# Rules of thumb for sizing an air pump piston?

I am working on a design for a small air pump that uses a single piston.

When choosing a piston I have two parameters diameter and stroke. So, I can make it fatter and shorter, or skinnier and longer, for the given volume of air that I want to move per stroke.

Are there any rules of thumb concerning how to make this choice?

• You could spend a lot of effort optimizing your design for various factors, but seeing that you don't need real high performance, my suggestion would be to use something off the shelf or made from standard size parts. Almost anything will work, the only consideration is power requirement - if you can provide the outlet pressure and flow rate someone here can help you with that. – Carlton Mar 23 '17 at 0:46
• If you are asking this question then you are almost assuredly better off buying a commercially available pump which would be higher performance, more reliable and most likely cheaper than what you can do yourself. – Eric S Feb 5 '18 at 15:52
• Are there any requirements for the pressure the pump needs to withstand? – masiewpao Mar 9 '18 at 15:19

Depends on a several factors.

Stroke length should just be application dependent. You should know how far you require it to actuate.

Diameter is a bit more complicated. Your $Force = \frac{Pressure}{Area}$ so if you know the required actuation force, you just need one more parameter. If space is an issue but air pressure isn't, then you would want smaller diameter (area) and higher pressure. If you want lower operating pressures and size isn't as much of a constraint; you would want a larger diameter/area to minimize pressure required.

• I don't care how far it actuates. I am using it as an air pump so the independent variable is the volume of air needed per unit time. – Wallace Park Feb 8 '17 at 19:48

When making a piston pump, it will depend on the torque and RPM of the motor you have selected, head pressure the pump is pumping against, and flow rate.

Smaller diameter means less volume per stroke and less motor torque required. Shorter stroke means less volume per stroke, and a shorter camshaft lever arm resulting in less torque required.

We will not be able to help much beyond those concepts without nailing down some variables like motor rpm, motor torque, flowrate, head pressure, etc.

• Okay, so if I use a fatter piston, then I can use a faster motor with less torque, is that the point? This is a small system that is moving about 5-10 liters of air per minute or so. – Wallace Park Feb 9 '17 at 19:34
• @WallacePark A fatter piston will mean a bigger diameter therefore more volume per stroke and more torque required. Skinnier and shorter requires the least torque (and also moves the least air, as per natures usual way of dealing with things). There are endless combinations of ways you could size the piston and motor to make this work. The variables all depend on what constraints you have. – JMac Feb 9 '17 at 19:46