Hydraulics Fundamentals: Transmission of Forces via Fluids

Hydraulics is the transmission of forces using fluids. It utilizes the property of fluids that they cannot be compressed. When a closed system filled with fluid is pressurized, this pressure is distributed equally throughout the entire system. This is the fundamental difference from pneumatics, the transmission of forces through gases.

Hydraulics Fundamentals: The Basic Principle

The hydraulic cylinder or linear actuator is only one of many components of hydraulic systems. However, its example provides a good illustration of how this technical principle works:

When two hydraulic cylinders are connected via an oil-filled line, the following occurs: If the piston rod of one cylinder is pushed in (thereby making it the “master cylinder”), the other piston rod (of the “slave cylinder”) extends. If the piston rods of both cylinders have the same cross-section, this movement is absolutely uniform: the slave cylinder moves at the same speed and distance as the master cylinder is moved.

However, if both cylinders have different cross-sections, the movement is no longer absolutely uniform. Nevertheless, the movements of the piston rods are proportional to each other, depending on the displaced volume. The volume displaced by the retracting cylinder is absorbed by the extending cylinder.

Hydraulics Fundamentals: Force Amplification Through Different Cross-Sections

If a narrower piston rod retracts into the hydraulic system, a piston rod with a wider cross-section will not extend exactly the same distance. However, since the pressure does not change, the force is increased accordingly. The force increases proportionally to the area: a circular cross-section with doubled radius has four times the area. Accordingly, the force at the slave cylinder becomes four times as great as the force applied to the master cylinder. The force amplification acts exponentially to the difference in cross-sections.

Basic Components of Hydraulic Systems

A simple hydraulic system therefore consists of:

• Master cylinder
• Slave cylinder
• Line
• Check valve
• Tank

With the help of the tank and check valve, it is possible to return the master cylinder to its original position without also retracting the slave cylinder. If the slave cylinder is to be extended to a desired distance, multiple strokes are simply performed on the master cylinder. To reset the system, the check valve is reopened. The slave cylinder retracts and the hydraulic fluid flows back into the tank.

These very simple hydraulic systems can be found, for example, in hydraulic car jacks or pallet trucks.

Components of Complex Hydraulic Systems

The range of components for hydraulic systems has now become very extensive. What they all have in common is that hydraulics can primarily generate and transmit very large forces. Since the combination of master cylinder and check valve is too bulky for many applications, a whole series of hydraulic pumps has been developed, for example. Likewise, an equally large number of hydraulic radial motors are available for generating rotational movements. For their control and regulation, the hydraulics industry has developed a vast selection of valves, orifices, branches, and many other components.

Hydraulics Fundamentals: Applications of Hydraulic Systems

Hydraulic systems are used wherever operating speed plays a lesser role than the force generated. Typical applications include the construction of hydraulic presses, kinematics of work equipment (excavators, forklifts), or drives for hydraulically operated agricultural equipment.