Hydraulic actuators can be classified according to how they operate. They operate on pressurized fluids, and there are two main groups, hydraulic cylinders and hydraulic motors.

- Hydraulic cylinders: according to their mode of operation can be single or double acting. The first fluid use force to go out and force other to contract. The double acting fluid power used both to go to contract. These types of cylinders are controlled by a relay which has a spring constant varies according to the passage of electric current.

- Dynamic Pressure Cylinders: carries the force of the cylinder in the ass. The cost of manufacture usually are reduced since there is no sliding parts.

- Single-acting: the rod is situated on one side of the piston. The piston is contracted by the force of a spring or by gravity. The load can be placed at only one end of the cylinder.

- Double acting cylinder. The load can be placed on either ends of the cylinder. Horizontal stress occurs by the pressure gradient between the sides of the piston.

- Telescopic cylinder: a rod having a tube that extends several stages with a fluid force. Longer strokes are achieved by comparing the length of the cylinder.

- Hydraulic motor: in this type of engine is the rotary motion produced by the fluid pressure. They can be classified into two groups: the first consists of gears driven by oil pressure and the second is oscillating, and the rotational movement is achieved by the motion of a piston. The latter is claimed to be more energy efficient. Let these engines more detail:

- Geared Motors: The pressurized fluid passes through the entrance that goes on the gear tooth face producing a torque on the gears. Engine is a very simple structure, so it is recommended for use in high speed applications.

- Piston motor: the pressurized fluid pushes the piston against the force of a flange and which is in the radial direction causes the shaft and the cylinder block rotate in the direction of that force. They are a widely used type of engines for large applications pressures and high speeds.

Axial piston engine: Its function is to absorb a volume of pressurized fluid and return it to the circuit pressure when required.