Displacement as a Basis for Flow Calculation

Displacement is the amount of hydraulic fluid that a hydraulic motor requires for one working stroke. This volume is used for the design of radial piston motors. Displacement is a type-specific piece of information. It must be provided by the manufacturer, as it is one of the foundations for calculating the entire hydraulic system.

Displacement in Radial Piston Motors

Hydraulic radial piston motors can have very different designs. The calculation of displacement therefore heavily depends on the motor’s design.

Displacement is also used for calculating pumps and turbines in water technology. Fundamentally, displacement is calculated based on the cavity in the radial piston motor that is completely filled with hydraulic fluid during one revolution.

Only the actually circulated amount counts towards the displacement. To ensure that the actual value of this characteristic is correct, for complex motors, the displacement is usually also determined empirically.

Calculated Dependencies of Displacement

The displacement V has calculated dependencies on the following hydraulic parameters:

• Power (P)
• Rotational speed (n) (Alternatively, cycle rate)
• Pressure drop Δp (depending on efficiencies)

Multiplying the displacement by the rotational speed gives the volumetric flow rate Q. There is a “pressure drop” between the incoming and outgoing hydraulic fluid. The pressure is very high before the motor and drops sharply afterwards. On the high-pressure side, it is referred to as “pump pressure,” and on the low-pressure side, as “tank pressure.” This implies that the hydraulic motor returns the hydraulic fluid to the tank.

Displacement in Practice

Displacement is a good theoretical basis for designing a hydraulic system. Nevertheless, it is only one of many parameters to consider when designing these systems. It is important to select the pump capacity in such a way that the hydraulic motor is not overloaded. Overloading risks damaged seals and high wear. To complicate matters, every additional component in the hydraulic system also increases friction. This leads to higher temperatures that must be dissipated. Displacement is therefore an important, but by no means the only, factor for the design of a hydraulic system.