pneumatic catapults

[Peyton Yeung]

Originally Posted by Cothron Theiss

So long story short, what methods do you all use to mount the catapult arm and/or use as a fulcrum?

This year we used steel rod through bushings for both the cylinder end pivots as well as the arm pivot.

3/4" bore 5" stroke cylinders i believe.

[GeeTwo]

Originally Posted by Cothron Theiss

So long story short, what methods do you all use to mount the catapult arm and/or use as a fulcrum?

This year, we used ball bearings, more out of the simplicity of doing so with versaframe than any requirement. Most years, we have used steel bolt(s) with bronze bushings.

[adciv]

Originally Posted by Cothron Theiss

So long story short, what methods do you all use to mount the catapult arm and/or use as a fulcrum?

Piano Hinge. Works great for a lot of applications.

[BrendanB]

Originally Posted by Cothron Theiss

So long story short, what methods do you all use to mount the catapult arm and/or use as a fulcrum?

Ours was made an added this year during our third district event so we drilled a 1/4" hole, threw a long bolt through the lexan frame of the catapult, and used some round 1/2" OD 1/4" ID tubing from Vexpro as spacers. The frame was pretty rigid so we made the spacers have a slightly loose fit. Just enough to keep it steady but allowed it to pivot freely.

Had it been a made in our shop we might have used bushings or bearings as a pivot.

[ollien]

Originally Posted by Billfred

- Multiple small cylinders will fill faster than one big one.

What's the intuition for this? If we have four .25 L cylinders, shouldn't it take just as long to fill those as it does to fill a single 1 L cylinder?

[Andrew Schreiber]

Originally Posted by ollien

What's the intuition for this? If we have four .25 L cylinders, shouldn't it take just as long to fill those as it does to fill a single 1 L cylinder?

It's a flow problem. At what point in the system is the flow most restricted?

[ollien]

Originally Posted by Andrew Schreiber

It's a flow problem. At what point in the system is the flow most restricted?

I would argue that the flow would be more restricted when going to many different outlets, but I could be totally off base.

[Andrew Schreiber]

Originally Posted by ollien

I would argue that the flow would be more restricted when going to many different outlets, but I could be totally off base.

Try drawing out the whole system.

The max flow into a 1" bore cylinder is going to be exactly the same as into a .25" cylinder because the input is the constraining factor on both of them.

[adciv]

Originally Posted by ollien

I would argue that the flow would be more restricted when going to many different outlets, but I could be totally off base.

Primary constraint is the solenoid flow rate (CV rating). If you use multiple cylinders, you can use multiple solenoids. This gives you a higher effective CV rating.

[GeeTwo]

addciv got the key point, but was probably not clear enough for many. Let me be a bit more explicit:

Originally Posted by Billfred

Some other notes we've found:

- Multiple small cylinders will fill faster than one big one.

Originally Posted by ollien

What's the intuition for this? If we have four .25 L cylinders, shouldn't it take just as long to fill those as it does to fill a single 1 L cylinder?

Multiple small cylinders filled through the same solenoid valve will fill no faster than a single large one (and perhaps a bit more slowly, given the tubing required). I presume that Billfred is providing this in light of the recurrent rule:

Originally Posted by 2016 Game Manual

R90 The outputs from multiple valves must not be plumbed together.

That is, multiple smaller cylinders, EACH FILLED THROUGH A SEPARATE SOLENOID VALVE, will fill faster than one big one filled through (as required by the rules) one solenoid valve.

[ollien]

Originally Posted by GeeTwo

addciv got the key point, but was probably not clear enough for many. Let me be a bit more explicit:





Multiple small cylinders filled through the same solenoid valve will fill no faster than a single large one (and perhaps a bit more slowly, given the tubing required). I presume that Billfred is providing this in light of the recurrent rule:

That is, multiple smaller cylinders, EACH FILLED THROUGH A SEPARATE SOLENOID VALVE, will fill faster than one big one filled through (as required by the rules) one solenoid valve.

Maybe I'm misinterpreting the rule and the conclusions you're drawing from it, but no inspector has ever faulted us for having one solenoid for two pistons.

Why would filling two pistons from one solenoid, vs 2 pistons from 2 solenoids, be faster?

[Knufire]

Originally Posted by ollien

Maybe I'm misinterpreting the rule and the conclusions you're drawing from it, but no inspector has ever faulted us for having one solenoid for two pistons.

One solenoid for two cylinders is legal. Two solenoids for one cylinder is not legal.

Originally Posted by ollien

Why would filling two pistons from one solenoid, vs 2 pistons from 2 solenoids, be faster?

There used to be a rule that limited the Cv (flow coefficient) of solenoid valves on the robot, causing the solenoid to be a bottleneck in the flow path. That rule has since been removed. However, larger Cv solenoid valves are generally larger and/or heavier, so multiple cylinders on multiple solenoids may be a good alternative depending on your design constraints.

[adciv]

Originally Posted by Knufire

There used to be a rule that limited the Cv (flow coefficient) of solenoid valves on the robot, causing the solenoid to be a bottleneck in the flow path. That rule has since been removed. However, larger Cv solenoid valves are generally larger and/or heavier, so multiple cylinders on multiple solenoids may be a good alternative depending on your design constraints.

To continue on this (sorry, too used to thinking electrical) the other counterpart to this is we are still constrained by the pneumatic size limits which act as an indirect Cv limiter. A fitting and tube withing the existing 1/8th inch port and 0.165 inch inner diameter limit your maximum air flow for each path the air takes.

By the way, watch out for the Cv rating on components. We've seen some come in where the solenoid was advertised at one Cv rating, but the fixtures had only a tiny hole for air to pass through and so the effective Cv rating was significantly smaller.

[GeeTwo]

Originally Posted by ollien

Why would filling two pistons from one solenoid, vs 2 pistons from 2 solenoids, be faster?

It wouldn't - it would be slower, assuming all of the solenoid valves are of the same type and that they are the limiting factor.