What are Hydraulic Piston Pumps?

Figure 1: Swash plate axial piston pump parts.

1-Slipper 2-Piston 4-Transmission shaft 6-Valve plate 7-Cylinder block 8-Centering spring 9-Outer sleeve 10-Inner sleeve 11-Steel ball 18-Piston 21-Dial 23-Shift fork 24- Axis pin 25-Swashplate 26-Pressure plate

1. Structure of the Hydraulic Piston Pump

The hydraulic piston pump is mainly composed of two parts: the power end and the hydraulic end, and is also composed of a belt pulley, a check valve, a safety valve, a pressure regulator, and a lubrication system. The following takes the single piston pump as an example to introduce the structure of hydraulic piston pump.

1.1 Power End

(1) Crankshaft (sometimes, a camshaft)
The crankshaft is one of the key components in the piston pump. Adopting the integral type of crankshaft, it will complete the key step of changing from rotary motion to reciprocating linear motion. In order to balance it, each crankshaft pin is 120° to the center.

(2) Connecting rod
The connecting rod converts the rotary motion of the crankshaft into the reciprocating motion of the piston.

(3) Crosshead
The crosshead connects the swinging connecting rod and the reciprocating piston rod, and has a guiding function.

(4) Machine base
The base is a force-bearing component for installing and connecting the power end and the hydraulic end. There are bearing holes on both sides of the rear of the base, and the front part is provided with a positioning pin hole connected with the hydraulic end to ensure the alignment between the center of the slideway and the center of the pump head. There is a drain hole on the front side of the base to discharge the leaking liquid.

Figure 2: The liquid end and power end of the piston pump.

1.2 Liquid End

(1) Pump head
The pump head is integrally forged from stainless steel, and the suction and discharge valves are arranged vertically. The suction hole is on the bottom of the pump head, and the discharge hole is on the side of the pump head, connected with the valve cavity, simplifying the discharge piping system.

(2) Piston
(3) Inlet valve and drain valve
The inlet and outlet valves and valve seats are of low-damping, conical valve structures, which have the characteristics of reducing viscosity and are suitable for conveying liquids with high viscosity. The contact surface has high hardness and sealing performance to ensure that the inlet and outlet valves have sufficient service life.

Figure 3: Single acting reciprocating pump diagram.

1.3 Auxiliary Parts

Auxiliary parts mainly include check valves, pressure regulators, lubrication systems, safety valves, pressure gauges, etc.

(1) Check valve
The liquid discharged from the pump head flows into the high-pressure pipeline through the low-damping check valve. When the liquid flows in the reverse direction, the check valve closes to prevent the high-pressure liquid from flowing back to the pump body.

(2) Pressure regulator
The high-pressure pulsating liquid discharged from the pump head becomes a relatively stable high-pressure liquid flow after passing through the pressure regulator.

(3) Lubrication system
Mainly, the gear oil pump draws oil from the oil tank to lubricate the crankshaft, crosshead and other rotating parts.

(4) Pressure gauge
There are two types of pressure gauges: ordinary pressure gauges and electric contact pressure gauges. Electric contact pressure gauge is an instrument system, which can achieve the purpose of automatic control.

(5) Safety valve
A spring micro-opening safety valve is installed on the discharge pipeline, which can ensure the sealing of the pump under the rated working pressure, and automatically open when the overpressure situation happens, playing the role of pressure relief protection.

Figure 4: Composition of a single plunger pump.

2. Types of Hydraulic Piston Pumps

Piston pumps are generally divided into single piston pumps, horizontal piston pumps, axial piston pumps and radial piston pumps.

2.1 Single Piston Pump

The structural components of single piston pumps, mainly include eccentric wheel, piston, spring, cylinder block and two one-way valves. A closed volume is formed between the piston and the cylinder bore.
When the eccentric wheel rotates for one turn, the piston moves up and down once, the downward movement absorbs oil, and the upward movement discharges oil. The volume of oil discharged per revolution of the pump is called the displacement, and the displacement is only related to the structural parameters of the pump.

2.2 Horizontal Piston Pump

The horizontal piston pump is a kind of hydraulic pump composed of several pistons (usually 3 or 6) installed in parallel. Its pistons are pushed through a crankshaft connecting rod slider or directly driven by the eccentric shaft to do reciprocating motion and realize the suction and discharge of liquid. They also all use valve type flow distribution devices, and most of them are fixed displacement pumps.

The emulsion pump in the coal mine hydraulic support system is generally a horizontal piston pump. The emulsion pump is used in coal mining to provide emulsion for hydraulic support. The working principle relies on the rotation of the crankshaft to drive the piston to reciprocate to achieve liquid suction and discharge.

2.3 Axial Piston Pump

Figure 5: Axial piston pump structure diagram.

Axial piston pump, as shown in the figure above, is a piston pump in which the reciprocating direction of the piston is parallel to the central axis of the cylinder. The axial piston pump works by using the volume change generated by the reciprocating movement of the piston which is parallel to the drive shaft in the piston hole. Since both the piston and the piston hole are round parts, high precision fit can be achieved during processing, so the volumetric efficiency is high.

2.4 Radial Piston Pump

Radial piston pump can be divided into two categories: valve distribution and shaft distribution. The valve distribution radial piston pump has the disadvantages of high failure rate and low efficiency. The shaft distribution radial piston pump developed in the 1970s and 1980s overcomes the deficiencies of the valve distribution radial piston pump. Because of the structural characteristics of the radial pump, the shaft distribution radial piston pump is more impact resistant, has longer life and higher control accuracy than the axial piston pump.

Figure 6: Radial piston pump structure diagram.

The variable displacement of the pump is realized by changing the eccentric distance of the stator under the action of the variable piston and the limit piston, and the maximum eccentric distance is limited to 5-9mm (depending on the size of the displacement).
The variable stroke is very short, and the variable mechanism is designed for high-pressure operation and controlled by the control valve, so the response speed of the pump is fast. The radial structure design overcomes the problem of eccentric wear of the slipper of the axial piston pump, which greatly improves the impact resistance.

3. How Does a Hydraulic Piston Pump Work?

The following takes the axial piston pump and the radial piston pump as examples to introduce the working principle of the hydraulic piston pumps.

3.1 Axial Piston Pump

Axial piston pumps include swash plate piston pumps and bent axis piston pumps. As shown in Figure 7, the bent axis piston pump consists of a transmission main shaft 1, a connecting rod 2, a piston 3, a cylinder block 4, a central shaft 5, a spherical valve plate 6, a casing and a rear cover (not shown in the figure) and so on. The cylinder block and the axis of the transmission main shaft form an inclination angle γ relative to each other, and the drive disc 7 at the end of the shaft is connected with each piston in the cylinder block by a universal hinge and a connecting rod.

Figure 7: The working principle of bent axis axial piston pump.

1—Drive spindle; 2—Connecting rod; 3—Piston; 4—Cylinder; 5—Central shaft; 6—Spherical valve plate; 7—Drive disc

When the prime mover drives the drive shaft of the pump to rotate, the connecting rod and a piston pair alternately drive the cylinder to slide and rotate on the valve plate. Because the main shaft and the cylinder axis have an included angle, when the piston moves from the bottom dead center to the top dead center, an oil suction stroke is obtained, and the oil is sucked into the cylinder block through the oil suction port and the valve plate.
When the piston moves from the top dead center to the bottom dead center, the compression stroke is generated, and the oil filled in the cylinder hole is discharged through the valve plate and the oil outlet.

Viewed from the direction of the drive shaft, if the pump rotates clockwise (right turn), the oil suction port is on the left side of the rear cover, and the oil compression port is on the right side of the rear cover. Still looking from the direction of the drive shaft, if the drive shaft rotates counterclockwise (left turn ), the oil suction port is on the right side of the rear cover, and the oil compression port is on the left side of the rear cover.

3.2 Radial Piston Pump

The working principle of a radial piston pump is shown in the figure below. This pump is composed of main parts such as stator 4, rotor (cylinder) 2, oil distribution shaft 5, bushing 3 and piston 1. The bushing fits tightly in the rotor bore and rotates with the rotor. The oil distribution shaft does not move.

Figure 8: Working principle diagram of radial piston pump.

1. Piston 2. Rotor 3. Bushing 4. Stator 5. Oil distribution shaft

When the rotor rotates clockwise, the piston is pressed against the inner wall of the stator under the action of centrifugal force or low-pressure oil. Due to the eccentricity e between the rotor and the stator, the piston extends outward when the rotor rotates in the first half of the cycle, and the sealing working volume in the radial hole gradually increases, forming a partial vacuum to suck the oil in the oil tank through b cavity on the oil distribution shaft.
When the rotor rotates to the second half, the piston pushes inward and the sealing working volume gradually decreases, and the oil is discharged from the c cavity on the oil distribution shaft to the outside. Each piston hole sucks oil and presses oil once for each revolution of the rotor. Moving the stator to change the eccentricity e can change the displacement of the pump.

The radial piston pump has a large radial size, complex structure, and poor self-priming ability. Besides, the oil distribution shaft is affected by the radial unbalanced hydraulic force, which makes it easy to wear, and limits the increase of its speed and pressure. Therefore, radial piston pumps are not widely used, and there is a tendency to be replaced by axial piston pumps.

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