Torque requirements for launching a soccer ball

[kmcmacki]

I am designing a soccer ball launcher (like the one in the attached picture) for school and am having trouble figuring out how much torque I need the motors to have.

I am trying to follow an energy analysis approach to get these calculations done. Attached is a file of my calculations so far. The idea behind them is that the wheels are already spinning with a tangential velocity equal to that of the ball launch speed so that there is then a transfer of energy from rotational energy in the wheels to kinetic energy in the ball. Then, the torque the motors must be able to provide is dependent on the time desired for the wheels to get balk up to speed after launching the ball.

Just looking for advice from anyone who can tell me if this is the correct approach and if not what the calculations should look like for this part of my design. Thanks.

 

[berkeman]

Welcome to PF, and fun project!

Will you be wanting to incorporate different wheel speeds to allow you to launch curved paths?

 

[kmcmacki]

Welcome to PF, and fun project!

Will you be wanting to incorporate different wheel speeds to allow you to launch curved paths?

Yes but for the calculations I will be assuming that the wheels will be rotating at the same speed for simplicity sake. To accommodate for this I have set the launch speed to much higher than necessary.

 

[Lnewqban]

Welcome!

I would sugggest to estimate the necesary rpm's of your motors first.
If direct drive, your selection would be based on available rpm's ranges of available motors and diameter of wheels.

 

[jrmichler]

The idea behind them is that the wheels are already spinning with a tangential velocity equal to that of the ball launch speed so that there is then a transfer of energy from rotational energy in the wheels to kinetic energy in the ball. Then, the torque the motors must be able to provide is dependent on the time desired for the wheels to get balk up to speed after launching the ball.

I did not look at your attached file. We prefer that you use LaTeX to put your calculations in your post.

That said, the approach you listed makes sense. The wheels will slow down, so the ball velocity will be the velocity after momentum is transferred. There is an assumption that wheel spacing and wheel to ball friction will be sufficient to get the ball up to the wheel velocity. Note that momentum is conserved, while kinetic energy is lost in this system.

If the wheels are several times heavier than the ball, then the wheel RPM drop will be small. You can then simplify your calculations by calculating only the torque needed to get the wheels back up to speed after throwing a ball.

Were you planning on driving will electric motors? If so, be aware that most electric motors heat up rapidly if the RPM drops more than about 2 to 3% from the no load speed.