Friction, Heat, Pressure drop

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Friction occurs in all devices and lines in a hydraulic system through which liquid passes.

This friction is mainly at the line walls (external friction). There is also friction between the layers of liquid (internal friction).

The friction causes the hydraulic fluid, and consequently also the components, to be heated. As a result of this heat generation, the pressure in the system drops and, thus, reduces the actual pressure at the drive section.

The size of the pressure drop is based on the internal resistances in a hydraulic system. These are dependent on:
• Flow velocity (cross-sectional area, flow rate),
• Type of flow (laminar, turbulent),
• Type and number of cross-sectional reductions in the system of lines (throttles, orifices),
• Viscosity of the oil (temperature, pressure),
• Line length and flow diversion,
• Surface finish,
• Line arrangement.

The flow velocity has the greatest effect on the internal resistances since the resistance rises in proportion to the square of the velocity.

Influence of flow velocity on pressure loss

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