has anyone tried to use a rack and pinion mechanism on their robot before? well, with a cylinder pushing the rack. not a motor spinning the pinion. that ive seen. also seen screw drives or w/e theyre called.
so, i would like to create a rack and pinion mech. few problems tho.
this is for the OCCRA competition. and in its rules it states NO precision tools allowed. ex) no cnc.
well, theirs lotsa rules, but mainily, unless we can hacksaw a flat gear rack from angle aluminum, we’ll hafta use a chain as the rack.
no welding.
so, first q.
how to mount a piece of chain securely with no slippage to a piece of flat or angle aluminum without welding… i was thinking zip ties, but i hope sumonell think of sumthing a bit more reliable.
next. how to keep the “rack” straight without falling off the sprocket. (remember, the "rack " is connected to a cylinder.)
and then the main question. is this gonna work?
so, anyone ever tried this idea of creating a rotary force with a cylinder before? we need a minimum of 140 degrees of motion. our cylinder we chose is a 2in stroke. so with a small sprocket, cant be too hard. except the chain may break? i dunno.
ps. no rotary cylinders in our kit…
well, we hope sumone can tip us on what would be difficult, improbable, easy “shortcuts” or even a picture of an example, it would be greatly appreciated.
main point… we want help!!! aaand oh, one more problem. our first competition is coming up REAL soon! :eek:
If you would be ok with using a cylinder with significantly the force that you actually need, you could wrap the chain part way around the sprocket with one end connected to the piston and the other end connected to a spring with a fixed mount. You would waste a lot of force (half the force?) tensioning the spring.
If you only need 140 degrees of motion, a better solution is probably to attach the cylinder to a spool of some sort with a cable. If i’m not mistaken, the maximum amount of force the cylinder produces will be applied through the whole 140 degrees as opposed to using a fixed linkage (like a cam).
how to mount a piece of chain securely with no slippage to a piece of flat or angle aluminum without welding… i was thinking zip ties, but i hope sumonell think of sumthing a bit more reliable.
There is a chain that has mounting points on it. From there you could bolt the chain to the piece of angle aluminum.
yes. i think thats a easy way. and then ill have to do sum math about the force of our cylinder. ill look into it. thanks!
If you only need 140 degrees of motion, a better solution is probably to attach the cylinder to a spool of some sort with a cable. If i’m not mistaken, the maximum amount of force the cylinder produces will be applied through the whole 140 degrees as opposed to using a fixed linkage (like a cam).
uhh. cant quite picture it. sorry. a simple paint sketch might help me. sorry im incapable.
and then i have to add, i want this compact.
There is a chain that has mounting points on it. From there you could bolt the chain to the piece of angle aluminum.
i doubt we are allowed any other kind of chain than the “normal” ones. sorry. OCCRA rules are pretty tight. thanks for the reply tho!
For this years FIRST robot (Daisy III) we actually used some 35 chain as a track to run our arm on. You can simply use screws or bolts (1 or 2 on each end) through the part of the chain that a gear tooth normall runs. Bolt it to some oak wood or alum. or the like and off you go! If the chain needs to be more taught, then use a router to put a recess in the material (wood or delran is nice) and then bolt it down.
Good luck with your project, Michael Greenley, Team 341
Try mounting th echain inside a piece of u shaped aluminum track. Something like what mounts to the wall for an adjustable shelf system. You can buy the track at most hardware / home stores. Cheap.
How about something like this? Putting the chain around an idler (which could be as simple as a little bronze bushing on a bolt) avoids the need for elaborate tracks and ensures the chain stays in contact with the sprocket. It should be pretty easy to put a bolt or something through one link and attach it to the end of the piston with a bit of aluminum angle.
And the same thing can be done with a cable and pulleys instead of chain and sprocket. The obvious advantages to cable are lighter overall weight, cheaper, and adjusting length is easier (use a turnbuckle). Are there any advantages to using chain?
You could use the chain in the traditional sense, with 2 sprockets and the chain running around those like you would with a motor powered sprocket then attach the rod of the cylinder to the chain in the middle span between the sprockets. This way the cylinder powers the chain in both directions without using springs, ect. If you want a good example of what I mean go look at the curtains on your stage… 2 pulleys, and your hand grabs one of the ropes, you can either pull down on it or push up on it (which actually pulls the rope up) this way your arm is powered in all directions of movement, which I have learned is a very good thing, gravity is nice, but with cables and such which can tangle when they become slack I prefer to power them in both directions.
Try this ms word picture. Hopefully it helps. Also, try a home store. Ask for small metal shelf hardware - slotted channel for the wall and metal shelf brackets. Should be easy to find.
If you complete a loop with the cable/rope, there is no problem with slack (rather, just about as much problem with slack as you would have with chain) So, again, I’m not sure why one would use chain in this situation.
Green,
There are several good suggestions, I like Chris’s chain in a “U” extrusion as a simple and effective approach. But a simple and lighter alternative is to wrap the chain around a sprocket (180 degree wrap) and terminate in a spring. The spring holds tension on the chain as the cylinder moves the sprocket. There is no need to have a bearing surface under the rack and no reason to hold the chain in alignment since that is automatic. This is similar to the loop approach discussed but doesn’t add the weight of the second sprocket, bearing, and extra chain.
One of the best chain rack and pinion designs I have seen in FIRST was on the ChiefDelphi 2000 robot. Its purpose was to entend and retract their boom-like arm. Here is a picture.
I wish I could find a picture of the back face of the robot. This view would show that 2 van door motors drove a chain up and down. The chain was attached to two aluminum tubes and was wrapped, serpentine-like, between sprockets on the 2 motors.
Can anyone from CheifDelphi give us a better picture of this?
This machine was and still is a thing of beauty. It is my favorite robot in all of FIRST, for all time. This design feature was just one of its strong points.
thanks everyone, we decided to use this idea and it worked flawlessly this is for our OCCRA robot, and it is as now in my opinion one of the top competing robots on the field as of now… and we wish to stay up there… we have the two highest scores in a single match. yay! the only difference from al’s plan is we used bungee to tension the chain rather than springs. easier to use. and lighter i think…
