Calculating force output

There is only one formula you need in pneumatics - that for calculating the force output from a cylinder. You will be given the formula sheet at the front of your GCSE answer book. So all you have to do is make sure you know how to use it!

The force you get out of a cylinder depends on the air pressure inside it, and the size or area of the piston. The formula is:

Working out the pressure and the area can present some difficulties, so here are some tips...

Working out the pressure

The trick to success is making sure you use the right units for pressure. In the formula, pressure must be given in Newtons/mm² (N/mm² ). So if pressure is shown in bar you must convert it to Nmm². To convert bars to N/mm² simply divide by 10. For example:

5 bar = 5 ÷ 10 = 0.5N/mm²

Working out the area

The area is the surface area of the piston. As every piston is round, you may have to calculate the area of the circular piston from the radius. To do this, use the formula for area of a circle -

π × radius² - taking π to be 3.14.

For example: if a piston has a radius of 30mm, the surface area of the piston is:

3.14 × 30² = 3.14 × 30 × 30 = 2826mm²

Calculating the force

The force you get from a piston is measured in Newtons (N). Here's a sample problem:

A piston has a radius of 20mm and the pressure in the cylinder is 4 bar. Calculate the force output from the cylinder.

First you need to convert the units of pressure: Pressure = 4bar = 0.4N/mm²

Next, work out the area: Area = 3.14 × 20 × 20 = 1256mm²

Then you can calculate the force output from the cylinder: Force = 0.4 × 1256 = 502.4N

Now try a Test Bite