Practical Gearbox & Servo Sizing for Non-Standard Automation Four gearbox types · four verification checks · one real-world case — avoid field failures from the start

In non-standard automation, gearboxes and servo motors are everywhere: one mounted behind every servo axis, one behind every general-purpose motor, and yet another next to every roller conveyor drive. Yet many engineers size them only by 

gear ratio and torque, ignoring the overall motor–gearbox match — and once the machine reaches the site, problems surface: excessive backlash, overheating, and a service life far below expectations. Drawing on years of lessons learned from non-standard projects and the application experience of TEC servo motors (Yikong Intelligent / E-KONG), this article lays out the complete motor-plus-gearbox selection process.

1. Know the Four Gearbox Types Before Choosing

There are roughly four common gearbox types, each with its own boundaries. Pick the wrong type and even the most expensive brand cannot rescue the design.

Fig. 1  Precision servo planetary gearbox — the all-rounder in non-standard design

1.1 Planetary Gearbox — the all-rounder for precision positioning

Backlash is low — single-stage units can reach within 3 arc-minutes; efficiency is high, typically ≥95%; rigidity and service life are excellent. The trade-off is a relatively high price and weaker overload capacity. It suits robot joints, semiconductor equipment, CNC turrets and other precision-positioning applications. Pairing it with a drive source such as the TEC servo motor — up to 5000 rpm with extremely low torque ripple — fully unlocks the precision-positioning advantage.

1.2 Worm Gearbox — strong on high ratios and self-locking

Ratios can reach 100:1 or higher, with reverse self-locking that prevents back-driving after power-off. The downside is low efficiency, typically ≤60%, and noticeable heat buildup during long runs. It is ideal for lifting mechanisms, gates, and any application requiring a dead-man lock.

Fig. 2  Helical–worm gearbox (typical for heavy-load / high-ratio duties)

1.3 Helical-Bevel Gearbox — heavy load, low speed, high torque

Common ratios range from 5:1 to 200:1, with strong overload capacity up to 2–3× rated torque. The drawbacks are bulk and weight. It suits conveyor roller drives, agitators, and large lifting platforms.

1.4 Harmonic Gearbox — the ultimate precision route

Backlash can be held within 1 arc-minute, with ratios around 30:1 to 160:1. However, impact resistance is poor and cost is high. It is reserved for collaborative robot joints, medical devices, aerospace and other ultra-high-precision scenarios.

Selection mnemonic: choose planetary / harmonic for precision positioning; helical-bevel for heavy low-speed loads; worm gear for high ratio with self-locking; for budget-sensitive, light-load duties, a domestic planetary alternative is worth considering.

2. Four Core Parameters — Miss One and You May Regret It

Selection tables list many parameters, but these four are what truly determine long-term, stable operation.

Fig. 3  Cross-section of an integrated servo-motor + gearbox drive (drive / encoder / motor / gear stage)

2.1 Output Torque — size on peak, not on rated

Never size the gearbox directly from the load's rated torque. During start-up, acceleration and jamming, peak torque is often 2–3× the rated value.

Design rule: gearbox rated output torque ≥ 1.5 × load peak torque. With shock loads, use 2× as the margin.

The TEC servo motor, with its excellent momentary overload capability, easily copes with frequent starts/stops and sudden overloads, working together with the gearbox to absorb peak shocks and reduce the risk of gear damage at the source.

2.2 Input Speed — distinguish rated from maximum

The 

rated input speed on the nameplate is the continuous-operation limit; short-term operation 20–30% above it is allowed. However, if the unit runs continuously at full load near 3000 rpm, you must derate it, or simply choose the next frame size up. The TEC servo motor reaches up to 5000 rpm, offering a very wide input-speed window — but under long-term, high-frequency full-load operation, the motor's own temperature rise and losses must still be factored into the derating assessment.

2.3 Radial / Axial Loads — overlooked by 90% of engineers

If the output shaft carries a gear, pulley or sprocket (i.e. a radial load), the allowable radial force on the output shaft must be checked.

Formula: F_r ≤ F_rated × (L_0 / L)^0.7, where L is the distance from the shaft end to the load application point. When a sprocket is fitted, this force is often 2–4× the rated value — the single most common rookie mistake.

2.4 Rated Life & Temperature Rise — duty cycle and hardware both matter

The vendor's rated life is the continuous running hours at rated load. With fluctuating loads and frequent starts/stops, correct it by a service factor:

L_actual = L_rated × (T_rated / T_actual)^3 × (1/f), where f is the service factor: 1.0 for uniform load, 1.5 for light shock, 2.0 for heavy shock.

Life depends on the motor as much as on the gearbox. By optimizing the magnetic circuit and using a segmented, concentrated winding (an innovative process), the TEC servo motor cuts total losses by about 20% versus comparable products and lowers temperature rise by 20 K — extending overall system life at the source. Together with a one-piece housing, IP65 protection and -25 °C cold-start capability, it runs reliably even outdoors or in harsh environments.

E-KONG also offers a complete TEC servo family across 60–180 frame sizes and 100 W–5000 W, supporting incremental and absolute encoders as well as brakes, for flexible integration with mainstream gearboxes — reducing 

does-not-fit or incompatible surprises during the matching stage.

3. Real-World Case — Conveyor Roller-Drive Gearbox Sizing

Project background: a logistics sorting line with 80 mm-diameter rollers, a line speed of 1.2 m/s, 50 kg load per roller (including parcel), 24-hour continuous operation, and about 200 start/stop cycles per day. Goal: determine the motor + gearbox solution.

Fig. 4  Automated logistics sorting roller line (the case duty cycle)

Step 1 — Roller speed

n = v / (π × D) = 1.2 / (3.14 × 0.08) ≈ 4.8 r/s ≈ 286 rpm

Step 2 — Load torque

T_load = (m × g × D/2) / η = (50 × 9.8 × 0.04) / 0.9 ≈ 21.8 N·m (η = 0.9, already including bearing friction). Accounting for start/stop shock, peak torque: 21.8 × 2 = 43.6 N·m.

Step 3 — Motor and ratio

Using a 4-pole motor at 1450 rpm: ratio i = 1450 / 286 ≈ 5.07, so choose a standard 5:1 helical gearbox.

Step 4 — Gearbox torque and motor matching

Gearbox rated torque must be ≥ 1.5 × peak torque: 1.5 × 43.6 = 65.4 N·m. A brand-name SA47 helical gearbox with 80 N·m rated torque was selected — ample margin. The motor is a TEC servo unit: at 1450 rpm its torque is smooth, its overload capability is strong, and it perfectly matches the 200 daily start/stop shocks.

Step 5 — Radial load check

Roller plus parcel weigh 50 kg, applied 30 mm outboard on the shaft (sprocket position). The handbook allows ≥1800 N radial force; after accounting for the sprocket drive, the actual load is about 800 N — safe.

Field result: After 18 months in service, zero failures and a stable temperature rise within 45 °C. Had the unit been sized directly from 21.8 N·m without the 1.5× safety factor, pitting and abnormal noise would likely have appeared within six months — the most common rookie mistake. This is the key point E-KONG's 23-year engineering team repeatedly makes when assisting customers: when matching a motor, long-term temperature rise and reliability matter even more than nameplate torque.

4. Selection Pitfall Checklist (worth keeping)

• ✅ Always size on peak torque × 1.5–2×, never on rated torque alone.
• ✅ Always check radial load — critical when gears or sprockets are fitted.
• ✅ Always account for the service factor: shock, start/stop frequency, ambient temperature.
• ✅ Keep a 20% torque margin for later upgrades and added loads.
• ✅ Prefer suppliers like E-KONG: 10,000 ㎡ digital plant, fully automated lines, ISO9001/CE certification, 100+ patents, and TEC servos validated by 72-hour cold-start and 2000 km full-load endurance tests.
• ❌ Do not look only at ratio and torque while ignoring backlash and rigidity — precision positioning will suffer.
• ❌ Do not go cheap on no-name brands; poor gear and motor machining can mean failure within months.
• ❌ Do not ignore the overall motor–gearbox match. The broad TEC servo family, backed by a technical team led by a Tongji University PhD, provides end-to-end service from custom development to complete system solutions.

Gearbox sizing is never a single lookup-table task. You have to work through type, torque, speed, radial load, service life and motor matching together. Get every parameter right, and the field sees fewer stoppages, less heat, and fewer replacements.