How to choose a hydraulic pump
The first thing that should be taken into consideration when choosing a suitable hydraulic pump for a specific application is the investigation of the amount of pressure and current required in the circuit, therefore, the flow and pressure curve must be calculated in a time cycle, and in this regard, the simultaneity of consumption in the factor different relevant ones should be determined to determine the maximum required current. To choose the right hydraulic pump, you should pay attention to the following:
- Adding ten percent of the flow rate to the flow rate determined in pump sizing
- In the safety valve or pressure relief valve, the regulation pressure must be equal to ten percent more than the working pressure of the system.
Finally, these two factors cause more power to be applied in the hydraulic injection system.
What acts as the driving force in hydraulic systems are combustion engines or electric motors. The speed range defined for electric motors is between 1200 and 1800 rpm; Of course, it should be noted that in air and space applications, the pump is also used with a speed of 2000 rpm. In cases where the system is mobile or there is a problem of lack of space and also the weight and volume characteristics of the pump are important, small units with high speed should be used.
Calculation of the required power for the electric motor driving the pump in hydraulic systems (by determining the working pressure and oil consumption flow rate and without considering the mechanical and volumetric efficiencies)
P (kilowatt) = [Q (lit/min) X p (bar)]/600 where in this formula P = power, Q = flow rate, p = pressure.
Example: the power of an electric motor with a working pressure of 120 bar and a flow rate of 30 liters per minute is: P = 30X120/600 = 6 kW
What is a hydraulic pump?
In every system, there is a part called the heart of the system, without which the system will fail. In a hydraulic circuit, it is this pump that if it fails, other components of the system will not be able to continue working.
Contrary to some wrong definitions, the pump does not create pressure in the circuit and its task is to produce and flow the desired liquid. In other words, the pump supplies mechanical energy to the hydraulic system with the help of electric, combustion, etc. engines. Basically, in a hydraulic system, the pressure represents the amount of resistance against the output of the pump. For example, if the output of a positive displacement pump is open to the atmosphere, while the fluid flow is established, due to the lack of resistance to the flow, there will be no pressure higher than the atmospheric pressure. On the other hand, if the pump output is completely blocked, theoretically infinite resistance is created against the flow because there is no space for the fluid to move; Therefore, in order to protect the components of the hydraulic system, it is necessary to use a safety valve. When the system pressure reaches the set value, the safety valve opens the flow path to the tank and limits the maximum pressure level in the circuit.
In simple terms, it can be seen that the output flow of the pump with positive displacement (regardless of the minor leaks inside the pump) is constant and does not depend on the system pressure. Therefore, if there is no place for the fluid to move (such as the valve is blocked or the actuator reaches the end of the stroke) or if the resistive load increases too much, the pressure control valve (safety valve) should be used to protect the pumps. A positive place is needed against extra pressure.
Pumps are classified into two categories: non-positive displacement and positive displacement.
Non-positive displacement pumps have domestic uses due to low liquid pressure and high output flow, which are mostly used for the initial transfer of fluid from one point to another and are not used in industry. Their maximum pressure capacity is limited to 250 to 300 pounds per square inch and they are classified into three categories: radial, axial and mixed flow. Centrifugal and axial pumps are widely used examples of non-positive displacement hydraulic pumps.
The second category of pumps are industrial pumps named positive displacement. These types of pumps are widely used in the hydraulic industry and send a certain amount of liquid to the hydraulic system for each revolution of the pump axis and the ability to overcome the pressure resulting from the mechanical loads of the system as well as the resistance created against the flow of friction. have
The advantages of pumps with positive displacement compared to the type with non-positive displacement can be mentioned as follows:
- Ability to work at high pressures (up to 1000 Psi and above)
- Small and compact dimensions
- High volumetric efficiency
- High flexibility (ability to work in a wide range of pressures and speeds)
These types of pumps are divided into three main types:
- Gear pump
- Vane hydraulic pump
- Piston hydraulic pump