Types of Resonances

The waveforms in a hydraulic pipe can be subdivided into two categories; uniform or opposite line Termination.

Resonance - closed-closed 300dpi.pngWhen resonant pulsation do develop in a hydraulic system, they can cause severe problems. Therefore, it is very important to know how pulsations behave in hydraulic systems, what pulsation sources exist in the system, and how the resonant frequencies of the piping network can be predicted.

Resonance - closed-open 300dpi.png

One important condition for the development of resonant frequencies is the type of impedance at the beginning and at the end of the piping. The impedances determine if the system will have quarter-wave or half-wave mode shape. 

Mode shapes of a simple hydraulic line can typically be broken into two categories common-boundary or opposite-boundary. Common-boundary spans of piping are open-open or closed-closed spans. Opposite-boundary spans of piping are typically described as open-closed respectively closed-open spans [1].

[1]  McKee, R. J.  and Broerman, E., 2009, "Acoustics in Pumping Systems",
Proceedings Of The Twenty-Fifth International Pump Users Symposium, Turbomachinery Laboratory, Texas A&M University, College Station, Texas, pp. 69-74.

Resonance - open-closed 300dpi.pngThe type of boundary-span determines at which position in the hydraulic line pressure nodes and pressure antinodes will develop. The knowledge of these antinode positions is essential for a successful implementation of damping devices.

The common-boundary "Closed-Closed" can be a pump that is connected by a hydraulic pipe to a valve, or it can be the pump motor setup of a hydro-static transmission.

Resonance - open-open 300dpi.pngThe common-boundary "Closed - Open" can be a pump that is connected by a hydraulic pipe to a reservoir (tank) or to a large cylinder.

The common-boundary "Open - Closed" can occur in automotive system, where road surface induced vibrations act onto a power steering cylinder or onto a pneumatic air suspension.

The common-boundary "Open-Open" can be the connection of two reservoirs (tanks) through a hydraulic pipe, or it can be a cylinder that is connected by a hydraulic pipe to an accumulator.

RohrLEx has prepared a flyer that gives an introduction to oscillation modes in hydraulic pipe systems.