1) Description of Internal Spur Gear

Internal spur gears, commonly referred to as internal gears, are a type of gear whose tooth profile is distributed along the outer surface of a cylindrical body. Their tooth form is straight and parallel to the axis. They mesh with an external spur pinion to form an internal gearing pair, enabling compact coaxial transmission.

2) Working Principle

Its operating principle is analogous to that of externally meshed gears, relying on continuous engagement between tooth profiles to transmit motion and power. The key distinction lies in the fact that when the external pinion, acting as the driving gear, rotates, it drives the internal gear to rotate in the same direction. This stands in stark contrast to the counter-rotation characteristic of externally meshed gears.

3) Characteristics of Internal Spur Gear

• Internal gear ring: The most distinctive feature is that all teeth are machined along the circumference of the part's internal bore.
• Coaxial design: The centre lines of the input and output shafts coincide or are extremely close, resulting in an inherently compact structure.
• Tooth tip and root: The pitch circle diameter of the internal gear is smaller than its root circle diameter, precisely the opposite of an external gear.

4) Advantages of Internal Spur Gear

• Extremely compact structure: Eliminates the large centre distance required for external meshing, significantly saving radial installation space.
• Co-directional shaft rotation: Input and output shafts rotate in the same direction, achievable without an idler gear.
• Low sliding rate: Minimal relative sliding between internal meshing tooth surfaces enhances transmission efficiency and wear resistance.

5) Application

Primarily employed in applications with stringent radial space constraints:

• Planetary gear mechanisms: Serving as the core ‘ring gear’ component, it constitutes a critical element in planetary gearboxes and automatic transmissions.
• Compact reducers: Suited for diverse light industrial machinery and robotic joints requiring small footprints and high speed ratios.
• Slewing bearings: Utilised in the rotating platforms of large-scale construction machinery, such as tower cranes.