Vex pnuematics

I was wondering if there was a way to limit how much the pnuematic piston extends or retracts. Like if you only want it to extend halfway or half stroke. Will this damage the piston if you stop it from extending fully. I don’t have a pnuematics kit but will be getting one soon so just wanted to know before I make any designs.


Rookie Year: 2012-2013 VRC Season

2012 Southern Ontario VEX Robotics Championship - Quarter-finalist, Judges Award
2013 St. Catharines VEX Robotics Championship - Tournament Champions (Thanks to 1114Z and 3314A), Judges award, 3rd seed
2013 Mississauga VEX Robotics Championship - Tournament Champions (Thanks to 1241Q and 1241Z)

“Show the world, win the world”

You can use spacers like this picture of my robot. Or you could make a lever. The lever would also increase your force but takes up more space. So depending on what you wanted to use them for…

Would this damage or effect the performance of the piston. Does limiting the distance the piston extracts allow less air to be used, so more cycles with the piston?
I am thinking of having 2 double acting pistons (used 5 times per game on average) for a transmission and 2 single acting pistons (used repeatedly to throw) for a thrower. How many air reservoirs should I use?

From experience my piston is still fully functional after going to 2 competitions and Worlds with that transmission of mine. So no it does not damage it. Or shouldn’t anyways.

I believe it uses less air when retracting but should use the normal amount when extending. But I am not 100% sure on this. If I am right you should be saving the distance of the spacers you added in air volume each time you retract. So in my case it was about 25% less air used compared to a normal double acting.

My robot had 9 air tanks and I ended matches with 60psi but I was using my 1 transmission piston and 4 double acting for the intake used constantly. So I would say you may get away with 3-4 tanks depending on how much pressure you need to throw the objects. I would always suggest adding functional weight aka air tanks. Each tank is about 27$ with fittings. But try to find a way to mount them without the huge nuts as they add up in weight. But 3-4 should be fine I would just say go with 4 because its better to have more air than not enough xD

Also another note. I ran 8 motors constantly on my robot and never tripped a cortex breaker even while pushing people across the field… you do not need a power expander. I ran without one at world’s and this saved me the weight of a battery xD which is about the same as the weight of an air tank!

The piston should still push with full power, and I don’t think it should harm anything. The threads on the tip should hold fine against the pounding of the spacers when you retract.
Also, limiting distance of the piston doesn’t dramatically decrease the amount of air each actuation consumes, because the solenoid still opens and equalizes the pressure on both sides.

Depending on how much power your pistons require at minimum to function properly and how much ‘repeatedly’ is… One is definitely on the low end, two is what I’d probably go with, and if you have a need for high power, three.

Thanks guys for the responses, I want to save weight as much as possible but I will most likely use a min of 3 tanks, 4 to be on the safe side. Because hanging will be crucial this year,

The reasoning for the using less air doesn’t add up to me. The way I see it, if you physically limit the stroke of the piston, less volume of air is going to be expelled when the solenoid lets go, meaning the actuation will use less air.

that is what I thought, it has a bit of logic behind it :smiley:

That is correct, there will be some dead air space that retains a little bit of air and won’t require quite as much on the next actuation, but it won’t be an appreciable amount since the remaining air will be at atmospheric pressure and not compressed.

When using a full stroke you use all the air, but only half stroke uses half the air in the piston. Would that mean would you get nearly twice more the strokes? Something to think about.

25% more like I originally stated xD

If you are using spacers to reduce the piston stroke by 50% then it takes 50% less air to retract it, but still takes more than 50% to actuate it. Take a flat tire off of your car and it appears inflated, but it takes air to get it to the desired pressure. The same thing with the way your cylinder is set up, it you have to pressurize the dead air space before you begin to move the piston.

Well extending would use the same amount of air that a piston with no spacers on it would. But retracting would be different it would only fill up until it hit the spacers.

So 1 full actuation we will call it open then close. Say it uses 4in^3 of air ( I know its not right just an example) for a full actuation. 2in^3 each time. Now if we add spacers to the piston so it only retracts halfway… we get the extending actuation that still uses the 2in^3 to extend. Then comes the difference the retracting action. This now only retracts half the distance it normally would. Thus it has less volume for air to fill so it now only uses 1in^3. For a total full actuation volume of air being 3in^3.
This gives a 25% air savings on a full actuation!

Another thing is if you are designing a transmission and use spacers I would recomend starting the match with the piston extended! This way you gain a slight amount more air because at the start of a match when you pump up your air it fills double acting pistons in 1 direction and the extended state in this case holds a bit more air! Probably won’t make a difference. But air is precious as I found out xD

It should be noted that single acting pistons are definitely going to be a lot more useful for a lot of applications because of the massive air savings you’ll get by using them. I would imagine that things like catapults will fit this definition.

This is certainly on the right track and accurate enough for government work. Retracting does not take quite as much air as extending since the shaft takes up a good portion of the space inside the cylinder on that side so it would leave less space to fill with compressed air. I don’t have the specs for the cylinder and can’t find my calipers so I am not sure what the exact Diameter of the shaft is. Even if the shaft takes up 1/2 of the space inside the cylinder on the retraction side that still accounts for a 12.5% savings, and those who have faced the challenge of trying to have enough reservoirs to complete a match know every little bit matters!