Well, our experience with unbag time has been bittersweet. At our build session last night and our 2nd piece of unbagging time getting ready to compete this week we bent the living ^%$# our of our extrusion at the top of our climbing mechanism. I have the aid of an awesome mentor from Team 67 Team Hot (Mike) which is helping me fix this issue but being a prior military member I want to have a plan B in case we have the same outcome.
If you see the pictures of our robot thus far on our profile you will see that the climbing mechanism, using a REVRobotics single stage extrusion COTS device, is rather tall and makes us very back heavy. Are there any ideas any of you have for a climbing mechanism which I could reuse the Redline 775 motor and 64:1 gearbox I have attached?
I have been trying to brainstorm overnight, although to be honest, I had nightmares of the events of yesterday, to be able to devise another climbing mechanism and looked some of the climbers from StrongHold but knowing my fellow mentors have had MUCH more experince then myself I thought I would ask this question. Any help is greatly appreciated!
That second pic you found was our first attempt a couple of weeks ago at a rung grabbing solution which bent badly. The pictures I just posted are one of the current stage of the robot and the damage from the second test last night.
It will be difficult to keep that top extrusion from twisting from the torque exerted by the weight of the robot. At the very least you should replace the corner gussets and perhaps use a stiffer/thicker material for them.
I would attempt to make a similar hook to what you have, but make it where it will rotate clockwise (based on uploaded pic) and allow the rung and hook to be aligned with your elevator so there is no moment applied to the extrusion. This could be done with a hinge or something similar.
I would also add some triangular bracing from the elevator to the other end of the base of your robot.
Your biggest issue is the twisting of the corner brackets.
If the elevator is ONLY for climbing, I’d put the hooks on the top of the cross bar. That will put the load directly under the elevator instead of in front of it.
Are those bolts on the corners going all the way through the extrusion, or are they connected with slot-nuts? All the way through would be much stronger.
I’d also recommend that the horizontal crossbar be bolted on top of the uprights instead of between. Though that’s more of a gut reaction inspired recommendation.
You may need to chop a few inches off the uprights to make the starting configuration height limit, but that’s not too much work.
Looks like you just need some new connecting brackets, preferably some beefy ones too from the looks of it, assuming the damage is limited to what I can see in those pictures. I would also STRONGLY recommend drilling the mounting holes for the brackets all the way through the extrusion and using longer bolts with washers and lock nuts to secure them.
For a “Plan B”, another option would be to attach a deployable hook of some sort to the top of the arm that releases once it attaches to the bar and connects to the robot with a cable, then use your 775Pro + 64:1 gearbox to drive a winch. If my math is right, that reduction should allow you to climb in under 10 seconds without any issue (and if you use a larger diameter pulley, say 1" diameter, you can climb in under 5 seconds without any problem).
My team uses a piece of 0.5"x0.375" aluminum bent into a hook shape to climb in a very similar way and it works quite well. Just make sure the cable you use is strong enough and have extra of it; you can get “Spectra” cord or Kevlar reinforced cord on Amazon Prime relatively inexpensively (though assume the load ratings are much lower than advertised), we use the 3mm Spectra cord and it works great (in our tests it has about 100lbs more load capacity than 3.5mm Kevlar cord, though your mileage may vary).
Also, if you don’t have the time/ability to fabricate an adequate winch pulley, give TheRobotSpace a call and explain what you’re trying to build, they should be able to recommend some off the shelf components (the new VexPro roller system comes to mind) that will work for a winch, and given your location, they will likely be able to deliver it next-day at regular ground shipping rates (note that their deliverys are picked up around 3pm, so get a hold of them asap if you decide to go this route).
I like the pivoting hook idea. It will take away the torsional load on the cross bar.
Also, do you really need two hooks? or would one do?
and might want to make the hook substantial…we had a problem with ours, it was a bit too flimsy (and closely resembled the construction of yours), and it buckled and dropped the robot, in a playoff round…not much time to fix it, but we did manage to have it working two matches later.
Replacing the top extrusion with a 1 x 1 x 1/8” wall tube will significantly improve your torsional stiffness.
For Sections In torsion:
Theta = (TL)/(GJeff)
T = applied torque
L = Length of the section
G = Shear Modulus (material property for 5052 Al this is ~26GPa)
Jeff = Effective Area Moment of Inertia or Torsion Constant for the section (I’m fuzzy on the terminology here)
Open sections do almost nothing for torsion, so we can effectively ignore the outside part of the extrusion and treat is as a 0.35” diameter solid aluminum tube (probably not completely accurate but will likely be better than the actual extrusion). For a solid section J = 1/2 pi *R^4.
Thin wall closed sections are represented by J = (4 t (Aenc)^2)/S where S is the circumference of the enclosed Area.
Running the numbers (I’d normally never use inches)
J_ext = pi/2 * 0.175^4 = 0.001473 in^4
J_tube = 4*(0.125)/(4)*(1x1)^2 = 0.125 in^4
That means the tube is ~ 84 times stiffer in torsion than the extrusion (please correct me if my math is way off - bottom line is it’s way stiffer)
Unfortunately this means all that load is going to go into the brackets… I think we can work through this though. As an alternate I think the detachable hook mentioned above is the easiest to implement given what you have in place, but I don’t think it’ll be necessary.
Hey, another rookie team from right down the road here. Personally im not a fan of elevator climbers because you need to line up near perfect and theres a lot of components to deal with. Im not sure if this idea is doable for you before competition but it took us less than half an hour of our bag time to completely redo our climbing mechanism (we tried using an 18" piston with hooks on the end at first, was too hard to line up).
What you can do it a winch and detachable hook. Set up your redline with the 64:1 up as a winch, then use a piece of 1x1 or 1x2 stock as an upright to 1" less than max height (gotta keep the inspectors happy). From there use a window motor from kop or a redline/775/550 with a gearbox to swing an arm with a detachable hook up to the bar, and then drive away to detach the hook and winch up.
I know that could be a little hard to visualize and take about an hour of your time, but its a very light mechanism and can be mostly done with your out of bag weight and then brought to competition.
7226 will be at Lansing at 4:45 right before pits open on thursday and you’re more than welcome to swing by if you have any questions.
that’s the main reason I suggested using only one hook, instead of two. Lots more tolerance for lining up. (although if you have two hooks, you have even more tolerance, if you realize that you only need to get one of them on the rung)
Another alternative is to have fixed hooks, but climb via a cable & winch system rather than by your elevator mechanism. Done right, this dramatically reduces the loads on your “hook deployment” mechanism. Plus it is easy to build and gear a winch to lift a 150 pound robot. We have a “buddy bar” on the robot and are capable of lifting a second robot. Plus you can gear the elevator for deployment, and the the winch separately for the required lifting.
We have a elevator design, with the hooks attached to the arms of the intake mechanism. We are using blue steel rope, which is velcro secured/routed down the intake arms, which is easier to route and work with than cable.
We created a rectangular box out of 80/20 extending to our frame perimeter. We connected it to the elevator with standard gussets from McMaster or 80/20. We gusseted the corners top and bottom and added a gusset tooth. We hang across the retangular bars. We use a dual input versa planetary geared 40:1 with 775 pros. Drop me an email for more info or photos.