im trying to figure out the mechanics of how to "load" a torsion spring down to a catch. im not sure of how much torque i'll need as i'm not sure of an easy way to measure this.
some quick thoughts were to butcher a ratchet strap / tie down to pull the arm back on the catapult(using a servo?), and use another servo to "release" the catch on the ratchet strap?
or come up with a slotted bracket that runs the length of the side of the "arm" with a linkage going back to a servo.
in all likelihood i'm overlooking a method that is fairly obvious.
my goal here is i dont want to have to touch the catapult, i want to stand back, hit a button, have it load a racket ball and fire,
Hang a bucket of water on the spring. Water weighs almost exactly 1kg per liter. You don't even need a bucket if you just want to guess - like maybe your spring looks like you could use 2 buckets or 4 or whatever.
Winding the spring stores energy. It's easy to calculate how much. Then think about how fast you want to wind it. You winder needs at least this many Watts to achieve that speed. Once you know the power, then you can think of mechanisms to do that. If you need a kilowatt then that is a totally different machine to what you have described so far.
Getting the ball to drop into the cup used to be very difficult. Now it's easy with an Arduino and a model airplane servo to feed the balls. There is still a unit of work to get that mechanism to work.
while that was a good read, i dont think i expressed what i was looking for clear enough.
i'm trying to mechanically devise a method to pull back the arm of a catapult if under tension down to a loading position and hold it there.(this can be two separate devices)
or a method to put a heck of alot of tension onto a spring prior to releasing the trigger mechanism.
I've thought about gear motors, but this entire project needs to be somewhat portable.
I'd implement something like a catapult was used in past decades. A rope winch to arm the catapult or spring, until it snaps into some holding device (bracket...). Then the rope is released again, until finally the bracket can be released for the shot. The winch can be geared to any required force.
brianfischman:
while that was a good read, i dont think i expressed what i was looking for clear enough.
i'm trying to mechanically devise a method to pull back the arm of a catapult if under tension down to a loading position and hold it there.(this can be two separate devices)
or a method to put a heck of alot of tension onto a spring prior to releasing the trigger mechanism.
I've thought about gear motors, but this entire project needs to be somewhat portable.
thanks
"Somewhat portable" doesn't tell us a whole lot. How much force do you want to put on the spring? How fast do you want to wind it? Do you have a motor or a servo that has enough power for that?
I'm still open to suggestions on either using a spring(torsion, or tension) in the case of tension, i was thinking of using a servo to pull the spring tight. if a torsion, was some way to pull the arm back into the loading position, this is where i am lost. i'm not sure how to design that aspect of a rachet with some type of reducer.
by portable, ideally the catapult would be around 3' by 15 inches or so, and as light as possible.
for a trigger, i think i have that part covered. i've got some old metal pcs liying around, and have an arm. i can either use a servo to move a pin through a catch on the arm, or use a limb on the servo to directly hold the arm down(im partial to the first option, as it will have more strength, and only require the servo to latterly move the pin and not have alot of side shear(or whatever its called) on the arms of the servo.
i'm also limited to 6 or 12v or a combination there of, i already have 2 6v rechargeable batteries.
Sorry, I thought you already had a spring. That makes it easier and harder.
How heavy is a racket ball and how fast do you want to throw it? That will tell you how much energy you need to get out of your spring. Remember a spring is not 100% efficient, so you will need to put more energy in. For a rough guess, assume your charging mechanism needs to put in twice the amount of energy as the racket ball gets.
brianfischman:
a true catapult, not a trebuchet or a ballista.
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All of those devices are called catapults - it's a category of weapon, notably using mechanical energy to throw a projectile. the device you are describing, where a wound spring is used to flip an arm that throws the rock, is called an onager. It's the thing you most often see in movies where barbarians are attacking a medieval castle.
Damn good engineers, those barbarians.
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I'm still open to suggestions on either using a spring(torsion, or tension) in the case of tension, i was thinking of using a servo to pull the spring tight. if a torsion, was some way to pull the arm back into the loading position, this is where i am lost. i'm not sure how to design that aspect of a rachet with some type of reducer.
by portable, ideally the catapult would be around 3' by 15 inches or so, and as light as possible.
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In my engineering 102 lab, the students are left to their own resources. The winning catapults (we have a competition) most often use twisted nylon twine as the spring. It's very light, and can hold more potential energy than an equal weight of metal spring.
Perhaps some sort of ratcheted winch would work.
ps, the winning catapults - for distance - are almost always trebuchets. Onagers do very well with the accuracy part.
OK, you are looking for a "linear actuator". These are typically used to move satellite station antennae to position and have servo control systems. If you arrange a trip which releases at the end of the motion, you could control it to pull back just short of that point, then on demand pull back further to release the trip, then wind back to the starting position to pick up the cord again.
Alternate version (talking large here) might be a garage door opener for a panel door.
if you really do have a catapult or trebuchet, your task is to pre-load the spring mechanism. either an actual spring, a spun cord or lifting weight.
depending what you have on hand, you can create some sort of pull. pulleys, gear motor, gears, spool with wrapped wire like a come-along or any such device would work.
you need to decide if you have to have it all done in one step, or if you can take two. pull it 3/4 the way, secure it, re-adjust your point, then pull it the rest of the way.
also, you need a trigger mechanism. I would highly suggest you de-couple the winder from the torsion from the trigger.
use your heavy gearing, pull the tension, lock on the trigger and get the gearing out of the way.
FWIR, a classic trebuchet would have a capstan or two to wind the unit. then a link that would hold the unit and wait for the trigger press.
the Arduino, what we do here, can power the motor, 'see' when it is wound, and maybe release some mechanism that releases the trigger.
the mechanical bits are what i'm having issue with. the controls are fairly easy(i program/troubleshoot/repair/design/install industrial/factory control systems, granted i use a PLC for that, but controls are controls)
my biggest mechanical issue is how to create a release mechanism that will be able to reattach to the "arm" of the catapult to pull it back into position, or if i leave it attached all the time and say just remove the power from the servo/gear motor, how much "drag" will this create against the spring. I do like the idea of using a torsion spring made from para-cord(550 nylon or whatever)
as far as a trigger, i have that covered(i think) i'm just going to use a piece of round stock attached to a linkage back to a servo(i've already tested this out on a piece of scrap material i have lying around)
a racquetball weighs about 40 grams, i'm not sure how far "ideally" to have it thrown, maybe 50-200'
edit: had wrong unit of measurement on weight
i'd love to get a prototype done in the next day or so.
brianfischman:
my biggest mechanical issue is how to create a release mechanism that will be able to reattach to the "arm" of the catapult to pull it back into position, or if i leave it attached all the time and say just remove the power from the servo/gear motor, how much "drag" will this create against the spring.
Drag is bad. Very bad for catapults. You should disengage everything that you can, to make the throwing arm lighter. That will put more energy into the thrown object.
