A conveyor gearbox selection lives or dies on two numbers: the steady-state torque the drive shaft must deliver and the belt speed the application requires. Get both right, with a 1.5–2.0 service factor on torque and a ratio sized to a 4-pole 1750 RPM motor baseline, and the conveyor runs cool; get either wrong and you trip on start, glaze bearings, or stall the line [S2].
Belt conveyors in bulk handling typically run 0.5–4.0 m/s at the belt, mobile conveyors and stacker/reclaimers use compact gearboxes between motor and head pulley, and screw conveyors demand high starting torque to break away from packed material. The right unit is the one whose rated torque, ratio range, mounting, and sealing match all four numbers at once, not just one of them [S1][S5].
Calculate Drive-Shaft Torque Before You Pick a Family
Belt pull equals the sum of product load, belt dead weight, friction, and any incline component. A 10° incline on a 500 lb product load adds roughly 87 lb of gravitational pull (500 × sin 10°); on a 6-inch-diameter drive pulley (3 in radius) that becomes 261 lb-in of additional torque the gearbox must supply on every start [S2].
Convert total belt pull to torque with T (lb-in) = Force (lb) × drive radius (in). A 200 lb steady-state belt pull on a 6 in diameter pulley yields 600 lb-in at the drive shaft. Apply a 1.5 service factor for normal duty or 2.0 for heavy or shock-loaded service and the gearbox rating rises to 900–1200 lb-in, which is the minimum mechanical rating to specify [S2]. Conveyors that must accelerate a full load from zero to running speed in 1–2 s commonly trip drives sized on steady-state only; the start transient is where most undersized selections show up first [S2][S3].
Horsepower ties torque and speed together: HP = (Torque × RPM) / 63,025. A drive shaft turning 50 RPM at 900 lb-in needs about 0.71 HP, so the next standard motor size is 1 HP. The reference sizing example in [S3] uses a 12 in/s top belt speed, a 12 in drive pulley (r = 6 in), 100 lb total load, friction coefficient 0.1, and a 1 s acceleration ramp, with a 2.0 safety factor entered into the motor sizing tool. The Bodine conveyor example in [S4] uses a 200 lb load, 12 in/s belt speed, 4 in rollers, 1 s acceleration, and arrives at a required motor speed of 57.3 RPM (N = 9.55 × V / r) with a 199 oz-in acceleration torque component.
Match Gearbox Family to Ratio, Duty, and Mounting
Four families dominate conveyor duty: worm, helical (parallel-shaft), planetary, and bevel (including helical-bevel right-angle). Worm units are compact and self-locking, useful where back-driving must be prevented, but efficiency drops as ratio rises. Planetary units pack the highest torque density per kilogram and are common on mobile conveyors and stacker/reclaimers where head-pulley space is limited; they also handle shock loads well because load is shared across 3 planet gears [S1]. Bevel and helical-bevel right-angle units are specified when the drive shaft must turn a corner, for example on transfer stations or reversible conveyors [S1][S5].
Gear ratio is the lever between motor speed and belt speed. A standard 4-pole AC induction motor at 1750 RPM paired with a 35:1 gearbox produces about 50 RPM at the output shaft, which on a 6 in drive pulley gives roughly 235 ft/min belt speed. The higher the ratio, the lower the output speed and the higher the output torque for a given motor power, with the inverse also true [S6]. Helical reducer ratio ranges of 1.26–33,000 are published for general industrial units, covering everything from low-ratio drum drives to very high-ratio mill feeders [S5].
Belt conveyors want consistent low-speed torque to prevent slippage; screw conveyors want high starting torque to break away from compacted material; bucket conveyors face irregular shock loads and benefit from strong radial load ratings on the output bearing [S5]. The duty description drives the family choice before any catalog number is opened.
Service Factor, Ambient, and Sealing: The Numbers That Get Ignored
Service factor is the single most-misunderstood number on a conveyor gearbox nameplate. For uniform load and under 10 hours/day operation a 1.0 factor is defensible, but most conveyors in mining, aggregate, and cement run 16–24 hours/day with shock peaks, so a 1.4–1.6 service factor is the working minimum, and shock-loaded or reversing applications should be specified at 1.8–2.0 [S5][S2]. Skipping the service factor is the textbook path to stripped teeth within the first year.
Ambient conditions change the lubricant and seal choices before they change the gearbox family. Bulk-handling conveyors typically see –10 °C to +40 °C ambient, often with dust, wash-down, or corrosive vapor, so sealed housings rated to IP54–IP55 with forced lubrication on continuous-duty units are common specifications; sealing class and oil specification should be written into the purchase order, not assumed [S5]. For high-horsepower or 24-hour conveyor drives, nameplate thermal rating must be checked against the installed ambient, because a gearbox rated for 30 kW at 25 °C may derate to roughly 25 kW at 40 °C with the same oil.
Efficiency per stage is the other quiet number. Helical stages commonly publish 96–98% mechanical efficiency, precision-ground helical-and-bevel sets can reach 98.5% per stage verified by factory load test, and a DIN 6 gear accuracy class is the typical published grinding tolerance that keeps vibration and noise below 85 dB at 1 m. Bearing L10 life of 25,000 hours is a common published benchmark for industrial conveyor gearboxes in continuous duty [S5].
Variable-Speed Conveyors: Why VFDs Change the Sizing Math
Adding a VFD to a 4-pole induction motor gives adjustable speed plus soft start, which lets a smaller gearbox survive start transients that would otherwise require a service-factor bump. Three-phase AC induction motors with VFDs are now the standard pairing for variable-speed manufacturing conveyors; servo motors are reserved for indexing or precision positioning where stop accuracy, often quoted at 0.25 in or tighter, matters more than torque density [S2][S3].
Variable-speed sizing has to evaluate both ends of the speed range. A belt that runs 12–24 in/s with a 1 s acceleration ramp needs a motor whose continuous torque covers the high-speed case and whose peak torque covers the low-speed, high-torque case; the same motor must also dissipate heat at the low-speed, high-torque point where fan cooling is weakest. That is why the sizing tool workflow in [S3] captures V1, V2, and acceleration time t1 as separate inputs rather than as a single average.
For belt conveyors, this matters less because the duty is mostly steady-state, but the same motor + VFD + gearbox combination frequently feeds multiple downstream machines, so the belt-conveyor drive is often the right place to standardize on a variable speed drive platform across a plant.
Comparison: Four Gearbox Families on a Belt Conveyor
Stacked against a typical 22 kW, 80 RPM drum drive (about 2625 Nm output, derived from T = 9550 × P / n2 where P = 22 kW and n2 = 80 RPM), the four families line up as follows [S1][S5]:
Worm: ratio range typically 5:1 to 100:1, efficiency 30–90% depending on ratio and lead angle, compact foot or flange mount, self-locking, best for low-power, intermittent, or hold-against-back-drive duty. Not the right call for a 22 kW continuous head-pulley drive because efficiency loss becomes heat that has to be rejected [S1].
The right call for the 22 kW drum drive when mounting and alignment are flexible [S1][S5].
The right call when mass and envelope dominate the spec [S1].
Helical-bevel (right-angle): ratio range roughly 5:1 to 400:1, efficiency 95–97%, solves 90° drive-shaft direction changes, used on transfer stations and reversible conveyors. The right call when the layout forces a right-angle drive [S1][S5].
Where Selection Goes Wrong
The four most common failure modes in conveyor gearbox selection are well documented. First, sizing on steady-state torque and ignoring the start transient; the conveyor trips on the first loaded start [S2]. Second, omitting the service factor; the gearbox survives a few months, then the teeth spall under combined shock and continuous duty [S2][S5]. Third, picking the family before the ratio; a worm unit on a high-ratio, high-continuous-duty conveyor runs hot and wastes 20–40% of input power as heat [S1][S5]. Fourth, ignoring thermal derate at the installed ambient; a gearbox that tests fine on the factory floor at 25 °C runs above its oil-temperature limit at 40 °C site ambient and fails the seal or the bearings first [S5].
Roller-chain conveyors sidestep some of these failure modes but introduce their own pitch-and-width sizing problem, which is covered separately in the #40 vs #60 vs #80 roller chain pitch reference when the conveyor is chain-driven rather than belt-driven.
Quick Sizing Workflow for a New Belt Conveyor
Step 1: total belt pull F = product load + belt weight + friction component + incline component. Step 2: drive-shaft torque T = F × drive-pulley radius, in consistent units. Step 3: apply service factor 1.5 normal, 2.0 heavy/shock. Step 4: target output speed from required belt speed and drive-pulley diameter, then ratio = 1750 / target RPM. Step 5: select gearbox family by mounting, efficiency, ambient, and duty. Step 6: verify thermal rating at installed ambient. Step 7: confirm output bearing radial load rating against belt pull and pulley weight [S2][S3][S5].
The output stage of this workflow feeds the same gearbox catalog that mobile and stationary conveyors share; for plants standardizing on a single supplier shortlist, the family-level selection often drives vendor qualification long before a specific ratio is chosen. In process plants where the conveyor sits inside a broader belt-conveyor system, the gearbox selection also has to be coordinated with the upstream belt conveyor design and the downstream take-up arrangement, otherwise the service factor is consumed by belt stretch rather than by the load it was meant to cover [S1][S2].