Planetary Gear System - Engineering Documentation
Providing maximum torque in a compact volume, the planetary gear system is designed to examine the epicyclic kinematic relationship between the sun, planet, and ring gears, load sharing principles, and precision gear manufacturing technologies.
Planetary Gear System Overview
Providing maximum torque in a compact volume, the planetary gear system is designed to examine the epicyclic kinematic relationship between the sun, planet, and ring gears, load sharing principles, and precision gear manufacturing technologies.
This structure is used for analyzing coaxial speed/torque converter systems in robotic actuators, automotive automatic transmissions, aerospace gearboxes, and wind turbines.
What You Will Learn
Torque Density and Load Sharing
Demonstrates why dividing the input torque among 3 (or more) planet gears allows it to carry up to 3 times more power than a parallel-axis gearbox of the same volume.
Gear Manufacturing and Profile Grinding
Highlights the importance of case hardening and ISO 1328 Class 6 precision gear profile grinding for quiet operation and zero backlash at high speeds.
Kinematic Transmission Ratios
Explains how reduction or overdrive ratios change depending on which element (Sun, Ring, or Carrier) is held stationary and which is driven.
Component Engineering Breakdown
Materials, manufacturing methods, surface conditions, tolerances, and functional roles of the major components.
Ring Gear (Annulus)
Outer Orbit Element
42CrMo4 (1.7225) Alloy Heat‑Treatable Steel | EN 10083‑3 | Mfg: Forging, Broaching, or Gear Shaping
Gas Nitriding surface hardness 50‑55 HRC, case depth 0.2‑0.4 mm | Tooth flanks Ra ≤ 0.8 µm
Gear quality: ISO 1328 Class 7 | Inner diameter radial runout: ≤ 0.02 mm | Cumulative pitch error: ≤ 0.032 mm (ISO 1328)
Functional Description
The outermost gear wheel with internal teeth that surrounds the orbit of the planet gears. Typically fixed to the gearbox housing to react the torque.
Critical Points
- Broached internal gear profile
- Anti‑distortion Gas Nitrided surface (50‑55 HRC)
- Housing mounting interface (radial runout ≤ 0.02 mm)
Cutting internal teeth is much harder than external teeth; therefore Broaching is used in mass production. To prevent heat distortion, lower‑temperature Gas Nitriding (500‑520 °C) is preferred over high‑temperature case hardening. Due to the thin nitrided layer, caution is needed under high impact loads.
Sun Gear
Central Torque Input Element
20MnCr5 (1.7147) Case‑Hardening Steel | EN 10084 | Mfg: Gear Hobbing and Profile Grinding
Carburizing (Case Hardening) 58‑62 HRC, effective case depth (Eht) 0.6‑0.9 mm | Tooth root Shot Peening (Almen A2 0.010‑0.014)
Gear quality: ISO 1328 Class 5‑6 | Concentricity: ≤ 0.01 mm | Spline/Keyway bore: H7 (ISO 286‑2)
Functional Description
Located at the exact center of the system, it takes power from the motor shaft and transmits it to the surrounding planet gears. It rotates at the highest RPM in the system.
Critical Points
- ISO 1328 Class 6 precision ground profile
- High RPM wear resistance (58‑62 HRC)
- Motor shaft input interface (Spline/Keyway H7)
Despite having the smallest diameter, it transmits all the torque, making its tooth roots highly susceptible to bending fatigue. It is case‑hardened to have a glass‑hard surface (58‑62 HRC) and a tough, shock‑absorbing core (30‑35 HRC). Shot peening induces compressive residual stress at the tooth root, increasing fatigue life by 2‑3 times.
Planet Gear 1
Orbital Torque Transmission Element
18CrNiMo7‑6 (1.6587) Case‑Hardening Steel | EN 10084 | Mfg: Gear Hobbing and Profile Grinding
Case Hardening 58‑62 HRC, effective case depth 0.5‑0.8 mm | Tooth flanks superfinished Ra ≤ 0.4 µm
Pitch and involute profile error: ≤ 0.005 mm | Weight difference among planets in the same set: ≤ 0.5 g
Functional Description
One of the satellite gears rotating between the sun gear and the ring gear, sharing the total load equally with the other planets.
Critical Points
- Dual‑contact involute tooth profile
- Needle roller bearing mounting bore
- Micron‑level load sharing tolerance
Because planet gears mesh simultaneously with both the sun and ring gears, their involute profiles must be perfectly ground. Otherwise, the 3 planet gears will not share the torque equally (Load Sharing Error); all the load will transfer to a single gear, leading to catastrophic failure. All gears in the same planet set must come from the same manufacturing batch and be weight‑matched with tight tolerance.
Planet Gear 2
Orbital Torque Transmission Element
18CrNiMo7‑6 (1.6587) | EN 10084 | Mfg: Hobbing and Grinding
Case Hardening 58‑62 HRC, Ra ≤ 0.4 µm tooth flanks
Profile error ≤ 0.005 mm | Weight difference ≤ 0.5 g
Functional Description
The second satellite gear ensuring load distribution in the system.
Critical Points
- Load Distribution Support
- Wear‑Resistant Case‑Hardened Surface
All planet gears must come from the same batch with tooth thickness equal at micron level. Weight difference among planets can cause vibration at high speeds.
Planet Gear 3
Orbital Torque Transmission Element
18CrNiMo7‑6 (1.6587) | EN 10084 | Mfg: Hobbing and Grinding
Case Hardening 58‑62 HRC, Ra ≤ 0.4 µm tooth flanks
Profile error ≤ 0.005 mm | Weight difference ≤ 0.5 g
Functional Description
The third satellite gear ensuring load distribution in the system.
Critical Points
- Radial Force Cancellation
- Case‑Hardened Involute Profile
The use of three planet gears makes the system statically stable and naturally cancels out radial (centrifugal) forces, reducing the load on the carrier shaft bearings.
Needle Roller Bearing
Rotary Bearing Element
100Cr6 (1.3505) Bearing Steel | ISO 683‑17 | Mfg: Precision Rolling and Superfinishing
Roller surfaces superfinished, Ra ≤ 0.05 µm | Rings ground and honed
Radial internal clearance class: C2 or C3 (ISO 5753) | Roller diameter tolerance: Grade G2 (ISO 12297)
Functional Description
The bearing group that allows the planet gears to rotate frictionlessly and at high speeds around their own pins (planetary shafts).
Critical Points
- High radial load capacity
- High‑speed capability in compact volume
- Superfinished rollers (Ra ≤ 0.05 µm)
Due to the restricted space inside planet gears, needle roller bearings are preferred over ball bearings because they have much higher radial load capacity for their size. These bearings do not carry axial loads. C2 clearance is for precision applications, C3 when thermal expansion allowance is needed.
Planetary Shaft (Pin)
Axis Fastening and Bearing Inner Race
42CrMo4 (1.7225) Heat‑Treatable Steel | EN 10083‑3 | Mfg: Turning and Centerless Grinding
Induction Hardening (bearing raceway surface) 58‑62 HRC, depth 0.8‑1.2 mm | Ra ≤ 0.2 µm
Outer diameter (OD): h6 (ISO 286‑2) | Cylindricity: ≤ 0.003 mm | Press‑fit end: m6 tolerance
Functional Description
The stationary bearing shafts pressed into the planetary carrier, upon which the planet gear bearings run. The pin does not rotate about its own axis.
Critical Points
- Precision surface acting as bearing inner race (Ra ≤ 0.2 µm)
- Press‑fit into carrier m6 tolerance
- High resistance to Shear force
Because the bearing needles roll directly on the outer surface of this pin, the pin surface acts as the inner race of the bearing. Therefore it must be hardened to 58‑62 HRC and mirror‑polished to Ra ≤ 0.2 µm. The press‑fit portion into the carrier is machined to m6 tolerance for a tight interference fit.
Planetary Carrier
Cage and Torque Output Element
Ductile Iron GJS‑500‑7 (GGG50) | EN 1563 | Mfg: Casting and CNC Machining Center
Phosphate coating + anti‑corrosion paint | Machined surfaces Ra ≤ 1.6 µm
Pin hole position tolerance (True Position): Ø0.01 mm (ISO 1101) | Output shaft concentricity: ≤ 0.015 mm | Balance quality grade: G6.3 (ISO 21940‑11)
Functional Description
The heavy‑duty cage that houses the pins of the planet gears and generally serves as the low speed / high torque output shaft of the gearbox.
Critical Points
- High torque output shaft
- Ultra‑precision CNC machined pin bores (True Position Ø0.01 mm)
- Rigid cast body with high Torsional resistance
Equal load sharing among the planet gears depends entirely on the flawless machining (True Position Ø0.01 mm) of the 3 pin holes on the carrier. If one hole is off by just 0.05 mm, the load on that specific planet gear can spike by 300%, causing catastrophic failure. The carrier must also be dynamically balanced (ISO 21940‑11 G6.3).
Circlip 1 (Retaining Ring)
Axial Fastening Element
C67S (1.1231) Spring Steel | DIN EN 10132‑4 | Mfg: Wire Forming and Heat Treatment
Phosphating and anti‑rust oil (DIN 471)
Shaft groove tolerance: JS9 (DIN 471) | Ring thickness tolerance: ±0.02 mm
Functional Description
A standard retaining ring holding the pins or planet gears axially in place on the shaft.
Critical Points
- Axial Lock feature
- DIN 471 compliant spring steel elasticity
Prevents components from moving axially (back and forth) and dislodging from the system. Groove depth must be sufficient to withstand centrifugal forces at high RPMs.
Circlip 2 (Retaining Ring)
Axial Fastening Element
C67S (1.1231) Spring Steel | DIN EN 10132‑4
Phosphating and oiling
Groove width fit: H11 (ISO 286‑2)
Functional Description
Another retaining ring in the system.
Critical Points
- Centrifugal Resistance
- Groove Lock
To prevent the ring from opening and flying off the groove due to centrifugal forces at high RPMs, its thickness must perfectly match the shaft groove, and the ring must be installed with the sharp edge facing the thrust direction.
Circlip Set (DIN 471)
Axial Fastening Element
C67S Spring Steel | DIN EN 10132‑4
Phosphating
Fully compliant with DIN 471 External Retaining Ring Standards
Functional Description
The external retaining ring set sealing the mechanism in its axial orbit.
Critical Points
- DIN 471 Industry Standard
- Requires special assembly tool
Special circlip pliers are mandatory for assembly and disassembly. Using incorrect tools will destroy the ring elasticity, making it non‑reusable.
Training Scope
This interactive module is intended for technical training, engineering visualization, component identification, and mechanical-system analysis. Numerical values, material selections, tolerances, and manufacturing details shown for individual components should be treated as representative technical information unless a specific manufacturer, drawing, or applicable standard is explicitly identified.