There are two common types of hydraulic cylinders- single acting and double acting.
But what is the difference between the two?
Simply put, a double acting cylinder has both an A and a B Port.
This means oil enters the cylinder via the A port, which pushes the piston down. When the control calls for the piston to retract, oil is diverted to the B port, which then pushes the piston up.
A double acting cylinder uses hydraulic power to both extend and retract. You can tell if a cylinder is double acting by looking at the number of ports. See the image below as an example.
In contrast, a single acting cylinder has an A port for running the piston down by pressure from the pump but is retracted with an internal spring.
See the image below as an example.
A double acting cylinder has pressing and pulling power and is generally controlled with a joy stick control. A single acting cylinder only has pressing power.