Undergrad Why does my ball launcher spin the ball and not launch it (air)?

[madeforarcade]

Hi,

We have tried to create various models of a "pingpong ball launcher" that works on a burst of air from a standard compressor.
Each version we tried to build results in the pingpong ball spinning inside the tube instead of launching out of the tube (bernoulli ?)
We varied the air pressure, pipe diameter, loading mechanism shape and direction of the air.

How can we launch a pingpong ball correctly using a burst of air without losing too much energy and without the ball spinning in place ?

tnx,
Pete

 

[Baluncore]

Welcome to PF.

How can we launch a pingpong ball correctly using a burst of air without losing too much energy and without the ball spinning in place ?

You might put a deep circumferential groove inside the launcher, that holds a rubber o-ring. The ball is loaded behind the o-ring. When you build up air pressure in a cavity behind the ball, and the pressure reaches a threshold, the ball will be pushed past the o-ring, and will be launched by the air expanding from the cavity behind the ball. It might work with an o-ring, against the ball, without the groove.

 

[A.T.]

Each version we tried to build results in the pingpong ball spinning inside the tube instead of launching out of the tube

Have you never tried shooting ping-pong balls with your mouth? It works great, and would have given you the clue, that leads to what @Baluncore wrote above.

 

[madeforarcade]

A friction fit with a pressure treshold unfortunately does not have our preference as there is a (very) slight difference in ball diameters and surface roughness with different colors. Also the construction becomes more complex (and more importantly prone to wear/tear or failure).
While we can sometimes launch (40mm diameter) balls out of a (40.5 mm diam.) tube with great force when we simply push a compressor hose into the tube the exact physics elude us as this is only works 20-30% of the time. At other times the ball just sits there spinning inside the tube.

We were hoping to find some solution in directing airflow a certain way, or disturbing air patterns so the spinning can never take place.

 

[Baluncore]

We were hoping to find some solution in directing airflow a certain way, or disturbing air patterns so the spinning can never take place.

I expect the compressed air is remaining in a jet, and not getting behind the ball, it goes around one side. Also, the volume of air required is taking too long to arrive through the air line.

To produce a single pulse of air, you need to use a big diameter ball valve, (20 to 25 mm), with a handle that turns through 90°. There needs to be a reservoir chamber to hold sufficient compressed air, to drive the ball up the barrel when the valve is opened.

 

[berkeman]

We were hoping to find some solution in directing airflow a certain way, or disturbing air patterns so the spinning can never take place.

Have you considered using some sort of sabot to give you a reliable "pusher" piece that separates from the ping pong ball once out of the barrel? You could make some cylindrical sabot things that fit well inside the barrel to give a good consistent seal, and place the ping pong ball on top of the sabot to be launched with the blast of compressed air.

I would use something like this type of trigger for the compressed air, like you see at vehicle gas/service stations for filling tires:

And I might try making the initial sabot pieces out of some plastic things that are the correct diameter to fit in your barrel: