I know this is a super silly question to ask but things really aren’t going so smooth for us lol, we’ve got a 32 inch long arm that will be lifting a hand that’s around 20 pounds or so. We haven’t been able to test it because our chains are taking really long to arrive and our captain is second guessing the idea. We have two neos with 320:1 gear ratios. And we’re not really sure if it’ll hold up during the competition, especially with that steel hex bar. Honestly not sure where our designer and captain were going with this but we need some tips. Would it hold? If not, what could we use to help it hold?
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though i don’t have all the data, there’s a helpful website that can do some of these calculations, to me it looks like it’ll work but i’d take that with a grain of salt
also if it ends up being too heavy you could add some lightening holes or use a lighter material for the shooter/intake
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Your robot looks very similar to Quokkas’ Ri3D! Check them out for possible live action as you wait for your parts.
Can you clarify a bit with this statement? Are you asking if the motors, chain+sprocket would be able to hold the arm upright?
Some quick/rough math with the JVN design calc shows that you should have the torque to make that arm move.
That being said, I would have some concerns about the steel hex shaft bending under such large loads, especially if the arm were to get hit or run into a wall.
Im not sure of your teams resources or timeframe, but replacing that hex shaft with a MAX Spline Shaft would be a pretty good improvement here.
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Looks good
Honestly both, we want to know if that steel hex can handle all that pressure and it our chains and gears can as well.
Hey there. From what I can tell in your image, your failure point is less likely to be the chain, sprocket or steel hex shaft and more likely to be the joint between the hex hubs you have connecting the shaft to the arm. Watch out for slotting in the aluminum tube wall where you fasten the tube to the hex shaft.
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So something you might be able to do to really strength up the pivot point with out TOO much work, is putting the sprockets on the outside of the pivot and screwing them directly to your moving arm. This would put most of the torque on the screws joining the sprocket to the moving arm and get it away from the pivot reducing the force that would be on smaller hub/steel hex shaft.
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Can I ask how you are getting that total reduction? The amount of torque through your steel shaft will be problematic, you are much better off making it a dead axle.
Source: this is our steel shaft last year with a single NEO on a 175:1 reduction I believe
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Yeah. Consider also that that torsion effect is magnified by the distance between the applied torque and the reaction torque. If you were to have much less distance between your motor and your rigid joint, the twisting would be reduced.
Welp seems like we’ve found a solution, from being almost impossible to lift with two hands it is not fairly easy to lift with one hand.If anyone can think of some issues this can lead to along the way it would help a lot. Currently it doesn’t interfere with our shooting at all
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Nice robot! I reckon the arm will work at first, though I do have a few concerns-
It appears as though your sprockets drive the hex shaft through hex hubs, which in turn drives the arm. If your hex hubs are anything like the ones I’ve used, there may be a bit of slop/backlash between the sprockets and the arm due to the loose fit of the hex hubs, which may impact shooting precision.
I suspect the steel hex shaft is not the biggest weakness here - On the original quokka bot we noticed the backlash in the arm greatly increasing over time. We had the arm tubes mounted directly to the sprockets with four bolts through the hub pattern, and we found that the holes begin to oval out with continued use, allowing some play between the arm and the sprocket. I fear that you’ll have a similar issue, likely worse due to using heavier aluminum in the intake plates instead of polycarbonate.
It seems like you could fix all of these things pretty easily, though- would it be possible to move those sprockets to the outside of the superstructure, on the inside of the arm? You’d obviously have to flip the motor plates as well. this way you arne’t relying on the hubs and hex shaft to transmit all the torque.
Furthermore, you can use the six outer holes on the sprockets to bolt through into the arm (with spacers for the width of the chain) to solve the issue of ovalling out. The larger radii’s to the center pivot means less shear force on the bolts.
Welp seems like we’ve found a solution, from being almost impossible to lift with two hands it is not fairly easy to lift with one hand.If anyone can think of some issues this can lead to along the way it would help a lot. Currently it doesn’t interfere with our shooting at all
The elastic bands and creative and will totally help, but I suspect you may still face some issues with some aggressive defense and whatnot.
Best wishes to 8871 this season!
It will work. We have similar dimensions to yousame arch and we got it working with a 288:1 reduction with 2 falcons. You just gotta be careful when setting up the motors so they dont twist off.
Th hex shouldnt twist if u drive it via a sprocket bolted onto the boxtube
Yes, that is what I mean by dead axle. The torque runs through the sprocket rather than through the shaft + hub
I wouldn’t be surprised if that steel hex shaft twists with that design. I would definitely have replacements on hand. If you can, I would bolt the arms to the sprockets using their outer screw holes. That will greatly reduce the torsion on the shaft, even if you maintain the live axle setup.
Your gearing is more than sufficient, it should be enough to lift the arm with only one motor, let alone two. We are using 120:1 on a arm that is 20 pounds and a little less than 24" right now.
Make sure your gearboxes are rigidly mounted to the chassis, because the mounts will see a lot of force.
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We have nearly exactly the same components as you and it works well. There is no problem moving the arm under power, but it will fall slowly while disabled/in brake.
Suggestions of trying to strengthen your axle are good - but the first thing I would do is take weight away rather than add strength. The load on the axle is a multiplier of weight and distance away from the shaft, so reducing all that mass far out would help significantly.
The biggest help would come from lightening the two heavy 1/4" aluminum plates. Polycarbonate is less than half as dense as aluminum, could you cut and replace them with poly? If that wouldn’t work, then drilling large holes at open spots in the plate would relieve stress on the arm without sacrificing strength. Some less intrusive options are shifting the 2x1 crossbar closer to the pivot, rotating the motors so the wires face the pivot, moving the motor controllers closer or even off the arm, etc. Any decrease in torque helps.
Once weight has been reduced, others’ suggestions like a deadaxle will increase your precision and rigidity. The good thing is that by minimizing the arm’s moving mass, its not only easier on your physical superstructure, but the pivot motors don’t have to work as hard, you can move the arm quicker, and your robot’s center of gravity is affected less by pivoting up and down. Good luck with your robot!
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One thing that you can do if worried about hex shafts is replace it with WCP’s spline XL shaft SplineXL – WestCoast Products
You can also use REV’s Maxspline if it is easier. They both make sprockets that fit directly on this shaft and with spline XL in our testing there is almost no backlash between the shaft and sprocket. You can buy bearings for it and rev sells shaft collars that work with both shaft shapes. We have used the spline XL this season and it works really well so far, let me know if you have any questions and hope this helps!