Spectrum 3847 | Build Blog 2023

I understand the motivation for using a continuous elevator - but any reason for the timing belt over just dyneema? Are you worried at all about tooth skipping?

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We have considered it and it’s one of the reasons we are testing this on the alpha robot. It’s possible this will change to a chain-driven cascade elevator at some point which would be pretty easy.

Continuous timing belt elevators have been done successfully by multiple teams 125 and 2910 in 2018 are two examples that quickly come to mind.


FYI, this finally got an answer:

We cannot rule absolutely on hypothetical ROBOT designs, and the final decision as to legality of a particular ROBOT lies with the Lead ROBOT Inspector (LRI) at each event. However, having a COMPONENT that passively moves is not a change to the ROBOT configuration.


Day 10 & 11: Design Recap

Every Tuesday we hold our design recaps

Elevator Test Video


Day 12: Alpha Elevator Explained

Our Alpha elevator uses a timing belt and idler pulleys as its motion system. It’s designed around the 0.5” hole spacing on the REV Maxtube and uses the REV Max tube plugs to allow us to bolt the idlers into the rail.

The main drive pulley is a 32 HTD 5mm hex pulley with a TheThrifityBot 3D print ½” hex insert in it.

The belt for the elevator has to be designed so that each of the vertical run is perfectly vertical, any angle in those runs will cause the belt path to change total distance as the elevator moves up and down thru its motion.

To allow these to be vertical paths we are 3D printing our own idler pulleys that are based on the width of the 5mm timing belt and the ½” hole spacing.

The 3D printed idlers are explained here

Tube Spacer Link: McMaster-Carr We sanded these and then cut them to length on our lathe.
These spacers may also work for this purpose but we haven’t tried them yet - McMaster-Carr

STLs and STEP files for the pulleys can be found here: Printables

3D printed Idler pulleys for 5mm belts with ½” hole spacing.

For Alpha, we are using 3D printed tube plugs with the ½” OD tube spacer and 3” long 10-32 bolts so they are supported through more of the tube and can’t bend as easily. We haven’t been able to test the shoulder bolt solution with the aluminum tube plugs yet as we are still waiting on that order to arrive. We will decide if we want to use the long bolts with tube spacers or the shoulder bolts in the coming weeks.

The belt is tensioned by mounting one end of the belt to a hex shaft that is spinning in a ½” ratcheting wrench (you can also do this with a ⅜” wrench and ⅜” hex shaft). So if we need to add tension you just turn the shaft a few clicks tighter.

The other end of the belt is secured to the elevator using a laser-cut polycarb bracket and zip ties.

We are currently using TheThrifityBot Constant Force spring and mount from the TTB Elevator Kit but we will likely move to a custom design for our competition robot.

The side bearing blocks on the Alpha Robot are the TTB Bearing Block Kit and for our competition robot, we are planning to use WCP Inline Clamping blocks since their hole patterns line up with the REV tube and tube plugs.


Day 13 & 14: It’s Always a Launching Game

A pretty short one today


Today we experimented a bit with launching cubes and made some progress. There is still a significant amount of work to be done, and we are still changing all the spacing to see how the cube reacts.

)](Week 2 - Spectrum 3847)

Cone Intake:

We found that softer rollers seem to grip onto the cones better. We can intake cones at really aggressive angles. More refinement is still needed.

)](Week 2 - Spectrum 3847)


It looks as though in a majority of your recent tests and set ups, you’ve opted for more of a block of wheels, whether it be 2” compliant or sushi rollers. Have you found a difference between using wheels in a denser fashion vs more spread out?

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“What the heck?” indeed. I need to do some diagramming to make me feel confident that about the repeatability of that point in action. But still a cool discovery.

8 am physics

Looks like the cross section of the cone is a rotated ellipse. And the high point will get more compression from the wheel… creating a greater compression into the carpet on that side, making it easier for the cone to “turn” that way. We’ll be testing that today!

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I wouldn’t recommend sushi rollers; we had one of them cut one of our cubes. I still am not convinced that it is better on the launcher. We are using them right now because those are the rollers we have.

On the Cone pincher, I have a running theory that the roller has to be soft enough to compress at the tip of the cone (allowing the cone to ride up the short roller) while still being rigid enough to compress the body of the cone. Spreading the compliant wheels apart artificially decreases the total durometer of the roller.


I guess it is then how soft is too soft? I sort of can see that theory. But I needed to purchase wheels and had a hard time deciding where to spend money. I am going with softest big wheel (4" green) and smaller (2" black 60A). I would have liked to 3d print wheels, but testing the TPU wheels and stars, they both were the worst.

My basic tug-of-war was to put one wheel in each hand and try to drag with even pressure to see which wheel types gripped better, and in all cases the softer wheel won. The bigger wheels also would win by flattening in having more surface area. My 3d printed TPU wheels were 95A. If going that route you’d want a softer filament.

Where are you sourcing your belt?

This is what we are using on Alpha, bought last fall -

Black 5M Type Opened Polyurethane Timing Belt 15/20/25/30mm Width PU with Steel Wire 5mm Pitch 1-10 Meters Synchronous Belt

We have fiber reinforced stuff coming, Cloudray HTD-5M Open-ended Timing Belt – Cloudray Laser bought it last week but it hasn’t arrived yet.


Day 15 & 16

Cone Intake Testing

  • We are pretty happy with the test version of the cone intake. It’s able to pick up tipped-over cones that have their tip ahead of the flange and the flange within the width of the intake.
  • When at the right height it can pick up standing-up cones as well.
  • There are more failed attempts on in our photo gallery.

Cube Intake Testing

Basic Charge Station Test

  • We are still building part of the station but got enough built to see if Alpha could drive up
  • This is using un-modified SDS MK4i modules in the standard low 2x1 mounting configuration.


Looks great!

Would you mind sharing the distance between the two intake rollers?

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Does your charge station have the weights in it? Those affected the difficulty of ours a bit, making us have to get higher on it to tilt it to our side (building both sections has a similar affect). We are using mk4is, and we need some speed to get up. Not that bad, just something to note

The center point distance is currently ~4.25 inches in order to fit both cones and cubes.

It currently does not. We are yet to attach the second panel; additionally, we are currently sliding the side of the charge station on the carpet. We are getting some FRP today.

Sounds good. It also looked like the front angle of your ramp was too steep. We found that if it is even a bit to steep (5 degrees), then higher speed interactions result in you pushing the ramp up instead of pushing it down, making it much more difficult to traverse


Sorry but to clarify, the center to center of the 30A wheels and sushi rollers is ~4.25in? What diameter are those gray wheels?

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That’s min distance (tangent to tangent) between rollers. We use that distance as it let’s us compare different roller diameters and setusp by using the gap distance and not compensating for wheel diameter by talking about C-C distance in prototypes. It’s more important to know what space the cone and cube are fitting through than the C-C distance.

The top rollers are WCP 3" flex wheels.