Understanding Skeet Thrower Mechanics: Spring Selection and Release Velocity

[Cam_P]

Hi guys!

I'm struggling to select a spring for a DIY skeet thrower i am making, it'll probably be a similar design to the one pictured.

I just don't understand how to relate the spring energy storage and the tension force from Hooke's law to the release velocity of the clay. or even the angular velocity of the spring arm.

 

[Dr.D]

This is essentially a machine dynamics question, and should not be too difficult (I think). I'm not personally familiar with this device, so I'm not entirely clear as to what I'm seeing in the picture. Do you have a top view or a schematic showing clearly how the device works? If so, I'm pretty sure I can help you.

 

[Cam_P]

this link may help

https://www.amazon.com/dp/B0714QHJ32/?tag=pfamazon01-20

 

[Cam_P]

https://outlook.office.com/owa/service.svc/s/GetFileAttachment?id=AAMkAGJiNGYzMzE2LTE2ZjctNDkwZS04YWU2LTFiZGNiNDZiNWExNABGAAAAAAAMsBEqZjKgS4Lp2EQQihiyBwBl1JHdDzN4S7H4NHxB29OKAAAAAAEMAABl1JHdDzN4S7H4NHxB29OKAAELotnqAAABEgAQAPHu44w2Xx9Gpbb%2BstqU2bw%3D&X-OWA-CANARY=rmkzbi613ESb6tabf406umDXG_7FFNUYjZegY9kXdkzDw3wdLWrlKiWqNFQngyKrC5VpkSAT5zA.&isImagePreview=True

 

[Cam_P]

Above is the sketch i have of the moving parts and relevant lengths

 

[berkeman]

I just don't understand how to relate the spring energy storage and the tension force from Hooke's law to the release velocity of the clay. or even the angular velocity of the spring arm.

For a first cut, assume about 75% of the stored energy in the spring goes into the launch velocity of the pigeon. Do you know about what you want for the Vo of the pigeon? Then just use 1/2 mv^2 for the KE of the pigeon compared to 1/2 kx^2 for the PE stored in the taut spring.

 

[berkeman]

Above is the sketch i have of the moving parts and relevant lengths

Not working so far... Are you using the Upload button? What is the file type?

 

[Dr.D]

You ask about determining the release velocity. I suggest to you that the simplest approach is to employ conservation of energy.

1. Determine the energy stored in the spring with the system cocked.
2. Express the system energy at the point of release. This will include (a) the kinetic energy of the arm, (b) the kinetic energy of the clay, and (c) any remaining energy stored in the spring.
3. Set these two energies equal and solve for the final velocity.
This approach neglects friction, but that may not be too important in this application. The resulting velocity will be a little bit higher than actual for that reason.

 

[Cam_P]

I want the release velocity to be about 15ms-1

 

[berkeman]

What is the mass of the pigeon?

 

[Cam_P]

0.12kg

 

[berkeman]

Well, what are the typical k values for the springs you are considering using? What is the amount of stretch you expect for your spring when in the loaded position.

Please show your work with the equations that Dr.D and I have been suggesting for you to use.

 

[Dr.D]

I cannot see any sketch with dimensions; all I see is . I'd be very interested in seeing what you have designed here. In particular, I'd like to know what causes the actual launch to occur? Does the arm hit a stop, allowing the clay to continue on, or is it some other mechanism?

If you use the energy approach that I suggested, you can assign the release velocity and solve for the spring constant K, and you can play games with the free length of the spring (all in a simple computer code).

In addition to the mass of the clay, you will need the mass moment of inertia of the clay about the spin axis. A substantial part of the kinetic energy of the clay will reside in the spin, so this is an important term.