Final CAD Aseembly with label components
The Air Tank
The chosen air tank needs to be rated for 125 psi to safely hold up to 100 psi, as that is the maximum shop air that ATA Engineering has access to. It also needs to have at least five ports: one inlet port, one outlet port, one port for a pressure transducer, one port for a passive relief valve, and one port for an active relief valve. The ports need to be accurately sized to fit the desired tubing necessary for the solenoid valve. In this case, a 1" port was desirable. After reviewing various air tanks, the final design choice was the Manchester Tank Universal Horizontal Air Tank. This tank has eight ports, one of which meets the 1" port requirement. It is rated for 200 psi, making it well-suited to the team's needs.
Manchester Tank Universal Horiztonal Air Tank
The Valves
The firing valve we choose needs to accommodate the maximum flow rate. This would occur when a 3 kg mass is used and the impact velocity is 13 m/s. To find the mass flow rate in this case, a free body diagram of the slug in the barrel was created, as shown to the right. The mass flow rate is solved to be 0.42 kg/s. Using this mass flow rate as well as the height and desired velocity of the impact, an actuation time can be estimated to be 77 ms, as shown here.
Multiple valves were ordered and tested as part of our risk reduction, as selecting the correct solenoid valve is vital in the mechanism's ability to achieve the maximum velocity. The final valve the team selected was the Directly Actuated 1” NPT US Solids Solenoid Valve. The 1" port diameter allows for the high mass flow rate needed, and the directly actuated Valve allows for the fast actuation time.
After initial testing, back pressure proved to be an issue. To solve this issue, a vent valve was added to the nozzle design and programmed to open at a variable delay after the firing valve is opened. This valve chosen for this is also a SMC Directly Actuated 1" NPT Solenoid Valve, as it can efficiently let out air and has already been tested in risk reduction.
Model to Find the Actuation Time of the Valve
The Nozzle
The nozzle refers to the interface between the solenoid valve and the barrel. It delivers the pressure force through the valve so that it acts onto the projectile, so it must be able to withstand 100 psi of compressed air. The nozzle must also serve as a mounting platform for the barrel and ensure proper stability for launch.
The final design of the nozzle is shown to the right. This bolted-on pipe flange enforces a clamping force from the bolts, keeping the barrel secure as the pressure force acts on the slug. The maximum clamping force is found by analyzing the free-body diagram of the barrel, and it is found to be around 314 lbf. Using the torque-fastener formula, six (insert size) bolts are needed in the final nozzle design.
Final Nozzle Design
The Barrel
The Barrel must be able to hold the mass and enable it to move up towards the impact test table with as little friction as possible. Because of this, a smooth inner diameter finish is required to limit the friction and ensure that the mass will reach the end of the barrel at the desired velocity. It is also required that the mass can fall back into the barrel after impact and realign itself, which requires a tight tolerance between the slug and barrel. After exploring different tubing finishes, a Honed Tube was selected for its extremely precise inner surface finish and tolerances.
Inside of Honed Tube
The Slugs
The Slugs are the masses that are being shot at the impact test table. The slug must not deform on impact, meaning that the impact stress must not exceed the yield strength of the material. The slug should also have a rounded top geometry so that the impact is always concentrated at the same spot. This ensures that the load produced from the slug hitting the testbed produces a repeatable result. The slug length must also be greater than the diameter to limit unwanted vibrations and wobbles as the slug moves throughout the barrel. Lastly, the slug must be easy to machine at the machine shop.
The material chosen for the slugs is 1144 Steel. This is chosen because it has a yield stress, so it should not deform on impact. It is also a steel that is easy to machine, making it ideal for working with at the provided machine shop. To determine the exact geometry of the rounded top and find the maximum impact stress, Hertzian Contact Theory is used. Hertzian Contact Theory analyzes the impact stress between two curved surfaces, and can be modified to find the impact stress between a curved surface and a flat surface. By iteratively running this set of equations, different impact stresses can be found for different radii at the contact point for the slug. The final slug radii geometry is shown to the right, with a curved surface of 1.25". This gives an impact stress of 16.5 ksi, which is well below the yield stress of 1144 steel of 95 ksi.
1 kg Test Slugs made of 1144 steel
Initial Testing
Initial testing was done with the 1 Kg slug to find the relationship between pressure and velocity. This was done by slowly incrementing the pressure and testing the firing mechanism to find the impact velocity each time. There seems to be a linear relationship between the pressure and the impact velocity, 4.14 PSI yielding around a 6 m/s impact velocity and 9.31 PSI yielding around a 14 m/s impact velocity.
However, after this testing, back pressure proved to be an issue. The tolerance was so tight between the slug and the barrel that pressure would not escape the barrel after the slug impacted the table. This led to the slug having repeated impacts with each test, as the pressure behind the slug would prevent the slug from falling back to the bottom of the barrel. This will skew the results of the shock test table, as the shock the test object experiences will change with each unwanted repeated impact.
Updated Design and More Testing
To prevent this, a venting valve was incorporated into the nozzle. This valve is programmed to open a variable delay after the firing solenoid valve opens. This valve will let the air escape from the bottom of the barrel, which will let the slug slide down the barrel after initial impact with the test table, and prevent repeated impacts. With this new venting valve, the same testing was done to find the new pressure and velocity relationship. A similar linear relationship is seen, with the impact velocity increasing an average of 0.57 m/s with each incremental PSI.
Pressure vs. velocity relationship from testing with the addition of the venting valve.
Testing Videos
25 PSI Acting on 1 Kg Slug with Initial Design. Slug has repeated impact with the test table, and eventually is held at the top as the result of back pressure
6.9 PSI Acting on 1 Kg Slug with the venting valve in the final design. Back pressure no longer causes repeated impacts, and the impact velocity is calculated to be 9 m/s