How can I calculate how many air tanks I need on a robot to run 2 pistons at 60 psi and another 2 at around 50?

The answer depends on additional factors:

- the number of actuations of each cylinder (e.g. how frequently you are cycling them throughout the entirety of the match)
- the bore and stroke length of each cylinder (this determines the amount of air required to cycle the cylinder each time)
- are the cylinders double-acting or spring return (or sprung out, although this is less common)?
- what compressor you are using / how frequently are you okay with it running. From the question, it sounds like you want to run the entire match without the compressor running?

Essentially, you need to calculate the total amount of air volume required at a particular PSI to actuate the cylinder the desired number of times. From that you can calculate the equivalent air volume at the stored PSI (120), which will tell you the number of accumulators required.

we’re planning on running the compressor throughout the match but my question is do i for example need 4 574ml air reservoir to be able to run 2 3cm diameter 53 mm stroke piston every 20 seconds at 60 psi and another 2 2cm diameter pistons every 10 seconds at 40 psi (both cylinders are double acting).

using a Viair 90C compressor. or do i need more than 4?

The formula for the volume of a cylinder (in in^3) is pretty simple:

- (piston area (in^2) - rod area (in^2)) x stroke (in)

Note this means the volume of air required to extend the cylinder is greater than the volume of air required to retract the cylinder (because the latter has the rod taking up some of the volume).

So calculate that for each cylinder, multiply by the number of cylinders and the number of actuations, and that gives you the total consumed volume.

Per Boyle’s Law, a decent approximation for converting this to the volume at a different pressure is simply linear (the actual relationship is more complex, as typically the gas will cool as it expands to the lower pressure). So 1000 in^3 at 60 psi is 500 in^3 at 120 psi.

I’ll let you do the math for your exact situation.

thank you for the help

You might be interested in AMBCalc’s pneumatics simulator, which does a lot of this math for you

One additional piece of information, if you want to see how the compressor will recharge the system… Viair publishes its compressor volumetric flow as a function of pressure:

Assuming the cylinders aren’t actuating, this becomes the rate at which n increases in PV=nRT .

Note this actually becomes a differential equation (since P is a function of n, and dn/dt is a function of P).

All of which means an online calculator may be the way to go