Scissor Design

[Tom McCurdy]

I was wondering if any team has had any experience using a scissor type arm. It currently seems to be our most likely design and I was wondering if it is something that can work. I am worried about how precise the parts must be made to keep the top level, and how we would need to make it slide at the top.

[jgannon]

I know that team 639 had a scissor lift last year to get onto the bar, and they won a regional with it. I would suggest searching for pictures of thier robot.

Mods, could you move this to the Technical Discussion forum?

[ChrisH]

Originally Posted by Tom McCurdy

I was wondering if any team has had any experience using a scissor type arm. It currently seems to be our most likely design and I was wondering if it is something that can work. I am worried about how precise the parts must be made to keep the top level, and how we would need to make it slide at the top.

Just about anything CAN work, but is it the best choice? If you really want to do a scissors (which I don't generally recommend) I suggest you go down to the local equipment rental place and take a really good look at the scissors-type man-lifts there. They have many subtle features that make them work well for their job.

A few other tips.

Make sure the joints cannot rotate out of plane
Make the sections very stiff
Make sure there is some distance between the line of action on the actuator and the pivots.
Be prepared to fall over a lot

Making the lift parts stiff enough can mean extra weight, generally in places you don't want it.

[Cory]

Team 100 used a scissors lift in 2000. It proved to be in general very unstable, as most of these designs are. It can't have any slop in it or it will start tilting left and right and as Chris said, you'll be on your back.

Most of the scissors linkages that I've seen have used very beefy material to help with the stiffness problem

[lsmith243]

I'm sorry this has NOTHING to do with the topic, but does anyone know how to start a new thread? It'd be REALLY healpful if someone would help me out... Kind of a CD rookie, if you can't tell.... Thanks a million.

-Lauren

[Arefin Bari]

I have made scissors on my own (not for FIRST robotics, but just in general)... it works ok, atleast serves the purpose. check out previous robots that has been designed with a scissor arm @ www.firstrobotics.net

- Arefin.

[Stormhammer]

only problem is that scissor lifts tend to be on the heavier side of things

[Gary Dillard]

OK, we're slowly getting our photo gallery back up and running. Here's a picture of our scissors last year:

http://www.spamrobotics.com/gallery2/nats04/P4150052

We used it to lift the hook to the bar and then used a winch to lift up the robot, so it didn't carry the weight of the robot.

When it worked "as designed" it worked well, most of the time that wasn't the case because it was too flexible out of plane - the problem with most scissors mechanisms. But it was definitely entertaining.

[Projectchon]

the way we put our Pneumatics are a little differebt that the other teams, the con about putting it side way is that it can start at a lower high, but it need some kick on the lift at the beging for about 6in, and it pop right out after that.

http://us.f3.yahoofs.com/users/41e70...WrX.BBNIEZvTJU

[coreyjon]

We had a scissor lift mechanism for "stack attack", and boy what a pain in the butt that was. Everything has to be kept in close in tolerance. We probably made the thing 3 times because of problems we had during manufacturing (student where just trying to get away with using the drill press - I was probably hogging the mill). We probably reassembled the thing a million times too, but it did work, and work well it did - until we smoked the fishy motors. we used a lead screw, and follower to actuate the mechanism, and there was so much torque at one point it bent the lead screw, and while it still worked, it went downhill from there. It's cool, if it doesn't have to go up very high, but its a heavy solution. Darn things weight a ton. Good Luck with your design. You could make he spans large, keeping the weight down, but then you would allow for a lot of flexing, and that could lead to disaster, with a lot of weight up high, it could shift easily, and cause the robot to tip on to its side.

[Rob]

In 1999 we made a scissors lift, and it actually worked quite well.

The members were made of marine grade 1/2" plywood
we used aluminum rods to hold its stiffness in plane.

we did one thing very differently than many others to get success with this mechanism. Typically people use some sort of linear motion to "pull" the bottom member and thus cause the raisin scissor action. Instead of this we used a cable and a pulley to lift the middle joint of the X part of the bottom member. We also never let the mechanism collapse all the way down, keeping the angle between each X stage more manageable.

I think I did a horrible job of expalining that, If you are interested in a better explanation, PM me and I will try to get you some pictures.

The only picture I could find is this:
http://www.firstrobotics.net/99Galle.../121-1_jpg.htm

There are more pics on www.rhodewarrior.org, but it is down right now because of bandwidth problems

Hope this was helpful.

PS...the lift we made lifted a load of about 15 pounds and the top of the lift reached over 10 feet.

[Yan Wang]

Our scissor lift from last year was very light and extremely simple. We gave it one function and planned on using it once per game. If you choose to do something similar for stacking tetras, pneumatics may be out of the question because recharging air will take 30-60 seconds. Before each match last year, we precharged using a compressor that we attached to our cart. Pics of it can be found in the Chiefdelphi pictures section under 2004 robots and 2004 Canadian Regional. Also at firstrobotics.net - our website is going reconstruction and has no pics of last year's bot.

[edit]

Just posted some pictures from last year:
http://www.chiefdelphi.com/forums/pi...gle&picid=9411
http://www.chiefdelphi.com/forums/pi...gle&picid=9412

Hope they're useful.

[/edit]