Ring Gear: Internal Gear Engineering Guide

The name ring gear hides how much engineering it carries. Cut teeth on the inside of a ring and it becomes the ratio-setting element of a planetary gearbox; weld a coarse ring of external teeth onto a flywheel and it becomes the interface between a starter motor and an engine. Both share one name, yet they are designed, loaded, and manufactured in completely different ways. This guide concentrates on the internal ring gear: its geometry, the ratio math, the interference traps that catch inexperienced designers, and the manufacturing routes available for it.

If your project involves small precision gears of any kind, our companion guide to MIM gears manufacturing compares net-shape and cutting processes side by side and will help you frame the sourcing decision.

What Is a Ring Gear? Two Meanings Engineers Should Know

In drivetrain and gearbox engineering, ring gear most often means the internal gear of a planetary gearset: an annulus with teeth cut on its inner diameter, inside which several planet pinions roll. The ring is usually the fixed or the slowest member of the set, and because it carries internal teeth, the planets mesh with it on the inside of the ring rather than the outside.

The second meaning belongs to engine engineering. A starter ring gear is a ring of coarse external teeth shrunk or welded onto the flywheel or flexplate of an internal combustion engine. When you turn the key, the starter motor pinion shoots forward and engages these external teeth to crank the engine. Once the engine fires, the pinion disengages. The starter ring gear is not part of a ratio transmission: it sees only brief, low-cycle cranking loads, and its tooth form is chosen for reliable engagement under misalignment rather than for smooth torque transmission.

Attribute	Internal (planetary) ring gear	Starter ring gear
Tooth location	Inner diameter of the ring	Outer diameter of the ring
Mesh partner	Planet pinions inside the ring	Starter motor pinion
Role	Sets the transmission ratio of the gearset	Cranks the engine; no ratio function
Typical loads	Sustained torque, contact fatigue	Brief, intermittent cranking loads
Common processes	Broaching, gear shaping, internal grinding, PM/MIM	Hobbing of the blank ring, stamping or rolling of teeth at volume

Everything below applies to the internal ring gear unless stated otherwise. Starter ring gears come up again in the FAQ.

Internal Ring Gear Geometry Basics

An internal gear pair consists of the ring gear with internal teeth and a meshing pinion inside it. Compared with an external pair of the same size, the geometry offers three practical advantages. First, both gears rotate in the same direction. Second, the center distance is short, roughly half of what an external pair with the same tooth counts would need, so the package is compact. Third, internal meshes run with a higher contact ratio and more favorable load sharing, which is why planetary stages are quieter and more compact than equivalent external-gear stages.

The basic proportions follow the same module system as external gears, and a 20-degree pressure angle is the most common starting point. The center distance of an internal pair is a = m(N_r - N_s)/2, where m is the module, N_r the ring tooth count, and N_s the pinion tooth count. Because the teeth face inward, the ring tooth shape is essentially the inverse of an external tooth, and the clearances at the tips and roots behave differently from what external-gear intuition suggests.

Planetary Gear Ratio: The Willis Equation

A simple planetary set has four members: a sun gear, a ring gear, several planet gears, and the carrier that holds the planets. The kinematic relation between them is the Willis equation:

(ω_s - ω_c) / (ω_r - ω_c) = -N_r / N_s

where ω is the rotational speed of the sun (s), carrier (c), and ring (r). The most common arrangement fixes the ring gear, drives the sun, and takes the output from the carrier. Substituting ω_r = 0 gives the classic reduction ratio:

i = 1 + N_r/N_s

Because N_r is always larger than N_s, the ratio is always greater than 2:1 in this configuration, and it grows quickly as the ring tooth count rises. This is the formula behind the deep reductions in automatic transmissions, planetary winches, and compact servo gearheads.

Ring-and-sun tooth count combinations

Sun teeth Ns	Ring teeth Nr	Ratio i = 1 + Nr/Ns (ring fixed)
36	60	2.67
24	72	4.00
18	72	5.00
30	75	3.50
15	90	7.00

Tooth counts are not free choices. For the planets to fit physically between the sun and the ring, the geometry requires:

N_r = N_s + 2N_p

where N_p is the planet tooth count. On top of that, if n planets are placed at equal spacing around the sun, the assembly condition requires that (N_s + N_r) is divisible by n, otherwise the teeth of successive planets will not phase correctly with the ring. When the numbers do not divide evenly, designers either change a tooth count, use unequal planet spacing, or accept an assembly sequence in which planets are timed individually.

The planets share the transmitted torque among themselves, which is why planetary stages carry so much torque for their size. A three-planet set splits the load three ways before fatigue ever reaches the ring teeth.

Internal Gear Design Risks: Interference and Profile Shift

Internal meshes fail in ways that external meshes do not. Four interference modes dominate the risk list:

• Involute interference: the involute profiles themselves foul each other near the pinion root when tooth counts are small or the difference between ring and pinion is tight.
• Trochoid interference: the tip corner of one tooth collides with the fillet trochoid of the mating tooth during the approach or recess part of mesh, a risk that grows as the tooth-count difference shrinks.
• Trimming interference: tooth tips trim material from the mating tips in certain tooth-number combinations, degrading profile accuracy over time.
• Radial assembly interference: if the pinion is inserted radially into the ring, the teeth can collide before the center distance closes; axial insertion or a slot in the ring avoids it.

The standard countermeasures are profile shift on the pinion, a shortened addendum on the ring teeth, generous tip clearance, and small amounts of tooth tip relief. None of these should be guessed: internal gear pairs should always be checked with a proper involute and trochoid interference analysis for the actual tooth numbers, pressure angle, and shift coefficients before tooling is ordered.

Tooth Difference: A Practical Screening Rule

How far apart can the ring and pinion tooth counts be? Geometry sets the floor at N_r - N_s = 2N_p, and the interference risks described above tighten as the difference shrinks. For gears with a 20-degree pressure angle, keeping the tooth-count difference above 9 teeth is a widely used first-pass screening rule: it keeps trochoid interference unlikely for standard, unshifted proportions and gives a clean starting point for a new design.

Treat this number as what it is: an empirical screening rule, not a universal law. Profile-shifted designs, modified tip forms, and carefully analyzed special cases can work with smaller differences, and a difference above 9 does not excuse a pair from a full interference check. Use the rule to shortlist tooth counts during concept design, then verify the final combination numerically.

Ring Gear Manufacturing Processes Compared

Ring gear manufacturing is constrained by one awkward fact: the teeth are on the inside of a ring, where chips are hard to evacuate and tools hard to support. Each process route trades volume, flexibility, accuracy, and cost differently, and the right choice depends on batch size, hardness state, and ring size more than on any absolute ranking.

Process	Best suited for	Strengths	Limitations
Broaching	Very high volumes, stable designs	Extremely fast per part, good accuracy and surface finish in one pass	Broach is a dedicated, expensive tool; design changes are costly; needs room to pull or push the broach through
Gear shaping	Prototypes, medium batches, design churn	Flexible, standard machines, can cut near shoulders and closed ends	Slower per part; per-part economics fade at very high volumes
Internal gear hobbing (skiving)	Medium to high volumes on dedicated equipment	Productive metal removal, good for moderate modules	Requires specialized machines and rigid setups that are not universally available
Internal grinding	Hardened rings, precision grades	Corrects heat-treat distortion, fine finish, tight accuracy	Slow and expensive; normally reserved for the final operation on critical rings
Powder metallurgy / MIM	Small rings in high volume, complex near-net shapes	Net-shape teeth, minimal material waste, integrated hubs and splines, high repeatability	Porosity limits fatigue strength and ductility versus wrought; part size limited (MIM typically 0.1-200 g)

Broaching wins when the design is frozen and the volume is large: a single pull of a multi-tooth broach finishes the ring in seconds. Gear shaping is the flexible default: a shaped cutter on a standard machine handles prototypes and revision-prone designs without dedicated tooling. Internal hobbing, including skiving-type processes on dedicated machines, closes much of the productivity gap but ties the job to equipment that many shops do not have. Internal grinding is the accuracy route for hardened rings, and it is usually the last operation because it removes the distortion that carburizing or nitriding leaves behind.

For small rings produced in volume, the powder metallurgy process family changes the equation: teeth are formed in the die or mold rather than cut, so the per-part cost stops depending on cutting time. Our MIM gears manufacturing guide covers where metal injection molding fits against machining for small gears in detail.

Ring Gear Materials

Ring gear materials follow the same logic as other power-transmission gears: match the steel and heat treatment to contact stress, shock loading, and distortion budget. The table lists the grades engineers reach for most often.

Material	Class	Typical use for ring gears
8620	Carburizing low-alloy steel	Planetary rings with impact loading, automotive-style duty
20MnCr5	Carburizing steel (European designation)	Small precision ring gears and pinions
18CrNiMo7-6	Heavy-duty carburizing steel	High-contact-stress industrial gearboxes
4140 / 42CrMo4	Quenched-and-tempered alloy steel	Moderate-duty rings where cost and availability matter
Nitriding steels (e.g., 31CrMoV9)	Nitriding grades	Precision rings that must stay straight through heat treatment
MIM-4605	Low-alloy MIM steel, heat treatable	Small net-shape rings and pinions produced by MIM

The MIM grades deserve a note: MPIF Standard 35 lists typical tensile strength around 520 MPa for sintered 316L stainless and around 1480 MPa for MIM-4605 after quenching and tempering, which puts the low-alloy MIM steels in the same conversation as wrought gear steels for small, moderately loaded rings. See our overview of MIM materials and the MIM-4605 high-hardness grade page for property detail.

Carburizing vs Nitriding: How to Choose

Carburizing diffuses carbon into a low-carbon alloy steel at austenitizing temperature; quenching then produces a hard, wear-resistant case on a tough, ductile core. The case is deep, contact fatigue resistance is excellent, and the tough core absorbs shock, which is why 8620, 20MnCr5, and 18CrNiMo7-6 dominate automotive and industrial planetary rings. The price is distortion: carburizing happens at high temperature followed by quench, so rings usually need hard finishing such as internal grinding afterward.

Nitriding diffuses nitrogen at a comparatively low process temperature, so the ring barely moves dimensionally and can be finish-machined before treatment. That makes nitriding attractive for precision rings with tight bore or tooth tolerances, and for parts where grinding after treatment is impractical. The trade-off is a thinner, shallower case that carries less contact load than a carburized case.

A practical selection logic:

• High contact stress, shock loads, or long fatigue life: carburize a low-carbon alloy steel such as 8620, 20MnCr5, or 18CrNiMo7-6, and budget for hard finishing.
• Precision geometry that must survive treatment without distortion: nitride, finish-machining before treatment and accepting a thinner case.
• Moderate duty with cost control: through-harden a quenched-and-tempered grade such as 4140 or 42CrMo4 and skip case hardening altogether.

MIM Internal Ring Gears at Emitech

For small internal gears and ring gears, metal injection molding offers a genuinely different route. Emitech has run MIM production since 2005 alongside powder metallurgy dating back to 1995, from a single Nanjing site with 17 MIM injection machines, continuous and vacuum sintering furnaces, and an in-house tooling shop. The MIM envelope covers parts from 0.1 to 200 g in weight, in MIM-4605 and 316L among other grades, with as-sintered tolerances of roughly ±0.3-0.5% and sintered densities of 95-99% of theoretical.

Where MIM earns its place on a ring gear drawing is integration: an internal ring gear with an integrated hub, splines, flats, or a mounting flange is molded as one part, with the teeth formed in the cavity rather than cut. Sintered MIM-4605 responds to quenching and tempering up to tensile strengths near 1480 MPa, and stainless options cover corrosion-exposed mechanisms. Tooling typically takes 15 to 20 days for a single-cavity mold, and drawings receive a DFM review with a quote within 24 hours. Production is specified against MPIF Standard 35, with feedstock controlled to ISO 22068 and sintered parts to ASTM B883. Our MIM tolerances page sets realistic as-sintered expectations, and every lot ships with documented quality inspection, including CMM checks and metallography.

Honesty about limits matters here: MIM rings carry a few percent of residual porosity, so for the largest rings or the most demanding fatigue duty, cut or ground gears from wrought blanks remain the safer choice. Inside the 0.1-200 g envelope, MIM internal ring gears routinely replace machined rings at a fraction of the per-part cost in volume. MIM 304 stainless steel parts illustrate the corrosion-resistant side of that envelope for mechanism hardware.

Ring Gear FAQ