3
\$\begingroup\$

A spring based catapult is at rest vertically. It gets pulled back to a horizontal position to arm, which activates a self-ball loader. On firing, it springs forward and launches ball.

The arming mechanism will be motor based but the issue I can't get past is the motor on the launch portion. My ideas include:

  • Bike chain and gearing.
  • Some sort of clutch on motor that releases on launch.
  • Some sort of indexed cog off the motor that pushes the catapult to the horizontal but at that point the catapult releases back forward and the cog spins around again to pick up.

I've considered stepper motors but I'd like to keep it as simple as possible.

Further Details -

The spring in question is actually a car engine belt tensioner. So we are talking roughly maybe 30 foot lbs to compress. It is bolted to a 3 ton jack stand which is bolted to a frame.

The frame right now is roughly 30 in long, the catapult arm length is still in question due to distance requirements of projectile.

I am considering making the arm length adjustable.

The ball feed mechanism is basically PVC with spring gate that gets compressed when arm is pulled down.

Projectiles are baseballs, softballs. snowballs etc....

I am currently stuck on finding the right motor. So far I've ripped apart a belt sander and an Oreck vacuum looking for answers, with no luck. I am also considering winch methodology or possibly chain hoist theory. Thanks for the responses.

\$\endgroup\$
  • 1
    \$\begingroup\$ How big are you planning on making it? what is the size of the projectile? \$\endgroup\$ – jsolarski Dec 22 '10 at 3:41
  • \$\begingroup\$ Do you have a design, and are looking for something specific, or are you looking for general ideas as to how to compress and release a spring? A sketch would help. \$\endgroup\$ – tyblu Dec 22 '10 at 18:11
  • \$\begingroup\$ Stepper motors = No! Stepper motors are horribly low torque for their size, and are complex to drive. Basically, you need a high-torque DC brushmotor of some sort. \$\endgroup\$ – Connor Wolf Jan 1 '11 at 10:33
1
\$\begingroup\$

You could probably do the whole thing with one motor and a solenoid using the same principle as a car's start motor / starter solenoid. Basically, mount a gear to the main arm. The solenoid would push the drive gear (which is attached to the motor with a worm gear) against the main arm gear to lower it down. To launch, disengage the solenoid.

Note that you can make your own simple solenoid with a simple electromagnet / spring in opposition.

I haven't thought a whole lot about the loading mechanism... Some more information on the size of the apparatus would be helpful.

| improve this answer | |
\$\endgroup\$
1
\$\begingroup\$

A good source for a drive for something like this would be a cheap electric drill/driver. Easy to control and plenty of torque.

| improve this answer | |
\$\endgroup\$
1
\$\begingroup\$

If you can get a motor with enough torque, I bet you can accomplish this the easiest by putting a larger gear on the motor shaft, and a smaller gear on the catapult arm's pivot point. The larger gear should have more teeth than the smaller gear. Then remove / grind off a few adjacent teeth on the larger gear.

When the motor spins the larger gear, the teeth are engaged with the catapult gear and causes it to move down, compressing the spring. Before the last tooth on the larger gear disengages from the smaller gear, the ball should get loaded. When the last tooth on the larger gear disengages, the catapult arm's gear is able to spin freely -- this allows the spring to uncoil and launch the object. If you get the gears sized properly, the catapult arm will come to rest before the next tooth on the larger gear re-engages the catapult arm's gear.

Just keep spinning the motor, and it'll just reload and launch over and over again.

Sounds like a great project to print on my 3D printer. I'll look into this. This also made me think of the geared motor used in the extruder -- you should have no problem finding a motor with enough torque to compress your spring (although I don't know what sort of spring rate you're using).

EDIT -- I didn't realize that you also might need info about the loading mechanism. Try drawing up something where the catapult arm, when lowered, hits a shelf with a ball on it. This makes the shelf tilt, and the ball falls into the catapult bucket. The act of tilting also prevents the other balls from coming out. When the catapult fires, the shelf tilts back the other way, allowing the next ball to advance. I think it's a pretty simple singulation device.

Your best bet is to prototype this with Lego.

EDIT #2 -- does it need to operate like a real catapult, i.e. angular motion? I think an even easier way is to use linear motion for the firing mechanism, with the same sort of gearing that I described above. I think this would simplify the ball singulation.

| improve this answer | |
\$\endgroup\$
0
\$\begingroup\$

Don't mind the updated name, had to register. Here are some of the answers so far. The spring in question is actually a car engine belt tensioner. So we are talking roughly maybe 30 foot lbs to compress. It is bolted to a 3 ton jack stand which is bolted to a frame. Frame right now is roughly 30 in long. Catapult arm length is still in question due to distance requirements of projectile. Considering making arm length adjustable. Ball feed mech is basically pvc with spring gate that gets compressed when arm is pulled down. Projectiles are baseballs, softballs. snowballs etc....

Stuck on finding the right motor at this point. So far I've ripped apart a belt sander and an oreck vacuum looking for answers, with no luck. Also considering winch methodolgy or possibly chain hoist theory. Thanks for the responses.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ This is not a forum. You should edit your question, not post additional info as an answer. \$\endgroup\$ – Connor Wolf Jan 1 '11 at 10:28
  • \$\begingroup\$ Having trouble getting into your account? \$\endgroup\$ – tyblu Jan 1 '11 at 10:34
  • \$\begingroup\$ @user2447, I can get you merged back into your original account, let me know if you need help. \$\endgroup\$ – Kortuk Jan 7 '11 at 14:58
0
\$\begingroup\$

Back to the gear idea.... wouldn't the large torque from the motor spinning the gear cause a large stress on the gear possibly breaking it when the gear teeth come back around to launch the item again?

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy