The hydraulic pump motor forms an integral part in driving the hydraulic pump with the aid of an electric motor. It draws hydraulic oil from the tank, pressurizes it, and delivers the pressurized oil to the actuators for mechanical motion. This process is very common in hydraulic systems.
Motor Drives Pump
The main purpose of the hydraulic pump motor is to convert electrical energy into mechanical energy to drive the hydraulic pump. The motor normally operates on a three-phase AC that creates a magnetic field inside the motor, causing the rotor to turn. This, in effect, applies the principle of electromagnetic induction. The ratings of motors used for driving hydraulic systems start from a few hundred watts up to hundreds of kilowatts, depending on the hydraulic pump’s requirements.
Generally, the rotational speed of hydraulic pump motors is stable within 1450 RPM to 3000 RPM. The higher the speed, the larger the displacement of the hydraulic pump and the more hydraulic oil is output. To ensure stable operation, the hydraulic pump motor usually requires high starting torque and good overload capacity. Many heavy machinery applications, including excavators and cranes, require that the hydraulic pump motor have overload capacity up to 2-3 times the rated power for a short period to cope with sudden high-load demands.
Hydraulic pump motors also require a good cooling system. The high-power operation produces a lot of heat in the motor; if it is not dissipated in time, the motor is likely to overheat and burn the coil. Therefore, many hydraulic pump motors are equipped with air cooling or water cooling devices to ensure long-term stability in high-load operation. For some extreme applications, such as mining or steel smelting equipment, the operating environment temperature of hydraulic pump motors may exceed 100°C, making the design of the cooling system particularly critical.
Hydraulic Pump Oil Suction
When the hydraulic pump motor drives the rotating components inside the pump (gears, vanes, or plungers), the pressure inside the pump drops, drawing hydraulic oil from the tank into the pump. This process of suction requires sufficient negative pressure inside the pump to effectively draw in the oil. Normally, the distance and height difference between the hydraulic tank and the pump affect suction efficiency, so the suction line must be shortened as much as possible to reduce pressure loss.
Hydraulic pumps, such as gear pumps, vane pumps, and plunger pumps, have different working characteristics in oil suction. For instance, gear pumps create negative pressure through the meshing action of a pair of rotating gears, vane pumps use centrifugal action from revolving vanes, and plunger pumps rely on reciprocating plungers, similar to the working principle of pistons inside a cylinder. Regardless of the type of hydraulic pump, oil must always pass through a filter before entering the pump body to remove contaminants that could damage the pump.
To ensure smooth oil suction, attention must be paid to the oil’s viscosity and temperature. The general requirement for hydraulic oil viscosity is between 10 and 100 centistokes (cSt). If viscosity is too high, suction resistance increases, reducing pump efficiency. If viscosity is too low, internal leakage may worsen. Similarly, the ideal hydraulic oil temperature is between 40°C and 60°C. Too high a temperature lowers viscosity, affecting system stability.
The design of the oil tank is also crucial. Normally, the tank volume is designed to be three times the maximum system oil volume to allow sufficient space for circulation during operation. This also aids in cooling the oil, lowering its temperature, and making it easier for the pump to draw oil.
Pressurizing Oil
After the hydraulic pump draws in the oil, the motor drives the rotating parts inside it, compressing or squeezing the oil within the pump chamber. The design of the pump chamber plays an important role in raising the oil’s pressure. Depending on the type of pump and application scenario, the outlet pressure range typically varies between 10 MPa and 50 MPa (100 to 500 bar), sometimes even reaching as high as 100 MPa. High-pressure devices, such as hydraulic presses and injection molding machines, often operate with output pressures exceeding 30 MPa to handle heavy loads.
The working pressure for gear pumps generally ranges from 10 MPa to 25 MPa, making them suitable for medium- and low-pressure hydraulic systems. Gear pumps are structurally simple and inexpensive, making them widely used in many standard hydraulic devices. Plunger pumps, with their more precise design, can achieve higher working pressures—typically up to 50 MPa—and are preferred for high-pressure hydraulic systems.
During pressurization, the hydraulic pump is subject to internal leakage and mechanical friction, both of which affect its working efficiency. If components inside the pump do not fit tightly, small gaps may allow pressurized oil to leak, reducing system efficiency. To address this, many hydraulic pumps incorporate compensation devices. Variable plunger pumps, for instance, automatically adjust the pump’s displacement based on the system’s pressure and flow requirements to avoid unnecessary energy loss.
Delivering Pressurized Fluid
The pressurized hydraulic oil is discharged through the outlet to hydraulic actuators such as hydraulic cylinders or motors. These components perform mechanical movements according to the pressure and flow of the oil. The hydraulic oil transmission in pipelines should aim to minimize pressure loss, so proper piping design and layout are essential. Common piping materials include steel pipes, hoses, and high-pressure plastic pipes, with the material chosen based on the application.
To prevent pressure pulsations or fluctuations during oil delivery, hydraulic systems often incorporate accumulators or dampers. Accumulators store hydraulic oil and release it instantly when needed to maintain stable pressure output. Dampers absorb pressure fluctuations, reducing system vibration and improving stability.
As the system operates, the oil temperature rises, so cooling is necessary to maintain system performance. Common cooling methods include air cooling and oil cooling, with the choice depending on the system’s working environment and temperature requirements. Oil cooling is typically used in high-power hydraulic systems. Coolers are mounted beside the oil tank, and circulating oil passes through the cooler to dissipate heat, ensuring the hydraulic oil remains within the appropriate temperature range.
Circulation in the Hydraulic System
The hydraulic system operates in a closed loop: the hydraulic pump motor drives the pump, which draws oil from the tank, pressurizes it, delivers it to the actuators, and finally returns it to the tank. Every component in this loop must work efficiently to avoid energy waste.
Control components play a vital role in regulating the system during circulation. Examples include relief valves, throttle valves, and directional valves, which manage oil pressure, flow, and direction. Relief valves automatically release excess oil back to the tank when system pressure exceeds a set limit, preventing overload. Directional valves control oil flow, determining the direction of actuator movement.
A filtration system is essential to prevent contaminants in the oil from damaging the pump and other components. Common filter precision ranges from 10 microns to 25 microns, and the appropriate precision should be chosen based on system requirements to ensure oil cleanliness.
Modern hydraulic systems often incorporate energy-saving technologies to improve overall system efficiency. Combining servo motors with variable pumps enables real-time adjustment of pump output based on system demand, reducing energy consumption. According to statistical data, hydraulic systems using servo control technology can reduce energy consumption by 20% to 30% compared to traditional systems.
Oil circulation in the hydraulic system is not only a process of transferring hydraulic energy but also one of regulating system temperature, pressure, and flow. A properly designed and controlled hydraulic system can operate efficiently and safely over extended periods.
