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Class Project: Tennis Ball Launcher

  1. Feb 15, 2017 #1
    What I'm here to find out
    I am in the early stages of planning for a tennis ball launcher I'm doing with my group for a class project. I want to find 2 cheap electric motors that will spin fast enough, and have enough torque, so that I can attach some wheels to the spinning motors for the purpose of launching a tennis ball a few feet. (See details.)

    Does anyone know where I can buy such a thing? One idea I had was to purchase 2 cheap fans and take the motors out of them. But I do not know if that would work. Not sure if I could control them well enough to hit the target distance (see details), or if cheaper motors are available (see details) that would work as well or better.

    Since it's a class project, we obviously can't buy an off-the-shelf unit. However, we can base our design on any existing unit we find as long as we mention the source in our final report.

    The requirements are that the ball must launch 6 to 10 feet (~1.83 to ~3.05 meters). No more, no less. It can initially land at less than 6 feet and then roll the rest of the distance. But the test will be done on grass, so we can't expect much, if any, rolling distance.

    We have a strict $75 budget for this project. All receipts must be kept and turned in at the completion of the project. So keeping cost down is a high priority. Since this is a class project, it doesn't have to be high quality, look good, or last a long time. In fact, it only has to work twice on the day of testing. After that, it can be scrapped. :)

    Note: We cannot just get a long piece of PVC pipe, drop the ball in at the top, have it roll down the pipe, go up an incline, and then launch out the top. The maximum height of the entire device is limited to 4 feet, so that wouldn't work even if it were allowed.

    The device must have some kind of launching system but cannot use fire, chemical explosions, or steam (boiling water). We can use motors (battery powered or extension cord.) Springs and slings are also allowed, but the device must self-reset, so I think a motor driven system would be the least problematic

    In order to minimize unpredictability in the launch distance (due to rolling), I am thinking a steep-ish launch angle would be a good idea. That way the ball would go up in an arc, and mostly stay put when it hits the ground. I'm not sure how much velocity will be lost due to air resistance, but a preliminary calculation tells me that with a launch angle of 70 degrees, I would need an initial velocity of about 5.5 m/s to go comfortably past 6 feet without worrying about going over 10 feet. (Again, due to air resistance, that may prove to be more like 6+ m/s.) But the point is, it doesn't have to go very fast.

    I'm open to any other ideas, but given the above details, and the little bit of online searching I've done, I think motors are the way to go. Since this is a popular project, I am hoping someone here has done this before and can offer some suggestions.
  2. jcsd
  3. Feb 15, 2017 #2


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    Staff: Mentor

    What is the exact project problem statement and what are the exact rules?

    I wouldn't use spinning wheels as my first choice. Way too hard to be accurate with that inexpensively. There are a couple other inexpensive ways to do short distance launchers for this project, as long as the rules allow it. What other light ball short range launcher mechanisms have you looked into?
  4. Feb 15, 2017 #3
    Thanks for the reply. I'd be interested in hearing what the other ideas are. As far as the rules, I'll outline them again here:
    1. $75 budget. Firm.
    2. Maximum height: 4 feet tall. (And also 4 feet wide and 4 feet deep.)
    3. Launch distance 6 to 10 feet. No more, no less.
    4. Cannot just use gravity. (Can't just roll the device down a tube.)
    5. No fire, boiling, or chemical reactions (including gas).
    6. Motors, electrical cords, batteries OK.
    7. Has to work 2 times on test day.
    8. Must self-reset. (Which is why I think springs and slings are a bad idea.)
    I can add this: The story behind the device is that it's intended to be used by a dog. (Though, it won't be.) So the ball needs to be placed into some kind of container that will be at a height of exactly 18 inches off the ground. So it doesn't need to have a large holding bin full of tennis balls that will launch rapidly. The ball will be placed into the container, and then launched. The idea being that the dog would retrieve the ball, carry it back to the launcher, drop it into the container, and off it goes again. Repeat until dog is bored.

    Those are the rules/requirements. Beyond that, we have a bit of creative freedom. If for some reason we needed the ball to be elevated to the maximum height of 4 feet, we can build that into our design. But I don't see any point in doing something like that. I would think the ball could be placed at the required height, and then it could roll into a pipe that would have the two spinning motors connected to wheels that would thrust it up the pipe at some angle that would allow it to launch at an arc to cover the required distance.
  5. Feb 15, 2017 #4


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    Staff: Mentor

    That's very helpful.

    Yes, I was thinking of a reset-able spring-based launcher. That would be much easier and more accurate for short-distance launching, IMO.

    BTW, I think I remember a video of a dog playing with such a ball launcher in the home of his owner. Have you seen it? It was pretty funny how much fun the dog was having retrieving the ball and dropping it in the launcher. I think it may have used spring-loaded arm launcher, but I'm not sure. I can try to find it if you haven't seen it already.
  6. Feb 15, 2017 #5


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    Staff: Mentor

  7. Feb 17, 2017 #6
    A simple slider crank mechanism is enough,first calculate how much velocity the ball should have to reach a height 4ft and distance 10ft , this could be done with projectiles calculations,-where from you could easily find the motor speed etc.,
  8. Feb 17, 2017 #7
    The ball itself doesn't have to reach any particular height. The 4' x 4' x 4' dimensions are the maximum size of the device. But we could launch the ball 20 feet in the air if we wanted, so long as it falls back down within 6 to 10 feet and doesn't roll backwards or forwards outside of that range. The final position is what will be measured, not the point where it first touches down. The ground will be flat, and it will be landing in grass, so the rolling distance shouldn't be much of an issue.

    But getting back to your statement about using a slider crank, I'm linking to this short animation below. I just want to make sure this is what you are talking about?

  9. Feb 17, 2017 #8
    Thats exactly what I mean,
    Position the slider inside a tube
  10. Feb 17, 2017 #9
    and makes the tube at angle of 45degree to the ground you could fire a ball to max distance
  11. Feb 18, 2017 #10


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    A spring loaded pusher with a motor driven retractor/release may be more effective and consistent.
  12. Feb 18, 2017 #11
    Why not look for old printer motors and the drivers
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