Custom Internal Ring Gears
Precision CNC Machined

An internal ring gear (also called an annulus gear or internal gear ring) is a cylindrical gear with teeth cut on its inner bore surface, pointing inward toward the center axis. The mating gear (planet gear or pinion) runs inside the ring, in the same direction of rotation. Internal ring gears are the fixed or rotating outer element in planetary gear systems, epicyclic gearboxes, and differential gear sets. They offer a higher contact ratio than external gear pairs and are more compact for a given reduction ratio.

Internal gear ring production cost depends on six variables: (1) bore diameter and module — a 300 mm internal ring gear in M3 costs more than a 100 mm in M2; (2) material grade — 20CrMnTi case-carburized steel is more expensive than 42CrMo4 through-hardened; (3) accuracy class — DIN 5 (ground) is roughly 2× the price of DIN 7 (hobbed only); (4) heat treatment — carburizing and post-hardening grinding add 25–40% to material cost; (5) order quantity — 1-piece prototypes cost more per unit than 100-piece batches; (6) integrated features — housings with bearing seats and oil ports cost more than simple ring inserts. Send your drawing for a free customized quotation — we typically respond within 48 hours with firm price and lead time.

Three ways to get a customized internal gear ring quotation from HXC: (1) Upload your CAD / PDF drawing through our quote form — fastest, response within 48 hours. (2) Email your specifications (module, tooth count, bore diameter, helix angle if any, material, accuracy class, quantity) directly to our engineering team. (3) Schedule a video call with our China factory for complex projects requiring DFM discussion. Every manufacturer quotation includes itemized cost breakdown, process recommendation, alternative material options, and firm lead time. Free, no obligation, no salesman markup.

"Ring gear" and "internal gear" are often used interchangeably, but there is a subtle distinction in usage. Internal gear is the precise engineering term for any gear with inward-facing teeth. Ring gear is a common name that most often refers to the annulus element in a planetary gear system, or to the large-diameter internally-toothed gear on an automotive flywheel (engaged by the starter motor). In practice, both terms — along with "internal gear ring" — refer to the same tooth geometry: teeth on the inner cylindrical surface.

Internal ring gears are manufactured by four main processes: (1) Gear shaping (most common) — a reciprocating cutter generates the internal teeth progressively; suitable for all bore sizes including blind bores. (2) Internal gear hobbing — faster for open-bore ring gears in production quantities. (3) Broaching — fastest for short face widths in high volumes. (4) 5-axis CNC milling — for complex ring gear housings integrating other features. After cutting, precision ring gears are case-hardened and internally profile-ground to achieve DIN 5–6 accuracy.

In a planetary (epicyclic) gearbox, the internal ring gear — the annulus — is the outermost element. It can serve three roles depending on which element is fixed: (1) Fixed ring gear (most common) — the carrier rotates, giving a compact speed reduction. (2) Input ring gear — drives the planet carrier, with the sun gear as output. (3) Output ring gear — rotates while sun and carrier are input/fixed. The ring gear's tooth count (always larger than the sun gear) largely determines the stage gear ratio. Ring gear tooth accuracy directly controls backlash, noise, and efficiency of the entire planetary stage.

Yes — internal helical ring gears are produced by gear shaping with a helical shaper cutter or by internal helical hobbing. They offer the same advantages over internal spur ring gears as external helical gears over spur gears: quieter operation, higher contact ratio, and greater load capacity. The helix angle of the internal ring gear must match that of the mating planet or pinion gear (same angle, same hand for external pinion inside an internal ring gear). Common helix angles are 15°–25° for planetary stages in servo actuators and precision gearboxes.

Accuracy class depends on the application: DIN 7–9 is sufficient for general industrial and automotive gearboxes where noise and efficiency are not critical. DIN 6 is standard for automotive passenger car transmissions and industrial inline reducers. DIN 5 is required for precision servo actuators, robot joints, and aerospace planetary stages where backlash must be controlled to <0.02 mm. DIN 5 ring gears require post-hardening internal profile grinding, which is more expensive than hobbing alone. We offer all classes — our engineers will recommend the right accuracy for your application and budget.

Material choice mirrors external gear practice but with one extra constraint: ring gear blanks are more difficult to heat-treat uniformly due to wall thickness variation. 20CrMnTi (carburized) is standard for automotive planetary ring gears — its predictable case depth and low distortion suit internal grinding after hardening. 42CrMo4 (through-hardened) is used for large-bore ring gears where case carburizing penetration would be insufficient. Al 7075-T6 for lightweight robot ring gears. SUS316L for food, medical, and cleanroom applications. POM, nylon, PEEK for low-load plastic internal ring gears in service robots and consumer products.

Unlike external gears (17-tooth minimum to avoid undercut at 20° pressure angle), internal gears have a minimum tooth count difference constraint between the ring gear and its mating pinion. To avoid involute interference (where the external pinion tooth tips foul the internal ring gear root), the tooth count difference must be at least 10–12 teeth for standard 20° pressure angle gears. For example, if the planet gear has 20 teeth, the ring gear needs at least 32 teeth. This constraint is less restrictive at higher pressure angles (25°). Our engineers check interference conditions for every custom ring gear design as part of the free DFM review.

HXC's internal gear ring factory is located in Dongguan, Guangdong, China — a major precision manufacturing hub in the Pearl River Delta. We have been a Chinese internal ring gear manufacturer since 2008, exporting to clients in Germany, USA, Japan, France, Italy, Korea, and 30+ other countries. All gear cutting, heat treatment, grinding, and Klingelnberg inspection are performed in-house at our China factory — no outsourcing to subcontractors. Factory visits and video audits are welcome for serious buyers and procurement teams qualifying new China internal ring gear suppliers.

External gears have teeth on the outer cylindrical surface and rotate in opposite directions when meshed. Internal ring gears have teeth on the inner bore surface and the mating gear rotates in the same direction — making internal gear pairs naturally suited to planetary systems where co-directional rotation is desired. Internal gear pairs also have a higher contact ratio and lower sliding velocity between teeth than external pairs of the same module, which means lower friction, quieter operation, and longer life at equivalent loads.

Internal ring gears are measured with specialised equipment: Klingelnberg or Gleason gear analysers fitted with internal probes measure tooth profile, helix deviation, pitch error, and runout directly on the internal teeth. For larger ring gears, CMM with a gear measurement module is used — a ruby-tip probe traces each tooth flank to build the profile deviation chart. Over-pin (or over-ball) diameter measurement is used for quick size checks on the shop floor. All HXC internal ring gears are measured on Klingelnberg equipment, and the full DIN 3962 measurement report is supplied with every order.

In a planetary gear system: the sun gear is a small external gear at the center, driven by the input shaft. The planet gears are intermediate external gears that mesh with both the sun and the ring. The ring gear (annulus) is the large internal gear at the periphery. Power flows in from the sun, through the planets, and out through either the carrier (for reduction) or the ring (for different ratio). The ring gear is always the largest and slowest-rotating element; the sun gear is the smallest and fastest. The ratio of ring to sun tooth counts determines the stage gear ratio.

The most common ring gear failure modes are: (1) Pitting fatigue — contact stress exceeds material limit, causing surface flaking on internal tooth flanks; prevented by correct hardness, module size, and lubrication. (2) Bending fatigue — cyclic tooth root stress exceeds endurance limit; prevented by correct module, profile, and shot peening of roots. (3) Bore distortion — thin-wall ring gears distort during heat treatment, causing uneven tooth engagement; mitigated by proper blank support during carburizing and post-hardening grinding. (4) Fretting — micro-movement between ring gear OD and housing bore; prevented by interference fit and proper surface finish on the OD. We address all these in our DFM review.

The key differences: tooth location — spur gears have teeth on the outer surface; ring gears have teeth on the inner bore. Rotation direction — external spur gear pairs rotate oppositely; ring gear and pinion rotate in the same direction. Geometry — internal gear teeth have a concave (hollow) involute profile on the tooth face, compared to the convex profile of external gear teeth; this requires different cutting tools and inspection methods. Application — spur gears are used in simple external gear trains; ring gears are the annulus element of planetary and epicyclic systems. Minimum tooth count — constrained by the tooth count difference rule, not the undercut rule.

Specify DIN accuracy class (5–9) based on your application speed and noise requirement. Additionally specify: bore diameter tolerance (typically H7 for press-fit housing installation or H6 for precision actuators); concentricity between bore ID and gear pitch circle (typically 0.01–0.02 mm TIR for actuators, 0.02–0.05 mm for industrial gearboxes); face runout (for axially located ring gears). If you are unsure, our engineers will recommend appropriate tolerances based on your planetary gear ratio, input torque, and speed — free with every quotation request.

Yes — we regularly machine ring gear housings that integrate internal teeth, bearing seats (housing bore H7 or P7 for press fit), oil ports, mounting flanges, O-ring grooves, and alignment spigots in a single part. 5-axis CNC milling ensures concentricity between the internal gear pitch circle and all housing bores within 0.01 mm. This eliminates assembly error between a separate ring gear insert and its housing. Integrated ring gear housings are common in compact planetary actuators for robotics and aerospace. Send us your drawing — we will review it as part of our free DFM analysis.

Lead times depend on complexity and process: Prototype (1–5 pcs), gear shaping + CMM: 5–10 days. Small batch (10–50 pcs), hobbing + standard inspection: 8–14 days. Precision batch (DIN 5–6), hobbing + hardening + grinding + Klingelnberg: 14–22 days. Large-diameter (>Ø500mm) ring gears: 18–28 days depending on material and heat treatment. We provide firm lead times with every quotation. Rush production is available for an additional charge — discuss your timeline when uploading your drawing.