Aircraft Bevel Gear – M0.15 to M2.2, AGMA 13, AS9100

What grinding achieves on spiral bevel teeth

After carburising and quenching, a gear's teeth distort by several micrometres to tens of micrometres from the thermal and transformation stresses of the heat treatment cycle. For a gear at AGMA 13 accuracy, this post-heat-treatment distortion would push the finished gear well outside the tolerance band unless corrected. Tooth grinding removes material from the distorted flank surfaces to restore the specified tooth form and spacing within the AGMA 13 tolerance, simultaneously achieving a tooth surface roughness (Ra) below 0.4 μm that is not achievable by any other post-hardening process on a bevel tooth.

The low surface roughness has a direct effect on the gear's fatigue life: surface micropits initiate at surface asperities, and a ground tooth surface has far fewer and shallower asperities than a lapped or as-hardened surface. In aviation applications where the gear is expected to run for thousands of hours before overhaul, this surface quality difference translates to a measurable increase in the inspection interval before pitting becomes detectable.

What this means for straight bevel gears in aviation

A straight bevel gear tooth, because it is generated by a different cutting geometry, cannot be finish-ground using the same equipment as a spiral bevel tooth. Lapping — running the pair together with abrasive compound — partially corrects tooth surface roughness but does not correct tooth spacing error or profile form deviation introduced by heat treatment distortion. The accuracy grade achievable on a hardened straight bevel gear is therefore lower than that achievable on a hardened and ground spiral bevel gear, regardless of the quality of the cutting operation before heat treatment.

This is the fundamental technical reason — documented in the product data for this series — why spiral bevel gears are becoming the standard in aviation applications, and why straight bevel manufacturing capability is declining. Korea Ever-Power produces straight bevel gears for aircraft applications where the drawing specifies this tooth form and the application's accuracy requirement is within the achievable range for hardened straight teeth. Where a new design is being specified, the spiral bevel form is recommended.

Aviation material selection for bevel gears involves a trade-off between strength-to-weight ratio, temperature performance, corrosion resistance, and the ability to survive both the operating environment and the certification process. The following covers the principal material families in use for aircraft bevel gears at small module.

Mill certificate and independent spectral analysis confirming alloy grade, chemical composition, and melt number. Issued with every aircraft bevel gear order. The melt number provides the first link in the traceability chain connecting the finished gear to the steel producer's original cast.

Tensile strength, yield strength, elongation, and Charpy impact energy from test pieces in the same heat as production parts. For aviation-grade alloy steels, minimum values are specified in the material standard (e.g., AMS 6265 for 9310) and the test results must meet these minimums before production parts ship.

Original time-temperature curves from carburising, quenching, and tempering furnace runs, traceable to the batch number. Includes case depth measurement result from cross-section metallographic inspection on sample pieces, confirming the effective case depth matches the design requirement.

Coordinate measuring machine verification of all critical dimensions: pitch cone angle, tooth spacing error, tooth profile form deviation, runout, bore diameter, hub dimensions. For AGMA 13 aircraft bevel gears, measurement is performed in a temperature-controlled metrology room (typically 20±1°C) to eliminate thermal expansion measurement error at this accuracy level.

For new aviation programmes, a first-article inspection report (FAIR) compares every drawing dimension against the measured value on the first production sample, confirms material and process compliance, and is submitted to the customer for approval before the production batch is released. Format follows the customer's requirement — AS9102 FAIR format is available on request.

Fluorescent penetrant inspection (FPI) and magnetic particle inspection (MPI) are available for 100% inspection of aircraft bevel gears where the application's criticality requires confirmation of surface and near-surface integrity on every part. Ultrasonic testing of the blank before gear cutting is also available per EN 10228-3 or the customer's specified standard.

🔎 Flight Instruments & Navigation

Gyroscope gimbal drives, altimeter and airspeed indicator mechanism gearheads, attitude indicator drives, and VOR/ILS receiver angle drives. These applications combine AGMA 13 accuracy requirements (transmission error directly corrupts instrument reading) with non-magnetic material requirements (brass, stainless, or aluminium — not carbon steel near magnetic flux instruments). Very small module — typically M0.15 to M0.8.

✈ Auxiliary Power Unit (APU) Gearboxes

APU accessory gearboxes use spiral bevel gear stages to drive oil pump, fuel pump, and starter-generator shafts from the APU turbine output shaft at 90° direction changes. Operating temperatures are elevated — the gearbox housing reaches 120–150°C under load — requiring alloy steel gears with heat-resistant surface coatings. Full traceability and first-article inspection are mandatory for APU gearbox components.

🚁 Helicopter Cockpit & Rotor Controls

Helicopter cockpit instrument drives, collective pitch mixing units, and tail rotor gearbox input stages use spiral bevel gears that combine compactness, high accuracy, and resistance to the high-frequency vibration environment of rotary-wing aircraft. 17-4PH stainless or 9310 alloy steel matched to the specific load and corrosion environment of the installation.

📷 Avionics Cooling & Actuators

Avionics bay cooling fan angle drives, antenna positioner drives, and cockpit panel actuators use small-module bevel gears at moderate accuracy (AGMA 12 typically) in aluminium or stainless for weight and corrosion reasons. These drives often operate continuously for the full flight duration and must meet defined MTBF (mean time between failures) targets as part of the avionics equipment specification.

🚢 UAV & Drone Propulsion

Fixed-wing UAV, tilt-rotor, and coaxial drone propulsion gearheads use bevel gear stages at the propeller hub or motor-to-rotor shaft junction. Weight minimisation is the dominant design driver: 7075-T6 aluminium hard-anodised, or carbon fibre composite housings with titanium gear inserts, are the directions being explored. Korea Ever-Power produces the aluminium and stainless gear elements for UAV drives at modules M0.5 to M2.2.

🔯 Military Aircraft & Targeting

Weapon aiming and stabilisation drives, targeting pod gimbals, and EO/IR turret angle drives use bevel gear pairs at the highest accuracy grades under shock and vibration environments that exceed civilian aviation standards. Non-magnetic materials (brass, stainless, aluminium) for electromagnetic compatibility. Full MIL-SPEC documentation support for procurement programmes with documented military standard compliance requirements.

Why are straight bevel gears becoming less common in aircraft applications?

Two factors drive the shift. First, the manufacturing equipment for high-accuracy straight bevel gears uses a different generating principle from spiral bevel machines, and this equipment is no longer being widely built or supported — expertise and tooling are concentrating in the spiral bevel world. Second, and more fundamental to the performance engineer: straight bevel gear teeth cannot be ground after heat treatment, while spiral bevel teeth can. Grinding corrects the dimensional distortion introduced by carburising and quenching, achieving tooth accuracy and surface roughness that lapping cannot match. For aircraft applications where AGMA 13 accuracy is required and where tooth surface quality directly affects fatigue life, the grindable spiral bevel gear has a clear performance advantage that the non-grindable straight bevel cannot overcome.

Can you provide AS9100-compatible documentation for aircraft bevel gear orders?

Yes. Korea Ever-Power's quality management system is structured to support AS9100-compatible documentation for aviation customers. This includes the material certificate, mechanical performance test report, heat treatment record, CMM dimensional inspection report, and — for new programmes — a first-article inspection report in the AS9102 format. Non-destructive testing (fluorescent penetrant inspection, magnetic particle inspection) can be added for safety-critical parts. Specify the required documentation package at the enquiry stage and we will include the scope and any effect on lead time in the quotation.

What is the typical lead time for a new aircraft bevel gear programme?

For a first-article sample on a new programme with a customer-approved drawing, the typical lead time is three to five weeks for gears at module M0.5 to M2.2 in standard aviation alloy steels or 7075-T6 aluminium. Very small modules (below M0.3) may require additional tooling lead time on the first order. The first-article inspection report, once the sample is produced, takes an additional one to two weeks to prepare and submit in the required format. For repeat production orders after first-article approval, lead time is typically two to four weeks. If you are attending a fair and need to exhibit an example of the gear, contact us with the fair date and we will confirm whether the sample lead time allows attendance.

Can you consolidate shipment of aircraft bevel gears with other components in one freight consignment?

Yes. Korea Ever-Power is willing to help customers consolidate their shipment to reduce freight costs, where the other components are also sourced from our facility or can be arranged through our logistics partners. For aviation supply chain customers who are sourcing multiple component types, discuss the consolidation requirement with our team at the order stage and we will advise on the logistics arrangement. Note that aviation parts documentation (airworthiness release certificates, material certs) must accompany the parts correctly — consolidation must not compromise the documentation integrity for any aviation-grade component in the consignment.

How quickly can a sample aircraft bevel gear be produced for evaluation?

For a simple sample at a standard module and tooth count in a stock material, we can typically complete a sample within one week of drawing confirmation. More complex samples — non-standard modules, non-standard materials, or gears requiring non-destructive testing as part of the sample package — will take two to three weeks. Submit your drawing (or sample gear for reverse engineering) and specify any documentation required with the sample; we will confirm the sample lead time with the feasibility response within two working days of receiving the enquiry.

Do you support wholesale or small-quantity purchases of aircraft bevel gears?

Where aircraft bevel gears in certain configurations are held in inventory — typically common module and ratio combinations in frequently ordered materials — small quantities can be supplied immediately from stock. For configurations produced to order, the minimum production quantity for new tooling is determined by the gear's module and complexity; for most aircraft bevel gears at module M0.5 to M2.2, single-piece to small-batch production is achievable without a minimum quantity constraint. Contact our team with your specification and quantity requirement and we will confirm availability and lead time.

"We produce gyroscope assemblies for aviation navigation systems. The M0.5 brass bevel gears in the gimbal drive require JGMA 1 accuracy and must be non-magnetic. Three other suppliers confirmed feasibility at JGMA 1 in brass but failed the dimensional inspection on the first samples. Ever-Power's sample met the tolerance on the first submission and the first-article report was in the correct format for our QMS approval."

Baek Jae-sung  |  Component Engineer, Daejeon Aviation Instrument Systems  ·  Q1 2026

"We manufacture APU accessory gearboxes for regional jet aircraft. The aircraft bevel gears in the oil pump drive stage operate at sustained 135°C oil inlet temperature. Ever-Power produced in 9310 alloy steel with the carburising and case depth documented to our process specification. Heat treatment records and CMM report were both in the correct format for our AS9100 supplier documentation folder."

Moon Hyeong-jun  |  Gearbox Programme Manager, Busan Aviation Components  ·  Q3 2025

"We design lightweight UAV propulsion drives for fixed-wing reconnaissance aircraft. Our propeller angle gearhead uses M1.5 7075-T6 hard-anodised aluminium bevel gears. Ever-Power produced to AGMA 12 accuracy, with the anodising specification we gave them. Weight per pair matched the material calculation to within 2%. The gearhead has completed 180 flight hours in field testing without gear failure or wear beyond the initial running-in."

Jang Min-seok  |  Mechanical Systems Engineer, Seoul Unmanned Platforms  ·  Q4 2025

"We needed to replace worn spiral bevel gears in a helicopter tail rotor feedback mechanism. The original manufacturer's documentation was no longer available. Ever-Power reverse-engineered from our worn sample, produced a complete production drawing for our approval, manufactured in 17-4PH stainless at AGMA 12, and included fluorescent penetrant inspection on every piece as required by our maintenance organisation approval. First installation passed the operational test without adjustment."

Park Soo-young  |  Maintenance Engineer, Gyeongnam Rotary Aviation Services  ·  Q2 2025

"We supply targeting pod gimbals for military aircraft. The bevel gear stage uses M0.8 17-4PH stainless at AGMA 13 — the entire mechanism must pass shock and vibration to MIL-STD-810 and be non-magnetic. Four suppliers declined. Ever-Power confirmed feasibility within three days, produced the first-article sample in four weeks, and the dimensional report met AGMA 13 across all measured parameters. The NDA for IPR protection was signed before any drawings changed hands."

Shin Byeong-cheol  |  Systems Integration Lead, Seoul Defence Optronic Systems  ·  Q1 2026