2073 Pneumatic Catapult

[pfreivald]

Lookin' good!

[buildmaster5000]

What are the specs on the cylinder? Specifically bore and stroke length...

[billbo911]

Originally Posted by buildmaster5000

What are the specs on the cylinder? Specifically bore and stroke length...

Really good question!
After trying a Bimba 178 (1.5"dia. X 10" stroke) we found we just didn't have enough height or distance out of our system. We run it at 60 psi which produces 106 Lb. of force.

So, our next thought was that the force was sufficient, but we needed more speed out of the cylinders. So, to maintain the force, we substituted in two Bimba 0910 (1 1/16"dia X 10" stroke). This yields the same 106 Lb. of force at 60 psi, and moves a lot faster.

An additional note here is that we are using low pressure storage tanks between the valve and cylinder. One per cylinder. This is done to maximize air flow to the cylinder by not restricting it as it flows through the valve.

[Birdie048]

Originally Posted by billbo911

An additional note here is that we are using low pressure storage tanks between the valve and cylinder. One per cylinder. This is done to maximize air flow to the cylinder by not restricting it as it flows through the valve.

Great addition. We were running into the same problem.
Thanks!

[JohnFogarty]

What valve are you using on there?

[billbo911]

Originally Posted by JohnFogarty

What valve are you using on there?

Honestly, I do not have the specific part number. The best way to describe it is The standard double solenoid valve from the KOP from previous years. You can also purchase them from VexPro right here. Their part # is 217-2948.

That said, in the configuration we are using it, it really doesn't matter what specific solenoid valve is being used. While the cylinders are extending, very little air is actually flowing through the valve.

[wilsonmw04]

Originally Posted by billbo911

Really good question!
An additional note here is that we are using low pressure storage tanks between the valve and cylinder. One per cylinder. This is done to maximize air flow to the cylinder by not restricting it as it flows through the valve.

Does this really work? Isn't the valve restricting the flow for everything "downstream" regardless of the volume?

[billbo911]

Originally Posted by wilsonmw04

Does this really work? Isn't the valve restricting the flow for everything "downstream" regardless of the volume?

Here are some additional details to help explain why it works.

Each cylinder (1 1/16" dia X 10") has a displacement volume of 8.87 in.^3. Each cylinder also has an accumulator tank (2"dia X 6"lng.) in line between it and the valve. The accumulator tanks have a capacity of 18.87 in.^3.

The cylinders are actually only retracted to 2/3 of their total length when the catapult is in the load position. The valve is then opened to allow the accumulators and cylinders to precharge up to 60 psi. The catapult is actually in a below top dead center (TDC) position, thus causing the arms to be forced against the stops and not allowing the arms to move. (There is the secret).
To launch, we just use a small dia. cylinder, or our hands during testing and in the video, to lift the catapult arms into an above TDC position, the rest is history.

So, as you can see, the air through the valves really doesn't move the cylinders during launching, it's the air in the accumulators that does.

BTW, this is not our original idea. We learned it in 2008. We have just modified it to our needs in this game. I wish I knew which team used this back then so we could give credit where it is due.

[wilsonmw04]

Originally Posted by billbo911

So, as you can see, the air through the valves really doesn't move the cylinders during launching, it's the air in the accumulators that does.

We get a similar effect with a different mechanic. I'll throw a collector in line with it tonight and take a before and after video so we can compare. I'm rather interested to see if it works.

[JohnFogarty]

I'm no master with Pneumatics, but could you draw a small diagram, OR take a picture of how you have it tubed together once you've tested it successfully? This could really prove useful to a couple of teams down here in South Carolina.

[seg9585]

Originally Posted by billbo911

Here are some additional details to help explain why it works.

Each cylinder (1 1/16" dia X 10") has a displacement volume of 8.87 in.^3. Each cylinder also has an accumulator tank (2"dia X 6"lng.) in line between it and the valve. The accumulator tanks have a capacity of 18.87 in.^3.

The cylinders are actually only retracted to 2/3 of their total length when the catapult is in the load position. The valve is then opened to allow the accumulators and cylinders to precharge up to 60 psi. The catapult is actually in a below top dead center (TDC) position, thus causing the arms to be forced against the stops and not allowing the arms to move. (There is the secret).
To launch, we just use a small dia. cylinder, or our hands during testing and in the video, to lift the catapult arms into an above TDC position, the rest is history.

So, as you can see, the air through the valves really doesn't move the cylinders during launching, it's the air in the accumulators that does.

BTW, this is not our original idea. We learned it in 2008. We have just modified it to our needs in this game. I wish I knew which team used this back then so we could give credit where it is due.

Fascinating, I never thought of doing it this way. I want to try this now (I'll give you credit, hah).

I assume the tubing diagram would look like this:

Compressor --> Upstream Accumulator (115 psi) --> Regulator --> Valve --> Downstream Accumulator (60 psi) --> Cylinder
T-junction connecting both the downstream accumulator and cylinder to the valve.

Mechanism it's supporting is against a hard stop with a pivot latch, and a second piston on its own valve connected to extend, retract the latch

[billbo911]

Originally Posted by seg9585

Fascinating, I never thought of doing it this way. I want to try this now (I'll give you credit, hah).

I assume the tubing diagram would look like this:

Compressor --> Upstream Accumulator (115 psi) --> Regulator --> Valve --> Downstream Accumulator (60 psi) --> Cylinder
T-junction connecting both the downstream accumulator and cylinder to the valve......

Absolutely correct!

One note:
To maximize the air flow from the downstream accumulators to the cylinders, use double ended accumulators and feed into one end, putting the "T" between the valves and accumulators, then run the hose from a straight fitting on the other end of the accumulator into the cylinder, also with a straight fitting. Avoid using the 90deg fittings if possible.

[Ether]

Originally Posted by billbo911

Here are some additional details to help explain why it works...

The cylinders are actually only retracted to 2/3 of their total length when the catapult is in the load position. The valve is then opened to allow the accumulators and cylinders to precharge up to 60 psi. The catapult is actually in a below top dead center (TDC) position, thus causing the arms to be forced against the stops and not allowing the arms to move. (There is the secret).
To launch, we just use a small dia. cylinder, or our hands during testing and in the video, to lift the catapult arms into an above TDC position, the rest is history.

Is the attached sketch a correct representation of what you are describing?

The blue line represents the launcher, the brown block represents the stops in the loaded position. The black circles are the pivot points.

In the loaded position (as shown), the launcher is being pressed against the stops. An upward force at the green arrow will move pivot2 above the line connecting pivot1 and pivot3, and the rest, as you said, is history.

[billbo911]

Originally Posted by Ether

Is the attached sketch a correct representation of what you are describing?

The blue line represents the launcher, the brown block represents the stops in the loaded position. The black circles are the pivot points.

In the loaded position (as shown), the launcher is being pressed against the stops. An upward force at the green arrow will move pivot2 above the line connecting pivot1 and pivot3, and the rest, as you said, is history.

Although the positions of the pivots aren't exactly where ours are, because of mechanical mounting options, functionally your picture and description are correct.

[SenorZ]

How are you reloading this? Once the cylinders are retracted don't they need to be forced into a locked position again?