Basics of Hydraulic Pumps

Basics of Hydraulic Pumps

Basics of Hydraulic Pumps

A hydraulic pump is a mechanical device that converts mechanical power into hydraulic energy. During operation, it performs two basic functions:

1. Vacuum Generation: Mechanical action creates a vacuum at the pump inlet, allowing atmospheric pressure to force the liquid from the chamber into the pump inlet line.
2. Conveying Liquid: Mechanical action transfers the liquid to the pump outlet and conveys it to the hydraulic system.

The pump produces fluid movement or flow; however, it does not produce pressure. Pressure occurs due to the resistance to the fluid flow in the system. For example, the pressure of the outlet fluid in a pump that is not connected to the system is zero, while the pressure in a pump that distributes to the system increases only to the level required to overcome the resistance of the load.

Classification of Hydraulic Pumps

All pumps can  be classified as positive displacement or non-positive displacement. Most of the pumps used in hydraulic systems are positive displacement.

• Non-Positive Displacement Pumps: They produce a continuous flow; however, the outlet flow changes significantly as the pressure changes because they do not provide a positive internal seal against slippage. Centrifugal and impeller pumps are examples of this group.
• When the outlet port is closed, the pressure rises, but the flow stops due to slippage.
• Positive Displacement Pumps: Displaces a fixed amount of fluid for each rotating cycle of the pump element. Thanks to the tight tolerance match between the pump element and the pump casing, constant distribution is ensured in every cycle. Fluid distribution per cycle is almost constant, regardless of pressure changes. Fluid drift is negligible.
• Positive displacement pumps can be either fixed or variable displacement. In a fixed displacement pump, a fixed distribution is achieved in each cycle, while in a variable displacement pump, the distribution can be adjusted by changing the geometry of the pump room.
• Other nomenclatures: positive displacement hydrostatic or positive displacement hydrodynamic pumps. In hydrostatic pumps, mechanical energy is converted into hydraulic energy with a relatively small fluid velocity; In the hydrodynamic pump, the fluid velocity and movement are more pronounced, and the outlet pressure depends on the speed at which the fluid flows.

Types of Positive Displacement Pumps

1. Linear Motion Pumps

In the alternative type pump, which is the most basic example of the positive displacement principle, when the piston moves forward, a partial vacuum is created in the pump chamber and this vacuum draws the liquid from the chamber through the inlet control valve and draws it into the chamber. When the piston is retracted, the inlet control valve closes; The force of the piston moves the outlet control valve, forcing the fluid through the pump into the system. Thus, the same amount of fluid is delivered in each cycle of back-and-forth motion.

2. Circular Motion Pumps

In a rotary type pump, the rotational motion of the rotor carries the liquid from the pump inlet to the outlet. These pumps are; It is classified as geared, lobe, vane or piston type.

• External Gear Pumps: Come with straight spur, auger or herringbone gears. Spur spur gears are the most common; Coil and herringbone gears, on the other hand, operate quieter but are costly.
• When the two gears are engaged, a partial vacuum is created at the inlet, the liquid is drawn in to fill the gap and transported on the outside of the gears. When the teeth rejoin at the exit end, the fluid is forced out.
• Gear pumps can have a volumetric efficiency of up to 93% under optimal conditions. However, at low speeds and flows, efficiency may decrease; Therefore, it is preferable to operate it at maximum rated speeds.
• Screw Pumps: It  falls into the group of axial flow gear pumps that work similarly to a rotary screw compressor. Single, two or three screw types are available.
• The liquid is transmitted axially using a spiral rotor or two parallel interlocking rotors. In screw pumps, the liquid is delivered thanks to the fact that the rotors work like worm pistons that constantly move forward; This ensures vibration-free and quiet operation.
• Internal Gear Pumps: There is an internal gear and an external gear. Low relative velocities occur in the internal gear due to the fact that one or two teeth are missing compared to the external gear. This results in a low wear rate.
• Crescent Seal Internal Gear Pumps: With the  crescent-shaped seal, the gap between the internal and external gear is kept under control.
• Gerotor Pumps: It works with a double gear design that is always in sliding contact; the sealing caused by the sliding contact of the teeth is ensured. In general, the tread is pressure-sealed, offering high efficiency at low speeds.
• Vane Pumps: Sliding blades are used in the slots in the rotor.
• During rotation, the fins perform both suction and discharge of the pumped liquid.
• Vane pumps; It can be of unbalanced or balanced types. Unbalanced vane pumps create a side load on the rotor and shaft. In balanced designs, two separate pumping areas are created with an elliptical body to balance the side loads.
• Unbalanced pumps with variable volume, the amount of displacement can be adjusted by external control devices. They are generally recommended to be operated above 600 rpm to operate efficiently at low speeds.

3. Piston Pumps

Piston pumps apply the principle of alternating motion in a rotary unit. Instead of a single piston, multiple piston-cylinder combinations are used.

• Axial Piston Pumps: The pistons move back and forth parallel to the centerline of the drive shaft. Fluid flow is usually provided with the help of check valves or port plates using multiple pistons.
• Sequential Piston Pumps: Reciprocating motion takes place thanks to the pistons connected to the swash plate rotated by the drive shaft. The stroke length is determined by the angle of the swashplate. In variable displacement models, the piston stroke length can be adjusted by controlling the angle of the swashplate.
• Inclined Axis Pumps: The  drive shaft is located angularly relative to the cylinder block, so the distance between the piston and the valve surface is constantly changing. During rotation, the pistons direct the fluid from the inlet chamber towards their own cylinder bores and outlet chamber.
• Radial Piston Pumps: The pistons are arranged radially in the cylinder block. Cylindrical or ball pistons can be used; The port arrangement can be in the form of a check valve or a pint valve. Fixed or variable displacement models are available.

Plunger Pumps: Although they work similarly to rotary piston types, the cylinders are fixed. The pistons are pushed back and forth by crankshaft or eccentric motion. These pumps provide a more positive seal between the inlet and outlet, allowing operation at high pressures; It also makes it possible for lubrication to be independent of the pumped fluid.

Measuring Hydraulic Pump Performance

The volume of liquid delivered by the pump per revolution is calculated depending on the geometric characteristics of the oil-bearing chambers. However, the pump never delivers the exact theoretical amount of liquid. Volumetric efficiency refers to the ratio between the calculated transmission and the actual transmission and varies according to the speed, pressure and structure of the pump.

Mechanical efficiency is also far from ideal; Because some of the input energy is lost due to friction. So, the overall efficiency of the hydraulic pump is equal to the product of volumetric and mechanical efficiency. Pumps are rated based on their maximum working pressure capacity and output (gpm or lpm) at specific drive speeds (rpm).

Matching Pump Power to Load

Pressure compensation and load sensing are features that improve pump performance. Although these concepts are sometimes used interchangeably, there are significant differences in their working principles.

For example, a fixed displacement pump that operates at a constant speed will only work efficiently when the load demands maximum power. The system prevents overpressure by directing high-pressure fluid into the tank with a relief valve. When the load is below the full flow or full pressure requirement, the excess liquid produced by the pump is converted into heat. In this case, the overall system efficiency may be 25% or lower.

In systems that operate multiple loads, flow and pressure matching characteristics can be impaired. In such cases, a solution is provided by using a proportional pressure compensator. The pump senses the load pressure thanks to the compensator spring; When the pressure exceeds a certain level, the pump displacement is reduced, thus avoiding unnecessary power consumption in the system.

Variable displacement, pressure compensated pumps, on the other hand, significantly reduce system horsepower requirements compared to a fixed displacement pump. The output flow of pumps of this type is adjusted according to the predetermined discharge pressure detected in the compensator. The two-stage pressure compensator control creates a pressure drop with the help of pilot flow, providing a more dynamic pump performance.

Load sensing is a control technology that has become popular recently. Load-sensing pumps operate in empty, running, and discharging modes:

• Idle Mode: Due to the absence of load pressure, the pump produces zero discharge flow at bias or empty pressure.
• When operating: Generates discharge flow according to load pressure, determined bias pressure or pressure drop.
• Maximum Pressure: When the system reaches maximum pressure, the pressure is maintained by adjusting the pump flow.

Such pumps are able to adjust the volumetric output according to load and flow requirements, thus increasing system efficiency while reducing power losses.

Load Sensing Gear Pumps

Load-sensing gear pumps offer the versatility of positive displacement axial piston or vane pumps at a lower cost. These pumps are:

• It provides high efficiency of load sensing without the high cost associated with piston or vane pumps.
• It goes from zero to full output flow in less than 40 milliseconds.
• It offers drive circuits that can operate at low discharge pressures.
• It provides priority and secondary flow, reducing standby and additional loaded power draw.
• It allows for interchangeability with load-sensing pumps without the need to change the dimensions of system components.

Load-sensing piston and gear pumps use a single load-sensing signal to control pump discharge pressure and flow. This signal, with the help of the hydrostat, measures the difference between the flow required to the system and the load pressure and adjusts the volumetric output accordingly. This saves power when the system is idling or non-operating, minimizing unnecessary heat generation.

In addition, thanks to the discharger control, power consumption in standby mode in the pump circuit can be reduced by up to 90%. This control works in parallel with the hydrostat and load sensing signal, and is activated if the pump discharge pressure rises above a certain rate.

Dual and Combined Controls

Combined control is achieved through the addition of a pilot relief valve, which allows the hydrostat to function like the main stage of a pilot-controlled relief valve. The load sensing signal limits the pressure in the remote sensing line, allowing both the hydrostat and unloader control of the pump to respond to the conditioned signal. This results in a design that is also suitable for high-displacement pumps.

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