Building The Best Arm

At the beginning of this season, I feel it’s safe to say that no one on our team had a really good idea of how to make the robot have a truly functional and sturdy arm mechanism. We had ideas, no doubt, but they all involved string or some other type of line which ran through a series of bearings and sliding aluminum channels/ drawer slides. While there is nothing wrong with this idea, and in fact we used this design in building Kenny last year (see video below, 1:20), it is simply not as a refined nor controlled way of building the arm as certain other alternatives.

Now I feel I have a much better understanding of how to assemble a well functioning robot arm system. Bear with me as I attempt to explain.

The Big Idea:
While there are some pretty linear truths about which arm design is better over all, there are certainly exceptions to these preconceived notions which all likely are a result of adapting to the task at hand. In other words – depending on what you’re trying to do with the arm, the “best” arm design may not be the prettiest or the sturdiest. Although there are definitely some changes I would make to the arm we had on last year’s robot to have a better chance at winning, I would be hard pressed to change the motion in which the arm is extended upwards. I don’t feel that such a long arm would be better served by a series of heavy rack and pinions, as perhaps this year’s challenge required. So to reach a conclusion on this matter, I’ll say this; figure out what you want the arm to do, and tailor every decision you make afterwards to fit that.

The Arm Itself:
The second decision a build team will face in creating the arm of their robot is what material the arm needs to be make out of. A lot of this will depend on what the arm is doing (if you’re confused about this already, reread the first step; otherwise continue). For instance, if the arm is trying to lift a 0.4oz plastic donut 3 feet in the air, the thickness and structural integrity of the arm (slides/ tubing) is less important than how much the arm actually weighs. believe it or not, gravity does have a pretty substantial effect on how high the Tetrix motors will let us lift stuff, so cutting weight here and there would be important in that scenario (cutting weight could even potentially mean removing sections of the interior of the arm through drilling or by use of a Dremel tool). On the other hand, if the robot is trying to lift something heavy, then the material you make the arm out of needs to structurally sound. Given the limits of the Tetrix motors, the arm should always remain lightweight to avoid overstressing the motors. In a nutshell, aluminum and plastic is your friend when creating the arm – thick steel is not.

There are a few different ways to make a robot arm move the way you want it to move. One of the simplest is a pulley system arm using bearings and some sort of string or cable.


This concept can be a very effective design if done correctly. As with all designs, this will work best with some sort of planning in CAD and CNC milling. the string or cable should be tested for toughness and should have a diameter greater than the thickness of 10 sheets of computer paper (this measurement is a guestimate, but I have found that using thin string or even heavy fishing line does not hold up to wear and tear as well as thicker cables/string). The pros of this system lie in its relatively simple design, and ease of construction. The cons are that the string or cable has the potential to slip, the system works best in the vertical application, and it is not suited for lifting heavy objects.
Another basic arm mechanism is the rack and pinion design.


The rack and pinion arm before the bucket was attached.

The rack and pinion arm before the bucket was attached.

The rack and pinion arm mechanism gives the operator absolute control and precision when operating the arm, due to its gear based construction. For FTC teams, I highly recommend the Tetrix rack and pinion system (not pictured). Although I generally dislike Tetrix parts, their rack and pinion system is a solid build, is surprisingly easy to assemble, and holds up to wear and tear. The pros of this system are its ability to lift heavier arm components, it works in both the vertical and horizontal application with no discernible detriment on either, and it is arguably less susceptible to wear and tear. cons are that the system is not easily made without a CNC or buy purchasing online (which can be expensive), the system can occasionally cause the gears in the (piece of shit) Tetrix motors to break (we broke four in one night this way), and in terms of the space it takes up, the system is not ideal for especially tall/ high reaching arm designs.


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