Kinematics – Actuation of Machines: Part II – Hydraulic Actuation

Fluids can store energy in form of pressure, transfer energy in form of fluid flow and can transform the stored energy (in form of pressure) into motion. These properties of fluids are harnessed by designing the actuators based on fluid power. The actuators using liquid as the actuating fluid are termed as hydraulic actuators and actuators using gases (commonly compressed air) are called as pneumatic actuators.

Hydraulic Actuators uses flowing liquid to transmit the energy from generation point to actuation point. The required fluid power is generated in the form of high pressure liquid by a pump, which can be driven by electric motor as in the case of industrial robots and by engines directly for heavy machinery used in oil well drilling or construction operations.

The pressure generated by the hydraulic pump is distributed to different actuators through control valves according to their requirement, which is proportional to the amount of load needed to be supported by them. A different configuration for hydraulic actuator is having a separate variable displacement pump for each actuator. Some commonly used hydraulic actuators are

Hydraulic Cylinder

It is a linear actuator, that is, it can generate linear motion when powered by pressurized hydraulic fluid. Hydraulic cylinders are used when loads are large and strokes are long. They are generally used to drive the linkages in heavy engineering machines such as excavators, heavy duty cranes and JCB machines.

click to enlarge

Hydraulic Motor

As the electric motors are similar in construction to those of electric generators, hydraulic motors are very similar in construction to hydraulic pumps. The functioning of an electric motor is just opposite to that of an electric generator. A generator converts rotation into electricity and a motor uses electricity to produce angular velocity and torque.

In the same way the working of a hydraulic motor is opposite of a hydraulic pump. A hydraulic pump produces fluid pressure and flow using rotation of shaft where as a hydraulic motor uses the fluid pressure and flow to produce torque and rotation of shaft.

A hydraulic actuation system is composed of hydraulic motors and pumps coupled in a circuit. Pumps generate the required fluid pressure and flow which is used by motors installed at different points for actuation.

click to enlarge

For a brake it is generally convenient to have the point of application or actuation point away from the point of control. Hydraulic brake assembly has a cylinder filled with hydraulic fluid (called brake fluid), hydraulic pipe, brake pedal and the mechanical brake. Brake is applied by pressing the brake pedal which pressurizes the hydraulic fluid, the fluid is forced through the pipes and force is applied at the mechanical brake.