Piston Gap Drawing

[dewey-1]

This drawing is made to give people an idea of what a 1mm gap looks equivalent to in a 1, 2 and 4 inch diameter.
Note dimensions are Imperial with Metric in brackets.

A 1 mm gap is quite large when you calculate the actual total.
The Red is piston OD.
The Green is the total equivalent area of a 1 mm gap around the piston.
The Blue is a 2 mm equalization hole for comparison.

For example; the equalization hole is .080" or 2mm in a typical 1" sprinkler valve.

This comparison should give you an idea how really large a 1 mm gap is!



Here is the drawing with .25mm or .0098" gap for comparison.
You will notice a huge difference.



edit; add .25mm gap drawing, fix first 4" circle gap to 3.9606" & calculations

[Technician1002]

That is a fantastic summary of why many piston valves fail to open. The size of the porting past the piston is underestimated. Well done. For close ratio pistons (seat diameter to OD), this leakage must be tightly controlled, otherwise, the pilot area can't drop in pressure enough to unseat the piston. The chamber pressure just blows into the pilot area.

[dewey-1]

Tech;

What is your equalization hole size in your 2" piston? I am guessing around
.0625" to .080". (1.59 to 2.00 mm)

I may do another drawing to show a .25mm or .010" gap for a more realistic picture.

[Technician1002]

Tech;

What is your equalization hole size in your 2" piston? I am guessing around
.0625" to .080". (1.59 to 2.00 mm)

I may do another drawing to show a .25mm or .010" gap for a more realistic picture.

My 2 inch piston is a QDV, so the piston is not using a pilot. The pilot area is never under pressure, so it can't be vented to fire the cannon. The QDV valve seat and piston OD is the same size.

The Mouse Musket is using a 1 inch barrel, so the piston is 1.5 in diameter. The EQ hole is 1mm.

The Mouse Musket is a 1 inch barrel 2 inch chamber coaxial. The piston rides in a very short length of 1.5 inch pipe in a 1.5 inch female adaptor swedged into the end of the 2 inch chamber. This is built that way to make the piston ratio a close ratio piston as the valve seat is larger than the bore of the barrel. The valve seat is an o ring on the outside of the barrel held in place by a coupler grooved to hold the ring.

Until it broke and I built the QDV, this was my best performing cannon. It put 1 inch gumballs through 1/2 inch sheets of plywood. The valve is very fast and opens with a POW.

The Dragon cannon, my other piston cannon is 2.5 inch porting with a 3 inch piston in a 6 inch chamber. A reducer is used to reduce the 6 inch to 4 inch. A female 3 inch adapter was used to hold the 3 inch section of pipe in the adapter for the piston, so the dragon is another close ratio piston. Due to the size, it tended to either not work or break things. An inverted 2.5 inch pipe cap was used for pistons. The PVC didn't like the forces and pistons were short in life expectancy. This cannon is retired due to the difficulties keeping it working. The Dragon used an o ring in the piston cylinder for a seal instead of on the piston.

A 3 inch QDV in a propane tank will replace the Dragon.

[dewey-1]

Tech;

My thinking for optimal operation for 1, 2, 3 & 4 inch pistons would have equalization hole sizes that would correspondingly be 1, 2, 3 & 4 mm with maybe a max of .25mm gap to allowed for grease/lubricant.

What do you think or any one else, before I do a realistic drawing?

Pistons with O-ring seals may require a slightly larger hole.

These sizes are just a starting point to prevent to large of a hole.

[jeepkahn]

other than for filling from the chamber side, what purpose does an EQ hole serve? A properly designed checking piston that allows filling past the piston is way too easy to construct...

[jrrdw]

other than for filling from the chamber side, what purpose does an EQ hole serve? A properly designed checking piston that allows filling past the piston is way too easy to construct...

It's for filling from the pilot side.

[dewey-1]

I updated the original post to show a .25mm piston gap to compare how much of a difference there really is!

[MrCrowley]

Great topic, you should put this on the Wiki for easier referencing.

[Technician1002]

other than for filling from the chamber side, what purpose does an EQ hole serve? A properly designed checking piston that allows filling past the piston is way too easy to construct...

As the chamber pressure changes as it is filled, without an eq hole or some intentional piston leakage, the pilot area starts without pressure. Without EQ it is impossible to pressurize the chamber as doing so will open the valve.

EQ can be provided either internal to the valve, usualy with a "leaky seal" or eq port in the piston. It can be external through an eq valving external to the cannon.

[King_TaTer]

Shouldn't the 4 inch piston in the first diagram have a diameter of 100.60mm instead of 99.60mm? It would also make the piston diameter in imperial change to 3.9606", no? Right now it's showing that there is a 2mm gap when it should only be 1, correct?

Good drawings though. I intend to use a check valve and o-rings for my barrel sealing piston, to alleviate the need for such precision.

[dewey-1]

Shouldn't the 4 inch piston in the first diagram have a diameter of 100.60mm instead of 99.60mm? It would also make the piston diameter in imperial change to 3.9606", no? Right now it's showing that there is a 2mm gap when it should only be 1, correct?

Correct!

Thank you for catching my blunder!

The drawing and the original posting have been just corrected.

[Insomniac]

Good drawings, but it isn't quite a fair comparison. The area of the gap and the drawn hole may be the same, but the narrow gap will restrict airflow alot more than a hole of the same area.

[Technician1002]

Good drawings, but it isn't quite a fair comparison. The area of the gap and the drawn hole may be the same, but the narrow gap will restrict airflow alot more than a hole of the same area.

That is true that the long slot will flow more poorly than a round hole. The point of this thread is to point out that the small slot can have a very large area and prevent the operation of piston valves in larger size cannons.

This thread reminds me of the design of steam whistles. They also have a very small gap.

Church pipe organs have a little larger gap, but they operate at very low pressure. Many operate in the 6 inch water column pressure range. Some operated with less than 3 inches water column pressure.

http://en.wikipedia.org/wiki/Pipe_organ