How to select reduction gearbox for electric motor?

How to Select a Reduction Gearbox for Electric Motor

What “Selecting a Gearbox for a Motor” Means

Most gearbox buying guides start from the load: “I have a conveyor that needs 60 rpm and 120 N·m, what reducer do I buy?” This article takes the opposite, equally common, starting point — you already chose the electric motor, and now you need a reduction gearbox that bolts onto it and delivers the speed and torque your machine needs. That single fact changes the whole workflow.

When the motor is fixed, three constraints dominate the choice:

• The mechanical interface is fixed. The motor flange pattern (B5 or B14), bolt circle, shaft diameter and shaft type decide which gearboxes can even be mounted. This is the first gate, decided before any torque math.
• The motor’s rated torque and speed cap the output. No matter how large the gearbox, the maximum usable output torque is the motor torque multiplied by the ratio and the gearbox efficiency. The gearbox cannot create torque the motor does not have.
• The motor type sets the matching rules. A 50 Hz three-phase induction motor, a brushless DC motor, a servo and a brushed DC motor each impose different inertia, commutation and control requirements on the gearbox.

This is distinct from our speed-reducer motor selection guide, which treats the geared motor as a single purchased assembly chosen from the load. Here the motor is given; we match the gearbox to it. For the broader types of speed reducers and the gearbox overview (pillar page), see those references.

How Motor–Gearbox Matching Works

A reduction gearbox is a speed-and-torque transformer. It trades rotational speed for torque while absorbing a small efficiency loss. Matching one to a motor is a three-step physical check.

The mechanical interface (flange, shaft, coupling)

The gearbox input must accept the motor’s shaft. In practice this means:

• Flange pattern. IEC motors use B5 (flange with threaded holes, no feet) or B14 (smaller flange with a spigot). The gearbox input flange must match the same pattern and bolt circle. Dimensions follow IEC 60034-1 and the IEC 60072 mounting series. See our note on motor flange dimensions.
• Shaft diameter and keyway. The gearbox input bore is sized for a shaft range (e.g. 14–19 mm). If the motor shaft is 19 mm, the gearbox must accept 19 mm directly or via the supplied collar/spigot. A mismatch here is a hard stop.
• Output side. The output shaft, hollow shaft or flange must fit the machine — and must carry the overhung (radial) and axial loads the machine applies (see the overhung-load rule in the geared-motor guide).

Torque and speed transformation

The two governing equations are simple:

• Reduction ratio: i = n₀ / n_out (motor speed ÷ required output speed)
• Output torque (theoretical): T_out = T₀ × i × η (motor rated torque × ratio × gearbox efficiency)
• Output power: P_out = (T_out × n_out) / 9.55 (with T in N·m, n in rpm, P in W)

The gearbox rated output torque must be at least the load torque multiplied by a service factor. But the output you can ever reach is still bounded by T₀ × i × η. If the motor cannot supply enough torque after transformation, a bigger gearbox frame will not help — you must change the motor or the ratio.

Inertia reflection (servo and high-dynamic drives)

For servo and stepper systems the gearbox also transforms inertia. The load inertia seen by the motor is reduced by the square of the ratio:

• Reflected load inertia: J_reflected = J_load / i²
• Inertia ratio: J_ratio = J_reflected / J_motor_rotor

Keep J_ratio at 5:1 or below (3:1 for precision positioning) so the servo stays tunable. This is why a servo matched with the wrong ratio either hunts or needs a far larger motor; the precision planetary gearbox page covers the servo case in depth.

Gearbox Type Comparison for a Given Motor

Once the interface fits and the torque is known, pick the gearbox type by duty. The table below maps motor + application to the right reduction principle.

Right-angle vs. in-line. If the motor shaft must turn the load 90°, choose worm, bevel or right-angle planetary. If space permits an in-line train, helical or planetary give the best efficiency and the lowest heat.

Engineering Data You Need From the Motor

Before sizing, pull these from the motor nameplate and datasheet. They are the inputs to every formula below.

Torque and power formulas

Efficiency by gearbox type and ratio

Efficiency falls as the ratio climbs because more stages are needed. A single-stage helical at 5:1 is ~97%; the same principle at 60:1 needs two stages and drops to ~92%. Worm units fall hardest — a 60:1 worm may be only 55% efficient, which is why they run hot under continuous duty. For a broader efficiency discussion tied to motor efficiency classes, see that guide. The efficiency you assume directly changes the torque the motor must supply and the heat the gearbox must shed.

Service factor table

These bands follow the AGMA duty classification (uniform / moderate / heavy shock) and match the ranges used by drive engineers; the gearbox catalogue then lists the rated torque for a given SF. Always multiply before comparing to the rated torque, never after.

Thermal and insulation limits

The motor winding insulation class (per IEC 60034-1) sets how hot the motor can run: Class B = 130 °C, F = 155 °C, H = 180 °C. The gearbox has its own thermal ceiling from lubricant and bearing limits — continuous S1 duty at high ratio can push oil above 80 °C and accelerate breakdown. Size for the thermal rating under the real duty cycle, not just the peak mechanical torque.

Best Applications and Typical Matches

Step-by-Step Selection — Worked Example

Given motor: a 1.5 kW 4-pole 3-phase induction motor, 1400 rpm, IEC 90 frame, B14 flange, 24 mm shaft. Machine: a packed-belt conveyor needing 60 rpm output and 90 N·m load, running 16 h/day with moderate impact.

Step 1 — Motor rated torque

T₀ = 9550 × 1.5 / 1400 = 10.23 N·m

Step 2 — Reduction ratio

i = 1400 / 60 = 23.3 → pick the nearest standard ratio, 24:1. Actual output speed = 1400 / 24 = 58.3 rpm (close enough to 60 rpm).

Step 3 — Required gearbox rating (service factor)

Moderate shock, 16 h/day → SF = 1.5. Required rated torque: 90 × 1.5 = 135 N·m. The gearbox must be rated at ≥ 135 N·m.

Step 4 — Check the motor can actually drive it

For a helical unit, η ≈ 0.95. Theoretical output: T_out = 10.23 × 24 × 0.95 = 233 N·m. This comfortably exceeds the 135 N·m requirement — the motor has headroom, and a size-90 helical gearbox rated ~200 N·m is the right frame.

Step 5 — Mechanical interface

IEC 90 / B14 flange with a 24 mm shaft. Select a gearbox whose input flange matches B14 and whose input bore covers 24 mm. A size-110 gearbox (140 mm input flange) would not bolt directly to an IEC 90 motor — an adapter would be required, which is why frame size tracks motor frame size.

Step 6 — Overhung load

The conveyor sprocket applies a radial load at the output shaft. Confirm P_radial ≤ the gearbox catalogue overhung-load rating at 58 rpm. If exceeded, move the support bearing outward or pick the next frame size.

Step 7 — Thermal / duty check

Continuous S1 at 16 h/day: verify the gearbox continuous-rated torque (not peak) ≥ 135 N·m and that oil temperature stays within limits. Output power P_out = (135 × 58.3) / 9550 = 0.82 kW, well within the 1.5 kW motor.

Common Engineering Mistakes

• Sizing torque but ignoring the flange. Engineers solve the torque math, then discover the chosen frame will not bolt to the motor. Check the B5/B14 pattern and shaft diameter first.
• Assuming a bigger gearbox gives more torque. Output is capped by T₀ × i × η. Oversizing the frame beyond the motor’s capability only adds inertia and cost.
• Using the rated torque instead of peak for servo. Acceleration and shock loads draw peak motor torque; size the gearbox against peak (with SF), or it will pit and spall in weeks.
• Forgetting inertia matching. On servo systems a poor ratio makes J_ratio exceed 5:1, causing hunting and long settling. Use J_load / i² to pick the ratio, not just torque.
• Confusing mechanical and thermal ratings. A worm unit that passes the torque check can still overheat in continuous duty because 40–50% of input power becomes heat. Verify the thermal rating.
• Skipping the overhung load. A sprocket or pinion on the output stub applies a radial force; exceeding the catalogue limit destroys the output bearing. Compute it before ordering.

Troubleshooting Table

FAQ

Why Choose Greensky for Matched Motor–Gearbox Packages

Greensky Power designs and manufactures custom electric motors and complete OEM actuator assemblies, so we treat the motor and the reduction gearbox as one matched system rather than two parts bought separately. For your program we supply:

• Standard and custom 24 V BLDC, BLDC, brushed DC and servo motors with IEC B5/B14 flanges and verified shaft dimensions.
• Helical, planetary, worm, cycloidal and micro gearboxes with matched input flanges and documented rated-torque, efficiency and backlash data.
• Pre-validated motor–gearbox pairs with the ratio, service factor and inertia checked before shipment — removing the flange-mismatch and thermal surprises described above.
• Engineering support for industrial motor selection and torque calculation.

Related Technical Resources

• Gearbox overview (pillar page)
• Speed-reducer motor selection guide
• Different types of speed reducers
• Precision planetary gearbox deep dive
• Worm gearbox selection guide
• Cycloidal reducer guide
• Harmonic drive vs. planetary gear
• Micro gearboxes explained
• Brushless DC motor (BLDC) basics
• 24 V brushless DC motor parameters
• Servo motor fundamentals
• Brushed DC motor overview
• BLDC vs. servo motor comparison
• What is the use of a DC geared motor?
• DC motor for rehabilitation robots
• Motor flange dimensions
• How to calculate motor torque
• Motor efficiency classes
• Selecting a motor for industrial use
• Motor controller guide
• Direct-drive vs. gear-motor
• DC motors category
• Custom electric motors
• OEM motor program

References and Standards

1. IEC 60034-1:2022 — Rotating electrical machines, general requirements (insulation & temperature classes, mounting). webstore.iec.ch/en/publication/65446
2. IEC 60034-30-1 — Efficiency classes of line-operated AC motors. webstore.iec.ch/publication/91195
3. ANSI/NEMA MG 1-2021 — Motors and Generators (definitions, service conditions, ratings). webstore.ansi.org/standards/nema/ansinemamg2021
4. NEMA — Motor and generator product guidance. nema.org/products/pages/motor-and-generator.aspx
5. U.S. DOE — Motor systems efficiency and load guidance. energy.gov — 10097517.pdf
6. IEA — Electric motors and industrial efficiency. iea.org/energy-system/industry/electric-motors
7. AGMA — Gear rating and service-factor methodology (uniform / moderate / heavy shock). agma.org
8. SKF — Bearing failures and their causes (overhung load, misalignment). skf.com — bearing failures
9. Siemens — SIMOTICS electric motors product range. siemens.com — electric motors
10. IEEE Xplore — Peer-reviewed motor / actuator design paper. ieeexplore.ieee.org/document/6342334
11. maxon — EC technology and gearhead selection. maxongroup.com — ec-technology
12. FAULHABER — Brushless DC motors and micro-drive know-how. faulhaber.com — brushless DC motors
13. Yaskawa — Motion and servo system technical downloads. yaskawa.com/downloads/search-index
14. Tsubaki — Gearmotor selection technical reference (service-factor method). tsubakimoto.co.jp — selection reference

This article was prepared by the Greensky Power engineering team as a practical selection reference. Standards and manufacturer documents are linked for verification; always confirm the latest edition and the motor/gearbox nameplate before finalizing a design.