Forklift Dilemma

[NorviewsVeteran]

We decided to try a forklift style mechanism this year, the polar opposite from our 2007 robot. After some deliberation and a lot of searching and looking at photos on here on CD, we're using 8020 for the sliding pieces and their linear bearings along with it. We have built it to the stage where you pull on the cord and it goes up (and barely bumps the ceiling in the shop at 9' 3") and it comes down needing only a little assistance. The next step is to motorize it, but we can't decide what medium to use: chain, rope/wire, or belts. As a side note, we've already determined to have it as a closed loop so it can be powered down.

Here's what we know about each medium:

With chain, it wouldn't need a drum to collect excess, it would just go past the driven sprocket and be on the other side, never slip (when properly tensioned), but it's heavy.

Rope/wire is lighter but the excess would need a drum to collect it, and if it's a closed loop it may slip.

We hardly have any experience with belts, and aren't sure how much it would stretch over an approximately 20' length.

If anybody has more knowledge or experience on the subject, please let us know what we got wrong.

Thanks!

[xSAWxBLADEx]

our team is building a forklift also and we are using a pully system with wire/rope (it worked well on your '09 elevator)

[McGurky]

Our team is using cable with pulleys to power our forklift, we are accounting for possible slip with a limit switch and clever programming.


While it may not be the best option, it is what we think will work the best.

[Tom Line]

Spectra is one option. It absolutely does not stretch. It's a polyethelene (plastic) based rope / line that many sailboat racers use in place of aradmids like kevlar due to it's high UV resistance and (comparatively) long life. It weighs next to nothing and is far stronger than steel rope. However, it does chafe and will wear through if it crosses sharp corners. It's also pricey compared to wire rope, but you can use tiny pulleys (less than 1 inch diameter).

Of course, the good old standby of cable works well too. Heavy, must have larger pulleys, but has excellent indicators of failure when strands begin to part.

Then there's chain.

You can always use chain to power the first stage, and spectra or wire for the others. That's what we did in '08.

[jsasaki]

rope will stretch, belt wont as much so its either going to be airline cable or chain. I'd go with the cable but that what I know

[mrmummert]

Hi...

This is Mr Mummert from 1610...We used a forklift in 2007. As some might recall we went to the finals with it (we lost by 7 points and because one of our alliance partners could'nt get on the ramp)

Anyway we used a outer lift tower made from 30x30 aluminium and a inner tower made also from 30x30 aluminium. We also used the slides with it. The outer tower had the slides fixed at the top between the two towers and the inner tower had the slides fixed to the inner tower at the bottom. We used No. 25 chain to move it up and down. The chain is in a closed loop. The chain was fixed to the bottom of the inner tower on the back side of the 30x30. It ran over two 18 tooth browning gears attached to the back side of the outer tower. We modified the Fisher Price gear box to drive the chain on a 20 tooth (I think its 20 tooth) gear at the bottom. The gear box is attached to a inner frame rail (30x30 again) and it drives a small chain that in turn drives two 18 tooth gears at the bottom.

To move the arm and claw up and down we made a 12 x 12 panel that is bolted to two pieces of 20x20 aluminium. The 20x20 has one sided 30x30 slides at the top and bottom on each peice. These in turn slide up and down on the inner tower. It moves up or down when the inner tower goes up or down as the panel is connected to two cables. These cables run over two 3 inch pulleys attached to the top of the inner tower. These are anchored to the body of the robot at the top of the tower braces behind the lift tower.

You can see pictures and video of it at our web site (look at the VCU regional team list and you see us there and click on our web link) We are
building a improved version of that robot for this year. We had some issues
with the arm and claw and will improve on it. It was one of the better tube
placing robots that year. One thing to remember about a Forklift design
is the high center of gravity when the lift is up. We were tipped I think three
times that year at VCU. (once when 843 pushed us over in the finals and was disqualified) So you have to be careful.
We used limit switches along the tower that stopped the driver from going up too far and doing damage to the tower
or motors.

[liam.larkin]

I most echo Tom Line's suggestion. We have used spectra cable for a number of years for various types of towers and forklifts. As he points out be wary of sharp corners but that can be done. Run the spectra through pulleys and youll be fine run it on a drum maybe powered by the barbimotor/gearbox. We have used this before as well with great success. Spectra comes in many different flavors and you should be able to find one for your needs.

Good luck

[JCharlton]

I've become a big fan of round urethane belting. It's light, cheap, and easy to make the length you need. That length can be designed so you slip instead of stalling the motors, which believe me, is a good thing.

The only disadvantage is finding the pulleys, but if you have a lathe you can easily make your own.

Another cool trick is making a "quarter-drive", two shafts at 90 degrees separated by a couple of inches or more. We used this is 2009 and it worked great on our ball-conveyor (the "series of tubes").

[Kevin Ray]

We have done fork lift on two previous bots and are very comfortable wth the results of the second year's bot, 2008. We used 3/32 cable and you can see in the first pic that we designed our own pullies because of the problem we had in the first forklift model. We made a recess to accommodate a bearing with a 1/4 in. bore in a small block of acetal (Delrin) and wrapped it in sheet metal. We then drilled an entry and exit hole in the Delrin for the cable and a 1/4 in hole through the side for a bolt for the bearing to ride on.
For the winch we used two FP's with a drum. Please note that you will ABSOLUTELY need a tension take up on the cable between the first pulley and the drum. If the slides are lubricated enough, you shouldn't need to close the loop (which just adds to the weight and headaches), gravity worked well enough for us.



If you'd like more detailed support, feel free to send a private message and I'll send you detailed pics if you'd like.

BTW, that forklift went to 11.5' in under 5 seconds

[dduran]

How do you prevent the motor from back driving when you want the elevator to lock in a particular position? Then how do you release to allow it to come down again?

[Bob Steele]

To prevent backdriving you are going to need some sort of brake. Our lift has a ratcheting system that is controlled by a small pneumatic.to stop it from backdriving the motors..

You could also make some sort of brake disk with a pad operated by a pneumatic.

Oh by the way, don't believe what everyone says about spectra... we use it...and it works great BUT it DOES stretch initially... so you have to pre-stretch it...after that you should be fine..

We have quite a few sailors out here that use it all the time and they have shown us how to splice it to make secure connections... you might want to learn that...or figure out some good knots...

[sdcantrell56]

We are using belt this year and so far it is working very well. It is much nicer to deal with than cable or chain and very light. Also belt essentially does not stretch. The belt we are using has a kevlar tension member.

My team this year is doing something similar, we're using super strong Spiderline fishing line (50lb test, no stretch), but we have issues with stops for the slides. Does anyone know of anything that works for stops on the standard KOP slides?

[joek]

you can get pre-stretched polypropylene rope, that has almost all of the stretch eliminated

No, I meant something that works well for slide stops, I already have some fishing line that appears to be indestructible.