Choosing the Design
Between my last blog post and now I have done a fair bit of thinking about which design concept to go with for my ping pong ball launcher. Check out my last blog if you want to find out what my three design concepts were. To come to my decision I obviously looked at which design might perform the best but also how feasible it would be to build the model with what I can find at home. I am going to walk you through my thought process and some comparisons of the different designs here.
First of all looking at how the three design concepts are likely to perform. Consistency is really important if I want to be able to hit the target as often as possible, therefore I needed to make sure that I am able to tightly control the force applied to the ball through the launcher. Although there would be a consistent volume of air in the pneumatic launcher through the use of the stopper, the force applied would be heavily dependent on the acceleration of the plunger (F=ma). Also having consistent volume assumes that the plunger would be removed completely after every launch which might slow down the rate of fire considerably. A potential solution to this would be to use an air compressor with an on/off lever rather than a plunger, as with the original concept, but there is no chance I’m going to be able to get my hands on one of those.
This is a big point in favour for the two spring launcher concepts as the force produced is much easier to predict through Hooke’s Law. This states that the force generated by the spring is directly proportional to its displacement by a factor called the spring constant. This is shown in the image below. When we apply a force through a distance on a spring we also know that there must be a equal force acting in the opposite direction keeping the spring stationary, this is called the restorative force of the spring.
Simply by how far back the spring is pulled (or how much it is compressed) it is very easy to predict the force applied to the ball and therefore also the acceleration and velocity (again F=ma).
Another disadvantage of the pneumatic launcher, which is more to do with feasibility than anything else, is that the launcher would need to be well sealed so that the air would not escape. This would be needed for any decent amount of pressure to be generated within the launcher. It could be a tricky task to do this especially with the limited equipment I have at home. For these reasons I am leaning towards the two spring launcher concepts. Choosing between these will likely again be due to feasibility.
I did find some small tension springs (4.5 cm length) at home which could work for the tension spring launcher. I was able to do some simple calculations to find out a bit more on how my launcher would likely work with these. The image below shows the springs with a pen for a better idea of scale.
I calculated the spring coefficient by putting incremental weights on the spring and measured the displacement. The results of this (shown on the graph below) actually came out surprisingly well considering I only had a kitchen scale, an old milk bottle and a plastic ruler to work with.
From the slope of the graph we can see that the spring constant is in fact a constant as the terms are linearly proportional. The average value for k I calculated was 172.3 N/m. This is a very high value meaning that there will be a lot of force produced even with a small displacement of the spring.
eg. For a displacement of 3cm a force of 5.16N would be applied to the ping pong ball
This might not be ideal for the launcher as I would not have a lot of stretch within the spring to play with before too much or too little force is generated. It would also not be very satisfying only to be able to pull back the launcher 2–3 cm before firing.
This is where I am at right now. I will likely try to find some sort of longer spring, either tension or compression, with a lower spring constant. If not I might have to make do with an unspectacular short draw launcher. Hopefully I’ll have finalised my design choice and at least started building it by the next time you hear from me here. As always you can find more regular updates on the Instagram page.
Thanks for reading through my thoughts and analyses, more to come real soon.
