How do you build telescopes/forklifts?

[Leav]

Hi CD,

I've been involved with FIRST since 2005, but never been involved in building any kind of telescoping arm or forklift device.

I got to thinking about it and couldn't think about simple ideas on how to solve the main problems:

what slides past what? for telescoping I would think it would be a square tube within a square tube with some wheels or rollers to allow movement. But most forklifts use two sets of tubes to improve rigidity so how do you link them and allow them to slide?

I can't imagine drawer slides are used extensively since they weigh so much.

regarding opening the telescopes and lifts: I'm guessing the cable+drum+pulleys system is the standard but with the telescoping arms some things just don't add up regarding the way the cables need to wrap.

I would appreciate some explanations and also pics of said systems would be very much appreciated. I couldn't find any closeups.

I know FRC233 (Pink) is very strong with telescopes. any other teams to look out for?

Thanks for your help in educating myself and hopefully others!

-Leav

[EricH]

It's been a while since 330 did one of their blue lifts...but the last time one of those broke, it was self-inflicted and it merely bent.

Lifts do not have to have their extending sections inside each other. In fact, it's a lot easier to do a cable-powered lift if they are not inside each other.

I'll mention cabling routes here...

There are 2 types: continuous and cascading. See this thread for explanation. (post #3) If cabling is used, it is best to ensure that there is a "gravity" cable to make sure that your lift can retract.
There are, of course, other means of telescoping.

If a 2-tube-set lift has the sections outside of each other, each "level" has a partner joined to it in the other set. Joints are typically at the bottom and on one side.

You might want to take a look at 25's section in the 2007 Behind the Design book, as well. There is another robot in there as well--19??--that had a telescoping arm, IIRC, but it was an internal-section one. Oh, and don't forget to look at 357 in the same book...

[AdamHeard]

Originally Posted by Leav

Hi CD,

I've been involved with FIRST since 2005, but never been involved in building any kind of telescoping arm or forklift device.

I got to thinking about it and couldn't think about simple ideas on how to solve the main problems:

what slides past what? for telescoping I would think it would be a square tube within a square tube with some wheels or rollers to allow movement. But most forklifts use two sets of tubes to improve rigidity so how do you link them and allow them to slide?

I can't imagine drawer slides are used extensively since they weigh so much.

regarding opening the telescopes and lifts: I'm guessing the cable+drum+pulleys system is the standard but with the telescoping arms some things just don't add up regarding the way the cables need to wrap.

I would appreciate some explanations and also pics of said systems would be very much appreciated. I couldn't find any closeups.

I know FRC233 (Pink) is very strong with telescopes. any other teams to look out for?

Thanks for your help in educating myself and hopefully others!

-Leav

254/968 had a solid lift design as well.

There is a manipulator presentation from 330 (and I've seen others as well with this information) that outlines the more common ways of routing cable/chain for the lifting motion.

173 had a very unique design in 05, I think they reused it in 07 as well.

[joek]

Originally Posted by Leav

Hi CD,

I've been involved with FIRST since 2005, but never been involved in building any kind of telescoping arm or forklift device.

I got to thinking about it and couldn't think about simple ideas on how to solve the main problems:

what slides past what? for telescoping I would think it would be a square tube within a square tube with some wheels or rollers to allow movement. But most forklifts use two sets of tubes to improve rigidity so how do you link them and allow them to slide?

I can't imagine drawer slides are used extensively since they weigh so much.

regarding opening the telescopes and lifts: I'm guessing the cable+drum+pulleys system is the standard but with the telescoping arms some things just don't add up regarding the way the cables need to wrap.

I would appreciate some explanations and also pics of said systems would be very much appreciated. I couldn't find any closeups.

I know FRC233 (Pink) is very strong with telescopes. any other teams to look out for?

Thanks for your help in educating myself and hopefully others!

-Leav

we did this in '08 with 80/20 and some sort of sliders, using pulleys and rope to lift it

[lbarger]

We've tried the 80/20 extrusion and Delrin sliders as well, but did not like the binding. That is why we went with draw slides.

With the extrusion/slider set up you typically need two sliders on each 'post' for stability. To minimize binding you want to keep the sliders as far apart as possible. This usually means mounting one slider at the top of the lower lift member and the bottom of the higher lift member. As the lift is extended, the slider on the upper member moves with it and gets closer to the the slider mounted on the lower member. Some experimentation can determine just how close the two sliders can approach without bind. You can then put in stops.

A general rule of thumb is bearing surfaces should be separated (lengthwise) about 3 times their diameter (or width). My experience suggests this may be a little close for 80/20 hardware so you may need 4 or 6 times the width. (For a 1" extrusion this means 4 to 6 inches between the slider pairs.)

One last though on bind: Aluminum is actually have fairly 'sticky' material. Can you substitute another material or cover the aluminum (plating, enamel, powder coat)? Dry lube sprays may also help.

[Teched3]

Just to add to some of the suggestions on multi-stage lifts, Team175 (Buzzrobotics) has used aluminum square tubing, aluminum round tubing, as well as PVC for the stages. Provide enough clearance for Delrin bearings on the aluminum so there is no metal-to-metal contact between stages, and also reduces friction between stages. Instead of using metal cable, we use Spectra braided Cord (1/8" - 1050 lb tensile strength) Which has lighter weight, and allows for much smaller pulley radii, saving weight and space. In addition, we used a double drum-one side to wind up cord for lift, and the second to wind down for retraction of the lift, driven off the same motor. We install an encoder or a multi-turn pot to control lift movement, and put tensioning rubber bands on the cable ends to provide some extra length that is needed due to cord overlap differences on the drums. This also keeps tension on the cord so they won't come off the pulleys. Good Luck prototyping cable routing.

[Al Skierkiewicz]

Leav,
We have used manufactured slides in the past. I think our Triple Play robot used slides from McMaster. In other years we have manufactured our own using sheet aluminum bent to nest and telescope. Almost exclusively we use Spectra cable I think from McMaster. It is pretty amazing stuff as far as strength vs. size. We manufacture cable reels using delrin and string them through each section of lift from top to bottom of each section. As the reel pulls on the cable it becomes shorter raising the sections. If you design for two sides to lift, I recommend using a dual reel so that the cables are drawn at the same speed and distance. We have generally used FP motors or window motors in the lifts in the past.

[Jeff Pahl]

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....

[ericzundel]

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.

[EricH]

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.

[Brandon Holley]

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

[Jon Stratis]

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.

[Jon Stratis]

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

[Mr_I]

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.

[PingPongPerson]

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.