In industrial hydraulic systems, the vast majority of hydraulic pumps are positive displacement pumps. The core characteristic of this type of pump is that, during each rotation cycle of the pumping element, a fixed volume of fluid is discharged; furthermore, regardless of how the system pressure fluctuates, the delivery volume per cycle remains essentially constant. This characteristic enables positive displacement pumps to provide a stable flow output for hydraulic systems, meeting the power requirements of most industrial operating conditions.
Based on the adjustability of their output flow rate, positive displacement pumps are further divided into two main categories: fixed-displacement pumps and variable-displacement pumps. There are significant differences between the two in terms of structural design, operating characteristics and applicable scenarios, making them key types that require careful distinction during industrial selection.
The defining characteristic of fixed-displacement pumps is 'constant flow'-at a given pump speed, the output flow rate per cycle remains unchanged, and the displacement cannot be adjusted via the pump's internal structure. The advantages of this type of pump lie in its simple structure, stable operation and extremely low noise levels, making it particularly suitable for scenarios with high noise control requirements, such as theatres and opera houses where environmental quietness is strictly mandated; screw pumps are one of the most representative types of fixed-displacement pumps. Fixed-displacement pumps have low maintenance costs and a low failure rate, making them suitable for medium- and low-pressure hydraulic systems with stable flow requirements and simple operating conditions.
Variable-displacement pumps, in contrast to fixed-displacement pumps, have the core advantage of 'adjustable displacement' – by altering the geometry of the internal volumetric chambers, the volume of fluid discharged per revolution can be flexibly adjusted; even at the same rotational speed, flow output can be precisely controlled according to system requirements. This characteristic makes variable displacement pumps particularly suitable for hydraulic circuits requiring variable speed and reverse operation, especially when paired with hydraulic motors, as they enable flexible adjustment of power output whilst achieving energy-saving benefits.
The selection between fixed-displacement and variable-displacement pumps hinges primarily on the hydraulic system's flow requirements and operational complexity: if the system's flow demand is fixed and the operating conditions are simple, fixed-displacement pumps should be prioritised to balance economy and stability; if the system requires flow adjustment in response to load variations and prioritises energy efficiency and flexible control, a variable-displacement pump is the superior choice. Understanding the core differences between the two is a crucial step in selecting the right hydraulic pump and forms the foundation for ensuring the efficient operation of the hydraulic system.
