A Hydraulic Pump is a mechanical source of energy that essentially transforms mechanical energy (rotary or linear motion) into hydraulic power. It brings about flow powerful enough to overcome the pressure created by the load at the pump outlet. A hydraulic pump, by mechanical action, creates a vacuum at the pump inlet, which then allows the atmospheric pressure to push the liquid in the reservoir through the inlet line to the pump. This liquid is then delivered to the pump outlet with the help of mechanical action and forced further into the hydraulic system.
The fluid has a higher velocity and pressure when exiting the pump as compared to the velocity and pressure it had while entering. It is, however, noteworthy that the pump does not produce pressure; it only creates liquid movement or flow necessary to develop the pressure.
Types of Hydraulic Pumps
Hydraulic Pumps can be broadly categorized into two archetypes, namely, Fixed Displacement Type, and Variable Displacement Type. Both of these have a different internal mechanism, but the underlying principle is the same in both cases. Let’s have a closer look at both of the archetypes of pumps and their examples.
Fixed Displacement Type
A Fixed Displacement Pump, regardless of its type, is the one which delivers the same amount of liquid or fluid in each cycle of its operation. It has a set rate of flow, that is, each stroke of the motor displaces the same amount of fluid. Fixed Displacement Pumps are:
- Relatively Inexpensive
- Easier to maintain
Let’s talk about the examples of a Fixed Type Displacement Pump.
These are the simplest and most common type of Hydraulic Pump used today. They are easy to maintain and highly economic. Gear Pumps displace fluid using the meshing of gears. They can further be divided into two categories – The Internal Gear type and The External Gear Type – based on the number and types of gears used. The internal gear pump uses an internal and external spur gear, whereas, the external gear pump uses two external spur gears to push and mesh the oil around the outer surface of the gear. It can withstand pressures up to 250 bars, which can further be increased up to 320 bars using cast iron designs.
Generator Rotor Pumps, or better known as Gerotor Pumps, use an external rotor gear meshing with an internal idler gear. The off-centre rotor seals to the idler to create different dynamically changing volumes between them by the geometry of the two rotors’ partition. This causes any given volume to first increase, creating a vacuum and thus sets off the intake, and then decrease, causing compression or pump of liquid/gas. These types of Hydraulic Pumps are generally used in low-pressure applications, where they are efficient enough yet less noisy.
Inside a common housing, two helical screws are intermeshed to create shafts. There is a drive end at one shaft. The passage of fluid in a linear direction through this pump gives rise to a constant displacement output. Screw pumps are usually very reliable and create low noise due to the gears being in continuous contact but the efficiency of the pump is low especially when the liquid is highly viscous.
Variable Displacement Type
A variable displacement type pump alters the rate of flow and outlet pressure by some means; whether electrical, mechanical or some combination of both, while operating. This consequence in pumps that are:
- More complex
- More expensive
- Able to perform a wide variety of operations
The following are the examples of Variable Displacement Type Hydraulic Pump.
Bent Axis Hydraulic Pump
These Hydraulic Pumps has a rotating cylinder with pistons acting around its edges. The pistons act at an angle with a thrust plate installed at the end of the shaft. As the shaft rotates, the pistons rotate as well relative to the body of the pump, which results in a change in the amount of fluid displaced in each revolution. The angle of the thrust plate is varied in order to change the displacement. This type of pump is highly efficient and reliable.
Axial Piston Pumps
These types of pumps are similar to the Bent Axis Hydraulic Pumps, differing in the mechanism of variable displacement. The shaft and piston are aligned axially which results in a compact, efficient and economically better design. It is also equipped with a wide range of function to control the pressure, flow and power in order to match the machine requirements.
Radial Piston Pumps
These types of pumps come in both fixed and variable displacement types. It has a rotating shaft surrounded by an odd number of radial pistons, enclosed within an eccentric ring. The rotation of the shaft changes the distance between the eccentric ring and the shaft centerline, which causes the piston to move through a cycle of suction and pressure. The displacement can be altered by altering the eccentricity, either manually or with adjustment screws. These are strong and reliable and can be used to perform high-pressure tasks.
Rotary Vane Pumps
The design of these types of Hydraulic Pumps is similar to the radial piston type except it uses vanes instead of pistons. These vanes form a seal in the rotating shaft, working against the eccentric ring. These are common in machine tool applications where the pressures are low and low noise is required.