When pneumatic cylinders are improperly sized, erratic, uncontrollable motion may (probably will) occur. There are at least two reasons why this is so.
- Moving the load requires the pressure in the cylinder to overcome the load and the internal friction of the cylinder.
- The maximum force exerted by the cylinder is not sufficient to manhandle the load.
- There is too little volume of air in the cylinder to make the flow controls work effectively at the given load.
Basically, when a cylinder is undersized, the tail is trying to wag the dog, so to speak.
How to size a cylinder
To size a cylinder, determine the force required to be exerted by the cylinder. Sometimes it is not easily determined, but a best guess can usually be made. In vertical arrangements, start out by calculating the weight of everything that will be lifted by the cylinder. In horizontal arrangements, either do some measurements on the load, if possible, or do calculations based upon load weights and frictions, etc.
Another component of moving the load is the inertia of the load. Sufficiently over-sizing the cylinder will normally take care of the load inertia. If you believe that the load inertia is unusually large, calculations to estimate the forces due to the inertia should be done. A large cantilevered load where a very small amount of cylinder motion causes a large load movement would be an example of a situation where the load inertia should be considered.
After determining a force requirement for the cylinder, I would double or triple the amount, unless there is a good reason not to. As an example, if a cylinder is going to be used to seat a part against another part to hold it in place for some operation, the force requirement may be very well defined, and exceeding that force may be detrimental to the operation. Also, in pressing applications, the tooling is usually fastened to the cylinder rod, and the force required to move the tooling is much smaller than the force to be applied to the part being pressed or held. In this case the cylinder may be sized more closely to the actual known force requirement.
Using a cylinder that is larger than required is normally more controllable in the end, so I normally favor sizing cylinder at 2 to 3 times the required force. A higher force cylinder by definition has a larger bore, which required more air to fill. The extra air volume has the benefit of making flow controls more effective in metering the air exiting the cylinder.
So, unless there is a good reason to use a closely sized cylinder, I favor playing it safe, and using cylinders that are larger than the calculations call for.
Controlling a Cyilnder
There are a number of things to consider when specifying the components used to control a cylinder. They will be discussed here.