Working Principle of the Internal Gear Pump
A comprehensive analysis of the design, operation, and functionality of one of the most efficient positive displacement pumps in industrial applications
Introduction to the Internal Gear Pump
The internal gear pump is a type of positive displacement pump that is widely recognized for its efficiency, compact design, and ability to handle a variety of fluids. Unlike external gear pumps, which use two separate gears meshing together, the internal gear pump features an inner gear that rotates inside an outer gear, with the teeth of both gears meshing together to create fluid movement. This unique design offers several advantages in industrial applications, making the internal gear pump a preferred choice for many hydraulic systems, lubrication systems, and fluid transfer operations.
One of the key benefits of the internal gear pump is its ability to maintain a consistent flow rate regardless of pressure fluctuations, which is crucial in applications where precise fluid delivery is required. Additionally, the internal gear pump operates with relatively low noise levels compared to other pump types, making it suitable for environments where noise reduction is a priority. The simplicity of its design also contributes to its popularity, as it requires fewer moving parts than many other pump configurations, resulting in lower maintenance requirements and increased reliability.
In this detailed explanation, we will explore the components, operating principles, and performance characteristics of the internal gear pump, providing a comprehensive understanding of how this essential fluid handling device functions in various industrial settings.
Anatomical Structure of the Internal Gear Pump
Figure 1-3: Internal Gear Pump Working Principle Diagram
Key Components Identification
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1
Driving Gear (Pinion): The smaller external gear that is driven by the pump shaft, providing the rotational force for the internal gear pump operation.
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2
Crescent (月牙板): A stationary component that separates the suction and discharge ports while maintaining the seal between the two gears in the internal gear pump.
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3
Driven Gear (Internal Gear): The larger gear with internal teeth that meshes with the driving gear and rotates in the same direction within the internal gear pump housing.
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4
Suction Chamber: The area where fluid enters the internal gear pump as the gears unmesh, creating a low-pressure zone.
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5
Pressure Chamber: The area where fluid is discharged from the internal gear pump as the gears mesh, creating a high-pressure zone.
Components of the Internal Gear Pump
The internal gear pump is composed of several key components that work together to achieve efficient fluid transfer. Each part plays a crucial role in the overall functionality of the internal gear pump, contributing to its performance, reliability, and durability. Understanding these components is essential for comprehending the complete working principle of the internal gear pump.
1. Driving Gear (Pinion)
The driving gear, also known as the pinion, is the smaller external gear in the internal gear pump assembly. It features external teeth that mesh with the internal teeth of the larger driven gear. The driving gear is connected directly to the pump shaft, which is typically driven by an electric motor or another power source. As the driving gear rotates, it imparts motion to the driven gear, causing it to rotate in the same direction. The size and number of teeth on the driving gear determine the displacement volume and flow rate characteristics of the internal gear pump.
2. Crescent (Separator)
The crescent, or separator, is a critical stationary component in the internal gear pump that fits in the space between the driving gear and the driven gear. Its curved shape follows the profiles of both gears, maintaining a close clearance with each to prevent fluid from leaking between the suction and discharge sides of the pump. The crescent's primary function is to separate the expanding (suction) and contracting (discharge) chambers within the internal gear pump, ensuring efficient fluid transfer without excessive internal leakage.
3. Driven Gear (Internal Gear)
The driven gear is the larger component with internal teeth that surrounds the driving gear in the internal gear pump. It rotates in the same direction as the driving gear but at a slower speed due to its larger diameter. The internal teeth of the driven gear mesh perfectly with the external teeth of the driving gear, creating a series of sealed cavities between them as they rotate. These cavities are responsible for trapping and moving fluid through the internal gear pump from the suction port to the discharge port.
4. Shaft and Bearings
The shaft transmits rotational power from the drive source to the driving gear in the internal gear pump. It must be precisely machined to ensure proper alignment and minimize vibration during operation. Bearings support the rotating shaft, reducing friction and allowing smooth rotation even under high-pressure conditions. The choice of bearing type (ball, roller, or sleeve) depends on the specific application requirements of the internal gear pump, including operating pressure, temperature, and fluid characteristics.
5. Side Plates (Port Plates)
Side plates, also known as port plates, are mounted on either side of the gear assembly in the internal gear pump. They form the end walls of the pumping chamber, creating a seal around the gears to prevent fluid leakage along the shaft. The side plates contain the suction and discharge ports that connect the internal gear pump to the fluid system. These plates are often made from wear-resistant materials to withstand the constant rubbing contact with the gear faces, ensuring long service life for the internal gear pump.
6. Housing (Casing)
The housing, or casing, encloses all the internal components of the internal gear pump, providing structural support and containing the fluid being pumped. It is designed to withstand the operating pressures generated by the pump and is typically made from cast iron, steel, or aluminum depending on the application requirements. The housing features flanges or connections for attaching the internal gear pump to the fluid system and may include mounting points for securing the pump to a base or frame.
Working Principle of the Internal Gear Pump
The operation of the internal gear pump is based on the principle of positive displacement, where a fixed volume of fluid is trapped, transported, and discharged with each rotation of the gears. This mechanism ensures a consistent flow rate that is directly proportional to the rotational speed of the pump, making the internal gear pump highly predictable and reliable in operation.
Basic Operational Concept
The internal gear pump operates through the meshing action of two gears - a smaller external gear (driving gear) and a larger internal gear (driven gear) - within a closely fitted housing. These gears, along with the crescent separator and side plates, form a series of sealed cavities that move fluid through the pump as the gears rotate. The key to the internal gear pump's operation is the controlled creation and movement of these sealed volumes, which transfer fluid from the suction side to the discharge side without allowing backflow.
As the driving gear rotates, it engages with the driven gear, causing it to rotate in the same direction. The point where the gears mesh creates a dynamic seal that prevents fluid from flowing backward from the high-pressure discharge side to the low-pressure suction side of the internal gear pump. This meshing action, combined with the crescent separator, ensures that fluid is positively displaced through the pump with minimal slippage.
Suction Process
The suction process in the internal gear pump begins as the driving gear rotates in the direction indicated in Figure 1-3. As the gears unmesh in the upper portion of the pump, they create an expanding volume between their teeth. This expansion of volume results in a reduction in pressure within that chamber, creating a partial vacuum.
Atmospheric pressure acting on the fluid in the supply reservoir forces the fluid to flow into this low-pressure area through the suction port, filling the expanding cavities between the gear teeth. The crescent separator ensures that this fluid is contained within these cavities as they move around the perimeter of the internal gear pump, preventing it from flowing back to the suction port.
Discharge Process
The discharge process occurs in the lower portion of the internal gear pump as the rotating gears begin to mesh again. As the teeth of the driving gear engage with the teeth of the driven gear, the volume of the cavities containing the fluid is progressively reduced. This reduction in volume increases the pressure of the fluid trapped within these cavities.
The increasing pressure forces the fluid out of the cavities through the discharge port and into the system. The meshing of the gears ensures complete evacuation of the fluid from each cavity, as the teeth effectively "wipe" against each other, leaving minimal residual fluid. This efficient displacement mechanism is what gives the internal gear pump its characteristic positive displacement behavior.
Rotational Dynamics
In the internal gear pump, both the driving gear and the driven gear rotate in the same direction, but at different speeds. The driving gear (pinion) typically rotates faster than the larger driven gear, with the speed ratio determined by the inverse ratio of their diameters. This relationship can be expressed as:
ω₂ / ω₁ = R₁ / R₂
Where: ω₁ = angular velocity of driving gear, ω₂ = angular velocity of driven gear, R₁ = pitch circle radius of driving gear, R₂ = pitch circle radius of driven gear
This rotational relationship ensures proper meshing between the gears throughout their rotation, maintaining the integrity of the sealed cavities that are essential for the internal gear pump's operation. The synchronized rotation, combined with the precise clearance between the gears and the crescent separator, minimizes internal leakage and maximizes volumetric efficiency in the internal gear pump.
Sealing Mechanisms
Effective sealing is critical to the performance of the internal gear pump, as any leakage between the high-pressure and low-pressure sides reduces efficiency and output. The internal gear pump employs multiple sealing mechanisms to ensure optimal performance:
- The meshing of the gear teeth creates a dynamic seal that prevents fluid from flowing backward through the pump
- The crescent separator maintains a close clearance with both gears, preventing cross-leakage between the suction and discharge chambers
- Precision machining of the side plates ensures minimal clearance between the gear faces and the plates, reducing axial leakage
- Radial clearances between the gear tips and the housing are carefully controlled to balance leakage prevention with the need for unimpeded rotation
These sealing mechanisms work together to maintain the pressure differential between the suction and discharge sides of the internal gear pump, ensuring efficient fluid transfer with minimal energy loss due to internal leakage.
Performance Characteristics of the Internal Gear Pump
The internal gear pump exhibits several performance characteristics that make it suitable for a wide range of industrial applications. These characteristics are directly influenced by its design and working principle, and understanding them is essential for selecting the appropriate internal gear pump for a specific application.
Flow Rate Characteristics
One of the defining features of the internal gear pump is its ability to deliver a relatively constant flow rate regardless of system pressure, which is a hallmark of positive displacement pumps. The theoretical flow rate of an internal gear pump can be calculated based on its displacement volume and rotational speed:
The actual flow rate of an internal gear pump is slightly less than the theoretical value due to internal leakage, often referred to as "slippage." This slippage increases with system pressure, as the higher pressure differential across the pump forces more fluid to leak through the various clearances. However, the internal gear pump typically maintains high volumetric efficiency across a wide range of operating pressures compared to other positive displacement pump types.
Pressure Characteristics
The internal gear pump is capable of generating significant pressure, with maximum pressure ratings typically ranging from 100 to 300 bar, depending on the specific design and construction materials. The pressure capability of an internal gear pump is primarily limited by:
- The strength of the housing and components
- The effectiveness of the sealing mechanisms
- The power available from the drive source
- The lubricating properties of the fluid being pumped
Unlike centrifugal pumps, the internal gear pump can operate against closed discharge conditions for short periods without damage, although this should be avoided in normal operation to prevent excessive pressure buildup and power consumption.
Efficiency Considerations
The internal gear pump typically exhibits high overall efficiency, combining good volumetric efficiency with excellent mechanical efficiency. Volumetric efficiency refers to the ratio of actual flow rate to theoretical flow rate, while mechanical efficiency relates to the power input versus the hydraulic power output.
The internal gear pump's efficiency is influenced by several factors, including operating speed, pressure, fluid viscosity, and temperature. Optimal efficiency is usually achieved at moderate speeds and pressures, with efficiency decreasing at extreme operating conditions.
Proper maintenance, including regular lubrication and replacement of worn components, is essential for maintaining the high efficiency of an internal gear pump throughout its service life.
Viscosity Effects
The performance of the internal gear pump is significantly influenced by the viscosity of the fluid being pumped. Unlike some other pump types, the internal gear pump is well-suited for handling high-viscosity fluids, which actually improve its volumetric efficiency by reducing internal leakage.
However, pumping high-viscosity fluids requires more power input and may necessitate reduced operating speeds to prevent excessive torque requirements and potential damage to the internal gear pump. Conversely, when handling low-viscosity fluids, the internal gear pump may experience increased slippage, reducing volumetric efficiency. Special designs with tighter clearances are available for applications involving low-viscosity fluids, ensuring that the internal gear pump maintains acceptable performance across the viscosity range.
Applications of the Internal Gear Pump
The internal gear pump's unique combination of features - including compact design, high efficiency, ability to handle a wide range of viscosities, and consistent flow characteristics - makes it suitable for a diverse array of industrial applications. Its reliable performance and relatively simple construction have established the internal gear pump as a workhorse in many fluid handling systems.
Industrial Hydraulics
The internal gear pump is widely used in hydraulic power units, providing pressurized fluid to actuate cylinders, motors, and other hydraulic components in machinery and equipment.
Automotive Industry
In automotive applications, the internal gear pump is commonly used for oil circulation, power steering systems, and fuel transfer, where its compact size and reliable performance are particularly valuable.
Chemical Processing
The internal gear pump is suitable for transferring a wide range of chemicals, including viscous and corrosive fluids, when constructed with appropriate materials resistant to chemical attack.
Food and Beverage
Sanitary designs of the internal gear pump are used in food processing for transferring viscous products such as syrups, sauces, and pastes, where cleanliness and gentle handling are essential.
Pharmaceutical
In pharmaceutical manufacturing, precision internal gear pump designs are used for metering and transferring sensitive fluids, where accuracy and contamination control are critical requirements.
Oil and Gas
The internal gear pump finds application in oil and gas production for lubrication systems, fuel transfer, and processing operations, where its ability to handle high-viscosity fluids is advantageous.
Advantages of the Internal Gear Pump
The internal gear pump offers numerous advantages over other types of pumps, which contributes to its widespread use across various industries. These benefits stem from its unique design and operating principle, making the internal gear pump a versatile and efficient solution for many fluid handling challenges.
Compact Design
The internal gear pump has a more compact footprint compared to many other pump types with similar flow capabilities, making it ideal for installations where space is limited.
High Efficiency
The internal gear pump typically achieves high volumetric and overall efficiency due to its positive displacement design and effective sealing mechanisms.
Smooth Flow
The internal gear pump produces a relatively pulse-free flow compared to some other positive displacement pumps, reducing system vibration and noise.
Bidirectional Operation
Many internal gear pump designs can operate in either direction by reversing the rotation of the driving gear, allowing for flexible system integration.
Viscosity Versatility
The internal gear pump can handle a wide range of fluid viscosities, from low-viscosity solvents to high-viscosity pastes and gels, with appropriate design considerations.
Self-Priming Capability
The internal gear pump is inherently self-priming, meaning it can evacuate air from the suction line and draw fluid into the pump without external priming assistance.
Low NPSH Requirements
The internal gear pump typically has low net positive suction head (NPSH) requirements, making it suitable for applications where suction conditions are less than ideal.
Simple Maintenance
The internal gear pump's relatively simple design with few moving parts makes maintenance and repair straightforward compared to more complex pump types.
Conclusion
The internal gear pump represents a sophisticated yet elegantly simple solution for fluid transfer applications across numerous industries. Its design, which features an internal gear meshing with a smaller external gear separated by a crescent-shaped separator, enables efficient positive displacement pumping with numerous advantages over alternative technologies.
The working principle of the internal gear pump, based on the controlled creation and movement of sealed fluid cavities between meshing gears, ensures consistent flow rates, high efficiency, and reliable performance across a wide range of operating conditions. From its ability to handle varying viscosities to its compact design and self-priming capabilities, the internal gear pump offers a versatile solution for challenging fluid handling requirements.
As industrial processes continue to evolve, the internal gear pump remains a critical component in fluid power systems, contributing to the efficiency and reliability of countless manufacturing and processing operations worldwide. Its enduring popularity is a testament to the effectiveness of its design and its ability to meet the diverse needs of modern industry.