A pneumatic cylinder is an actuator that turns compressed air energy into straight-line motion. In robotics, it is often used to push, pull, clamp, lift, sort, or eject parts quickly and reliably. Its main parts include a cylinder barrel, piston, piston rod, seals, and air ports.
Understanding the cylinder helps students connect pressure, area, force, and motion in a real mechanical system.
When air enters one side of the cylinder, it pushes on the piston and creates a force along the piston rod. In a single-acting cylinder, air usually drives the piston in one direction while a spring or external load returns it. In a double-acting cylinder, air pressure can drive the piston in both directions by switching which port is pressurized.
The output force depends mainly on the air pressure and the effective piston area, so bore size and pressure are key design choices.
Key Facts
- Cylinder force is found from F = P A, where F is force, P is pressure, and A is piston area.
- For a circular bore, piston area is A = pi d^2 / 4, where d is the bore diameter.
- A double-acting cylinder can extend and retract using compressed air on opposite sides of the piston.
- A single-acting cylinder uses compressed air for one stroke and a spring or load for the return stroke.
- Retract force is often smaller than extend force because the piston rod reduces the effective area on the rod side.
- Airflow rate affects cylinder speed, while pressure and piston area set the ideal output force.
Vocabulary
- Pneumatic cylinder
- A pneumatic cylinder is a device that uses compressed air to create linear mechanical motion.
- Piston
- The piston is the moving disk inside the cylinder barrel that air pressure pushes against.
- Piston rod
- The piston rod is the shaft connected to the piston that transfers the cylinder motion to an external mechanism.
- Bore
- The bore is the inside diameter of the cylinder barrel and determines the piston area.
- Seal
- A seal is a flexible part that reduces air leakage between moving or stationary cylinder surfaces.
Common Mistakes to Avoid
- Using diameter as area in F = P A is wrong because pressure acts over surface area, not length. First calculate A = pi d^2 / 4 before finding force.
- Forgetting unit conversions gives incorrect forces because pascals, square meters, bars, millimeters, and newtons must be consistent. Convert pressure and bore size before substituting into equations.
- Assuming extend and retract forces are equal is wrong for many double-acting cylinders. The rod takes up area on the retract side, so the effective area is smaller.
- Ignoring friction and leakage overestimates real cylinder performance. The equation F = P A gives an ideal force, while actual force is lower due to seals, side loads, and air losses.
Practice Questions
- 1 A pneumatic cylinder has a bore diameter of 40 mm and is supplied with air at 600 kPa. Calculate the ideal extension force in newtons.
- 2 A double-acting cylinder has a 50 mm bore and a 20 mm rod diameter. If the supply pressure is 500 kPa, calculate the ideal retract force using the annular area.
- 3 Explain why a robot gripper might use a double-acting pneumatic cylinder instead of a single-acting cylinder when it must both clamp and release a part quickly.