Throwing Problems/Motors

[BBray_T1296]

Originally Posted by stretch851

Yes our pivot is three foot off the ground. our arm is curved so that it's height/length(frame to pivot, dy) is about two feet. Oh what does the 1:1 ratio do? We figured a 1:1 ratio was good.

You are already running a net reduction of 8.45:1, from CIMs to arm, which is lacking torque needed to throw the ball reasonably.
If you changed one of the pulleys in the setup by factor of 2 (doubling the highest pulley, or halving the lowest pulley, you will change the ratio to a 16.90:1, halving the "top speed" but doubling the torque. You are never making it to the mathematical top speed anyways, because the arm is unable to accelerate to that number in the 120 degrees.

What you want to do is optimize the ratio of top speed to torque; having enough torque to reach the top speed, while having enough speed to throw the ball effectively.

Let's get into the math: You want to shoot for roughly the 4000RPM range on the CIM side of the gearbox. With your current setup, that would (in physics land) get the arm moving at 475rpm, with the tip of the arm (2' long) moving at 100 feet per second. Now obviously that would never happen and is basically completely impossible. Now the ball is not at the arm tip but is probably about 18 inches from the fulcrum (let's assume this). I don't know what speeds you were looking for, but the 15-20 feet per second range would definitely make it a competitive shooter (depending on the launch angle).

To achieve this speed on the arm, you will likely want to adjust the ratio on your pulleys from 1:1 to between 3:1 and 5:1. Of course I am working from a mathematics perspective so it may be best to buy 2 or 3 different ratios worth of pulleys and swap them in and out to get the best ratio.

[stretch851]

So besides changing the pulley ratio, how much would the numbers change if we changed our gearbox ratio to 5:95? I'm sorry but I couldn't follow how you produced all your calculations.

[EricH]

Originally Posted by stretch851

So besides changing the pulley ratio, how much would the numbers change if we changed our gearbox ratio to 5:95? I'm sorry but I couldn't follow how you produced all your calculations.

5:95 is 1:19 gearbox, which is an overdrive gearbox (increase speed, decrease torque) and will not help anything.

I think you meant to say 95:5, or 19:1.

Let's walk through the math (I'll try to keep it simple).

You have 8.45:1 and 1:1 ratios. These combine by multiplication to a 8.45:1 overall ratio. Note that a 1:1 ratio just means that you're moving the same power to a different place (ignoring efficiency losses). Take the CIM free speed (without load), and divide that by the ratio (8.45) to get the current theoretical RPM of the shooter. (Torque gets multiplied by the same ratio.) Using circle math, you can compute a linear speed for the tip, as well as force on the arm (and ball, by Newton's First Law) and the torque applied.

But, we know that an 8.45:1 reduction just isn't enough. What BBray suggested was to change the 1:1 reduction to a 3:1 or a 5:1, something like that. Using a 3:1 reduction (3x the teeth on the pulleys where the arm flips) results in (8.45*3): (1*1), or 25.35:1. Run through the numbers again for that reduction.

I'm sure someone can go more in depth at another time with gearing theory, but this should get you started.

[chigskonkwo]

Our team also had our catapult shooter powered by motors and it seemed to be powerful enough, but with the ball it didn't have enough power to get it high or far enough. The team decided to change the entire design to pneumatics (with one week left ) but got everything on and working in 3 days, and it works great , better than the motors.