Gear Pumps: Principles, Design, and Applications
A comprehensive technical guide to the operation, characteristics, and variations of gear pumps in hydraulic systems, including the essential role of the hydraulic pump gearbox.
Gear pumps are positive displacement pumps that use the meshing of gears to pump fluid by displacement. They are widely used in various industrial applications due to their simplicity, reliability, and cost-effectiveness. The hydraulic pump gearbox is a critical component in many of these systems, ensuring efficient power transmission and operation. This guide covers the fundamental principles, design characteristics, and different types of gear pumps, providing a thorough understanding of their functionality and applications.
I. Working Principle of External Gear Pumps
The working principle of an external gear pump is illustrated in Figure 3-3. Within the pump housing, there is a pair of externally meshing gears, enclosed by end covers on both sides (as shown in Figure 3-3). The housing, end covers, and the tooth spaces of the gears form numerous sealed working chambers. The hydraulic pump gearbox typically provides the rotational force needed for these gears to operate in synchronization.
When the gears rotate in the direction indicated in Figure 3-3, the oil suction chamber on the right side experiences a gradual disengagement of the meshing teeth. This disengagement causes the volume of the sealed working chamber to increase, creating a partial vacuum. Oil from the tank is drawn into this space, filling the tooth grooves completely. As the gears continue to rotate, they carry the oil to the pressure chamber on the left side.
On the pressure side, the teeth gradually engage with each other, causing the volume of the sealed working chamber to decrease continuously. This reduction in volume forces the oil to be displaced out of the chamber. The suction and pressure zones are separated by the meshing teeth and the pump body itself. This fundamental operating principle is what makes the external gear pump, along with its integral hydraulic pump gearbox, such an efficient fluid transfer device.
The simplicity of this design contributes to the reliability and popularity of external gear pumps in various applications. The hydraulic pump gearbox ensures that the gears maintain proper timing and rotational speed, which is crucial for consistent fluid displacement and system performance.
Figure 3-3: External Gear Pump Working Principle
II. Displacement, Flow Calculation and Flow Pulsation
The accurate calculation of the displacement of an external gear pump should be based on meshing principles. For approximate calculations, the displacement can be considered equal to the sum of the volumes of the tooth spaces of its two gears. This displacement value is crucial for determining the pump's performance characteristics, especially when paired with a properly sized hydraulic pump gearbox.
Displacement Calculation Fundamentals
The theoretical displacement (V) of a gear pump is the volume of fluid displaced per revolution. For external gear pumps, this can be approximated using the formula:
V = 2 × π × m² × Z × B
Where: m = module, Z = number of teeth, B = face width of gears
This formula provides a reasonable approximation, but actual displacement may vary due to manufacturing tolerances, gear geometry, and operating conditions. The hydraulic pump gearbox ratio directly affects the pump's output flow by controlling the rotational speed, as flow is proportional to both displacement and speed.
Flow Rate Calculation
The theoretical flow rate (Qₜ) of a gear pump is calculated by multiplying its displacement by its rotational speed:
Qₜ = V × n
Where: V = displacement (m³/rev), n = rotational speed (rev/s)
However, due to internal leakage, the actual flow rate (Qₐ) is always less than the theoretical value. The volumetric efficiency (ηᵥ) is the ratio of actual flow to theoretical flow:
ηᵥ = Qₐ / Qₜ × 100%
Flow Pulsation
One characteristic of gear pumps is flow pulsation, which refers to the variation in output flow during each revolution. This pulsation is inherent to the gear meshing process and can affect system performance, particularly in applications requiring smooth fluid delivery.
The amplitude of flow pulsation depends on factors such as the number of teeth, gear geometry, and operating conditions. Pumps with more teeth generally produce lower pulsation. The hydraulic pump gearbox can sometimes be designed to mitigate pulsation effects through damping or by optimizing the speed characteristics.
Figure 3-4: Flow Pulsation Characteristics
III. Structural Characteristics, Advantages and Disadvantages of External Gear Pumps
External gear pumps offer a unique set of structural characteristics that make them suitable for various applications. When integrated with a properly designed hydraulic pump gearbox, they provide reliable performance in many industrial settings. Understanding their advantages and disadvantages is crucial for selecting the right pump for a specific application.
Advantages
- Simple and compact design, making integration with a hydraulic pump gearbox straightforward
- High mechanical efficiency when properly maintained
- Relatively low cost compared to other pump types
- Ability to handle a wide range of viscosities
- Consistent performance when paired with a suitable hydraulic pump gearbox
- Easy to maintain and repair
Disadvantages
- Flow pulsation can cause noise and vibration, especially with smaller numbers of teeth
- Susceptibility to wear, particularly when handling abrasive fluids
- Limited high-pressure capabilities without special modifications
- Need for close tolerances, increasing manufacturing complexity
- Potential for cavitation if inlet conditions are not properly maintained
- Sensitivity to hydraulic pump gearbox misalignment, which can cause premature wear
1. Trapped Oil Phenomenon
For a gear pump to operate smoothly, the contact ratio of gear meshing must be greater than 1. This requirement inevitably results in a situation where two pairs of teeth are always in mesh, trapping a portion of oil between the closed cavity formed by these two pairs of teeth, as shown in Figure 3-5. This phenomenon is known as the trapped oil phenomenon.
As the gears rotate, the volume of this trapped cavity changes. When the volume decreases, the oil inside is compressed, leading to a significant pressure increase. This can cause noise, vibration, and even damage to pump components. Conversely, when the volume increases, a partial vacuum may form, leading to cavitation and its associated problems.
To address this issue, manufacturers incorporate relief grooves into the pump housing or end plates. These grooves provide a path for the trapped oil to escape, mitigating pressure spikes and cavitation. Proper design of these relief features, combined with a well-matched hydraulic pump gearbox, ensures smooth and efficient pump operation.
Figure 3-5: Trapped Oil Phenomenon
IV. Measures to Increase Pressure of External Gear Pumps
To increase the pressure capability of gear pumps, it is essential to reduce end face leakage. This is typically achieved using an automatic compensation method for the end face clearance. The development of higher pressure gear pumps has been instrumental in expanding their applications, particularly when paired with a robust hydraulic pump gearbox capable of handling increased loads.
End Face Clearance Compensation
Figure 3-6 illustrates the principle of end face clearance compensation. A specially designed channel directs pressure oil from the pump's high-pressure chamber to the outside of the bushing, creating a hydraulic force that presses the bushing against the gear end face.
This force must be greater than the force acting on the inside of the bushing from the gear end face. This ensures that under all pressure conditions, the bushing remains automatically pressed against the gear end face, minimizing leakage through the end face and thereby increasing the pump's pressure capability.
This compensation mechanism is critical for maintaining efficiency at higher pressures. When combined with a properly engineered hydraulic pump gearbox, it allows external gear pumps to operate effectively in systems requiring elevated pressure levels, expanding their range of applications beyond traditional low-pressure uses.
Figure 3-6: End Face Clearance Compensation Principle
Key Components of Compensation System:
- Pressure oil channel directing fluid to bushing
- Compensation bushing with controlled movement
- Spring or hydraulic force mechanism
- Precision machined contact surfaces
- Sealing elements to control pressure distribution
Additional Pressure Enhancement Techniques
Material Selection
Using hardened materials for gear surfaces and wear plates reduces deformation under high pressure, maintaining critical clearances. This is particularly important when the hydraulic pump gearbox operates at higher torque levels.
Precision Manufacturing
Tighter tolerances and improved surface finishes minimize leakage paths. Advanced machining techniques ensure gear tooth profiles optimize meshing under high-pressure conditions.
Bearing Improvements
High-capacity bearings capable of handling increased radial and axial loads extend pump life at elevated pressures, working in harmony with the hydraulic pump gearbox.
V. Internal Gear Pumps
Internal gear pumps feature a different configuration from their external counterparts while operating on similar principles. These pumps are particularly valued in applications where compact size and quiet operation are important, often working in conjunction with a specialized hydraulic pump gearbox designed for their unique characteristics.
Internal gear pumps are available in two main types: those with involute tooth profiles and those with cycloidal tooth profiles (also known as gerotor pumps). Both types share the same working principles and main characteristics as external gear pumps but offer distinct advantages in certain applications. The integration of an appropriately designed hydraulic pump gearbox enhances their performance in specific operational contexts.
Operating Principles
Figure 3-7 shows the working principle of an internal involute gear pump. The sealed volume formed by the meshing pinion gear (1), internal gear (3), and side plates is divided into two parts by the crescent-shaped separator (2) and the gear meshing line, creating the suction and pressure chambers.
When the drive shaft rotates the pinion gear in the direction shown in Figure 3-7, the internal gear rotates in the same direction. In the upper half of the diagram, the teeth disengage, causing the sealed volume to increase, forming the suction chamber. In the lower half, the teeth engage, reducing the sealed volume and creating the pressure chamber.
The crescent separator maintains the seal between the suction and pressure chambers while supporting the gears. This design allows for smoother operation and reduced noise compared to many external gear pumps, especially when paired with a well-matched hydraulic pump gearbox that optimizes rotational characteristics.
Figure 3-7: Internal Gear Pump Working Principle
Advantages of Internal Gear Pumps
- Compact design with high power density, making them suitable for space-constrained applications with limited room for both pump and hydraulic pump gearbox
- Smoother operation and lower noise levels compared to many external gear pumps
- Self-priming capabilities, reducing the need for complex suction arrangements
- Ability to handle viscous fluids more effectively than some other pump types
- Simple construction with fewer moving parts than many other positive displacement pumps
- Compatible with a wide range of hydraulic pump gearbox configurations for flexible system design
VI. Screw Pumps
Screw pumps represent a specialized type of positive displacement pump that can be considered a form of external meshing cycloidal gear pump. Their unique design offers distinct advantages in certain applications, particularly when paired with a precisely engineered hydraulic pump gearbox that optimizes their rotational characteristics.
These pumps are classified based on several criteria. By the number of screws, they can be categorized as single-screw, twin-screw, triple-screw, quadruple-screw, and quintuple-screw pumps. Based on the cross-sectional profile of the screws, they can be divided into three types: cycloidal, cycloidal-involute, and circular arc tooth profiles. Each configuration offers specific performance characteristics that make it suitable for particular applications when combined with an appropriate hydraulic pump gearbox.
Figure 3-8: Triple-Screw Pump Configuration
Working Principles of Screw Pumps
Screw pumps operate on the principle of fluid being trapped between the meshing threads of rotating screws and transported axially through the pump. In multiple-screw designs, one screw acts as the driver, while the others are driven by their meshing with the driver screw. This meshing creates sealed cavities that move along the length of the screws as they rotate.
The hydraulic pump gearbox provides the rotational force needed to drive the main screw, with precise speed control essential for maintaining consistent flow rates. As the screws rotate, fluid is drawn into the suction port and trapped in the cavities formed between the screw threads and the pump housing. These cavities move axially toward the discharge port, where the fluid is released at the system pressure.
Single-Screw Pumps
Featuring a single screw rotating within a stationary rubber stator, these pumps excel at handling viscous fluids and those containing solids. They require a specialized hydraulic pump gearbox to accommodate their unique torque characteristics.
Twin-Screw Pumps
Utilizing two intermeshing screws, these pumps offer high efficiency and can handle a wide range of viscosities. They are often used in lubrication systems where the hydraulic pump gearbox must maintain precise speed control.
Triple-Screw Pumps
Consisting of a central driver screw meshing with two idler screws, these pumps provide smooth, low-pulsation flow ideal for high-pressure hydraulic systems when paired with a robust hydraulic pump gearbox.
Advantages of Screw Pumps
Screw pumps offer several advantages that make them suitable for specific applications. Their design inherently produces very low flow pulsation, resulting in smooth operation and reduced noise levels compared to many other positive displacement pumps. This characteristic, combined with a properly matched hydraulic pump gearbox, makes them ideal for applications where system stability is critical.
They can handle high viscosities efficiently and maintain good performance across a wide range of flow rates. The continuous nature of their fluid displacement minimizes pressure fluctuations, reducing stress on system components. This gentle handling of fluids also makes them suitable for shear-sensitive materials.
When integrated with an appropriately designed hydraulic pump gearbox, screw pumps can operate at high pressures while maintaining efficiency and reliability. Their robust construction allows for long service life with minimal maintenance, making them a cost-effective solution in many industrial applications despite their higher initial cost compared to some gear pump alternatives.
Gear pumps, including external, internal, and screw variants, play a crucial role in countless industrial applications. Their simplicity, reliability, and efficiency make them a cornerstone of fluid power systems worldwide. The hydraulic pump gearbox is an essential component that enables these pumps to operate at optimal speeds and torque levels, ensuring efficient fluid transfer across various operating conditions. Understanding the principles, characteristics, and capabilities of each type allows for informed selection and application, maximizing system performance and longevity.
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