Involute Internal Gear – 42CrMo, DIN Cl.7, Per Drawing

Involute internal gears — also called ring gears or annular gears — are circular gears in which the teeth are cut on the inner cylindrical surface following a standard involute tooth profile. The gears on the ring gear are internal: the mating external pinion runs inside the ring, with both gears rotating in the same direction. The involute tooth form is the universal standard for precision gears because the involute profile guarantees a constant velocity ratio throughout the mesh cycle regardless of small variations in centre distance — a property no other tooth form can match. Korea Ever-Power's involute internal gears in alloy steel and stainless steel to module M4.0 and inner diameter 30 mm serve the most demanding precision applications: planetary gearboxes and automotive drives where the accuracy and load capacity of the ring gear directly determine the system's performance.

Korea Ever-Power has focused on CNC machining since 2007. All steps are handled in one location — CNC machining, turning, and sheet metal fabrication — making the process speedy, cost-efficient, and well-controlled. Every production order for involute internal gears is logged into a computerised system and can be followed closely from order receipt to shipment. MOQ 50 pieces.

Constant Velocity Ratio

The instantaneous angular velocity ratio is constant throughout the mesh cycle — exactly equal to the inverse ratio of the tooth counts. This is the law of gearing, and the involute profile is the only profile that satisfies it inherently. For a planetary gearbox or automotive transmission where the ring gear's transmission ratio must be accurate, the involute profile guarantees this accuracy by geometry, not by manufacturing perfection.

Centre Distance Insensitivity

Small variations in the centre distance between the ring gear and its mating pinion (from manufacturing tolerance, thermal expansion, or bearing deflection) do not affect the transmission ratio — the involute pair maintains the same velocity ratio across a range of centre distances. This makes the involute ring gear tolerant of the dimensional variations that are unavoidable in production gearboxes, where achieving exact theoretical centre distance is not practical.

Conjugate Action

Two mating involute gears act as conjugate pairs — the shape of one tooth uniquely determines the shape of the mating tooth that produces smooth, constant-ratio transmission. This conjugate property means that any external involute gear (pinion) will mesh correctly with any internal involute ring gear of the same module and pressure angle, provided the interference and minimum tooth count difference conditions are met.

Interchangeability

Involute gears of the same module, pressure angle, and standard (ISO/DIN/ANSI/JIS) are interchangeable regardless of tooth count. A Korea Ever-Power M2 DIN involute internal ring gear will mesh with any M2 DIN external pinion from any manufacturer worldwide. This interchangeability is the industrial standard that makes the involute system the universal gear tooth form — and it is why Korea Ever-Power's involute internal gears are specified by designers who need global supply chain flexibility.

30 Sets CNC Gear Shaping

Internal gear teeth are cut by CNC gear shaping machines using profile-ground form cutters — the standard process for internal involute gears. The shaping machine's cutter reciprocates axially while rotating in synchrony with the ring gear blank, generating the involute tooth profile on the inner bore. Korea Ever-Power's 30-set CNC fleet provides the capacity for both prototype quantities and production batches without scheduling conflicts.

Computerised Production Tracking

The entire production process is logged into a computerised management system. Every step — from receiving the order, material procurement, CNC setup, gear cutting, heat treatment, inspection, and shipment — can be tracked in real time. Customers who cannot visit the factory receive weekly digital photo and video reports showing the processing progress. This transparency eliminates uncertainty about delivery schedule.

QC Department — Start to Finish

Korea Ever-Power's QC department controls quality from the start of production to the completion of goods. Incoming material inspection, in-process dimensional checks at each CNC operation, post-heat-treatment hardness verification, final gear profile measurement, and visual inspection are all performed in-house. In the event of any quality issue, the QC records allow rapid identification and isolation of the non-conforming production step.

Quality Non-Conformance Resolution

In the rare event of a non-conformance in delivered involute internal gears, Korea Ever-Power will not risk producing inferior products and will rework or re-manufacture to the correct specification. The QC inspection record from the original production batch is the baseline for root-cause analysis. The cost of re-manufacture and re-shipment for confirmed quality non-conformances is borne by Korea Ever-Power.

Planetary Gearboxes

Alloy steel involute internal ring gears M2–M4 as the fixed annulus in servo planetary gearboxes, robot joint planetary reducers, and industrial servo drive planetary stages. The involute tooth form's centre-distance insensitivity is particularly valuable in planetary stages where the ring gear bore must accommodate the planet gear bearing tolerance stack.

Automotive Drives

Alloy or stainless involute internal ring gears M1–M3 in automatic transmission planetary stages, EV motor planetary reduction drives, and seat adjustment mechanism planetary gear sets. The standard involute form ensures interchangeability with OEM planet and sun gears from any standard source.

Medical Instrument Drives

Stainless steel involute internal gears M0.5–M2, ID ≤ 30 mm in surgical tool planetary drives and infusion pump planetary stages. Stainless material meets the corrosion resistance and sterilisation compatibility requirements; involute profile meets the transmission ratio accuracy requirement at the inner diameter and module range.

Electric Screwdrivers and Power Tools

Alloy steel or POM involute internal ring gears M1–M2 in electric screwdriver planetary torque limiters, cordless drill planetary speed reduction stages, and rotary tool planetary drives. The compact inner diameter (≤ 30 mm) suits the small-body power tool housing envelope.

Bicycle Components

Stainless or alloy involute internal ring gears M1–M2 in internally geared bicycle hub transmission planetary stages (e.g. Shimano Nexus equivalent designs) and e-bike mid-drive planetary reduction gear sets. Small OD, precise involute profile, and high material efficiency suit the compact bicycle hub envelope.

Pumps and Positioning Equipment

Involute internal ring gears in gear pump internal rotors (gerotor pump ring gear), positioning equipment planetary drives, and fluid metering system drives. The involute gerotor ring gear's inner tooth profile is the critical accuracy element determining the pump's volumetric efficiency and flow uniformity.

Is Korea Ever-Power a trading company or a manufacturer?

Korea Ever-Power is a manufacturer. All machining steps — CNC internal gear shaping, CNC turning, sheet metal fabrication, heat treatment, and final inspection — are handled in one location. This integrated single-facility production makes the process speedy, cost-efficient, and quality-controlled, eliminating the communication delays and quality handoff risks of multi-supplier production chains. The facility has focused on CNC machining since 2007.

What information is needed for a quotation?

Korea Ever-Power needs four elements to provide an accurate quotation: the product drawing (DWG, IGS, STEP, UG, PDF, or SolidWorks format preferred), the material specification, the required quantity, and the surface treatment requirement. If no drawing is available, Korea Ever-Power can design the involute internal ring gear from the application requirements — provide the required module, inner diameter, tooth count, material preference, and operating conditions, and the design team will produce a drawing for approval.

Can I understand production progress without visiting the factory?

Yes — Korea Ever-Power provides a detailed production plan at the start of the order and sends weekly reports with digital pictures and videos showing the processing progress at each stage. The computerised production management system logs every step from order receipt through shipment. Customers who cannot make a factory visit receive the same production visibility as those who can. If a specific production milestone needs confirmation (e.g. heat treatment completion, final inspection pass), Korea Ever-Power will provide the relevant documentation or photo evidence.

What is the internal gear tooth cutting process and why does the ID limit matter?

Internal involute gear teeth are cut by CNC gear shaping — a form cutter reciprocates axially inside the ring gear bore while synchronously rotating with the workpiece to generate the involute tooth profile. The minimum inner diameter that can be gear-shaped is limited by the cutter size: the cutter must fit inside the bore and have enough clearance to reciprocate without fouling the bore wall. The maximum inner diameter (ID ≤ 30 mm) for this product series reflects the smallest bore size at which Korea Ever-Power's gear shaping capability produces the required involute profile accuracy. For larger inner diameters, see Korea Ever-Power's broader ring gear range. Contact our team with your specification.

"We manufacture servo planetary gearboxes and source M2 alloy steel involute internal ring gears from Korea Ever-Power. The weekly production photo reports confirmed heat treatment and gear shaping completion on schedule. Profile accuracy confirmed at our incoming gear analyzer — within our DIN 7 tolerance on all 50 pieces in the first batch. Single-facility production eliminates the quality handoff risk we had with our previous multi-supplier chain."

Kim Sang-min  |  Servo Gearbox Design Engineer, Incheon Precision Drives Co.  ·  Q1 2026

"We design e-bike mid-drive planetary stages and specified stainless steel involute internal ring gears M1.5, ID 28 mm from Korea Ever-Power. The involute profile's centre-distance insensitivity compensates for our housing tolerance at the planet pin locations. Product drawing submitted as PDF — Korea Ever-Power produced a DWG confirmation drawing for approval before production. MOQ 50 pieces matched our first production run quantity."

Lee Hyun-seok  |  E-Bike Drivetrain Engineer, Gyeonggi Electric Bicycle Systems Co.  ·  Q4 2025

"We make electric screwdriver planetary gear sets and needed M1 alloy involute ring gears ID 18 mm — at the fine pitch/small bore boundary. Korea Ever-Power confirmed their shaping cutter capability for this combination, produced 50 sample pieces on schedule, and provided a QC inspection report with tooth profile measurement. All gears assembled into our planetary carrier without dimensional adjustment."

Park Jun-ho  |  Power Tool Gear Design Engineer, Busan Precision Tool Co.  ·  Q2 2026

"We distribute involute internal gears to automotive and medical equipment manufacturers. Korea Ever-Power's computerised production tracking means we can give our customers an accurate delivery date — not an estimate. Weekly photo reports passed directly to our customers who specify Korea Ever-Power as approved supplier after seeing the production visibility. No quality non-conformances in 12 months of supply."

Choi Sang-jun  |  Industrial Gear Distribution Manager, Daejeon Components Co.  ·  Q3 2025

"We had a quality concern on a non-conforming delivery from a previous supplier. Korea Ever-Power's QC department provided a root-cause analysis report identifying the non-conformance as a heat treatment temperature deviation, corrected the process, and re-manufactured the affected batch within 2 weeks. Replacement gears confirmed to specification on our incoming inspection."

Yoon Byeong-jun  |  Planetary Gear Procurement Manager, Ulsan Automation Systems Co.  ·  Q1 2025