The difference between hydraulic motors and hydraulic pumps

Oct 23, 2023

Hydraulic pumps and hydraulic motors are energy conversion components in hydraulic transmission systems. What are the differences between the two? How to distinguish? Where are they applied separately?

The similarities between hydraulic motors and pumps:

1. In principle, hydraulic motors and pumps are reversible. If driven by an electric motor, the output is pressure energy (pressure and flow), which is the hydraulic pump; If the input pressure oil outputs mechanical energy (torque and speed), it becomes a hydraulic motor.

2. From a structural perspective, the two are similar.

hydraulic pump

3. Hydraulic motors and hydraulic pumps have the same basic structural elements - a sealed and periodically variable volume and corresponding oil distribution mechanism. The working principle of hydraulic motors and pumps is to use changes in the sealing working volume for oil suction and drainage.

For hydraulic pumps, when the working volume increases, oil is sucked in, and when the working volume decreases, high-pressure oil is discharged. For hydraulic motors, when the working volume increases, high-pressure oil enters, and when the working volume decreases, low-pressure oil is discharged.

Differences between hydraulic motors and pumps

1. A hydraulic pump is a conversion device that converts the mechanical energy of an electric motor into hydraulic energy, outputting flow and pressure, with the hope of high volumetric efficiency; Hydraulic motor is a conversion device that converts the pressure energy of liquid into mechanical energy, outputting torque and speed, hoping for high mechanical efficiency. Therefore, hydraulic pumps are energy devices, while hydraulic motors are executive components.

2. The steering of the output shaft of the hydraulic motor must be able to rotate forward and reverse, so its structure is symmetrical; However, some hydraulic pumps (such as gear pumps, vane pumps, etc.) have clear regulations for steering, and can only rotate in one direction and cannot change the direction of selection at will.

3. In addition to the inlet and outlet ports, the hydraulic motor also has separate oil leakage ports; Hydraulic pumps generally only have inlet and outlet ports (excluding axial piston pumps), and the leaked oil inside is connected to the inlet port.

4. The volumetric efficiency of hydraulic motors is lower than that of hydraulic pumps.

5. Usually, the working speed of hydraulic pumps is relatively high, while the output speed of hydraulic motors is low.

6. In addition, the oil suction port of the gear pump is large and the oil discharge port is small, while the oil suction and discharge ports of the gear hydraulic motor are of the same size.

7. The number of teeth in a gear motor is greater than that in a gear pump.

8. The blades of the vane pump must be installed diagonally, while the blades of the vane motor are installed radially; The blades of a vane motor rely on a swallow shaped spring at the root to compress them onto the surface of the stator, while the blades of a vane pump rely on the pressure oil and centrifugal force at the root to compress them onto the surface of the stator.

In terms of working principle, both hydraulic motors and hydraulic pumps operate based on changes in the volume of the sealed working chamber. However, due to their different purposes of use and structural differences, they cannot be directly interchangeable.

Classification of hydraulic pumps

According to structure, there are three major categories: plunger pumps, gear pumps, and vane pumps.

According to whether the displacement can be adjusted, it can be divided into fixed displacement pump and variable displacement pump.

According to the direction of oil discharge, it is divided into one-way pump and two-way pump.

Classified by pressure level: low pressure, medium pressure, medium high pressure, and ultra-high pressure pumps.

Gear pump: smaller in size, simpler in structure, less strict in oil cleanliness requirements, and cheaper in price; But the pump shaft is subjected to unbalanced force, severe wear, and significant leakage.

Gear pumps are widely used in industries such as mining equipment, metallurgical equipment, construction machinery, engineering machinery, and agricultural and forestry machinery.

Vane pump: divided into double acting vane pump and single acting vane pump. This type of pump has uniform flow rate, smooth operation, low noise, higher operating pressure and volumetric efficiency than gear pumps, and a more complex structure than gear pumps. High pressure vane pumps are used in the hydraulic systems of lifting and transportation vehicles and construction machinery.

Plunger pump: High volumetric efficiency, small leakage, capable of working under high pressure, mostly used in high-power hydraulic systems; But the structure is complex, with high requirements for materials and processing accuracy, expensive prices, and high requirements for oil cleanliness. A plunger pump is commonly used in cycling diesel engines to deliver high-pressure fuel.

Classification of hydraulic motors

Classified by structural form: gear type, blade type, and plunger type.

Classified by speed and torque range: high-speed motor and low-speed motor.

Geared hydraulic motors have a simple structure and are inexpensive, and are commonly used in situations with high rotational speed, low torque, and low requirements for motion stability. Such as driving grinding machines, fans, etc.

The blade type hydraulic motor has small rotational inertia, sensitive action, low volumetric efficiency, and soft mechanical characteristics. It is suitable for situations above medium speed, low torque, and frequent starting and reversing requirements.

Axial piston type motors have high volumetric efficiency, wide adjustment range, good low-speed stability, poor impact resistance, and are commonly used in high-pressure systems with high requirements.

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