Hello, so our team is working on our off-season bot right now. Originally the design called for a pvc spool but it was switched to a 3d printed one because of sourcing problems.
When we were running the elevator up with a battery it stopped 3/4 of the way up. After closer examination it is from the shaft key literally tearing into the plastic.
We then found a metal spool that fit the shaft perfectly. The one for belts on the Kop drive trains. But, we can’t figure out a way with our limited manufacturing abilities to attach the webbing to the spool.
Do you have access to files, a drill, and taps? If so, this is a possibility which I have seen done this year. Here is a very rough sketch not to scale:
If you have a file, you could file a flat on one side of the pulley, drill and tap holes, make a little washer plate (regular washers would also work), then fasten with two button heads. It might be a good idea to melt the edges the the hole with a lighter, or even make the holes with a soldering iron to prevent any fraying. Also, you might want to find a way to increase the size of the flanges to prevent the webbing from tracking off.
Have you considered using an appropriate piece of timing belt, cut down to the circumference of the sheave, then rivet your webbing onto it in such a way that the webbing both holds the ends of the belt fragment together, and grips the webbing for the lift?
Another alternative might be to use a high-strength flexible adhesive (gorilla glue or shoe goo) to do the same thing.
Or, of course, both…
Edit: and yes, Mike’s suggestion of larger flanges is definitely in order!
2nd edit: as you can 3-d print a spool, a totally different solution would be to 3-d print a spool that accepts a COTS metal hub with some bolts, then engages with the webbing, and provides the needed flanges.
This seems like the most robust option. We have taps but I don’t think anyone has ever used them should be an interesting learning experience if we choose this route.
Also yea we have these little PVC Plates I am going to drill the middle out of and use as flanges.
In the past my team has bolted the webbing onto a spool. You do want to melt the hole through the webbing I’ve used the chuck end of the appropriate sized drill bit heated up with a propane torch. You will want to use a button head bolt and a big thick washer or plate just make sure that there are no sharp edges that will cut the webbing or wear through over time. I’d also recommend a single bolt and a larger washer/plate than two bolts side by side. You also want at least 1 1/2 wraps on the spool before the webbing is loaded. That way a lot of the force is carried by the webbing cinching on itself and not on the bolt and hole.
My thought was to attach the webbing to the belt only. You also want to go all the way around the pulley, and use enough webbing so that the rivet is under shear stress, not extension. If you thought you might have tension stress with a normally blind rivet holding two pieces of rubber together, you could add a washer on the other side to prevent pull-out. If you do that, you’re going to need a sprocket without a flange, at least on one side, so you can assemble the setup and slide it onto the sprocket. Then you would add your larger flange to keep the webbing aligned.
Also, Mr. V is correct in that having a couple of wraps that never come off will reduce the stress on your attachment point(s) significantly. As you have already modified your sheave, your simplest route at this point is to add a couple of wraps of webbing to reduce the end stress.
There are many ways of lifting an elevator.
If you look at some of the robot release videos you will see teams that lifted the elevator with a belt and with chain. Some lifted from the center and some teams lifted from both sides of the elevator.
We chose to use chain from both sides in a loop, to balance the lifting force side to side and to pull the elevator down, to reduce the chance of the elevator jamming. We also located our motor in the frame to lower the robot’s center of gravity.
I understand that you are trying to keep things simple and quick to build.
So my suggestion follows a tow rope design. We used 2 AndyMark flanges with a small spacer and bought some thin high tensile rope and tied a loop. Inside the loop we tied our tow rope. I think it was a 1/4" Dacron (sailboat) rope. The sides of the flanges were enough to keep the rope from slipping off.
If you want to go with the belting, you can fold the belt over and sew it closed, then pass the thin rope thru and tie it to the flange. If the flanges are too small to contain the wraps, rivet a disk with a hole cutout to the inside of the flanges.
To keep things really simple, just use spectra line. Its real thin and super strong or just use the Dacron rope (line).
Don’t think so, from the top the rollers should make an “X”
If you plan on wrapping a few times with the webbing, you could just cut a slot into the pulley and put the end of the webbing in prior to wrapping it. The tighter the fit, the better it will hold. If it’s loose or you don’t trust it, add epoxy to seal the deal. The pulley will be ruined for it’s original purpose anyway if you file or drill it, you might as well really ruin it by making a big cut all the way across. We used this method for 2013 on our climbing belts and they lifted 140 lbs at least 100 times without failure.
Looking down from the top, the rollers should point towards the very center of the robot.
So the far right wheel is correct.
Just the two on the near side need to be swapped.
I thought you meant that our wheels were on the wrong side of the chassis. We cant count how many people who have walked in and told us that… After we explained mecanum wheels they got it but… yea.
Thanks for catching that though before our programmers go crazy.
Make sure that you have sufficient space between the spool and the top cross piece to accommodate your webbing once the carriage is raised as high as you want it to go. From how it looks in your first picture (zoomed in on the motor and spool), it looks like you may get only 2~4 wraps around the spool before it all jams against the top cross piece.
You may also want to use something other than nuts as spacers between the cross piece and the gear box. I have noticed that the tolerances on the thickness on hardware like nuts and washers can be quite large. The contact area of the nuts is also quite small so you may want to replace them with a thick plate or a stack of fender washers to prevent crushing the top side of your top cross piece.
You will be putting quite a bit of force on two of the four mounting bolts as you raise the carriage, especially when loaded. Over time, this will likely crush the lower side of your cross piece, making the bolts go loose. You may want to make sure your top cross piece is thicker walled material and since it is aluminum, you may want to add steel fender washers or a 1/16 inch thick steel plate to spread the forces out.
It may be better to rotate the gear box on it’s axis by 90 degrees then attach it to the front and rear faces of the cross piece with two plates (I am assuming that there are mounting holes on the other side of the gear box too). You can then easily get any spacing you want between the spool and the cross piece. The fasteners would then be in shear rather than in compression (and possibly crushing your tubing).
Lastly, you may want to add a shallow U-shaped brace between the two vertical posts, about half way up, attached to the outside surface of the vertical posts. We used the same (or similar) carriage wheels on our practice robot and the carriage wheels to slip between the tubes when the carriage is loaded. There is only about 3/16 inch of overlap of the carriage wheels on the vertical tubes so the carriage twisting slightly will pry the two tubes apart and allow the carriage wheels to slip in between the tubes. You may want to grab the two tubes and try spreading them to see how rigid your tubes are. At Robot Remix, this past weekend, the Finalist Alliance Leader robot had two long woodworking clamps to prevent this from happening to their robot
What we ended up doing was 3d print the spool and guide.
On the spool where the two bolts goes through to assemble the two halves we put in a large enough cut out to accommodate the strapping as well.
You just sew the end of the belt to make a small loop for the bolt to go through when you assemble the spool the belt is on.
It keeps the belt quite flat when spooling since the loop is below the barrel where the strap winds up.
It can feed either direction. The second part is a guide to keep the belt centered while spooling. Though the sides were high enough that it wasn’t really necessary.
Here is a picture of the spool on the robot, you can zoom and hopefully see.
I would recommend using a full belt or chain so that you are powering both the up and down direction.
Gravity does work… just not that well in some situations. You can get belt that you basically cut to length. Then I recommend having the ends be on the carrier portion of your elevator.
This loop style is also beneficial as you can put the motor down in the belly thus lowering your center of gravity (which is generally a good thing).
You can see the recess for the bolt and looped end of the strap.
The spool is printed to use the 1/2" hex directly out of the transmission.
We basically mated two Vex Pro 2 CIM, 2 speed transmissions together. one used the motors and two speed selection really helped when the load got heavy.
The second transmission was setup to shift between a free wheel spin and a one way ratcheting system. We just used a ratcheting wrench and cut a small hole for the reset of the wrench it to stick out of housing.
If you are interested I can post pictures of the assembled winch system.
Or CAD files if you like.
Also as mentioned it would be a good idea to get that motor and spool down into the chassis. Yes the strap will be longer but it will not affect the amount of strapping you need to winch…
Our system was gravity return and with the weight of the tool we never had an issue with the lift lowering when empty.
Decided to just wait to comment on this thread until we finished. We chose the first option of tapping the holes and using a soldering iron to burn holes through.
It was a great experience learning how to tap holes and it allowed us to end with a much more robust product.
Here is a video of the lift working.
Here are some pictures of the finished spool
We are going to do some finishing touches like adding the ramp (which is why the motor is not lower in the Chassis and possibly adding intake rollers.
Thanks everyone for your suggestions. This is the first bit of success our team has had in years. A lifting robot may not mean much to a lot of you but to us its awesome.
There is going to be a reveal video sometime late this week!
That’s awesome! I am glad you got it working. Do you know what you’re using for a tote grabber yet? With that belt design, you might be only limited to one that opens and closes as opposed to “Indiana clips” since it’s only powered up.
