-like design using a chain as a rack, in a manner similar to the metal bands in rolamite bearings, and a driven sprocket paired with an idler sprocket/roller acting like the rollers in a rolamite bearing. I don't remember where I saw it, but I do believe it was on an FRC robot (which I saw in pictures only, so it was sometime between ~'93-'08). I'm not sure how well it worked then, but by using large chain, it could be made incredibly strong.

Re: How do you build telescopes/forklifts?
 

More from the Team 1379 First Overdrive robot experience. We had one square section of aluminum permanently embedded in another. Everyone laughed when I tried to take it out one day.

We machined blocks of Delrin as sliders, but the tolerances were still very tight even with liberal amounts of grease and had to be re-machined a couple of times if I remember correctly. I wouldn't dismiss the 80/20 linear sliding solutions so quickly. Besides the bearings, they have pre-engineered rollers or you could make your own to fit in those slots.

Re: How do you build telescopes/forklifts?
 

330, when they used lifts, used Al C-channel. We'd weld two pieces into an I-beam if we needed extra height over what we could get out of our base pair of sections, and then put the I-beam in between the C-channels. The bottom ends would be fastened together (we used a 2-tower system); the tops would be fastened on the lower side. Each piece had a delrin roller at top or bottom, 2 rollers to a sliding pair. The cabling was steel cable, properly crimped at the ends, cascading run, IIRC. Oh, and we had a gravity cable to make sure the lift came down. 2 FPs through stock gearboxes and a winch drum.

Re: How do you build telescopes/forklifts?
 

One simple comment I will make here is to never underestimate friction. When designing something like a lift where you know you will have a surface needing to rub, roll or slide on another, do not underestimate friction. The amount of resistive force (to lifting) that can result from a poorly designed sliding interface can be staggering.


Also, try to counterbalance your lifter with springs or weight if possible. It will not only make the lift easier to operate (ie: less load on the system), it will also make the system much more consistent and repeatable.


-Brando

Re: How do you build telescopes/forklifts?
 

I know I have some better close up pictures at home, PM me your e-mail address if you'd like them. This is from our Overdrive robot:

http://www.chiefdelphi.com/media/photos/30270
http://www.chiefdelphi.com/media/photos/30267

The whole thing is pretty straight forward:
- Each stage fits inside of another stage, loose enough that it doesn't bind, but tight enough that it doesn't wiggle loose.
- At specific points, there are brackets that hold small wheels on (you can see one of the wheels at the top of the first picture) in order to hold the whole thing together. The outer most stationary section has the wheels bolted to the top, and the middle section fits inside of those wheels. Then the middle section has wheels bolted to the bottom that wrap around the outer section. As you pull the middle section up, the wheels get closer together. As you can imagine, this means there's more stability when it's all the way down, versus all the way up... but still stable enough for our purposes.
- We used a big drum at the bottom, powered by a motor, to lift the arm, and allowed gravity to lower it as we ran the drum in reverse. The cable routing went from the drum to a fixed pulley at the top, down to a pulley attached to the bottom of the middle stage, up to a fixed pulley at the top of the middle stage, and finally down to terminate at the bottom of the inner stage. If required, you could run a cable opposite this one on the same drum to pull it down.

Re: How do you build telescopes/forklifts?
 

As requested, here are the best close ups I have of that robot:





Also, you can see it in action at TBA:
http://www.thebluealliance.net/tbatv/match/2008mn_f1m2
Check out 0:48, 1:06, 1:36, and 2:03

Re: How do you build telescopes/forklifts?
 

found the picture, didn't remember it as well as I thought I did, still a cool idea:

Re: How do you build telescopes/forklifts?
 

In Overdrive we used Bosch (similar to 80/20) and steel cable, and it worked very well (except that the cable would get tangled if something caused the lift to move too quickly, or bumped it upward -- which happened a few times).

Last year for our lift we used Bosch again, only this time we used a 3' four-start screw drive (direct-driven with a CIM, with a shaft coupler and thrust bearing from McMaster) along with aluminum carriages equipped with delrin sliders. This worked *extremely* well for us.

Indeed, it worked so terrifically well that I think that we'd have to have a very compelling reason not to use a similar setup again if required to do any heavy lifting over any real linear distance.

Re: How do you build telescopes/forklifts?
 

Leav,
In our 10 year history, we tried a telescoping pole only once: in 2004, our robot lifted a grappling hook 15' up to grab the bar. We used multiple lengths of PVC pipe of various diameters, so that each section nestled into the next, and we used cascading cabling using 600 lb test Kevlar cord (as we hadn't discovered spectra cable yet). The results are why we never used telescoping poles since.
The first stage cabling was under such high stress that it snapped repeatedly, forcing a complex restringing. We used a dual-diameter capstan pulley to pull the poles back down, since the pole was pivoted to aim for the bar, and as such we couldn't rely on gravity. We designed and built the capstans ourselves to match the geometry of the pole. We used a window motor to do the lifting and retracting.

If we ever have cause to lift again, we will hopefully never use that design again. Even though the pipe sections slid nicely, friction was a big issue. Spectra cable with (pulleys around turns!) is a must, unless you go a chain & sprocket route. And I would concur with others that draw glides are a good approach, as they are likely manufactured to higher tolerances than you can do in the shop in 6 weeks.

Re: How do you build telescopes/forklifts?
 

You mentioned fitting square tube inside itself. Though it is doable, I know that it has a tendency to bind up a lot unless perfectly attached.

Re: How do you build telescopes/forklifts?
 

Team 1675 has built a telescoping lift twice: 2005 and 2008. Both years we used a pair of three nesting aluminum tubes (two stage lift). We did it a little bit different from a traditional telescope though where we had the largest diameter tube on the outside. The smallest diameter tube was our rigid tube welded to the frame. I believe we used 2", 2.5" and 3" tubes, all 3/16" wall (the first year we ordered the wrong thickness and actually had to lathe four of the tubes!!). Depending on your use, 1/8" or maybe even .090" would be sufficient. The first year we simply lubed the tubes and didn't have too many friction problems. The second time we used 1/8" delrin strips mounted to the outside of the inner two pipes 120 degree intervals. The sliding was extremely smooth and our lift was actually angled about 15 degrees.

Both systems were cascading. The first year we used a kit gearbox (essentially today's AM Toughbox) and CIM to drive a 3" drum. We used 1/4" sailing rope (very strong, very light, and quite colorful!) and machined pockets into the tube for the cables to run between stages. We also use Harkin Micro Pulleys. For 2008 we switched to spectra cable and tried to make our own pulleys but eventually installed the Harkin pulleys. This one used a 2" drum and used a dual FP winch. We only used our lift in competition once (at IRI) due to mechanism problems (not lift problems).

You can see a picture of our lift from 08 here. The brown stage is the outer stage and rides up on the gold stage. The gold stage also would raise up over a black stage (not visible) and have a total lift height of about 8'. The black stripes are the delrin slides. The wood was there for stability since we weren't using the lift (the gripper arm would keep stability normally).

Good luck to you!