How to Select a Planetary Gearbox for Conveyor Head Drive Applications?

Conveyor Drive Sizing — The 3-Step Methodology

Step 1: Calculate the running torque at the head drum. Running torque (Nm) = (total effective belt tension in N × head drum radius in metres). For a 150 kW conveyor drive with a 600 mm diameter drum at 1.5 m/s belt speed: P = T × ω; ω = belt speed / drum radius = 1.5 / 0.3 = 5 rad/s; T = 150,000 / 5 = 30,000 Nm. This is the running torque at the drum.

Step 2: Apply the service factor. Conveyor duty service factors range from 1.25 (light aggregate, moderate starting) to 2.0 (heavy material, frequent starts, reversing). For a standard aggregate conveyor with 8 starts per hour: service factor 1.5. Design torque = 30,000 × 1.5 = 45,000 Nm. This selects the EP311L frame (40,000 Nm rated) as the minimum — or EP313L (55,000 Nm) with a safety margin.

Step 3: Confirm the gear ratio from motor speed to drum speed. Motor: 1,450 RPM (4-pole, 50 Hz). Target drum speed: 5 rad/s = 47.7 RPM. Required ratio: 1,450 / 47.7 = 30.4:1. The EP311L3 (3-stage inline) covers this ratio range within the 311 frame.

Frame Selection Reference — EP300L for Conveyor Applications

Why Inline Planetary Outperforms Helical-Bevel for Conveyor Drives

The inline planetary configuration is preferred over the helical-bevel alternative for conveyor head drives for two measurable reasons. First, the inline (coaxial) output aligns directly with the drum shaft — eliminating the offset and overhang loads that a helical-bevel gearbox places on the drum shaft bearing when the gearbox output shaft is not on the drum centreline. This overhang load is a primary cause of drum shaft bearing failure on helical-bevel-driven conveyors. Second, the planetary architecture delivers 96–98% transmission efficiency per stage versus 85–92% for the equivalent helical-bevel unit — on a 150 kW conveyor running 6,000 hours per year, the energy saving exceeds 5,000 kWh annually.

Browse the full inline planetary gearbox series covering 1,000–500,000 Nm, or see the 311 series planetary gearbox for 35,000 Nm conveyor drive applications with inline and right angle configuration options.

Service Factor Explained — Why the Rated Torque Is Never the Design Torque

The service factor for a conveyor application is the multiplier applied to the calculated running torque to account for the torque peaks that occur throughout normal operation but are not captured in the steady-state torque calculation. The service factor is not a safety margin — it is an acknowledgment that the running torque is only the average torque, and the actual torque the gearbox experiences varies considerably around that average in ways that determine how quickly gear surface fatigue accumulates.

The main sources of torque variation on a conveyor head drive gearbox are:

• Starting torque under full load: A fully loaded conveyor belt at rest has significant static friction. The starting torque is typically 2.0–2.5× the running torque for a standard electric motor start. Conveyors using direct-on-line (DOL) motor starters experience the full starting torque spike on every start cycle. Conveyors with variable frequency drives (VFD) or fluid couplings reduce the starting torque multiplier to approximately 1.3–1.5×, which justifies a lower service factor and a smaller gearbox frame when VFD control is specified at the design stage.
• Blocked chute or jammed material: If the feed chute onto the belt blocks and material builds up, the belt load increases progressively until the motor stall torque is reached. The stall torque for a standard induction motor is 2.5–3.5× the rated torque. Whether the gearbox experiences this load depends on whether the conveyor drive has a slip coupling (shear pin or fluid coupling) between the motor and gearbox. Without a slip coupling, the gearbox experiences the full motor stall torque during a jam event.
• Harmonics from belt splice: Belt splices create a periodic variation in belt tension as the splice passes over the head drum. This produces a cyclic torque variation at the gearbox output shaft that has a period equal to the belt length divided by the belt speed. The amplitude of this variation depends on the quality of the splice — poorly made splices create torque variations of 15–25% of running torque on every belt revolution, which accumulates fatigue faster than a smooth belt would predict.

Installation Best Practices — Preventing the Most Common Conveyor Drive Gearbox Problems

Three installation decisions at commissioning have a greater impact on conveyor drive gearbox service life than any single maintenance task applied later:

1. Motor-to-gearbox shaft alignment: The coupling between the motor output shaft and the gearbox input shaft must be aligned within the coupling manufacturer’s specified tolerance — typically 0.05–0.10 mm parallel offset and 0.05°–0.10° angular misalignment for standard jaw couplings. Misalignment beyond these tolerances imposes radial loads on the gearbox input shaft bearing that are not included in the gearbox’s rated load calculation and accelerate bearing wear in proportion to the misalignment magnitude. Laser alignment is the standard method; dial gauge alignment is adequate if performed correctly.
2. Gearbox mounting bolt torque and retorquing schedule: The gearbox mounting bolts must be torqued to specification using a calibrated torque wrench, not an impact driver. Impact driving creates inconsistent bolt tensions that allow micro-movement of the gearbox housing under cyclic torque loading — a movement that polishes the mating surfaces (fretting) and progressively loosens the bolt over 500–1,000 hours. Retorque all mounting bolts to specification after the first 200 hours of operation.
3. Backstop installation on inclined conveyors: Any conveyor inclined more than approximately 5° requires a backstop (anti-reverse device) between the gearbox and the drive pulley to prevent the belt from running back when the motor is switched off. The planetary gearbox does not self-lock and does not provide any reverse resistance when the motor is de-energised. Operating an inclined conveyor without a backstop results in belt runback events that impose severe reverse torque shocks on the gearbox — typically much higher than the rated forward running torque — and cause rapid planet carrier pin failure.

Browse the full inline planetary gearbox series for conveyor drive frame selection, or see the 311 series planetary gearbox for crane and conveyor drives at 35,000–40,000 Nm with inline and right angle configurations. Send your conveyor specifications to [email protected] for a frame selection, ratio confirmation, and formal quotation within 24 hours.

Sourcing a Replacement Gearbox for an Existing Conveyor Drive — What to Measure

When a conveyor head drive gearbox has failed and needs to be replaced on a running operation — as opposed to a new installation — the specification process starts with the existing unit rather than the conveyor design documentation. In many cases the original design documents are unavailable or have been lost after years of plant ownership changes. The replacement gearbox can still be correctly specified from the existing unit using four measurements:

1. Output shaft diameter and keyway dimensions: The output shaft must match the head drum bore exactly. Measure the shaft diameter with a micrometer — not a calliper — because a 0.5 mm undersized shaft will not transmit full torque before fretting at the interference fit. Record keyway width, keyway depth, and keyway position.
2. Output shaft length from mounting face: This determines whether the output shaft protrudes beyond the drum end face. An excessively long shaft will hit the drum housing wall during installation; an excessively short shaft will not have sufficient bearing length in the drum hub bore.
3. Motor mounting flange dimensions (IEC or NEMA frame size): Measure the motor shaft diameter, motor flange pilot bore, and motor flange bolt circle. This confirms the correct input adaptor specification for the replacement gearbox without requiring the motor to be removed from service.
4. Gear ratio (read from existing nameplate or calculated from drum RPM and motor RPM): If the nameplate is illegible or missing, measure the drum speed (RPM) with a non-contact tachometer while the conveyor is running at normal belt speed. Divide the motor nameplate speed by the measured drum RPM to calculate the gear ratio. A slight deviation (e.g., calculated ratio 29.7 versus a standard ratio 30:1) confirms a standard ratio and not a custom-engineered ratio that would require special manufacturing.

Send these four data points — plus a photograph of the existing nameplate if legible — to [email protected]. We confirm the correct EP300L frame size and stage count within 4 hours and provide a dimensional drawing before the order is placed.