Rubber Band Assist for a 2-bar

Hello fellow engineers!

(Thank you to auckland robotics for the photo)

We have a simple arm (see above) on our Sack Attack robot and while it manages to raise about 7 sacks with ease, it begins to struggle as we go higher. The gearing is already very high in torque and we are already using 4 393 motors on high torque mode. Is there any way to add rubber band assist to a simple arm like this in order to allow us to lift more weight? If so, how?

Thank you in advance for your help!

The easiest was would be to add a standoff on the arm and on the back of your upright (gray in the picture) then run rubberbands (orange) from one to another, making sure they go over the back of your arm. This picture shows it better than I can say.

You could use a longer bar for the arm, extending the back (gray bar), and attach a rubber band in the back. If you raise the bar (so the gray back is as low as possible) when you tie on the band, you’ll get maximum tension (upward lift). Optimally, you want just enough tension so the weight of the lift (unloaded) keeps the arm down initially, but when the arm is “pushed up”, it stays put. This video and document also give more detail.

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We took a short piece of angle and made the channel an “L” shape at the pivot point and then used standoffs to connect the rubber bands. We essentially doubled our lifting power and with the rubber bands placed in this location they provide more help throughout the entire lifting process. We have our robot apart at the moment or I would take a picture. I will get you a picture later today. below is a jpg file I did in paint to illustrate what I am talking about.

Some gotchas to look out for:

  1. Sharp edges of the arm or gear or metal pieces - can be alleviated sometimes by going over something smoother like a standoff (but they can get in the way of the arm or need to be higher than your current tower)

  2. Too much help from the rubber bands - can you say catapult? This hurts on the way down as well as unpowered positions like the starting position. It can make it so you no longer fit in the 18" box. Adding and removing rubber bands can be easier to tune than a long piece of black band. So add them each side until the arm is freely balanced is the typical starting load.

  3. Sudden rubber band release pokes an eye out - look at how you can hold the rubber bands on so you don’t hurt anyone around you. Had some close calls on this one before. 1x25 pieces to make a release zone can work for this.

  4. Rubber bands wear out - replace them often. They are dirt cheap. However the black bands wear better. Leaving your robot in the stretched position is the main reason for this.

  5. Stretched rubber bands are more linear in their force than near their slack point. Meaning start further stretched than slacked. Get your rubber bands to help in the right arm lift zone and be slack when the motors don’t need it can be a fine tuning part of this. You may move that standoff a few holes to get the right balance.

Your center of gravity shifts as you lift so you need the rubber bands help in some arc positions more than others. That can be a trick unto itself. You want the rubber bands to be more like a Hybrid SUV - they help at start off the line tremendously and not at all at highway speeds.

Have fun and don’t poke your eye out.

As stated before make sure that the bands don’t run over anything that is sharp or they will break. This is the most common thin that i see in our club. As stated before don’t store your robot in the “down” position or you will ware the band out without even using them. I would replace the band before and in the middle of each day of competition(or more depending on how many games you have). The most assistance is needed at the 90 degree point in the path of the lift, so keep that in mind.

good luck!

id say 4080A had the best design plan. once i made a catapult for a school assignment and thats how we set the bands up so i know that gets you a lot of strength.:D:D

If your robot is still together can you take a picture of this? I’m having a hard time figuring it out.

Here you go. We took the parts we neeeded off of it but did not take it completely apart.

Keep in mind, the longer the bar coming down from the pivot point, the more help you will get from the rubber bands. We only came down about 2" because we were using 4 motors and didn’t need too much more help.

Nice clean implementation of rubber bands. looks good. That is exactly how you should do it.