Technical Criteria for Selecting Axial Piston and Gear Pumps

In hydraulic system design, pump selection is a critical engineering decision with a direct effect on overall system efficiency, operating pressure and long-term operating costs. As the source of hydraulic energy, pumps use different design mechanisms to convert mechanical power into fluid power. The two principal pump technologies in the HMLIFT product family—axial piston pumps and gear pumps—have been developed to address different operational profiles.

Operating Principles and Structural Characteristics of the Technologies

Gear Pumps

Gear pumps transfer hydraulic oil through the rotational movement of two precisely meshing gears inside the housing. A vacuum is created in the suction zone where the gears separate, drawing oil into the pump; along the line where the gears mesh again, the available fluid volume decreases and, because the fluid is incompressible, it is pressurised and delivered to the system.

Key characteristics include structural simplicity with few moving parts, high tolerance of demanding field conditions, ease of maintenance and a broad flow range. They are sized according to the application’s volumetric requirement through group classifications such as 25, 30, 35 and 40.

Axial Piston Pumps (Bent-Axis Type)

Axial piston pumps of the bent-axis type contain a cylinder block positioned at a defined angle to the shaft axis and pistons that reciprocate inside this block. In the HMLIFT PEX7 series, the bent-axis angle is optimised at 40°, so the rotational movement of the shaft causes the pistons to perform a linear stroke. This mechanism alternately performs suction and delivery with high precision.

Key characteristics include high pressure capability, high volumetric efficiency and stable flow generation over variable speed ranges.

Comparison of Technical Parameters

The main technical differences that shape the choice of pump technology can be grouped under the following headings:

• Pressure Limits: Owing to their structural design, axial piston pumps can reach high pressure levels—350-400 bar continuously and 400-450 bar at instantaneous peaks in the PEX7 series. Gear pumps generally provide an optimum balance at continuous operating pressures between 170 and 260 bar.
• Volumetric Efficiency: Thanks to precise internal sealing and tight tolerances, piston pumps operate with high volumetric efficiency under suitable conditions. This helps minimise energy losses and heat generation, particularly at high pressure.
• Speed Flexibility: Axial piston models such as the PEX7 can adapt to a broad operating-speed range, from a minimum of 300 rpm to high intermittent speeds. This flexibility is advantageous with variable-speed power take-offs (PTOs).
• Fluid Sensitivity: Because of the precision of their internal mechanisms, piston pumps are more sensitive than gear pumps to hydraulic oil quality and filtration standards. Gear pumps may offer somewhat greater tolerance of field contamination, but correct filtration remains essential to the service life of both technologies.

Special Configurations: Bi-Rotational and Tandem Designs

Special pump configurations are used in hydraulic systems to meet the diversity of application requirements:

• Bi-Rotational Pumps: Available in selected gear-pump groups—25, 30 and 35—this feature allows the pump to operate both clockwise and counterclockwise. Where the rotation direction of a vehicle-mounted PTO varies, installation can be simplified by changing the orientation of the inlet and outlet ports.
• Tandem Pumps: These designs combine two separate pump housings on a single drive shaft and are available in Group 40. They allow two independent hydraulic circuits to be supplied from one PTO connection, simplifying system architecture when several independent functions must operate on the vehicle.

Conclusion and Initial Start-Up Practices

Choosing between an axial piston pump and a gear pump is not a question of one technology being superior; it depends entirely on correct analysis of the application requirements, including operating pressure, flow demand, speed characteristics and system conditions. Whichever technology is selected, the pump must not run dry during initial start-up, the system must be filled with oil, and the first movement should be performed at low speed without pressure. These practices are important for mechanical service life. HMLIFT technical documentation can be used to determine the correct configuration.