Plastic Internal Gear

A plastic internal gear (also called a plastic internal ring gear or annular gear) is a gear whose teeth are cut on the inside surface of a cylinder or ring — rather than on the outside as in conventional external gears. When a small external pinion meshes inside the ring gear, both gears rotate in the same direction (unlike external gears which rotate in opposite directions). This same-direction rotation and the compact coaxial arrangement make internal gears the standard component in planetary gearsets and in compact, high-ratio single-stage drives. Korea Ever-Power's plastic internal gears are available in POM-C natural, PA, PPA, PBT, and PEEK; module M0.15–M2.0; diameter Φ3 mm–Φ120 mm; meshing grade ISO6, JGMA 1, JIS 6, AGMA 12–13, DIN 5–6. Applications: automotive, military, aircraft, mechanical, industrial, and medical.

Korea Ever-Power's plastic internal gears cost 50–90% less than equivalent metal internal gears and require no secondary finishing — the injection-moulded precision is achieved directly from the die. The lighter plastic internal ring gear seat reduces the overall planetary gearset mass. ODM/OEM supported; ISO 9001 / TS16949 certified; vacuum-packed in plastic trays; DHL/UPS delivery; sample available.

Nylon Planetary

Ring Gear Set

Custom Ring Gear

Plastic Internal Gear Details

M1 Standard Range — POM-C Natural (Sample)

Note: Exact dimensions per Korea Ever-Power drawing on request.

Inside-Out Tooth Form

The internal gear has teeth cut on its inner cylindrical surface — the tooth flanks face inward. The mating pinion (external spur gear) rotates inside the ring and meshes with these inner teeth. Both the ring gear and pinion rotate in the same direction — opposite to external gear pairs where the two gears rotate in opposite directions. The same-direction rotation is the key kinematic signature of internal gear drives: it is always present and is often exploited in the drive design (e.g., same rotation direction maintained through the drive train without additional idler gears).

Higher Contact Ratio

The internal gear mesh produces a higher contact ratio than an equivalent external gear pair at the same module and tooth count. This is because the concave internal tooth flank and the convex external pinion tooth have a more favourable curvature relationship at the contact point — the sum of principal curvatures is lower, producing a larger contact ellipse and distributing the tooth force over a larger area. The higher contact ratio means more tooth pairs are simultaneously in contact, reducing the peak tooth force and producing smoother, quieter operation.

Compact Coaxial Arrangement

When a pinion meshes inside a ring gear, the input and output shafts can be placed on the same axis (coaxial) — the output shaft runs through the centre of the ring gear, concentrically. This coaxial arrangement is far more compact than an external gear pair of the same ratio, which requires offset parallel shafts with centre distance between them. The coaxial arrangement is why planetary gearsets (which all use internal ring gears) are the most compact gear reducers per unit of torque capacity — and why plastic internal gears appear in miniature robot joints, camera zoom mechanisms, and compact industrial reducers.

Gear Ratio from Internal Mesh

For a simple internal gear drive (fixed ring gear, pinion drives the output): the ratio is Z_ring / Z_pinion — the ring has more teeth than the pinion. A 60T ring gear driven by a 20T pinion gives ratio 60/20 = 3:1. In a planetary gearset with fixed ring gear: ratio = 1 + (Z_ring / Z_sun). The ability to achieve compact ratios from 2:1 to 10:1 in a single coaxial stage makes internal gear drives much more compact than equivalent multi-stage external gear trains at the same ratio and output torque.

Plastic vs Metal Internal Gear

Plastic internal gears cost 50–90% less than equivalent metal internal gears. Internal gears are among the most difficult metal gears to manufacture — they require internal broaching, gear shaping (cannot be hobbed), or internal grinding — all slow, expensive operations. Plastic internal ring gears are injection-moulded in one shot with no secondary operations needed. The precision available from POM-C injection moulding (to DIN 5–6 for fine-pitch gears) meets the accuracy requirements of planetary gearsets in consumer electronics, automotive accessory drives, and light industrial equipment.

Plastic Internal Gear Shaft Features

The shaft of the plastic internal ring gear includes a stepped portion that engages a gear on its periphery — this stepped portion comprises straight-edge and inclined sections that enable the rack (or external drive element) to engage the ring gear's outer profile while the planet gears mesh on the inner teeth. The free end of the shaft is free to move along the output track, enabling the internal gear to function as the ring element of a planetary stage or as a linear actuator ring gear in rack-driven mechanisms.

50–90% Lower Cost

Metal internal gears require internal gear shaping or broaching — the most expensive gear manufacturing operations. Plastic internal ring gears are injection-moulded in one shot at very low per-part cost at volume. The cost saving of 50–90% vs metal internal gears is the primary driver for using plastic internal gears in consumer electronics, automotive accessories, and medical devices where the internal gear's load and temperature requirements are within plastic's capability.

No Secondary Finishing Required

Metal internal gears require secondary operations (deburring, shaving, grinding) after the primary gear cutting operation. Plastic internal ring gears need no secondary finishing — the injection-moulded tooth profile is the finished surface directly from the die. This eliminates process steps, reduces lead time, and makes the plastic internal gear a true net-shape part that can be used directly from the injection moulding machine after basic visual inspection.

Lower Overall Mass

The plastic internal gear seat is significantly lighter than the metal equivalent — POM density 1.41 g/cm³ vs steel 7.85 g/cm³. In planetary gearsets where the ring gear is the outermost and therefore largest-diameter component, the mass saving is proportionally large. For battery-powered portable devices (cameras, power tools, medical equipment), this mass saving directly extends battery life by reducing the rotor inertia that the motor must accelerate and decelerate.

Greater Precision Potential

Due to the inherent properties of plastic gears moulded from high-quality POM-C, they can be made with greater precision than many metal internal gears at the same cost level. A high-quality POM-C injection mould with precision tooling can maintain tooth profile accuracy to DIN 5–6 — matching the accuracy achievable with expensive internal gear shaping of metal. This precision-to-cost advantage is particularly marked for fine-pitch internal gears (M0.15–M0.5) where metal shaping cost is prohibitive.

Planetary Gearsets — Robotics

POM-C and Nylon internal ring gears as the fixed outer element of planetary gearsets in servo motor gearboxes, robot joint drives, and compact actuators. The plastic ring gear enables a lightweight, compact planetary reducer at a fraction of the cost of an all-metal planetary — suited for educational robots, collaborative robots, and consumer-grade servo actuators where cost and weight are primary design drivers.

Automotive Applications

POM and PPA internal ring gears in electric seat adjustment planetary drives, electric window lift planetary reducers, and automotive electronic throttle body gear stages. PPA (polyphthalamide) provides higher temperature resistance than standard POM for underhood automotive applications where temperatures to 120°C are encountered. TS16949 certification for automotive OEM supplier qualification.

Medical Equipment

PEEK internal ring gears for high-temperature sterilisable surgical instrument drives; POM-C for infusion pump planetary reducers and diagnostic equipment positioning stages. ISO 9001 certification and complete material traceability for medical device quality system requirements. Vacuum-packed delivery protects from contamination before installation.

Military and Aircraft

PEEK internal ring gears for aircraft accessory drives and military equipment position actuators where weight reduction is critical and environmental resistance (chemicals, temperature, UV) must be maintained across extreme conditions. PEEK's combination of high temperature resistance (250°C continuous), chemical resistance, and good fatigue life suits these demanding application environments. AGMA 12–13 meshing grade required for aerospace applications.

Consumer Electronics

Fine-pitch POM-C internal ring gears (M0.15–M0.5) in camera zoom lens drives, phone camera module actuators, and AR/VR headset optical adjustment mechanisms. The miniature internal ring gear enables a compact planetary reduction with coaxial input/output arrangement — ideal for the space-constrained optics modules in modern consumer electronics where every 0.5 mm of housing depth is valuable.

Industrial Machinery

PA and PBT internal ring gears in light industrial conveyor drives, packaging machine planetary reducers, and industrial instrument positioning systems. The corrosion resistance and self-lubrication of engineering plastic internal ring gears suit food industry and chemical processing machinery where metal gear drives require expensive stainless steel or surface treatment to resist the cleaning and product contact environments.

Why are plastic internal gears 50–90% cheaper than metal internal gears?

Metal internal gears can only be manufactured by internal gear shaping (a slow, expensive process using reciprocating cutting tools) or broaching — neither process is fast, and neither benefits from the "near-net-shape" economy of injection moulding. Plastic internal ring gears are injection-moulded in seconds per piece at high volume, directly producing the finished tooth form without secondary machining. The cost saving reflects the fundamental process efficiency difference between mass-produced injection moulding and slow metal cutting operations.

What is the difference between POM-C and POM-H for internal ring gears?

POM-C (copolymer acetal) has a slightly lower melting point and better chemical resistance than POM-H (homopolymer acetal / Delrin). For internal ring gears, POM-C natural is the standard specification — its good chemical resistance, excellent machinability (for post-machining corrections), dimensional stability, and lower moisture absorption make it the most common choice. POM-H (Delrin 500P) provides slightly higher tensile strength and stiffness for higher-load applications. Korea Ever-Power produces both grades; the standard is POM-C natural.

Can Korea Ever-Power supply a complete plastic planetary gearset (ring + planets + sun + carrier)?

Yes — Korea Ever-Power can supply complete plastic planetary gearsets including the internal ring gear, planet gears, sun gear (or pinion), and carrier plate — as a complete matched set or as individual components for customer assembly. For plastic planetary gearsets, all tooth elements must be matched for consistent module, pressure angle, and backlash: supplying all four elements from Korea Ever-Power ensures this consistency. Contact Korea Ever-Power with your ratio, torque, and size requirements.

"We design servo motor gearboxes for educational robotics and switched from metal to POM-C plastic internal ring gears (M0.5, 60T, DIN6) from Korea Ever-Power. Cost per ring gear: 82% lower than the previous metal part. Weight per gearbox: 34 g → 21 g (the ring gear contributes most of the mass saving). Precision check: profile deviation within DIN6 tolerance for all 50 pcs in first batch. Planetary gearset backlash: 0.04 mm — unchanged from metal spec. Running noise at 200 RPM sun: 28 dB(A)."

Kim Jae-in  |  Robot Gearbox Engineer, Incheon Educational Robotics Co.  ·  Q1 2026

"We manufacture infusion pumps and source POM-C internal ring gears (M0.3, 40T) from Korea Ever-Power for our peristaltic pump planetary drives. Biocompatibility: Korea Ever-Power provided FDA-listed POM-C material certification — accepted by our regulatory affairs team. Dimensional check at incoming: all 200 pcs within Φ+0.02/−0.01 mm on internal pitch diameter. Vacuum-pack delivery maintained cleanliness for medical-grade assembly. TS16949 certificate available — matches our quality system requirements."

Lee Sang-woo  |  Medical Device Mechanism Engineer, Gyeonggi Medical Devices Co.  ·  Q4 2025