Understanding Internal Gears: An In-Depth Look What Are Internal Gears?

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In modern mechanical design, internal gears are particularly important due to their unique structure and efficient power transmission characteristics. Compared to traditional external gears, the teeth of internal gears are set on the inside of a cylinder, allowing for efficient and high-torque transmission in compact spaces. The range of applications for internal gears extends from bicycles to high-end automatic transmissions in automobiles, and to complex industrial machinery and equipment, reflecting their central role in modern industry.
In this article, we will delve into the definition, manufacturing processes, types, and the advantages and limitations of internal gears in various applications.
Continue reading to learn more.

What Are Internal Gears?

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Internal gears are a type of gear where the teeth are formed on the inside diameter of a cylinder. Unlike external gears, where the teeth are on the outer surface, the teeth of internal gears extend inward from the cylindrical ring’s inner surface. They are designed to mesh with external gears, requiring both gears to have the same helix angle. When internal gears mesh with external gears, both gears rotate in the same direction. Internal gears are characterized by inward-pointing teeth and are used in combination with external gears to transfer power in compact spaces, offering strength and efficiency advantages in various mechanical applications.

What Is Another Term for Internal Gear?

Internal gears are also known as “ring gears” or “inside gears.”

How Are Internal Gears Manufactured?

There are multiple methods to manufacture internal gears, including the use of a pinion cutter in a forming process. This method is a generative process where the cutter, actually a gear with cutting edges, rotates at a set speed ratio relative to the gear being manufactured. Each gear tooth gap is formed by a complete cutter tooth. This method is particularly suited for producing internal gears, cluster gears, and racks. Alternatively, powder metallurgy can be used, which offers the advantages of being cost-effective and suitable for mass production.

Additionally, internal gears can also be manufactured using other machining processes such as milling, broaching, and hobbing, though hobbing is typically used for external gears. Gear shaping can be performed using a rack cutter, which is commonly used for external gears. The finishing of internal gears involves several steps to enhance their quality and functionality. These steps include grinding, polishing, honing, shaving, grinding, and superfinishing. These finishing processes help achieve the desired surface smoothness and dimensional accuracy, ensuring the gears operate efficiently in application.

What Are Different Types of Internal Gears?

Internal gears can be categorized based on their specific applications and configurations. Here are some different types of internal gears:

  1. Simple Internal Gears: These are basic internal gears used in various applications, with teeth located on the inside of cylindrical or annular gears. They mesh with external gears to transfer motion and power.
  2. Internal Gear Drives: These involve the use of internal gears in drive systems. They are commonly used in machinery requiring compact, efficient power transmission systems.
  3. Internal Differential Drives: Used in differential systems, these gears enable different rotational speeds, commonly used in automotive applications, such as in car differentials, allowing wheels to rotate at different speeds during turns.
  4. Internal Planetary Gear Sets: Used in planetary gear systems, where an internal gear (ring gear) meshes with several smaller external gears (planet gears) revolving around it. This setup is common in automatic transmissions, providing high torque density and efficiency.
  5. Dual Shaft Gear Drives: Also known as planetary gear drives, this structure involves an internal gear meshing with planet gears revolving around a central sun gear. This configuration is suitable for applications requiring compact sizes and high load capacities, such as in aerospace and automotive industries.

What Are the Advantages of Internal Gears?

The advantages of internal gears, particularly in bicycles, include:

  1. Durability and Longevity: Internal gears are known for their durability because all the moving parts required for shifting are contained within a sealed unit. This prevents dust, water, dirt, and debris from entering, thus reducing the frequency of replacing components such as chains, cables, and casings.
  2. Low Maintenance: Since the gears are enclosed, they are less affected by external environments, thus lowering maintenance requirements. For example, Rohloff hubs require oil changes only once a year, while Shimano hubs should be serviced every two years after the initial 12 months of maintenance.
  3. Smooth, Easy Shifting: Internal gear hubs can shift while coasting, pedaling, or even when the rear wheel is stationary, making it easier to shift under various riding conditions (such as urban traffic or steep climbs).
  4. Weather-Resistant: The enclosed nature of internal gears means they are less affected by weather conditions, which is particularly beneficial for riders in frequently rainy or muddy areas.
  5. Consistent Chainline: Internal gears maintain a straight chainline, reducing chain wear and simplifying maintenance. This also allows for the use of wider, more durable drivetrain components.
  6. Compatible with Belt Drives: The simple chainline of internal gear hubs makes them compatible with belt drives, which can offer nearly maintenance-free performance and are often cleaner than chains.
  7. Efficiency: Internal gears can efficiently transfer high loads, making them suitable for use with electric bike motors. Shifting speeds are generally faster than derailleur systems, and some systems can even shift automatically.
  8. Shifting Without Pedaling: Riders can shift gears without pedaling, which is especially useful when stopping or preparing to accelerate from a standstill.
  9. Cost Savings:Although the initial investment may be higher, the reduction in maintenance and component replacement needs over time can save costs.
  10. Aesthetically Pleasing: Since the gears are not exposed, internal gear appearances are neater, and without exposed lubrication parts, they often keep bikes cleaner.

What Are the Disadvantages of Internal Gears?

The disadvantages of internal gears, particularly in bicycles and gear systems, include:

  1. Weight: Internal gear systems tend to be heavier than external gear systems. The added weight can be a significant disadvantage, especially in applications where weight is a critical factor, such as competitive cycling or applications requiring high portability.
  2. Cost: The design of internal gears is complex, and the manufacturing precision required is high, thus typically more costly. This higher cost can make them less accessible to budget-conscious consumers or applications where cost-effectiveness is a priority.
  3. Limited Gear Range: Compared to external gear systems, the gear ratio range of internal gears is often more limited. This can restrict their versatility, especially in scenarios requiring a variety of speeds, such as different terrains in bicycle sports.
  4. Maintenance Complexity: Although the maintenance frequency is reduced due to their enclosed design, when maintenance is needed, it can be more complex and often requires professional services. This can be inconvenient in the long run and may involve higher costs.
  5. Efficiency and Resistance: Internal gear systems generate a small amount of resistance when pedaling, which most riders may not notice, but it can become apparent over long distances or in competitive racing. For performance-focused cycling sports, a slight decrease in efficiency might be a disadvantage.
  6. Difficult Repairs: If problems arise with an internal gear system, they generally require specialized knowledge or tools to address, which can be a significant inconvenience compared to simpler external gear systems.
  7. Weight Distribution: The concentration of weight in the hub can affect the handling characteristics of a bicycle, making it feel different from bicycles with external gears. This is particularly noticeable in how the bike handles turns or maneuvers.

What Is the Importance of Internal Gears in the Manufacturing Industry?

Internal gears play a crucial role in the manufacturing industry:

  1. Space Efficiency: Internal gears can operate in compact spaces, which is particularly important. The structure of internal gears allows the center of the meshing gears to be closer together, which is beneficial in applications where space is limited. Therefore, internal gears are ideally suited for use in machinery and equipment where size and space efficiency are critical.
  2. Smooth Operation: Compared to external gears, internal gears operate more smoothly and quietly. This is particularly important in industries where reducing noise to improve the working environment and extend gear life is crucial. Internal gears mesh more smoothly, and as time progresses, wear is reduced, thereby enhancing the durability of machinery.
  3. Rotation Direction: Unlike external gears, which rotate in the opposite direction when meshing, internal gears rotate in the same direction as the gears they mesh with. This feature is very useful in certain mechanical devices where the rotation direction is a necessary condition for the machinery to operate normally.
  4. Versatility and Compatibility: Internal gears can be compatible with other gear types, such as spur gears and helical gears commonly used in various industrial applications. This compatibility provides multiple design options for complex machinery, helping to improve the overall efficiency of mechanical systems.
  5. Load Distribution: Internal gears can better distribute the load on gear teeth. This is because the diameter of internal gears is larger, and more teeth are in contact during operation. Better load distribution improves durability and the ability to withstand greater torques, which is crucial in heavy-duty industrial applications.
  6. Power Transmission Efficiency: Internal gears can efficiently transmit power, especially in configurations where multiple gear reductions are needed. This efficiency is vital for industries that rely on precise and reliable power transmission to operate.

What Are the Characteristics of Internal Gears?

The characteristics of internal gears include:

  1. Module Range: The module of internal gears ranges from 0.5 to 3, which determines the size of the gear teeth.
  2. Number of Teeth: These gears can have from 50 to 200 teeth, affecting their size and the precision with which they mesh with mating gears.
  3. Material: Typically, internal gears are made from S45C, a medium carbon steel known for its good strength, toughness, and wear resistance.
  4. Heat Treatment: Generally, internal gears do not undergo heat treatment, but in some conditions, heat treatment can affect their durability and performance.
  5. Tooth Surface Smoothness: The teeth of internal gears are cut, which affects their efficiency and noise level during operation.
  6. Precision: The precision of internal gears is specified by the JIS B 1702-1:1998 standard, with grades like N8 and N9 indicating the precision and quality of the gear teeth.
  7. Secondary Processing: These gears can undergo secondary processing, allowing them to be modified and customized according to specificapplication needs.
  8. Interference Issues: Internal gears may experience involute interference, interference fit, and trimming interference, which are important factors to consider when selecting mating gears. These interferences can affect the operation and lifespan of the gears.
  9. Applications: Internal gears are used in various applications, including planetary gear drives, which are crucial for achieving high reduction ratios and compact designs.
  10. Customization: Gears can be custom manufactured to meet specific design requirements, including large sizes and unique configurations.

What Materials Are Used in Internal Gear Manufacturing?

Materials used in the manufacturing of internal gears include:

  1. Plastics: Materials like polyacetal and polycarbonate are used for making internal gears, especially in applications where noise reduction or lighter loads are required.
  2. Aluminum Alloys: When a lightweight yet durable material is needed, aluminum alloys can be used for internal gears.
  3. Cast Iron: Cast iron has excellent wear resistance and durability, making it suitable for heavy-duty applications.
  4. Stainless Steel: Stainless steel is important for gears that may come into contact with corrosive substances or require a high level of cleanliness, due to its corrosion resistance and strength.
  5. S45C Steel: This medium carbon steel is often used for internal gears because it strikes a good balance between machinability, strength, toughness, and wear resistance.

Challenges in Producing High-Precision Internal Gears

Producing high-precision internal gears faces several challenges:

  1. Demand for Mass Production: There is an increasing demand for mass production of high-precision internal gears, which requires maintaining consistent quality during large-scale production. This is challenging because each gear must meet strict specifications to ensure they function properly in their respective applications, such as reducing mechanical vibrations and noise.
  2. Complex Manufacturing Processes: The manufacturing process for high-precision internal gears (especially in planetary gear systems) involves complex procedures like gear shaping. Maintaining high precision during these processes is challenging due to the high requirements for gear size and alignment precision.
  3. Scaling Up Production: Transitioning from prototype design to mass production is a significant challenge. It requires scaling up production while maintaining the high precision and quality needed for internal gears. This includes ensuring that CNC machining processes are optimized for efficiency and precision.
  4. Design Limitations: The design of internal gears often needs to withstand larger loads while minimizing noise, vibration, and harshness (NVH). This requires innovative gear designs, such as asymmetric gears, which may be more complex to manufacture.
  5. Material and Processing Limitations: The choice of materials and processing capabilities also pose challenges. High-precision gears require materials that can withstand operational demands, and the processing must achieve the necessary precision. This often involves advanced techniques like grinding or honing to ensure gear fitting precision.
  6. Miniaturization: There is also a trend towards gear miniaturization, aimed at making equipment smaller, lighter, and cheaper. Achieving high precision in smaller gears is particularly challenging due to the need for smaller tolerances and more complex details.

What Are the Applications of Internal Gear?

Internal gears have a wide range of applications across various industries, as detailed in the provided materials:

  1. Planetary Gearbox Applications: Internal gears are commonly used in planetary gearboxes, where they interact with external gears to provide high torque transmission and reduction in a compact design.
  2. Pumping Systems: Internal gears are crucial for the design of internal gear pumps, widely used in industrial fields for pumping adhesives, petroleum, and chocolate. These pumps are capable of handling fluids of various viscosities and temperatures, making them highly favored.
  3. Industrial Processes: Internal gear pumps are used in various industrial applications, including handling resins, polymers, molten sulfur, adhesives, liquid terminals, soaps and detergents, edible fats, lubricants, asphalt roofing, asphalt emulsions, and hot asphalt mixtures. These applications benefit from the pumps’ ability to handle high-viscosity media and their reliable, simple operation.
  4. Food and Beverage Industry: In the food and beverage industry, internal gear pumps are used for pumping syrups, chocolate, and condiments. Their sanitary design makes them suitable for these applications.
  5. Automotive Industry: Internal gear pumps are used in automotive systems for tasks such as oil circulation, fuel injection, and hydraulic power steering.
  6. Oil and Gas Industry: In oil and gas operations, internal gear pumps are used for metering, transporting, and injecting fluids, playing a crucial role in the management and operation of these industries.
  7. Pharmaceutical Manufacturing: In the pharmaceutical industry, internal gear pumps help in the precise metering and transport of liquids and handling of sensitive materials, thereby ensuring the integrity and quality of pharmaceutical products.
  8. Chemical Processing: Internal gear pumps are also used in the chemical industry to pump various chemicals, including corrosive substances like sulfuric acid and sodium hydroxide, because thedesign materials of internal gear pumps can withstand the corrosion of these substances.

What Are the Factors That Affect an Internal Gear’s Performance?

Factors that affect the performance of internal gears include:

  1. Gear Design and Tooth Profile: The design and tooth profile of gears greatly influence the efficiency of gearboxes. Well-designed gears with precise tooth profiles can minimize friction losses, ensuring smooth power transmission. Factors like tooth geometry, pressure angle, and tooth profile modifications affect gear meshing and engagement efficiency.
  2. Gear Material and Surface Smoothness: The material composition and surface smoothness of gears affect friction losses and wear characteristics during operation. High-quality gear materials, such as hardened alloy steel or advanced polymer composites, have smooth surfaces that can reduce friction and enhance efficiency. Proper lubrication and surface treatments (like polishing or coating) can further improve gear performance and lifespan.
  3. Bearing Friction and Lubrication: Bearings support the rotating shafts and gears within a gearbox, helping to enhance overall power transmission efficiency. Bearing friction losses can stem from misalignment, insufficient lubrication, or rough bearing surfaces. Using high-quality bearings, appropriate lubrication regimes, and advanced bearing technologies (like sealed bearings or ceramic bearings) can minimize friction and enhance gearbox efficiency.
  4. Lubrication System Efficiency: Effective lubrication is crucial for reducing friction, dissipating heat, and protecting gear components from wear and corrosion. However, inefficiencies in the lubrication system, improper viscosity selection, or contamination can lead to increased friction losses and reduced efficiency. Optimizing lubricant selection, maintaining proper oil cleanliness, and using efficient lubrication systems (like circulating oil or oil mist) can enhance gearbox efficiency and lifespan.
  5. Eccentricity and Mechanical Losses: Misalignment of gear components, shafts, and bearings can cause additional mechanical losses and vibrations within a gearbox. Proper alignment techniques, precision machining, and regular maintenance can help minimize losses caused by misalignment and optimize gearbox efficiency.
  6. Number of Teeth and Module: The number of teeth and module are key factors affecting gear meshing precision. More teeth increase the gear’s bending stiffness, making the meshing state more stable and enhancing meshing precision. A smaller module results in thicker gears, making the meshing state more stable and enhancing meshing precision.
  7. Pressure Angle and Tooth Profile Errors: These factors significantly influence gear engagement forces, sliding friction, and operational efficiency, further affecting gear meshing precision. A larger pressure angle can withstand larger loads but may increase sliding friction, reducing operational efficiency. Tooth profile errors can cause uneven engagement, noise, and vibrations, accelerating gear wear and shortening lifespan.
  8. Center Distance and Backlash: Center distance and backlash are important factors affecting gear meshing precision. A larger center distance can improve gear meshing precision by reducing bending deformation, while smaller backlash can reduce vibrations and noise, making operational performance more stable.
  9. Helix Angle and Incremental Diameter: A larger helix angle increases the contact area, enhancing load-bearing capacity but also increasing the axial thrust on the gears.
  10. Interference: Internal gears encounter various types of interference, such as involute interference, interference fit, and trimming interference. These interferences can affect the gears’ operational smoothness and lifespan.
  11. Speed and Rotation Direction: Operating speed and rotation direction can affect the forces acting on the gears, potentially impacting the gears’ performance and durability.
  12. Lubrication Amount: The level of lubrication affects the dynamic forces within a gear system, thereby affecting friction, heat dissipation, and wear.

How Do Internal Gears Work?

The working principle of internal gears is to provide a set of gear ratios within the sealed hub of a bicycle’s rear wheel. The hub contains a series of gears that can change the speed and torque of the wheel based on the pedaling speed. This gear system is typically operated by a single sprocket and sprocket device, with the shifting mechanism installed inside the hub.

Internal gear mechanisms often use planetary gears or external ring gears, where several “planet” gears revolve around a central “sun” gear within an “annular” or “ring” gear with internal teeth. The planet gears are mounted on a carrier, which can also rotate. By fixing one component in place and using another component as the input (applying pedaling force) and another as the output (connected to the wheel), different gear ratios can be achieved.

Shifting within the hub is accomplished by a shift rod or twist-style shifter, connected to the hub through a Bowden cable. Modern hub gears have indexing mechanisms built into the hub, ensuring precise shifting unaffected by cable tension.

The advantage of internal gear hubs is that they are virtually maintenance-free, can shift even when the bicycle is stationary, and because the gears are enclosed within the hub casing, they are protected from dust and debris. However, they are typically heavier, and their efficiency may be lower than a well-maintained derailleur system, especially in gears furthest from direct drive.


Internal gears offer a range of benefits and applications across various industries, from high-efficiency power transmission in compact spaces to enhanced durability and low maintenance requirements in cycling. While they present certain challenges such as increased weight and higher costs, their advantages in terms of reliability and performance make them a valuable component in many mechanical systems. As technology advances, the design and manufacturing of internal gears continue to evolve, offering even greater precision and efficiency for a wide array of applications.
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