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ALARM Robotics
01-23-2008, 04:13 PM
We are making a forklift this year and have come up with a potential design. The design has stages mounted in front of eachother and will use drawer slides or igus linear slides. Do you think there is too much force on the slides if we do a 4 foot stage with 10 inch overlap? Should we go with an imbedded u-channel design instead and how can we do this? Here is the design:
http://alarmrobotics.wikispaces.com/space/showimage/Forklift.PDF
Thank you very much!

Elgin Clock
01-23-2008, 04:16 PM
It should be noted for future browsers on this thread that your attachment is a PDF file, and not an image so that's why it's not showing up with the image command.

http://alarmrobotics.wikispaces.com/space/showimage/Forklift.PDF

^ correct link.

That said, it certainly looks feasable. 10" should be more than enough overlap with a 3 level system.

1" overlap = very bad & unstable!

Your 10" should be about right if not more than enough imho.

Good luck!

Al Skierkiewicz
01-23-2008, 04:51 PM
We have used similar designs in the past and I don't see any glaring errors in the drawing you provided. Always consider the mass of the ball when you are working on drive code. A big ball, high up, will make the robot unstable as the center of gravity moves further from the floor. Practice, Practice, Practice!

KTorak
01-23-2008, 04:59 PM
Looks pretty solid. We did one like that before, but if you want to, you could put each successive level inside the other (like a commercial forklift) instead of in front and having cross bracing.

Molten
01-23-2008, 05:08 PM
KTorak's idea would be good, but I suggest against it. It is simply too time consuming for the season. The actual design looks good. The main problem our team came against last year was that our motor was strugling against the friction/weight. Just keep these in mind while wiring/chaining and you should be fine.

ALARM Robotics
01-23-2008, 05:19 PM
Thanks for everyone's responses so far and correcting the PDF thing, I will make a note of how to do that.

Jon Stratis
01-24-2008, 08:41 AM
Looks pretty solid. We did one like that before, but if you want to, you could put each successive level inside the other (like a commercial forklift) instead of in front and having cross bracing.

I second this notion - It's our teams design for this year, and really isn't all that much more time consuming. We had a prototype made out of wood after the first week and it works very well and is extremely durable, which is something that all teams should think about when going for a tall, hurdling bot - With your CG so high, you do risk tipping over no matter what you do, and if you tip, you have to consider the fact that other teams will be "gently" pushing you out of the way, which could put stress on the lift design where it's not designed for...

Daniel_LaFleur
01-24-2008, 09:13 AM
Your design looks good although I'm a little concerned on your overlap. 10" is about 21% overlap. Considering the weight and combersomeness of the trackball, I'd look for more like 25-30% overlap.

You'll also want to look at the lever forces on your slide pieces to make sure that they will slide (side friction) and not "pull out" (deform) should you get "gently bumped" while you are extended. Also is it powered down as well or are you using gravity?

Also watch your CG when the forklift is extended. I dont see the drive base so I dont know how far forward your CG is. You may find yourself tipping if you drive once extended.

All-in-all, while you have a few challanges to overcome, it looks good.

ALARM Robotics
01-24-2008, 02:09 PM
I think we have the CG issue under control because we are going to attach the forklift to a pivot point so that it can tilt back, getting the weight more centered on the robot. Although, I am very concerned about the lever force pulling the slides apart and the added friction. We could definitely do more stages but thought it might add too much weight/complexity. Ideally, we had wanted to do an embedded U-channel type design but don't see how you could run cables between the stages if you go with drawer slides? Also, the lift will be powered down as well as up.

Jon Stratis
01-24-2008, 02:30 PM
I'll try to remember to take a picture tonight for you of our prototype, it actually works out pretty well. Like you, we have a three stage design, but ours looks a little like:

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The center stage is attacked to the forks, and moves completely within the middle stage. The middle stage moves up to the top of the outer stage (with some overlap) and (not pictured here) the outer stage has a support beam at the top that goes behind the middle stage. It was actually pretty simple to hook up pulleys in the expected locations (the top and bottom horizontal support beams) for each stage - they just stick out the back a little instead of all being directly inline with the plane of the rack.

ebarker
01-24-2008, 08:29 PM
A year or so ago I saw a powerpoint presentation on a team website that talked a LOT about different forklift designs.

What team was that ? Where is the powerpoint ?

JesseK
01-25-2008, 09:03 AM
To figure out what motor to use, I'd suggest using this spreadsheet as a rough guide:
http://www.chiefdelphi.com/media/papers/2059

Go to the "linear mechanism" tab. The two #'s 1885 set requirements for was the "Time to move travel distance under load" (< 3 seconds) and "Max loaded current draw" * 12V < half of the peak power rating of the motor. Miraculously, we found 2 perfect matches of motor/gearbox/sprocket combos!

Note that the output current cannot constantly exceed the fuse for that particular motor, for whatever your load is. Actually, if it exceeds a certain amperage consistently, you will burn out the motor. Also note that with a 2-stage elevator, your load is either ~0.9(2*lifted weight) or 1* the weight but you'll have to move 2 times a single stage's travel distance, depending on how you hook it up.

We're lifting ~28lbs including the ball over 3 stages, so we're accounting for 90 lbs and have a slightly larger sprocket ready if we decide it's doing well enough to add some extra speed.