Quote:
Originally Posted by Lil' Lavery
Why opt for the "passive" chain rotation of the second joint over, say, a 4-bar linkage?
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We tossed around the pros and cons of the two concepts. There were a few main points that made us lean towards using chain as a virtual 4-bar:
- That design plays to our fabrication strengths. The central tube is the only fabricated item, and the only precision dimension on it is the center to center distance between the holes. Minimizing fabricated parts count was big for us this year, as most of our mechanical team are rookies. For the 4 bar, we'd need to make 4 tubes and 8 end plugs for each arm. Before we ran out of money and weight budget, we planned to have two arms. Throw in a practice bot, and you're looking at 48 fabricated parts. For the chain arm, that drops to 4.
- Mechanical play. In my experience, for linkages to work well, they need to have bearings with absolutely zero play. In this case, that would probably mean good quality rod ends. For 4 arms that's 48 rod ends. Expensive.
- Actuation. Although the prototype had pneumatic actuation, we suspected you'd need more complex control than a bang-bang cylinder to do the things we want to do with the arm. Actuating the chain style arm is as simple as connecting a COTS gearbox to the tube. Driving the 4 bar arm isn't nearly as straightforward.
The nice thing is that the arm ended up with a weight of about 14 pounds. If we come up with a better way (cheaper and lighter), we can swap it out at our first regional.
Quote:
Originally Posted by Lil' Lavery
Chain failure seems like a higher risk than linkage failure, to me.
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Well, that's simple engineering design. If the tension on the chain exceeds its rated strength it may fail. The links, on the other hand, have lots of failure modes, tensile stress, buckling, thread pullout, etc. Used properly, both links and chain will work fine. Used improperly, they will both fail. As I say to the students, "Engineers don't need to guess. You can run the numbers and KNOW. That's what all that math you guys are learning in class is actually used for in the real world."