Estimated reading time: 24 minutes

Selection of hydraulic pump

The hydraulic pump is an energy conversion device, which converts the mechanical energy of the driving motor into the pressure energy of the oil to meet the needs of the actuator to drive the external load.

Question 1: Hydraulic pump performance

The main performances of commonly used hydraulic pumps are shown in Table.

Question 2: Selection of hydraulic pump type

The principle of selecting the hydraulic pump is: according to the working condition of the main engine, the size of the power and the performance requirements of the system, first determine the type of the hydraulic pump, and then determine its specification and model according to the pressure and flow required by the system. Hydraulic Pump Parameter

• For general machine tool hydraulic systems, double-acting vane pumps, and pressure-limiting variable vane pumps are often used; while road construction machinery, port machinery, and small construction machinery, gear pumps with strong anti-pollution capabilities are often selected; In occasions, plunger pumps are often chosen. Hydraulic Pump Parameter
• Pay attention to whether the flow of the hydraulic pump is adjustable. Generally speaking, if the hydraulic power is less than 10kW, the working cycle is on-off, the hydraulic pump can be completely unloaded when not in use, and the hydraulic pump is required to output full flow under most working conditions, you can consider using a fixed pump; if the hydraulic power If the flow rate is larger than 10KW, the variable pump can be considered. Hydraulic Pump Parameter
• Pay attention to parallel pumps and series pumps. Gear pumps and vane pumps can also be made into several pumps connected in parallel, and use a double pump or triple pump with the same drive shaft, or can be connected in series to form a multi-stage pump. When the hydraulic system-the flow rate changes greatly within a working cycle, multiple pumps can be used. Multiple pumps usually have one oil suction port and multiple oil outlets. The pressure oil from each oil outlet can supply oil to different actuators of the system, or can be combined to supply a certain actuator. Hydraulic Pump Parameter

The hydraulic pump parameter calculation

Question 1: Calculate the maximum working pressure of the hydraulic pump

The maximum working pressure P_p of the hydraulic pump depends on the maximum working pressure of the actuator (hydraulic cylinder or hydraulic motor), namely

P_p≥P₁+ ∑△p

Question 2: Calculate the maximum flow rate of the hydraulic pump

The maximum flow q_p (m³/s) of the main hydraulic pump depends on the flow q_v required by the system. Hydraulic Pump Parameter

For a system where multiple hydraulic cylinders or hydraulic motors act at the same time, the maximum flow rate of the hydraulic pump should be

q_p≥q_v＝K( ∑q )max  

Question 3: Calculation of small pump flow when calculating double pump oil supply

If the oil supply of the hydraulic pump is selected according to the working conditions (for example, when two pumps are used for oil supply, the small flow pump supplies the flow of the working conditions), the minimum overflow of the overflow valve should also be considered when calculating the oil supply. Hydraulic Pump Parameter

Question 4: Select the rated pressure of the hydraulic pump to determine

The pressure of the hydraulic pump obtained by the above calculation is the static pressure of the system. In the dynamic working process of the system, the dynamic pressure of the system far exceeds the static pressure due to pressure overshoot or periodic pressure pulsation during the transition process. Therefore, when selecting a hydraulic pump, the rated pressure of the hydraulic pump should be 25%~60% higher than the maximum calculated pressure of the system (a high-pressure system takes a small value, medium and the low-pressure system takes the larger value) so that the hydraulic pump has a certain pressure reserve. Hydraulic Pump Parameter

• Generally speaking, in fixed equipment, the rated pressure of the hydraulic pump should be about 25% higher than the normal working pressure, and the rated pressure of the hydraulic pump for vehicles should be about 45% higher than the normal working pressure to ensure the pump has a sufficient life. If the reliability requirements of the hydraulic system are particularly high, the rated working pressure of the hydraulic pump can be selected higher. Hydraulic Pump Parameter
• If the hydraulic system only requires the supply of high-pressure and small-flow working medium in a short period, the maximum working pressure of the hydraulic pump can be selected smaller at this time (when selecting a hydraulic pump, it is not advisable to reduce the specifications of the large-capacity hydraulic pump. It is because when the working pressure and working flow of the hydraulic pump are less than its rated value, the overall efficiency of the hydraulic pump will decrease). Hydraulic Pump Parameter
• The highest working pressure on the sample is the allowable pressure during short-term impact. Do not have impact pressure in every cycle, otherwise, the life of the hydraulic pump will be significantly shortened or even damaged. Hydraulic Pump Parameter

Question 5: Select the rated flow of the hydraulic pump to determine

As for the flow rate of the hydraulic pump, the maximum flow rate required by the system can be selected when selecting, and it should not exceed too much to avoid excessive power loss. After the structure of the hydraulic pump is drawn up, the specifications and models of the hydraulic pump can be determined by referring to the relevant technical manuals or product samples according to the maximum working pressure and maximum output flow of the hydraulic pump.

In the actual selection, because the pump displacement, speed range, and output flow under different pressures at typical speed are usually given on the product sample. Therefore, when the required flow q_v of the system is known, the flow rate of the pump q_p (L /min), speed n (r/min), and displacement V (mL/r) should be considered comprehensively. In fact, since the output flow of the pump is

    q_p＝Vnη_v×10^-3

• The output flow of the hydraulic pump should include the flow required by the actuator (the total flow can be obtained from the time chart when there are multiple actuators), the minimum overflow flow of the overflow valve, the sum of the leakage of each component, and the motor reverse rotation (usually 1r/s or so), the flow reduction caused by the reduction in efficiency of the hydraulic pump after long-term use (usually 5% to 7%). Hydraulic Pump Parameter
• The flow rate of the hydraulic pump is related to the working condition, and the maximum flow rate of the hydraulic pump is determined by the maximum flow rate in the system working condition diagram. When selecting the flow rate of the hydraulic pump, it should not be less than or equal to the maximum flow rate when the hydraulic system is working. Hydraulic Pump Parameter
• The flow rate of the hydraulic pump is related to the speed of the prime mover. When the prime mover and the hydraulic pump are connected by a coupling, the speed of the prime mover should be within the working speed range of the hydraulic pump, otherwise, a speed increasing or decelerating device should be installed between the prime mover and the hydraulic pump. Hydraulic Pump Parameter

Question 6: Regarding the speed and efficiency of the pump

• The maximum pressure and the maximum speed of the hydraulic pump should be used at the same time to extend the service life of the hydraulic pump. The product manual provides a more detailed guiding chart of hydraulic pump parameters. When selecting, you should strictly follow the regulations in the product manual.
• The higher the pressure and the lower the speed, the lower the volumetric efficiency of the hydraulic pump, and the volumetric efficiency of the variable pump will decrease when the displacement of the variable pump is adjusted. When the speed is constant, the total efficiency of the hydraulic pump is the highest at a certain pressure, and the total efficiency of the variable pump is the highest at a certain displacement and a certain pressure. The total efficiency of the hydraulic pump has a great influence on the efficiency of the hydraulic system. Therefore, it is taboo to choose a low-efficiency hydraulic pump. You should choose a high-efficiency hydraulic pump, and try to make the hydraulic pump work in an efficient working condition area.
• Consider the problem from the perspective of improving energy conversion efficiency. Don’t just consider the rated working conditions, but should make the efficient working range as wide as possible. When choosing a hydraulic pump, do not reduce the specification of a large-capacity pump. This is because when the pump’s working pressure and working flow are less than its rated value, the overall efficiency of the pump will decrease. Generally speaking, the volumetric efficiency of the hydraulic pump is more important than the mechanical efficiency. Because the hydraulic pump can obtain power from the prime mover to compensate for the mechanical friction loss, but the leakage loss in the hydraulic pump cannot be compensated by increasing the power input. Of course, the rotational speed of the hydraulic pump should not be too high or too low, otherwise, its mechanical efficiency will decrease. It is generally believed that when the hydraulic pump works in the range of 1000~1800r/min, it has little effect on the overall efficiency. Hydraulic Pump Parameter
• In the case of using an engine to drive the hydraulic pump, low speed is prohibited when the oil temperature is low, otherwise, it will be difficult to absorb oil, and there will be a risk of seizing failure due to poor lubrication; at high speeds, cavitation, vibration, abnormal wear, and flow must be considered The possibility of instability and other phenomena, to avoid drastic changes in the speed of the hydraulic pump from having a huge impact on the strength of the internal parts of the hydraulic pump. Hydraulic Pump Parameter

Question 7: Calculate the drive power of the hydraulic pump

If the pressure and flow rate of the pump are relatively constant during the working cycle (that is, the p-t curve and q-t curve on the working condition diagram change more smoothly), then the hydraulic pump drive power P_p Should be calculated as follows Hydraulic Pump Parameter

P_p＝P_pq_p/η_p

Question 8: The choice of motor

• When selecting the electric motor of the hydraulic pump, the influence factor of speed must also be considered. The synchronous speed of the electric motor cannot be higher than the rated speed of the hydraulic pump.
• Hydraulic pumps of fixed equipment are usually driven by electric motors. According to the power calculated above and the rotational speed of the hydraulic pump and its use environment, select its model specifications from the product catalog or manual (rated power, rotational speed, power supply, structure (vertical, horizontal, open, closed, etc.) ], and calculate the overload capacity to ensure that the peak overload of the motor is less than 25%~50% in each working stage. Hydraulic Pump Parameter
• Since the hydraulic pump usually starts under no load, there is no excessively high requirement on the starting torque of the motor, the load changes relatively smoothly, and the number of starts is small, so a cage-type three-phase asynchronous motor can be used. But if the power of the hydraulic system is large and the grid capacity is not large, a wound rotor motor can be used. For hydraulic pumps that adopt variable frequency flow adjustment schemes, AC asynchronous motors controlled by variable frequency speed regulation or electromagnetic speed regulation should be used to drive the hydraulic pump. Hydraulic Pump Parameter
• Motors of the same type with the same capacity (power) usually have different speeds for selection. Low-speed motors have a large number of magnetic poles, large dimensions and weights, high prices, and require pumps with a large displacement (under a certain flow rate), while high-speed motors are the opposite. Therefore, the speed of the motor should be considered comprehensively with the flow and displacement of the pump. Hydraulic Pump Parameter
• The working environment of the hydraulic pump is different, and the protection form of its drive motor is required to be different: an open motor (protection mark is IP11) should be used in a clean and dry environment; a protective motor should be used in a cleaner and clean environment (protection mark is IP22 and IP23) drive; humid, dusty, high temperature, corrosive or weather-prone environments should be driven by enclosed motors (protection mark IP44); explosion-proof motors should be used in explosive environments. Hydraulic Pump Parameter

Question 9: The choice of internal combustion engine

Most of the hydraulic systems of mobile machinery use internal combustion engines to drive hydraulic pumps.

When the hydraulic pump is driven by an internal combustion engine, in one case, the hydraulic pump is only a part of the internal combustion engine’s drive load; in another case, the entire power of the internal combustion engine is used to drive the hydraulic pump. In the former, the power of the internal combustion engine is large, which can always meet the power required by the hydraulic pump. The speed of the internal combustion engine should match the optimal speed of the hydraulic pump.

High-speed internal combustion engines usually have a deceleration device to make the hydraulic pump work within the optimal speed range. In the latter, the system in which the entire power of the internal combustion engine is used to drive the hydraulic pump is called a full hydraulic drive system. The full hydraulic drive system usually adopts a variable pump or variable motor volumetric speed control system to meet the requirements of large changes in the speed of mobile machinery.

The maximum speed of the internal combustion engine should meet the maximum flow required by the system and not exceed the maximum allowable speed of the hydraulic pump. If the engine speed is too high, a deceleration device should be installed. The maximum power of the internal combustion engine should be slightly greater than the maximum power required by the hydraulic system. Hydraulic Pump Parameter

Hydraulic pump circuit design

Question 1: Closed system design

A closed hydraulic system that uses a manual servo variable pump to control the commutation of the actuator should avoid self-priming and oil replenishment. As shown in Figure 1-1, in the process of using a manual servo variable pump to control the commutation of the actuator, the variable mechanism of the servo variable pump must pass the zero position (that is, the position when the variable pump has no flow output). The variable mechanism of the pump is set to zero.

From the variable principle of the servo variable pump, it is known that pressure oil must be available to realize the variable. However, when the variable mechanism is in the zero position, no pressure oil can be generated when the self-priming oil is used. Hydraulic Pump Parameter

Note that corresponding measures should be taken to form a closed hydraulic system with a variable pump and a single-rod hydraulic cylinder. In Figure 1-2, the variable pump and the single-rod hydraulic cylinder form a closed hydraulic system. When the piston rod is extended, there will be no problem. When the piston rod is retracted, the hydraulic cylinder discharges more oil than it enters. The amount is much larger. If no measures are taken, the hydraulic cylinder cannot move at this time. Hydraulic Pump Parameter

When the hydraulic motor is operating as a hydraulic pump, avoid the main pump from being subjected to working pressure difference. Figure 1-3 In a closed system composed of a variable pump and a hydraulic motor, when the variable mechanism of the variable pump is set to zero, the motor operates as a meter under the action of external power. Adjusting the overflow valve can control the motor to operate as a pump.

The working pressure difference also acts on the main pump, but at this time, the variable mechanism of the main pump is set to zero as a stop valve. However, the high pressure cavity of the main pump has to bear high-pressure, and the relevant parts of the high-pressure cavity are equivalent to bear static high pressure, which is very disadvantageous. In addition, when the variable mechanism of the main pump deviates from zero due to vibration and other reasons, the system pressure will suddenly drop and cause an accident. Hydraulic Pump Parameter

When designing the pump-motor closed system, pay attention to the situation of heavy objects falling at the moment of driving. As shown in Figure 1-3, the heavy lifting closed system is composed of a hydraulic pump and hydraulic motor. When the brake of the motor is released, the heavy object forms an excess torque to drive the hydraulic motor to rotate, so that the hydraulic motor is in working condition of the hydraulic pump.

With the increase of the falling speed of the weight, once the torque of the pressure oil at the motor outlet is not enough to balance with the load torque of the weight acting on the hydraulic motor, the falling speed of the weight will be out of control, causing a crash accident.

Because the pressure built up by the heavy object causes oil compression and oil leakage, which are the main reasons for the sliding of the heavy object at the moment of driving, a one-way valve can be installed in the high-pressure pipeline of the system close to the hydraulic motor to connect the hydraulic motor with other The components are separated,

and the pressure signal of the heavy object can also be used to control the variable mechanism of the hydraulic pump so that the outlet pressure of the hydraulic pump automatically tracks the pressure established by the weight at the motor outlet so that the sliding speed of the heavy object can be significantly reduced at the moment of driving.

The sliding distance is greatly reduced, and the working reliability of the hydraulic system is also improved. Hydraulic Pump Parameter

The variable pump determines the amount of oil supplement for the closed system of the main pump. In a closed system where the variable pump is the main pump, it is inevitable to supplement the oil. However, only the actual flow rate of the variable pump in the system is used as the basis for determining the replenishment, which sometimes results in insufficient replenishment. If the rated speed of the variable pump is 1500r/min, and the speed of the variable pump in the system is only 1000r/min,

or the actual maximum flow of the system variable pump is only 90% of the rated flow of the pump, etc., insufficient oil supplement will occur. Hidden dangers. Of course, when determining the replenishment amount of a closed hydraulic system, the leakage caused by pipelines, valves, and actuators still needs to be considered. Hydraulic Pump Parameter

The oil in the replenishment circuit should not be returned to the oil without cooling. If the oil in the replenishing circuit uses uncooled return oil, the temperature of the system oil will rise, which will adversely affect the system. If the leakage in the components and the system increases, the sealing device rapidly ages and deteriorates, loses the sealing performance, accelerates the oxidation of the oil, causes the oil to deteriorate, and reduces the service life of the oil. Therefore, the oil in the replenishment circuit should not be used. Cooled oil return. Hydraulic Pump Parameter

Question 2: Hydraulic pump circuit design taboos

The built-in overflow valve of the hydraulic pump is not suitable for system pressure adjustment, but should only be used as a safety valve. A simple, direct-acting relief valve is installed in some types of hydraulic pumps, and the valve has a small diameter. It is generally not suitable for system pressure adjustment, but only as a safety valve. The structure of the relief valve and the safety valve are similar,

but the diameters of the two types of valves are different, and the purpose of the use is also different. The relief valve used for system pressure regulation requires a large valve diameter, which is mainly used to stabilize the system pressure, and the flow rate varies. The safety valve is mainly used to protect the system pressure from overloading.

Usually, the diameter is small and the flow rate is small, and the system pressure cannot be returned to normal. Therefore, the built-in relief valve of the hydraulic pump should not be used for system pressure adjustment, but should only be used as a safety valve. Hydraulic Pump Parameter

The constant power speed control circuit should not be used alone. In Figure 1-5, the constant power pump and the variable motor form a so-called constant power speed control loop. Generally, the speed control loop is required to have greater speed rigidity. But the speed rigidity of this circuit is proportional to the square of the motor displacement, but when the displacement of the variable motor is reduced, the speed rigidity will drop sharply, and the torque that can be output is also reduced, so there are few such circuits Use alone. Hydraulic Pump Parameter

Excessive pressure loss in the pipeline causes the output flow of the constant power variable pump to fail to meet the expected requirements. As shown in Figure 1-6, the outlet of the constant power variable pump is connected to the hydraulic cylinder through a one-way valve and a reversing valve, and the return oil of the hydraulic cylinder also needs to flow to the oil tank through the corresponding valve. Because some valve specifications are selected too small, the pressure loss is too large. As a result, the output flow of the pump is much smaller than expected, which fails to meet the requirements of the hydraulic cylinder to move quickly. Hydraulic Pump Parameter

When multiple actuators adapt throttling speed regulation, avoid using only one hydraulic pump for oil supply. When there are multiple actuators in the system, each agency cannot always maintain the coordination of flow and pressure requirements. However, the use of a throttling speed control circuit inevitably has overflow loss and throttling loss. At this time, if only one pump is used to supply oil, the pump must be selected based on the highest pressure and flow required by the system, which cannot adapt to the low-pressure small flow and high-pressure large flow conditions, resulting in low efficiency of the system, as shown in Figure 1- 7 shown. Hydraulic Pump Parameter

The diameter of the stop valve at the suction port of the hydraulic pump should not be too small. To facilitate maintenance, a shut-off valve is generally installed at the inlet of the hydraulic pump, which will increase the flow resistance of the pump. Therefore, the diameter of the shut-off valve must be large enough to prevent the vacuum of the hydraulic pump from being too high. The shut-off valve should have sufficient pressure resistance. When selecting the shut-off valve on the high-pressure pipeline, in addition to the maximum working pressure of the system, the instantaneous pressure change of the system should also be estimated to prevent damage. Hydraulic Pump Parameter

The speed of the hydraulic pump should not be too high. The hydraulic pump has a rated speed, and it is not allowed to rotate too high during use, to avoid problems such as insufficient oil suction, increased wear, and reduced life of the hydraulic pump. Generally, the maximum speed of gear pumps and vane pumps is 2000r/min, while plunger pumps should work at a speed of 1000r/min, which should not exceed 1500r/min. Hydraulic Pump Parameter

Question 3: Self-priming problem

When used in an open hydraulic circuit, the hydraulic pump needs to have a certain self-priming capacity. The occurrence of cavitation may not only damage the hydraulic pump, but also cause vibration and noise, which will make the control valve and executive components operate poorly, and have a bad influence on the entire hydraulic system. Hydraulic Pump Parameter

Taboos to ensure the self-priming ability

• While confirming the self-priming capacity of the hydraulic pump used, it is necessary to determine and design the installation position of the hydraulic pump relative to the oil tank level based on calculating the resistance of the suction line in consideration of the operating temperature conditions of the hydraulic device and the viscosity of the hydraulic oil. Suction line. When calculating the self-priming capacity of the hydraulic pump, a sufficient margin should be left.
• The distance from the center of the hydraulic pump to the oil surface is not more than 500mm, and the suction pressure should be within -125mmHg, otherwise cavitation is prone to occur, causing damage to parts, noise, vibration, and other failures. Hydraulic Pump Parameter
• Install a 150-mesh oil suction filter on the suction line. Some plunger pump manufacturers stipulate that it is not allowed to install an oil filter on the suction line, but this should have strict requirements on the cleanliness of the oil in the tank. Install the oil filter on the hydraulic pump, but this should have strict requirements on the cleanliness of the oil in the oil tank. Install a pipeline oil filter with a filtering accuracy of 25 mesh on the outlet side of the hydraulic pump.
• Note that the speed of the hydraulic pump cannot be greater than the rated speed.
• Pay attention that the diameter of the suction pipe is not less than the recommended value (see the product catalog of installation dimensions), and the suction pipe is at most one elbow joint.
• If the oil distribution plate needs to reduce the deflection angle of the swash plate when starting, it cannot guarantee self-priming. If the user needs a small flow, he should use the variable mechanism to change the flow after the hydraulic pump is started at full deflection. Hydraulic Pump Parameter
• For hydraulic pumps with poor self-priming, avoid installing filters on their suction pipes. As shown in Figure 1-8(a), if a hydraulic pump with poor self-priming is equipped with a filter at its suction port, as the pressure drop of the filter increases, the minimum suction pressure of the hydraulic pump will not be guaranteed. As a result, the hydraulic pump is insufficient to absorb oil, the volumetric efficiency also drops sharply, and vibration and noise occur until the hydraulic pump is damaged. It should be changed to the structure shown in Figure 1-8 (b). Hydraulic Pump Parameter

The problem of backflow and self-priming

• For pumps with a flow rate greater than 160L/min, it is recommended to adopt backflow self-priming.
• The diameter of the suction pipe should not be less than the recommended value, and the diameter of the shut-off valve should be twice the diameter of the suction pipe.
• The suction pipe length of the oil pump is L <2500mm, the pipe elbows should not be more than two, the distance from the suction pipe to the tank wall is H1>3D, the distance from the suction pipe to the bottom of the tank is H≥3D, and D is the diameter of the oil pipe.
• When the lowest oil level of the oil tank is 300mm higher than the center of the oil inlet of the oil pump, the hydraulic pump can start self-priming at a small deflection angle.

The self-priming problem of the vertical installation oil pump

• The distance from the suction port of the oil pump to the lowest oil level is not more than 500mm.
• The oil filling joint on the oil return pipe should be higher than the bearing lubrication line of the oil pump (the end face of the shaft end flange cover).

Question 4: Cooling problem

Pour oil into the hydraulic pump and motor housing for cooling to avoid high pressure in the housing. Sometimes it is necessary to vent oil to the casing of the hydraulic pump and motor to cool the hydraulic pump and motor. This is necessary, but remember not to take the pressure of the oil in the casing too high, otherwise, it will cause the hydraulic pump and motor shaft seal At the same time, some plunger pumps will increase the resistance of the plunger in the suction chamber to extend outward.

When the hydraulic pump often runs at zero eccentricity or the system working pressure is lower than 8MPa so that the leakage of the hydraulic pump is too small and the pump body heats up, the oil circuit can be forced to circulate and cool.