Planetary gearbox selection in 2026 is a torque-first, ratio-second exercise, with backlash, stage count and shaft configuration resolving the remaining trade-offs. The current DirectIndustry product set shows single-, two- and three-stage coaxial units covering 5 Nm to 1,000 Nm at gear ratios from roughly 3:1 to 350:1, with rotational speeds up to 18,000 rpm and backlash as low as 0.02° on precision servo lines [S1][S2][S3][S4].
For automation, AGV/AMR and servo-motion buyers, the decision pivots on four numbers: nominal output torque, maximum output speed, required gear ratio, and permissible backlash in arc-minutes. A reducer is essentially a torque amplifier — output torque scales with ratio against motor torque — so specifying a frame size without first sizing the load will produce either an oversized, expensive unit or a short-lived one [S2][S7].
Definition and Operating Envelope of Modern Planetary Units
A planetary (epicyclic) gearbox distributes torque across multiple planet gears meshing with a central sun and an outer ring gear, producing a coaxial input-output layout with high power density per stage [S9]. This architecture explains why current catalogues concentrate on the 5-1,000 Nm envelope: with three planet gears sharing the load, each tooth carries roughly one-third of the transmitted force, allowing the housing diameter to shrink versus a comparable coaxial parallel-shaft unit [S1][S4].
The 2026 commercial envelope from leading OEMs covers nominal torque from 5 Nm (Bosch Rexroth GTE single-stage) to 1,000 Nm (EPL series three-stage) and peak speeds from 1,050 rpm to 18,000 rpm across the same series [S4]. Ratio coverage splits cleanly: economy lines such as the Eisele EPL deliver ratios from about 3:1 to 100:1 in one to three stages, while precision servo lines such as the A2V SLF060/SLF080/SLF090 stretch to 350:1 in two- and three-stage configurations [S1][S2].
Selection Criteria: The Four-Number Sizing Workflow
Start with the load's continuous torque requirement and apply a service factor of 1.25-2.0 depending on shock load, duty cycle and number of starts per hour — only then select a frame size whose rated nominal torque exceeds the factored value [S7]. The Eisele EPL line publishes nominal torque bands in stepped ISO-like classes (10-20, 20-50, 50-100, 100-200, 200-500, 500-1,000 Nm) that map directly to motor frame sizes IEC 63-132, simplifying the motor-to-gearbox pairing [S1].
Calculate the required gear ratio from the motor's nominal speed divided by the driven shaft's required speed; verify the resulting output torque (motor torque × ratio × efficiency) does not exceed the catalogue nominal, and that output speed stays inside the gearbox's maximum rpm rating [S2][S7]. For example, a 3,000 rpm servo driving a 10 rpm conveyor needs a 300:1 ratio, which forces a two- or three-stage build because single-stage planetary units rarely exceed 10:1 economically [S2][S3].
Backlash specification must come from the application's positioning repeatability, not from marketing. A 0.1° backlash unit (Eisele EPL) is adequate for packaging and general automation; an RPC+ class unit at 0.02° is reserved for machine-tool feed axes and indexing tables where lost motion directly degrives part accuracy [S1][S2].
Configuration Matrix: Coaxial, Hollow-Shaft, Flange and Right-Angle Options
Coaxial solid-shaft planetary units dominate the catalogue count because the inline architecture is mechanically simplest and lowest-cost per Newton-metre [S1][S3][S4]. Bonfiglioli's TQW series adds a flange-mount face and a heavy-duty wheel interface specifically for AGV/AMR drive wheels, holding 0.17° backlash at 6,000 rpm and ratios 9:1 to 100:1 in a compact housing designed for direct chassis mounting [S3].
Hollow-shaft versions (EPL-H series) accept a through-shaft from the driven machine, eliminating a coupling and shortening the overall drivetrain length; right-angle planetary/bevel combinations reach ratios under 5:1 that pure planetary stages cannot, which is the architectural reason a right-angle section is bolted on the front of RPC+ type precision units [S1][S2]. For applications where corrosion, weight or electrical isolation matters, POM plastic planetary gearboxes (ZHAOWEI MEPLGE) trade steel's strength for self-lubrication, low noise and non-magnetic operation, typically in sub-20 Nm micro-motion roles [S6].
Comparison of Main Planetary Types on Decision Criteria
For a buyer choosing between economy coaxial (EPL), precision servo coaxial (SLF/GTE/RPC+), AGV/AMR wheel-drive (TQW) and micro plastic (MEPLGE) units, four criteria resolve the choice. Nominal torque coverage: economy 5-1,000 Nm, precision 5-810 Nm, AGV 200-500 Nm class, plastic typically under 20 Nm. Backlash: economy 0.1-0.17°, precision 0.02-0.18°, AGV 0.17°, plastic not specified. Maximum output speed: economy 6,000 rpm, precision up to 18,000 rpm, AGV 6,000 rpm, plastic application-dependent. Ratio range: economy 3-100, precision 3-350, AGV 9-100, plastic sub-100 [S1][S2][S3][S4][S6].
Where the decision forks is duty cycle and accuracy: a packaging line running 16 h/day at 0.5° repeatability should land on an economy coaxial unit; a CNC rotary axis at 0.02° lost-motion budget needs a precision servo line; an AGV drive wheel needs flange-mount and life-lubrication over backlash [S1][S2][S3]. For higher output loads on the EPL platform, Eisele specifies the EPL 50X-155X higher-stiffness range and the EPL-Q series with a squared output flange, both carrying the same torque bands up to 1,000 Nm [S1].
Real Use Cases: AGV/AMR, Servo Motion, Linear Actuator Pairing and Industrial Winches
Automated Guided Vehicles and Autonomous Mobile Robots have become a defining application class in 2026, driven by the Bonfiglioli BlueRoll platform (Basic/Advanced/Compact) that puts a TQW planetary gearbox inside the drive wheel itself to save chassis space and simplify cable routing [S3]. Bosch Rexroth's GTE planetary line pairs directly with MSK servo motors for handling and rack-and-pinion automation, with backlash reduced enough for pick-and-place repeatability under 0.1° [S4].
Linear-actuator manufacturers such as isel match their planetary gearboxes to LES and LEZ linear actuators specifically to increase available thrust and to inertia-match the actuator screw to the driven load, which is the typical pattern when a stepper or servo cannot deliver the required force at the rated speed [S5]. For heavier industrial work, hydraulic winch and travel-drive builders (INI Hydraulic) integrate planetary gearboxes rated to 200 Nm continuous at 2,000 psi hydraulic pressure, with DNV/CE certification for marine and lifting duty [S8].
Limitations, Failure Modes and Sourcing Constraints
The single most common selection error is specifying a single-stage planetary gearbox where the application actually requires a two- or three-stage ratio above 10:1 — efficiency drops and heat rises sharply when a single stage is pushed beyond its ratio limit [S2]. Backlash values are quoted under no-load at the output; under reversing torque load the effective lost-motion can be two to three times the catalogue figure, which matters for bi-directional servo axes [S1][S2].
Temperature rise inside the gearbox housing is a function of input speed, ratio and torque; precision lines such as the RPC+ cite a positive influence of their product design on temperature development, meaning their internal gear geometry is chosen to keep the oil below 90°C at rated load [S2]. For plastic POM planetary units, the thermal ceiling is the polymer limit (typically below 80°C continuous) and the torque ceiling is below 20 Nm, which removes them from any heavy-industrial specification from the outset [S6]. Sourcing risk sits mostly in custom bore adapters and motor-flange interfaces: standard IEC/NEMA motor frames are widely supported, but servo-motor-specific adapters must be confirmed at the order stage, particularly for low-inertia stepper-motor pairings such as the EPL-SA [S1].
Standards, Sourcing Logic and Cross-Reference
Planetary gearbox mechanical ratings in 2026 are typically published to internal OEM standards rather than to a single ISO gearbox-efficiency class; frame sizes, however, follow IEC 72-1 motor-flange conventions so that a planetary unit drops onto a standard servo or induction motor face without an adapter plate [S1][S3]. The Eisele EPL line states compliance-relevant features — lifetime lubrication, any-mount position, honed toothing, integral ratios — that map to typical continuous-duty industrial specifications rather than to a single ISO gear-quality grade [S1].
For buyers comparing catalogues, the most reliable cross-reference is the published nominal torque band, the maximum rpm, the available ratio list and the backlash arc-minute value, all of which appear on the four reference lines in this guide [S1][S2][S3][S4]. Two trackable signals to watch in the second half of 2026: Bonfiglioli's TQW AGV/AMR platform expanding with new BlueRoll sizes ahead of Intermat Paris (21-24 Apr 2027) [S3], and the precision-servo category pushing single-stage backlash below 0.05° as helical gear-grinding processes mature across the KPLF and RPC+ lines [S2]. For drivetrains where a planetary gearbox feeds a linear screw, the matched actuator pairing logic parallels the bearing-selection workflow discussed in deep groove ball bearing selection, where bore, series and seal choice dominate the spec before the lubricant question arises. Load and geometry trade-offs covered in tapered roller bearing selection follow the same logic a process engineer should apply to a planetary gearbox: torque, speed and life first, then configuration details.
For component-level specifications, see gearbox, planetary reducer, and linear guide.