Reduced Mass Piston:

[click for larger 28KB]

Diagram:

1] Gasket material: soft neoprene rubber taken form a 3" Fernco Qwick Cap [pic]

2] 1/4" x 1-1/4" fender washer

3] 1/4" x 3-1/2" bolt and nut

4] 3" PVC coupler sanded to fit loosely inside 4" pipe (.5 mm clearance between pipe and piston) To reduce mass, 1/4" and 1/2" holes were drilled in the coupler in a specific pattern.

5] 1/2" strips of 3" cellular core PVC to support front and back plates

6] Front and back plates: 3-1/2" x 1/16" steel with 1/4" hole in center

7] 4" sch 40 PVC pipe. This creates a cylinder for the piston to travel in.

8] 1/4" x 3/4" Washer

 

Design:

This piston design has two special features which will hopefully increase performance when used in a valve. The first is a reduction in mass and the second is an oversized specially shaped gasket that seals the barrel port and the piston's cylinder.

The idea behind reducing the mass of a piston can be seen using Newton's second law of motion: F = ma. The equation below can be derived from the second law of motion and is used to find the time it takes for the piston to move a certain distance.

Time = sqrt ( [mass x distance] / force) )

The current mass of this piston is .40 kg, the distance it travels is .05 m, and the force acting on the piston at 100 psig is ~5590 N. The time needed to travel the distance is ~1.89E-3 seconds. Before the extra PVC material was removed the mass was .45 kg and would have taken 2.01E-3 seconds to move the same distance.

The faster a valve opens the faster the gas can transfer from the chamber to the barrel. This increases the time a projectile is acted upon by the full force of the gas and more energy is transferred to the projectile.

The enlarged portion of the diagram shows the shape of the gasket where it meets the cylinder walls. The gasket diameter is is slightly larger than the bore of the cylinder. When pushed forward the shape of the gasket flattens and seals the cylinder. The same is true when the piston travels backwards. This action eliminates the need for o-rings.

 

Pictures:

This is the first prototype of the piston. The small washer holding the gasket was replaced with a larger washer because the gasket was pulled off the bolt during tests at 100 psig.

 

This shows the coupler with the hole pattern drilled. The flanges that hold the the front and rear plates are also seen.

 

 

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