Final Design

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. 

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. 

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. 

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.