Internal Gear

What Is an Internal Helical Gear

An internal helical gear is a cylindrical gear with angled teeth machined on its inner bore surface. The helix angle, typically between 15° and 30°, achieves the same highly efficient, progressive tooth-meshing characteristics found in external helical gears. Helical gear engage progressively to smooth torque transmission, reduce noise levels, and distribute the load across multiple contact points simultaneously.

What sets internal helical gears apart from their external counterparts is the direction of tooth curvature and the resulting mesh geometry. When an external helical pinion meshes inside an internal helical gear, both components rotate in the same direction. This is a fundamental geometric difference from external gear pairs, where rotation directions are always opposite. This property is critical in planetary gear systems.

Product Specifications

Parameter	Details
Tooth Form	Involute helical, internal (ring gear)
Helix Angle	15° – 30° (custom angles available)
Pressure Angle	20° standard
Module Range	Metric and imperial standards available
Accuracy Grade	ISO 6 – ISO 9 (application dependent)
Material Options	Case-hardened alloy steel, stainless steel, engineering plastics, through-hardened carbon steel, aluminum alloy, bronze / brass
Manufacturing Process	Internal gear shaping (slotting), broaching, grinding
Customization	Bore diameter, face width, helix hand, tooth count, housing integration
Quality Documentation	Material certificates, CMM inspection reports available on request

Material Selection for Internal Helical Gears

Key selection factors include operating load and torque, running speed, environmental exposure, required service life, and whether weight or cost is a constraint. Here’s how the most commonly specified materials compare.

Material	Strength	Wear Resistance	Corrosion Resistance	Weight	Cost	Best Application Fit
Case-Hardened Alloy Steel (e.g. 20CrMnTi, 8620)	Highest	Excellent	Requires coating	Heavy	Moderate	Heavy-duty planetary gearboxes, industrial drives
Through-Hardened Carbon Steel (e.g. 4140, 4340)	High	Good	Requires coating	Heavy	Lower	Medium-load gearboxes, general power transmission
Stainless Steel (304 / 316)	Moderate–High	Good	Excellent	Heavy	Higher	Food processing, pharmaceutical, marine environments
Aluminum Alloy (6061 / 7075)	Moderate	Moderate	Good	Lightest	Lower	Robotics, automation, lightweight precision drives
Engineering Plastics (Nylon, POM, PEEK)	Low–Moderate	Moderate	Excellent	Lightest	Lowest	Low-load instrumentation, quiet auxiliary drives
Bronze / Brass	Moderate	Good	Good	Moderate	Moderate	Low-speed, high-friction environments, worm-mating stages

Key Advantages of Internal Helical Gears

• Compact co-axial design

Input and output shafts share the same axis, enabling significantly more compact gearbox packaging than equivalent external gear arrangements.

• Higher power density

The internal mesh geometry allows a larger contact area and longer tooth engagement, supporting higher torque transmission within the same envelope size.

• Smooth, quiet operation

Helical tooth geometry ensures progressive engagement, reducing impact noise and vibration even at elevated speeds and loads.

• Same-direction rotation

The co-rotating mesh is essential for planetary gear configurations.

• Reduced bearing loads

Internal gear mesh geometry generates lower separating forces compared to external pairs, reducing radial loads on supporting bearings.

Typical Applications

Internal helical gears provide compact, high-efficiency, co-axial power transmission in demanding applications.

Planetary gearboxes

The ring gear in planetary gear systems is almost always an internal gear. Helical tooth profiles add smoothness and load capacity to these high-performance reducers.

Automotive transmissions

Automatic and dual-clutch transmissions use internal helical gears extensively in their planetary gear sets for smooth, efficient ratio changes.

Industrial automation & robotics

Compact servo gearboxes and rotary actuators rely on internal helical gear stages for high torque density and precise motion control.

Wind turbine gearboxes

Multi-stage planetary gearboxes in wind energy applications use large-diameter internal helical ring gears to handle the extreme torque demands of rotor input stages.

Custom Internal Helical Gears

Standard catalog internal gears rarely match the exact bore diameter, face width, tooth count, or integration requirements of a custom drivetrain. Hansheng manufactures fully custom internal helical gears to your precise specifications — from prototype single pieces through to production volume supply.

Our engineering team supports you from initial design review through to final inspection, ensuring your internal helical gear optimizes for your specific load conditions, speed range, and housing constraints.

FAQ

What is the difference between an internal helical gear and an external helical gear?

An external helical gear has teeth on its outer surface. An internal helical gear has teeth cut on its inner bore. Unlike external pairs, when meshed together they rotate in the same direction.

What are internal helical gears most commonly used for?

Internal helical gears are primarily used in planetary gearboxes, automotive automatic transmissions, industrial servo gearboxes, and wind turbine drive stages.

How are internal helical gears manufactured?

Internal helical gears are produced using internal gear shaping with a reciprocating pinion-type cutter, or broaching for shorter face widths. Post-cutting, teeth may be ground for tighter accuracy grades.

Can Hansheng manufacture custom internal helical gears to my specifications?

Yes. Hansheng produces fully custom internal helical gears tailored to your exact requirements, including module, helix angle, tooth count, bore diameter, face width, material grade, and accuracy class. Share your application details and our engineering team will provide a design recommendation and competitive quote within 24 hours.