Fixeddisplacement

Fixed displacement refers to hydraulic devices in which the volume of fluid moved per revolution remains constant across the operating range. These devices deliver a fixed amount of fluid per rotation, in contrast to variable-displacement units that adjust displacement to regulate flow. Fixed-displacement configurations are common in pumps and motors used in heavy machinery, where simplicity and robustness are valued.

In a fixed-displacement pump, the displacement D is fixed. The flow rate Q is proportional to speed, described by Q = D × n, where n is the rotational speed. This means that increasing pump speed increases flow linearly, while system pressure is determined by the load and limited by relief or pressure-control devices. Conversely, a fixed-displacement hydraulic motor converts a constant volume of hydraulic fluid per revolution into mechanical work, with output torque related to the pressure drop across the motor and its displacement.

The main advantage of fixed displacement is simplicity, high efficiency at a given speed, and predictable flow characteristics, which simplifies system design. Limitations include limited intrinsic flow control; to throttle flow, external valves or pressure control must be used, potentially wasting energy when loads vary widely. Temperature rise and efficiency losses at off-design points can also occur under heavy or fluctuating loads.

Common examples include gear, vane, and piston pumps with fixed displacement, and fixed-displacement hydraulic motors used in winches, conveyors, and other machinery. These devices form the backbone of many traditional hydraulic systems, particularly where a straightforward, robust flow source is required.