Planet carrier

01

Understand how the carrier affects load sharing

A conference paper from the US National Renewable Energy Laboratory (NREL) explains that epicyclic gear sets carry identical gears, called planets, equally or nearly equally spaced on a carrier. This symmetry splits the input torque into parallel paths, with each planet ideally carrying an equal share. In practice equal loading is not always achieved because of manufacturing and assembly errors such as carrier pin position error, variation in planet size and gear runout, and sensitivity to position errors rises with the number of planets. [1]

In wind turbine gearboxes, NREL found that unequal load sharing also comes from rotor moments, gravity and the carrier bearings moving within their clearance. On two three-planet gearboxes it measured load-sharing factors as high as 1.16 at extreme rotor moments and 1.08 with no moment where the carrier ran on cylindrical roller bearings. With tapered roller bearings the factor stayed below 1.10. Reducing the planetary system mass and the carrier bearing clearance, even modestly, improved load sharing. [1]

Editorial calculation: a load-sharing factor of 1.16 means the heaviest-loaded planet carries about 16% more than an equal share.

Editorial calculation: with three planets, an equal share is one third of the input torque, about 33.3%; a load-sharing factor of 1.16 puts the heaviest planet at about 1.16 × 33.3 ≈ 38.7%. [1]

Give the supplier the gearbox model or drawing, the number of planets, the planet pin bore positions and tolerances, the bearing seats, the material and heat treatment, the process (casting, forging or machining from bar), the inspection required (dimensional report, ultrasonic or other NDT, material certificate), the finish, and the quantity.

02

Specify accuracy, material and supply form

The NREL paper compares measured load sharing with the mesh load factor requirements in the AGMA 6006 and IEC 61400-4 wind turbine gearbox design standards. Those standards are not freely available, so ask the gearbox designer which mesh load factor and pin position tolerance the carrier must meet. [1]

A JARA-ENERGY article from RWTH Aachen's Institute for Materials Applications in Mechanical Engineering puts nacelle cast iron use at about 20 t per megawatt installed, and names the torque arm and planet carrier of the main gearbox among the classic cast components. It groups the grades used into ductile ones such as GJS-400-18LT and less ductile, high-strength ones such as GJS-700-2, whose strength comes from an almost purely pearlitic matrix. It adds that high-strength cast irons do not reliably meet low-temperature toughness requirements at some sites, so a fracture mechanics assessment may be needed in design. [5]

Silbitz Group, a German foundry, lists planet carriers (planet spiders) in EN-GJS-700-2, an alloyed spheroidal graphite iron, from 20 to 45,000 kg per piece, delivered machined with optional primer. It casts bearing housings, torque arms, flanges and covers for the same planetary gearboxes in EN-GJS-400-18LT and EN-GJL, and offers mould-filling and solidification simulation. [6]

Suppliers listed below offer carriers in different forms. A foundry lists cast wind turbine planet carriers alongside wind turbine hubs; a reducer maker lists planet carriers among its gear components; and a loader parts supplier lists planet carriers among drive axle parts. [2] [3] [4]

Editorial note: when the gearbox designer specifies a pearlitic grade such as EN-GJS-700-2 for strength, confirm whether the site's lowest operating temperature calls for a low-temperature grade or a fracture mechanics check, and require the foundry's solidification simulation and test-bar results on the purchase order. [5] [6]

03

Carrier body, pin bores, carrier bearings and the gears and castings around them

Two of these rows — the planet pin bores and the carrier bearings — are where NREL traces unequal load sharing between planets. [1]

04

Planet count, pin accuracy, load-sharing factor, iron grade, casting weight and process for a carrier

The hard numbers here are NREL's measured load-sharing factors and Silbitz's 20–45,000 kg weight range; the pin position tolerance has to come from the gearbox designer, since AGMA 6006 and IEC 61400-4 are not freely available. [1] [6]

05

Send the gearbox, accuracy and inspection data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Gearbox: [application; model; drawing]
Planets: [count; pin size]
Accuracy: [pin position tolerance; bearing seats]
Material: [grade; heat treatment]
Process: [casting; forging; machining]
Inspection: [dimensional; NDT; certificates]
Quantity: [pieces]

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