GEAR APPLICATION GUIDE · RING GEAR · R03

Ring Gears for Excavators:
Swing Ring Gears, Final Drive Ring Gears and Replacement Specification

Excavator ring gears perform two of the most mechanically severe functions in any construction or mining machine — the swing ring gear enables the 360° rotation of the excavator’s upper structure around its undercarriage under the combined weight of the boom, arm, bucket, and counterweight assembly at potentially high angular deceleration, while the final drive ring gear transmits full track drive torque to the sprocket through each of the two planetary final drive gearboxes. Understanding the material, dimensional, and wear characteristics specific to each ring gear position is essential for both machine OEMs selecting specifications for new excavator designs and for maintenance engineers procuring replacement ring gears for existing fleet machines.

42CrMo4 · 18CrNiMo7-6 · 50Mn · Case Nitrided
M8–M20 · Swing + Final Drive · 5T–70T Class
Komatsu · CAT · Hitachi · Doosan · Volvo

SWING RING OD

0.8 – 3.5 m

Excavator swing ring gear OD range. Mini excavator (1–3T): 0.8–1.2 m. Compact (5–10T): 1.0–1.6 m. Medium (15–30T): 1.4–2.2 m. Large (50–70T): 2.0–3.0 m. Mining shovel (100T+): 2.5–3.5 m. All swing ring gears use internal tooth form — the swing drive pinion gears on the upper structure mesh with internal teeth on the ring fixed to the undercarriage

SWING RING MODULE

M8 – M16

Excavator swing ring gear module range. Mini: M8–M10. Compact (5–10T): M10–M12. Medium (15–30T): M12–M14. Large (50–70T): M14–M16. Material: 50Mn (Chinese standard) or 42CrMo4 QT + case nitrided for medium/large excavator swing rings, through-hardened for mini excavators

FINAL DRIVE RING

M5 – M10

Final drive internal ring gear module — smaller than swing ring because the planetary final drive operates at higher speed and lower torque per tooth. 18CrNiMo7-6 case carburized HRC 58–62 mandatory for final drive ring gears in all but the lightest mini excavator applications

SWING TOOTH FORM

Internal Spur

Excavator swing ring gears use internal straight-tooth spur form — not helical. The zero helix angle eliminates axial thrust that would be undesirable in the large-diameter swing ring bearing. The internal spur tooth also allows easy visual inspection of the tooth surface condition during field maintenance inspection

Excavator Ring Gear Engineering — Swing Rings vs Final Drive Rings

The excavator contains two distinct and very different ring gear applications that share only their basic ring gear geometry — an annular ring with internal or external teeth. The swing ring gear (the large-diameter ring that allows the excavator superstructure to rotate relative to the undercarriage) and the final drive ring gear (the small ring inside each track final drive gearbox) differ in module (M8–M16 vs M5–M10), material (manganese steel or nitrided alloy steel vs carburized alloy steel), tooth form (internal spur vs internal helical on many designs), load type (combined radial and moment bearing loads vs pure planetary tangential tooth load), and failure mode (surface fretting and edge contact from ring gear tilt vs contact fatigue from planet gear load cycling).

Korea Ever-Power’s ring gears for excavators are manufactured in two distinct product lines matching these two applications: 50Mn or 42CrMo4 QT + case nitrided for swing ring gears (covering the 5T through 80T excavator size range), and 18CrNiMo7-6 case carburized for final drive ring gears (covering mini through large excavator final drive gearboxes). The swing ring gears are manufactured as complete slewing ring assemblies (ring gear + bearing raceway + ball or roller bearing elements + sealing ring) for the OEM market, and as ring-gear-only replacements for the aftermarket service market where the existing slewing ring bearing is still serviceable but the gear teeth are worn. The final drive ring gears are supplied as matching internal ring gear sets for planetary final drive replacement, paired with the planet gears, sun gear, and planet carrier as complete stage kits compatible with the specific excavator model’s gearbox. For planetary final drive complete drive unit specification, see the planetary gearbox design resource for excavator final drive stage arrangement and torque calculation.

The swing ring gear wears through a fundamentally different mechanism than a standard ring gear in a gearbox. The excavator’s upper structure rotates the swing ring gear relative to the swing drive pinions through a limited angular range in the majority of work cycles — digging typically involves repeated left-right swings of 60°–120° to move material from the dig face to a truck or stockpile. Each swing cycle reverses the swing ring gear direction, introducing a cyclic load reversal at the mesh contact point. Over time, the tooth surfaces in the frequently-used angular sector (the 60°–120° sector engaged during the dig-and-swing cycle) accumulate far more contact fatigue cycles than the remaining circumference, resulting in localised pitting and spalling in the active sector while the remainder of the ring is relatively unworn. This characteristic partial-circumference wear pattern is the defining diagnostic sign of swing ring gear fatigue — and it directly informs the replacement decision, because the amount of remaining usable tooth surface is the key life-extension variable.

planetary ring gear excavator final drive Korea Ever-Power — Korea Ever-Power 18CrNiMo7-6 case carburized internal ring gear for excavator final drive planetary gearbox — M6, 84 internal teeth, OD 310 mm, bore dimension and OD ground to H5 for housing press-fit. Case carburized to effective case depth 0.9–1.3 mm, HRC 59–62 tooth surface, shot peened tooth roots. Hobbed internal teeth to DIN 7 quality class. Used in 20T class excavator final drive second stage — paired with 20CrMnTi carburized planet gears and sun gear to form the complete second-stage planetary set. B10 life 12,000 hours at rated operating cycle per ISO 6336-6 with service factor 2.0. 100% magnetic particle inspection on all internal tooth surfaces after carburizing and before assembly. Dimensional inspection report supplied. Replacement supply for Komatsu PC200-series, Caterpillar 320-series, Hitachi ZX200-series and Doosan DX225-series final drives.

Swing Ring Gear Specification, Wear Assessment and Replacement

The swing ring gear is one of the most expensive single components in an excavator — a replacement swing ring gear for a 30T excavator costs €5,000–€15,000 depending on the machine make and model, compared to €200–€800 for a typical final drive planet gear set. This cost makes the swing ring gear replacement decision an important economic and engineering choice that should be based on measured wear criteria rather than on elapsed hours alone. The OEM-published wear limit for swing ring gears is typically 15–20% reduction in tooth height (measured from the original tooth height at the pitch circle) before replacement is mandatory — at this wear level, the tooth strength has been reduced to the point where the contact fatigue safety factor S_H falls below 1.0 under the maximum swing acceleration torque, and tooth fracture under peak dynamic load becomes a credible risk.

Swing Ring Gear Wear Assessment Procedure

Excavator swing ring gear wear should be measured at each major service interval (every 4,000–6,000 hours) using a gear tooth vernier caliper or a purpose-made tooth caliper gauge. Measure at minimum 12 positions equally spaced around the full ring circumference, including at least 4 measurements in the actively-worn sector (the 60°–120° sector engaged during the machine’s typical dig-and-swing cycle, identified by the visible surface contrast between worn grey and unworn golden-coloured tooth flanks). Compare each measurement to the original tooth height specification (typically OEM-published in the machine service manual, or calculable from the module: full tooth height = 2.25 × module for standard addendum and dedendum).

Record the measurements and the angular position of the minimum tooth height — if the minimum is below 85% of original (15% wear), replacement is recommended. If minimum is between 85% and 90% of original, begin planning procurement of the replacement ring and schedule installation at the next major service window. If minimum is above 90%, continue operation and re-measure at the next service.

Swing Ring Gear Life Extension — Rotation Method

The partial-circumference wear pattern of excavator swing rings creates an opportunity for life extension not available with uniformly-worn gears — if the ring is rotatable relative to its mounting bolts, the worn sector can be rotated away from the active position and an unworn sector positioned in the active mesh zone. This ring rotation extends the effective life of the ring by the ratio of the ring circumference to the worn sector circumference — for a ring with 90° worn sector, rotating by 90° brings a fully fresh tooth sector into service, potentially doubling the ring’s total service life before replacement.

Requirements for ring rotation to be effective: (1) The bolted mounting holes in the ring must allow rotation — most OEM swing ring designs have equally-spaced bolt holes (every 10°–15°) that allow rotation in 10°–15° increments. (2) The ring must be removed, rotated, and reinstalled — a 2–4 hour maintenance job with overhead lifting equipment. (3) The remaining unworn tooth surface must be sufficiently intact to justify the maintenance cost — this should be confirmed by tooth height measurement before committing to the rotation. Korea Ever-Power provides swing ring gear rotation guidance documents for the most common excavator models on request.

alloy steel internal ring gear excavator swing ring Korea Ever-Power — Korea Ever-Power 42CrMo4 QT + case nitrided internal ring gear for excavator swing slewing ring application — M12, 120 internal teeth, OD 1,520 mm, wall thickness 35 mm, internal tooth profile cut and then gas nitrided (HV 650–720 at depth 0.25–0.40 mm). The gas nitriding process is applied after the ring gear has been through-hardened and precision machined — it provides a hard, fretting-resistant tooth surface without the dimensional distortion that would occur with case carburizing at this large diameter. Mounting holes: 48 equally-spaced M20 bolt holes on a 1,460 mm bolt circle for the bolted connection to the slewing ring housing assembly. Blank manufactured from 42CrMo4 forged ring blank (not plate-cut), UT tested after forging. 100% MT inspection of all internal tooth surfaces after nitriding. Used in 20–30T class excavator swing ring replacement programmes for Komatsu, Hitachi, CAT, Doosan, and Sany excavator brands.

Korea Ever-Power manufacturing workshop excavator ring gear swing ring production — Korea Ever-Power excavator ring gear manufacturing facility — large-diameter swing ring gear hobbing and tooth profile inspection. Excavator swing ring gears present a challenging manufacturing problem at the intersection of large diameter (0.8–3.5 m) and high precision (the gear tooth must maintain consistent pitch accuracy around the full 360° circumference to prevent speed variation in the swing drive that would be felt by the operator as jerky swing motion). Korea Ever-Power uses large-diameter vertical hobbing machines with table diameter capacity up to 4,000 mm for excavator swing ring production, with 100% pitch measurement at 24 equally-spaced positions around the ring circumference on a large-ring CMM after hobbing. Swing rings failing to meet the DIN 8 pitch tolerance at any position are rejected and re-hobbed, not accepted with local deviation — a single tooth spacing error above tolerance creates a detectable speed pulse in the swing drive that operators experience as a “knock” at a fixed angular position during swing, one of the most common field complaints leading to premature swing ring replacement.

Frequently Asked Questions — Ring Gears for Excavators

Q 01

Our 25T excavator swing drive is making a grinding noise during rotation. Is this the swing ring gear or the slewing ring bearing? How do we diagnose and is immediate repair needed?

Distinguishing swing ring gear noise from slewing ring bearing noise is critical for correct repair planning: (1) Bearing noise characteristics: slewing ring bearing defects (ball or roller damage, raceway spalling) typically produce an intermittent clunking or crunching noise that varies with the angular position of the ring — if you rotate the ring slowly by hand (with the swing brake released) and the noise occurs at specific, repeatable positions, bearing element damage is likely. Bearing noise is also often accompanied by increased swing free-play (the upper structure can rock slightly in the horizontal plane when the swing brake is applied — this lateral rocking indicates bearing race wear). (2) Ring gear noise characteristics: swing ring gear tooth damage produces a rhythmic clicking or grinding that is proportional to swing speed (faster swing = faster clicking) and occurs throughout rotation rather than at specific positions. Gear tooth damage is also often accompanied by metal particles in the swing gear grease (if the swing ring is grease-lubricated). (3) Immediate repair assessment: a grinding noise that is new (not present at the last service check) indicates active material removal from either the gear or bearing — the machine should be operated with reduced swing acceleration until inspection can be performed. If the noise is accompanied by vibration visible to the operator, the machine should be taken out of service immediately. Swing ring gear damage that progresses to tooth fracture can cause the upper structure to seize in rotation during a heavy swing — a potentially dangerous condition when the boom is fully extended with a loaded bucket. Korea Ever-Power recommends a qualified excavator mechanic inspect the swing ring gear grease port (accessible without ring removal on most machines) by sampling the grease for metal particle content as the first diagnostic step.

Q 02

What is the lead time and documentation for a swing ring gear and final drive ring gear replacement set for a Komatsu PC300 excavator?

Korea Ever-Power replacement ring gear supply for Komatsu PC300-7/PC300-8: Swing ring gear (internal, M12, approx. 1,380 mm OD, 42CrMo4 nitrided): lead time 35–50 days from order confirmation. The PC300 swing ring is manufactured from a forged 42CrMo4 ring blank, through-hardened, precision machined, and gas nitrided — the total process chain including NDT inspection and dimensional verification is the primary lead time driver. The PC300 swing ring is one of Korea Ever-Power’s most stocked sizes — if an emergency shipment is needed, contact Korea Ever-Power to check current inventory of ring blanks at different process stages. Final drive ring gear (internal, M6, 18CrNiMo7-6 carburized, for stage 1 and stage 2): lead time 20–30 days from order confirmation, including carburizing, quench, grind, and 100% MT inspection. These are available as individual ring gears or as complete stage kits including planet gears, sun gear, and planet carrier. Documentation: 42CrMo4 forging certificate, heat treatment record (QT + nitriding cycle), MT inspection certificate, dimensional inspection report, and ISO 9001:2015 quality system certificate are supplied with the swing ring. 18CrNiMo7-6 material certificate, carburizing cycle record, case depth and surface hardness measurement, MT certificate, dimensional report supplied with final drive ring gears. All documentation is formatted for inclusion in the machine’s maintenance record and, if required by the owner, for submittal to a third-party inspection agency (Bureau Veritas, SGS, TÜV) for the machine’s statutory inspection record.

Q 03

Our excavator fleet includes 12 machines of the same model. Can Korea Ever-Power provide a framework supply agreement for ring gears at a committed price and lead time?

Yes — Korea Ever-Power operates framework supply agreements for excavator fleet operators, mining companies, and equipment rental fleets with multiple machines of the same or similar models requiring recurring ring gear replacement. A framework supply agreement (FSA) specifies: (1) Part numbers and specifications: the swing ring gear and final drive ring gear part numbers for the agreed machine models, with the Korea Ever-Power equivalent specification (material, module, tooth count, heat treatment) confirmed by first-article inspection against the original OEM parts. (2) Price commitment: fixed pricing for a 12-month or 24-month period, protecting the customer from steel and energy cost fluctuation within the agreement period. (3) Lead time commitment: Korea Ever-Power pre-orders and holds ring blank inventory for FSA customers, reducing lead time from the standard 35–50 days (swing ring) to 20–30 days by eliminating the raw material procurement step. For the final drive ring gears, Korea Ever-Power may hold finished goods stock for the most common sizes under the FSA, enabling dispatch within 5–10 days from order. (4) Order flexibility: FSA orders can be placed in individual units (one ring gear per machine service event) without minimum order requirements, since the production schedule is pre-agreed in the framework. Contact Korea Ever-Power’s key accounts team with the excavator model list, current annual ring gear consumption per machine, and preferred delivery lead time to receive a framework supply agreement proposal.

Explore Korea Ever-Power Gear Categories

Seven precision gear product lines for excavator, construction, mining and heavy equipment applications worldwide.

Spur Gears

M8–M20 · excavator · swing drive pinion

Helical Gears

Heavy machinery · gearbox · mill drive

Engrenages coniques

Right-angle · pump · auxiliary drives

Worm Gears

Self-locking · actuator · slow drive

Ring Gears

Swing ring · final drive · M8–M16

Planetary Gears

Final drive planet · swing stage · kit

Plastic Gears

Sensor · instrument · light auxiliary

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Need Ring Gears for Excavators?

Korea Ever-Power manufactures 42CrMo4 nitrided swing ring gears (OD 0.8–3.5 m, M8–M16) and 18CrNiMo7-6 carburized final drive ring gears (M5–M10) for all excavator brands — Komatsu, Caterpillar, Hitachi, Doosan, Volvo, Sany, Liebherr, and Hyundai. Complete final drive stage kits including planet gears, sun gear, and carrier. Framework supply agreements for fleet operators. MT inspection certificate included. ISO 9001:2015 certified.

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Ring Gears for Excavators: Swing Ring Gears, Final Drive Ring Gears and Replacement Specification