Hydraulic Spring

The term “Hydraulic Spring” is ambiguous. It can be applied to two areas:

1. The Hydraulic Spring Constant
2. Hydromechanical Damping Elements

Hydraulic Spring as a Hydraulic Spring Constant

The basic principle of hydraulic systems is based on the assumption that liquids are incompressible. This property allows them to distribute applied pressure evenly within a system. However, this only applies to “ideal liquids,” which are more of a theoretical construct than physically existing fluids.

“Real liquids” have certain properties that contradict these theoretically ideal assumptions. These include a certain compressibility and thus also internal friction. In practice, these properties of real liquids only play a role under specific circumstances.

The compressibility of hydraulic oil or other liquids is extremely low. It is primarily based on the compressibility of dissolved gases, which cannot be entirely avoided in hydraulic systems. Nevertheless, considering them can become important in highly stressed systems. Especially the internal friction in pressurized hydraulic fluids can lead to thermal problems if not accounted for. Also, large hydraulic cylinders do not behave entirely as expected under the assumption of ideal liquids.

Calculation of the Hydraulic Spring Constant

The general formula for spring constants can be used to calculate the hydraulic spring constant “C”:

C = A² x E / V

C = specific spring constant of a hydraulic fluid
A² = cross-sectional area of the test cylinder
E = modulus of elasticity of the hydraulic fluid
V = the pressurized volume

The compressibility of real liquids can lead to insufficient volume in very large systems. The spring constant can thus also be used to calculate the expected volume loss under pressure.

Hydraulic Spring as Hydromechanical Damping Elements

Hydraulic shock absorbers are occasionally also referred to as “hydraulic springs.” This rather colloquial term describes components with hydraulic and mechanical elements.

Oil-pressure shock absorbers are considerably stiffer than the much more common gas-pressure shock absorbers. They offer less comfort and are primarily used in motorsports.

Hydraulic shock absorbers consist of two nested, oil-filled tubes. Additionally, a spiral compression spring is located inside the inner tube. The oil is used as a damping element. The internal spiral spring serves to reset the component when the load is removed.

In a hydraulic shock absorber, a piston under load pushes a standing column of oil through small channels into a reservoir. When the load is removed, the spiral spring pushes the shock absorber apart again and draws the oil back. The damping properties are based on the viscosity of the hydraulic oil used. The damping characteristics of the components can be varied by changing the preload or the hydraulic fluid.