Why Servo Motors Need a Planetary Gearbox
A servo motor excels at speed control and positioning accuracy, but it produces relatively low torque at high speed. Most servo-driven loads — conveyor rollers, robot joints, ball screws, rotary tables — require higher torque at lower speed than the motor delivers on its own. A planetary gearbox bridges this gap by trading speed for torque at efficiencies above 96%, while keeping the motor and load on the same axis in a compact package.
Beyond torque multiplication, the planetary reducer also reduces the reflected load inertia seen by the servo amplifier. This improves the dynamic response of the entire motion system: the motor accelerates and decelerates faster, settling time shortens, and the servo loop remains stable under varying loads. The inertia reduction follows the square of the gear ratio — a 10:1 reducer cuts reflected inertia by a factor of 100, which is why selecting the right ratio is critical for achieving responsive servo control.
Selecting the wrong planetary gearbox — whether undersized, oversized, or mismatched on backlash — degrades all of these benefits. An undersized unit fails prematurely under thermal and mechanical stress. An oversized unit wastes budget, adds unnecessary mass, and may create installation clearance problems. A backlash grade tighter than necessary inflates procurement cost by 3–5 times without improving system performance. The six steps below take you from your servo motor’s nameplate data to a confirmed specification, with formulas, worked examples, and the most common pitfalls to avoid at each stage.
Step 1 — Calculate the Required Gear Ratio
The gear ratio determines how much the planetary gearbox reduces the motor speed (and proportionally increases torque). It is the first parameter to calculate because it drives every subsequent selection decision.
Motor rated speed: 3,000 rpm. Required output speed: 150 rpm.
i = 3,000 ÷ 150 = 20:1
Since 20:1 is not available as a single-stage ratio (max ~10:1), a two-stage unit with a 20:1 ratio is required. Check your target manufacturer’s catalogue for the exact available ratio — Korea Ever-Power offers 20:1 as a standard two-stage option across all frame sizes.
Step 2 — Calculate Required Output Torque
The output torque determines the frame size of the planetary gearbox. You need to calculate it from the motor’s rated torque and the gear ratio, then verify it falls within the gearbox’s rated capacity.
Motor: 750 W, 3,000 rpm → Tmotor = (0.75 × 9,550) ÷ 3,000 = 2.39 N.m
Gear ratio: 20:1. Efficiency (two-stage): 0.94
Toutput = 2.39 × 20 × 0.94 = 44.9 N.m
This output must be less than the rated torque of the selected gearbox frame. For an EP-PL80 at 20:1, the rated output torque is 110 N.m — well within the safe operating margin.
Step 3 — Select Frame Size by Motor Power
Planetary gearbox manufacturers designate frame sizes by the output flange diameter in millimetres (e.g., Frame 60, 80, 90, 120). Each frame accepts a specific range of motor shaft diameters and motor power levels. The table below shows the Korea Ever-Power standard mapping:
| Frame Size | Motor Power Range | Motor Shaft (mm) | Rated Torque (N.m) | Typical Motor Brands |
|---|---|---|---|---|
| 60 / 70 | 50–400 W | 6–19 | 13–60 | Panasonic A6B, Delta ECMA-C |
| 80 / 90 | 200–1,000 W | 8–24 | 27–150 | Mitsubishi HG-KR, Yaskawa SGM7J |
| 120 | 400–2,000 W | 14–35 | 63–280 | Siemens 1FK7, Mitsubishi HG-SR |
| 145 / 160 | 750–4,500 W | 19–42 | 170–720 | Yaskawa SGM7G, Siemens 1FL6 |
Frame sizes vary by series. EP-PL/PF: 60, 80, 120, 160. EP-PBL/PBF: 70, 90, 120. EP-HAB: 60, 90, 115, 145. Input coupling: OP1 (integral) for standard series, OP2 (split-type) for precision series.
The frame size must satisfy both constraints: the motor shaft must fit within the accepted diameter range, and the calculated output torque from Step 2 must fall below the frame’s rated torque. If the motor shaft fits but the torque exceeds the rating, move up one frame size. For our 750 W example, Frame 80 or Frame 90 is the correct match.
Step 4 — Choose the Right Backlash Grade
Backlash is the angular dead zone at the output shaft when the input reverses direction. It is measured in arcminutes (arcmin), where 1 arcmin = 1/60 of a degree. Lower backlash means higher positional precision — but also higher cost. Specifying tighter backlash than your application actually needs wastes budget without improving performance.
Robot J1/J6 joints
Optical positioning
Laser cutting
AOI inspection
Packaging
General automation
Winding machines
Non-positioning drives
Korea Ever-Power maps backlash grades to specific product series: the EP-PL/PF standard planetary gearbox delivers ≤8 arcmin at the most competitive price point. The EP-PBL/PBF high precision series reaches ≤5 arcmin with IP65 sealing. The flagship EP-HAB achieves ≤3 arcmin with the highest torsional stiffness in the catalogue.
Step 5 — Decide Between Inline and Right-Angle Output
The output shaft direction determines whether you specify an inline planetary gearbox (motor and output on the same axis) or a right-angle unit (motor perpendicular to the output). This decision is driven almost entirely by the available installation space behind the output face.
An inline planetary gearbox with a 750 W servo motor extends approximately 280–320 mm behind the output face. A right-angle unit at the same power redirects the motor sideways, reducing the depth behind the output to approximately 140–170 mm — a 40–50% reduction. However, the right-angle configuration adds a spiral-bevel input stage that increases backlash by approximately 3 arcmin and reduces efficiency by about 2% per stage compared to the equivalent inline unit.
The decision rule is straightforward: choose inline whenever physical space permits, because it offers the best efficiency, tightest backlash, and lowest cost. Switch to right-angle only when the available depth behind the driven axis is physically insufficient for the inline motor-gearbox assembly length, or when the motor must be oriented perpendicular to the load axis for clearance, cabling, or maintenance access reasons.
Motor and output shaft on the same axis. Highest efficiency (≥96%), tightest backlash (≤3 arcmin available), and lightest weight. Choose inline whenever the installation has sufficient axial depth behind the output face to accommodate the motor-gearbox assembly length. Series: EP-PL/PF, EP-PBL/PBF, EP-HAB.
Motor perpendicular to the output shaft. Reduces machine depth by 40–50%. Specify when axial space is constrained (AGV wheel hubs, wall-mounted conveyors, panel actuators) or when the motor must clear an adjacent structure. Efficiency is ~2% lower than inline due to the bevel input stage. Series: EP-WPL/WPF, EP-WPBL/WPBF.
Step 6 — Verify Inertia Match
Inertia matching determines how well the servo motor can control the load through the planetary gearbox. The reflected load inertia at the motor shaft should ideally be within 1:1 to 10:1 of the motor rotor inertia (Jload/Jmotor). A well-matched system responds crisply; a poorly matched system oscillates, overshoots, or fails to settle within the required time.
Load inertia: 0.005 kg·m². Motor rotor inertia: 0.0001 kg·m². Gear ratio: 10:1.
Jreflected = 0.005 ÷ 10² = 0.00005 kg·m²
Ratio: Jreflected / Jmotor = 0.00005 / 0.0001 = 0.5:1
This is within the ideal 1:1–10:1 range. The servo system will respond well.
If the inertia ratio exceeds 10:1 even after accounting for the gear ratio, consider increasing the gear ratio (which reduces reflected inertia by i²) or stepping up to a larger motor with higher rotor inertia. The planetary gearbox itself adds a small amount of inertia (the planet carrier and gears), but this is typically negligible compared to the load and motor inertia.
Selection Summary — One Table, All Six Steps
The table below consolidates all six selection steps into a single reference. For each step, confirm that your calculated value falls within the acceptable range before proceeding to the next step.
| Step | Parameter | Formula / Method | Check |
|---|---|---|---|
| 1 | Gear ratio | i = Nmotor ÷ Noutput | Match to nearest catalogue ratio |
| 2 | Output torque | Tout = Tmotor × i × η | Must be < gearbox rated torque |
| 3 | Frame size | Motor power → frame table | Shaft & torque both within range |
| 4 | Backlash grade | Application mapping table | Don’t over-specify |
| 5 | Output direction | Measure installation depth | Inline preferred if space allows |
| 6 | Inertia match | Jload/i² vs Jmotor | Ratio 1:1 to 10:1 |
5 Common Selection Mistakes to Avoid
The planetary gearbox must handle continuous duty torque, not brief acceleration peaks. Size on rated motor torque × ratio × efficiency. Check that motor peak torque × ratio does not exceed the gearbox emergency stop torque.
A ≤3 arcmin unit costs 3–5× more than ≤8 arcmin. If the application is unidirectional or tolerates 0.5 mm positional uncertainty at the working radius, ≤8 arcmin is sufficient. Over-specifying backlash is the most common source of unnecessary cost in servo gearbox procurement.
The output shaft bearings have permissible radial and axial load ratings. If the load applies significant side force (belt tension, cantilever weight), verify the load falls within the gearbox specification. Exceeding the rating shortens bearing life dramatically.
Each frame size accepts a specific shaft diameter range. A 750 W motor from Brand A may have a 14 mm shaft while the same power motor from Brand B has a 19 mm shaft. Always confirm the exact motor shaft dimension against the gearbox input coupling specification.
Each additional stage adds length, weight, cost, and approximately 2% efficiency loss. A two-stage unit covers ratios up to 100:1. Only specify three stages if you genuinely need ratios above 100:1 (e.g., 160:1, 200:1, 512:1) and can accept the cumulative backlash increase.
Frequently Asked Questions
Tell us your servo motor model, required output speed, and torque. Our application engineers will return a confirmed model number, dimensional drawing, and quotation within one business day — no obligation.
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