Hydraulic Pressure Explained

Pressure is generally a force acting on a surface. Hydraulic pressure is accordingly a force transmitted through a fluid that acts on the internal walls of a hydraulic system. The special characteristic of hydraulic pressure is that it is transmitted with virtually no compression losses and distributes evenly throughout the entire system. This makes hydraulic pressure particularly useful.

Generation of Hydraulic Pressure

Hydraulic pressure is technically generated by a retracting piston in a hydraulic tank or by a hydraulic pump.

Small, manual systems work with the piston solution. Typical examples are hydraulic jacks, transmission jacks, hydraulic cylinders, pallet trucks, or manual hydraulic presses.

Motor-driven hydraulic systems, on the other hand, generally work with hydraulic pumps. The power of the motor, the size, and the design of the pump determine how high a hydraulic pressure can be generated. The weakest hydraulic pumps are vane pumps. They consist practically only of a small impeller that circulates the hydraulic fluid. The most powerful hydraulic pumps are radial and axial piston pumps. They can generate hundreds of bar and are the standard components used in demanding applications, such as construction vehicles.

Force Amplification

Loss-free pressure transmission is not the essential unique selling point of hydraulic systems, but rather their ability to amplify force.

Since pressure is defined as a force acting on a surface, remarkable effects can be achieved with this simple formula: The force with which a hydraulic cylinder extends increases exponentially with its cross-section. Since a circular area is defined as pi × radius squared, the effective area quadruples with each doubling of the diameter.

A slave cylinder four times larger therefore has a sixteen times greater effective area than its master cylinder. Accordingly, the force with which the slave cylinder extends increases. Although the extension distance and extension speed decrease correspondingly, neither is a primary requirement for hydraulic systems. These are exclusively concerned with the efficient generation of very high forces.

Arbitrary Force and Pressure Distribution

Since hydraulic pressure distributes evenly in the system’s piping, its configuration is virtually unlimited. Pumps, lines, and consumers can be placed at almost any location. They always function with equal reliability. This is a significant advantage of hydraulic systems over purely mechanical systems.

Requirements for Using Hydraulic Pressure

Hydraulic pressure requires two conditions for optimal long-term performance:

• Leak-free operation
• Gas-free operation

If hydraulic fluid leaks from a system at any point, the internal pressure drops very quickly. This causes the entire system to weaken rapidly and eventually stop.

When gas is trapped in the piping system, pressure transmission is no longer reliable. Instead of smooth movements, every action becomes jerky or fails completely.

The larger a hydraulic system becomes, the more complex and demanding it becomes, as the mentioned and other parameters gain increasing importance.