Hydraulic pumps are classified into several types, each with its own set of capabilities and limitations. It can be challenging to decide what type of pump you need for a hydraulic system application, but having a basic understanding of the most common types of hydraulic pumps is a good place to start.

 

Basics of Hydraulic Pumps


A hydraulic pump aims to move hydraulic fluid through a hydraulic system, acting as the system's beating heart. All hydraulic pumps share two characteristics: 

 

  1. They provide hydraulic flow to other components (e.g., rams, hydraulic motors, cylinders) within a hydraulic system, and 

  2. They produce flow, which generates pressure when there is resistance to flow. 

 

Furthermore, most hydraulic pumps are motor-driven and include a pressure relief valve as overpressure protection. Gear, piston, and vane pumps are the three most common hydraulic pumps in use today.

 

Gear Pumps


The hydraulic fluid becomes trapped between the pump's body and the areas between the teeth of the pump's two meshing gears in a gear pump. The driveshaft powers one gear while the other sits idle until it meshes with the driving gear. Because each shaft rotation displaces the same amount of hydraulic fluid at the same pressure, these pumps are classified as fixed displacement or positive displacement. External and internal gear pumps are the two basic types of gear pumps, which will be discussed further below.

 

 

Gear pumps are small and compact, making them ideal for applications with limited space. They are also easy to repair and maintain due to their simple design. It's worth noting that gear pumps are most efficient when running at full speed. External gear pumps, in general, can produce higher pressures (up to 3,000 psi) and higher throughput than vane pumps.

 

  1. External Gear Pumps


External gear pumps are frequently found in close-coupled designs in which the gear pump and hydraulic motor share the same mounting and shaft. Fluid flow around the outside of a pair of meshed external spur gears occurs in an external gear pump. The hydraulic fluid circulates between the pump's housing and the gears, causing the alternating suction and discharge required for fluid flow.

 

External gear pumps can deliver very high pressures (up to 3,000 psi), run at high speeds (3,000 rpm), and operate more quietly than internal gear pumps. When gear pumps are designed to handle even higher pressures and speeds, they will be very noisy, and special precautions may be required.

 

External gear pumps are frequently used in powerlifting applications and other situations where electrical equipment would be too bulky, inconvenient, or expensive. External gear pumps are also used to power agricultural and construction equipment hydraulic systems.

 

  1. Internal Gear Pumps


The meshing action of external and internal gears combined with a crescent-shaped sector element generates fluid flow in an internal gear pump. The teeth of the outer gear point inward, while the teeth of the inner gear point outward. As these gears rotate and enter and exit the mesh, they create suction and discharge zones, with the sector acting as a barrier between them. A gerotor is an internal gear pump that uses trochoidal gears to create suction and discharge zones instead of a sector element.

 

Unlike external gear pumps, internal gear pumps are not designed for high-pressure applications, but they produce flow with very little pulsation. They are not as common in hydraulics as external gear pumps; however, they are used with lube oils and fuel oils and perform well in metering applications.

 

Piston Pumps


Reciprocating pistons are used in a piston pump to generate suction and discharge alternately. The Denison piston pumps are classified into two types based on whether their piston is axially or radially mounted and whether their displacement is fixed or variable.

 

Even with comparable displacements, piston pumps can handle higher pressures than gear or vane pumps, but they are more expensive in terms of the initial cost. They are also more susceptible to contamination; however, adhering to strict hydraulic cleanliness guidelines and filtering any hydraulic fluid added to the system can address most contamination issues.

 

  1. Axial Piston Pump


The pistons in an axial piston pump, also known as an inline axial pump, are aligned with the pump's axis and arranged within a circular cylinder block. The inlet and outlet ports are on one side of the cylinder block, and an angled swashplate is on the other. The pistons move in and out of the cylinder block as the cylinder block rotates, causing alternating suction and discharge of hydraulic fluid.

 

Axial piston pumps are ideal for high-pressure, high-volume applications and are frequently used to power mission-critical hydraulic systems on jet aircraft.

 

  1. Bent-Axis Pumps


The pump in a bent-axis piston pump (which many consider a subtype of the axial piston pump) consists of two sides that meet at an angle. The driveshaft on one side turns the cylinder block, which contains the pistons that match up to bores on the opposite side of the pump. The distances between the pistons and the valving surface change as the cylinder block rotate, resulting in the required suction and discharge.

 

 

These pumps are made for heavy-duty applications such as hydrostatic transmissions and power machinery.

 

  1. Radial Piston Pump


The pistons in a radial piston pump are arranged radially around an eccentrically placed cam perpendicular to the pump's axis. The cam moves and pushes the spring-loaded pistons inward as it passes them as the drive shaft rotates. Each piston has its inlet and outlet ports, which lead to a chamber. This chamber contains valves that control hydraulic fluid release and intake.

 

Radial piston pumps are commonly used in machine tools and power sources for hydraulic systems such as cylinders.

 

Fixed Displacement vs. Variable Displacement


The amount of fluid discharged in each reciprocation of a fixed displacement pump is the same volume. A change in the angle of the adjustable swashplate, on the other hand, can increase or decrease the volume of fluid discharged in a variable displacement pump. This design allows you to change the system speed without changing the engine speed.

 

Vane Pumps


When a vane pump's input shaft rotates, rigid vanes mounted on an eccentric rotor collect hydraulic fluid and transport it to its outlet. The area between the vanes expands on the inlet side as hydraulic fluid is drawn inside the pump and contracts on the outlet side as hydraulic fluid is ejected through the output port. As with piston pumps, vane pumps can have fixed or variable displacement.

 

Vane pumps are still used in utility vehicles (such as those equipped with aerial ladders or buckets), but gear pumps have largely replaced them. This does not imply that they are no longer in use. They are not designed to withstand high pressures, but they can produce a strong vacuum and even run dry for short periods.

 

Selecting a Pump


There are other key aspects to choosing the right hydraulic pump beyond deciding what type is best adapted to your application. These pump characteristics include the following:

  • The type of hydraulic fluid that will be used

  • Operating speed in rpm

  • Maximum operating pressure

  • Fixed or variable displacement

  • The flow rate (which is related to pump speed in rpm, pump efficiency, and displacement)

  • Torque ratings and power curves

 

However, your starting point will always be the type of motor that you need.

 

Conclusion


Selecting a pump can be difficult, but a good place to start is considering the type of pump you require. Vane pumps have largely been replaced by compact, durable gear pumps, with external gear pumps performing best for high pressure and operating speeds and internal gear pumps capable of producing flow with very little pulsation. On the other hand, Vane pumps can still create an effective vacuum and can run even when dry for short periods. Piston pumps, in general, are more powerful, but they are also more prone to contamination.

 

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