Specifying a helical gear reducer for a conveyor comes down to four inputs: motor RPM (1450 at 50 Hz, 1750 at 60 Hz), output RPM at the head pulley, conveyor archetype with hours-per-day duty cycle, and material shock-load character [S2].
Published 2026 OEM lines cover torque from 10 Nm up to over 10 kNm and power from 0.06 kW to 220 kW across single-, double-, and triple-stage builds, organised by shaft orientation into coaxial, parallel-shaft, and right-angle families [S1]. The selection lever is not catalog kilowatts; it is whether the unit meets AGMA 6034 service factor for the duty, fits the ratio bracket, matches the conveyor archetype, and passes thermal-plus-overhung-load verification.
Service Factor Set by AGMA 6034, Not by Rule of Thumb
AGMA 6034 service factor for a 24-hour shock-loaded conveyor is 2.0, not the 1.4 most catalog defaults list [S2]. The class definitions are: Class I, minimum SF 0.8 for steady load 8 to 10 h/day; Class II, minimum SF 1.4 for steady 24 h/day or moderate shock (up to 1.25x rated) at 8 to 10 h/day; Class III, minimum SF 2.0 for moderate shock 24 h/day or heavy shock (above 1.25x rated) at 8 to 10 h/day [S2].
Design HP equals Prime Mover HP times the applicable SF, so SF 2.0 doubles the design figure versus SF 1.0. Conveyors generate starting torque well above running torque during breakaway, and the SF multiplier is the catalog expression of that headroom; sizing to running torque at SF 1.0 is the dominant failure mode observed on field-replaced bearings [S2].
Stage Count and Ratio Envelope: A Hard Physical Bound
Per ISO 6336 / DIN 3990 cylindrical-gear geometry, a single-stage helical caps near i = 7:1 before center-distance and tooth-profile constraints force a second stage; two-stage reaches roughly 50:1, and three-stage extends past 200:1 [S2][S4]. A 20:1 or 29:1 ratio is two-stage; above 50:1 needs three [S2].
Each stage takes 2 to 5 percent off mechanical efficiency, so a three-stage helical at 94% per stage delivers roughly 83% overall [S2]. A 29:1 single-stage helical quote is physically incorrect; a 29:1 two-stage worm at 65% is correct on ratio but wrong on energy economics. Helical inline (R/MD) units cover 5:1 to 300:1 at above 95% per-stage efficiency, parallel-shaft (F/MP) units mount hollow on the drive-pulley shaft, and bevel-helical (K/MJ) units fold 90° for inclined or compact transfer stations [S2].
Match the Conveyor Archetype to a Gearbox Family
The archetype decides which family fits the layout before any duty rating is checked. The mapping is stable across vendors: straight horizontal belt fits inline helical (R/MD) at 0.12 to 160 kW, 30 to 150 RPM; shaft-mount on head pulley fits parallel-shaft helical (F/MP) at 0.12 to 200 kW, 20 to 100 RPM; inclined, curved, or right-angle layouts fit bevel-helical (K/MJ) at 0.12 to 200 kW, 5 to 60 RPM; heavy-duty mining and steel lines up to multi-MW at 5 to 30 RPM; packaging and light-duty falls to worm (NMRV) or helical-worm (S/MN) below 2.2 kW [S2].
Shaft-mounted helical reducers like Hansen HSM use a 25-degree pressure-angle gear set with keyless taper-grip bushing mounting and integrated backstop, covering 5:1 to 220:1 at 0.2 to 220 kW, which is the practical upper limit for monolithic helical boxes [S1]. The SMRA shaft-mount design installs via a hanging torque arm, eliminating separate mounting bases, couplings, and alignment procedures, and the housing is high-strength gray cast iron selected for vibration damping and dimensional stability [S5].
Torque Class Bands and Bearing Life on 24-Hour Duty
Published OEM torque bands cluster into three practical tiers. Compact / instrument-class reducers span 10 to 200 Nm and are typically single-stage, right-angle, flange- or hollow-shaft mounted; mid-range industrial units span 200 Nm to 5 kNm and cover the bulk of conveyor, mixer, and extruder applications; heavy-duty industrial units start above 5 kNm and run beyond 10 kNm [S1].
Bearing L10 life is the practical limit on continuous-duty service: published ratings sit at above 25,000 hours under design load [S4]. A typical break-in schedule calls for the first oil change at 100 to 150 hours to flush debris, then annual or condition-based intervals thereafter [S3].
Thermal and Sealing Constraints in Dusty or Washdown Plants
Conveyor plants run hot and dirty. Ambient temperatures from -10 °C to 40 °C with high humidity or dust accelerate gear wear, and IP54 to IP55 sealed housings with optional forced lubrication stabilise oil temperature on long-duty lines [S4].
Passive natural-convection cooling through the cast iron housing is adequate for most belt-conveyor ratings; the material's thermal conductivity transfers operational heat to surrounding air without forced-air fans or liquid cooling circuits [S5]. Double-lip skeleton seals integrate sealing and dust-proofing in one component, and heavy-duty double seals are specified for mining, quarry, and aggregate service where conventional seals fail prematurely from abrasive particle infiltration [S5].
Decision Grid: Inline, Parallel-Shaft, Bevel-Helical, and Shaft-Mount Compared
Across the four selection criteria that matter on a conveyor (efficiency per stage, ratio envelope, mounting method, thermal path), the four common architectures line up as follows [S1][S2][S5]:
Inline helical (R/MD): above 95% per stage, 5:1 to 300:1 ratio, foot- or flange-mounted, convection or forced lubrication; suited to horizontal belt conveyors with a separate bedplate.
Parallel-shaft helical (F/MP): 94% to 96% per stage, 5:1 to 220:1 ratio, hollow output bore mounts on drive-pulley shaft; the retrofit-friendly default because no foundation rebuild is needed.
Bevel-helical (K/MJ): 93% to 95% per stage, 5:1 to 200:1 ratio, right-angle housing; the only practical answer for inclined, curved, or compact transfer stations.
Shaft-mounted (SMRA, HSM): 94% to 95% per stage, 5:1 to 220:1 ratio, hanging torque arm on the conveyor drum shaft with no separate coupling or alignment; minimal component count and shortest install time.
What Not to Use a Helical Reducer For
Helical gear reducers are wrong for two specific cases. A packaging or light-duty line below 2.2 kW with a budget-driven spec is more economically served by a worm (NMRV) or helical-worm (S/MN) unit, accepting the lower single-stage efficiency in exchange for the lower first cost [S2].
Single-stage helical units are also wrong for ratios above about 7:1, where the geometry becomes physically unrealistic; forcing the ratio into a single stage produces either noise, vibration, or bearing-life loss, and a 29:1 single-stage helical quote is a flag to recheck the spec [S2]. For higher ratios in the same envelope, designers step to helical-bevel or move into planetary helical units, with published planetary-helical lines such as GEARKO TB covering 50 Nm to 5 kNm at 2,000 to 10,000 RPM [S1].
Verification, Standards, and Sourcing
Conveyor gearboxes from the major Chinese OEMs are built to GB/T 3480, DIN 3990, and ISO 6336 geometry rules, and each model is load-, heat-rise-, and shake-tested before shipment, with carburized and ground gears reaching up to 98.5% mechanical efficiency at DIN 6 gear accuracy and under 85 dB(A) [S4].
The two verification steps that catch most field failures are thermal rating and overhung load on the output shaft. A unit sized correctly on torque and ratio can still cook itself on a 24-hour dusty duty if the thermal rating is not checked against ambient and enclosure, and the output bearing can still fail early if the overhung load from the pulley and belt tension is not verified against the catalog figure. The work to cement plants and steel mills follows a similar shape, with the spec gate shifting toward thermal and dust-load verification, as laid out in the cement-plant helical reducer spec map and the steel-mill F, MJ, K series spec map.
Trackable signals for the next design cycle: confirm whether your conveyor service factor has been written into the spec as AGMA 6034 Class II or III, not the catalog default 1.4; verify that the ratio bracket matches stage count physically, with two-stage for 20:1 to 50:1 and three-stage for above 50:1; and require the OEM thermal and overhung-load curves on the same datasheet as the torque rating before purchase order release.
Spec-level background on the components involved: material handling, and storage handling.