A screw conveyor gearbox must turn the screw when material is moving normally and when the conveyor starts with material inside it. Selecting only by motor kW can miss the second condition. The useful starting points are screw speed, running torque, starting torque and operating hours. Material behaviour matters too: dry grain, damp powder and a sticky bulk material can put very different demands on the same screw. Units throughout: mm, kW, rpm and Nm.
1. Start with material load and screw speed
The torque required depends on what the screw must move and how the conveyor is built. Give the designer the material, bulk density, required capacity, screw diameter, pitch, length, inclination and expected fill level. Bearings, seals and material rubbing against the trough add resistance. A screw used as a feeder under a full hopper needs separate attention because the material pressure at its inlet changes the load.
Material weight alone does not give conveyor torque. A screw conveyor does not simply lift its complete contents at one fixed radius. Use a conveyor design calculation, reliable machine data or suitable measurements to establish running and starting requirements. Note whether the conveyor normally starts empty, partly loaded or full.
Once torque and speed are known, the power at the screw follows this relationship: power in kW = torque in Nm × rpm / 9550. This is power delivered to the screw. Motor power must also cover gearbox losses and any other losses between the motor and screw.
2. Calculate the ratio and check the motor
Example only: suppose the screw requires 800 Nm at 40 rpm, and the motor runs at 1440 rpm at the intended operating condition. The nominal reduction ratio is 1440 / 40 = 36:1. Screw power is 800 × 40 / 9550 = approximately 3.35 kW.
If gearbox efficiency is assumed to be 80% for this example, gearbox input power is 3.35 / 0.80 = approximately 4.19 kW. That assumed efficiency is not a guaranteed value for a worm or helical-bevel gearbox. Obtain the efficiency for the proposed unit at its ratio, speed and load before selecting the motor.
A standard gearbox ratio may differ from 36:1, so calculate the resulting screw speed. Where a variable-speed drive is used, state the full speed range. Check low-speed motor cooling, available starting torque and the gearbox input-speed limits. A larger motor also needs checking because it may apply more torque than the screw or gearbox can safely accept.
3. Apply service factor to the actual duty
Service factor allows for the difference between steady catalogue duty and the application. Frequent starts, long working hours, uneven feeding, compacted material and shock loads can change the selection. State starts per hour as a number. Also state whether starts are empty or loaded; ten loaded starts can be more demanding than ten empty starts.
Example only: if the supplier specifies a service factor of 1.5 for an 800 Nm running load, the corresponding mechanical rating requirement is 1200 Nm at the selected operating condition. The factor is an assumption for this example, not a recommendation for every screw conveyor.
Ask how the supplier applies service factor so it is not counted twice. Starting and peak torque still require separate checks against permitted short-duration loads. A service factor cannot make a blocked conveyor safe. Suitable overload protection and a defined procedure for clearing blockages are separate requirements.
4. Choose hollow output or foot mounting
A shaft-mounted gearbox with hollow output can fit directly onto a suitable conveyor shaft. It can reduce the need for a separate output coupling and base. Check shaft diameter, key dimensions, engagement length and the available space. Its torque arm, restraint and supporting arrangement must follow the gearbox design; the gearbox must not be left free to rotate around the shaft.
A foot-mounted gearbox sits on a base and commonly drives the screw through a coupling. This arrangement allows a separate support layout, but needs alignment, a sufficiently rigid base and access for maintenance. Check the coupling rating and the permitted loads on the gearbox output shaft.
In either arrangement, identify which bearings carry screw thrust and material loads. Do not assume the gearbox output bearings can carry these loads without confirmation.
5. Compare worm and helical-bevel drives
A worm gearbox provides right-angle reduction in a compact arrangement and can suit some slow conveyor duties. Selection must include efficiency and thermal capacity, especially for continuous operation. Mechanical torque capacity alone does not establish whether the unit can dissipate the heat generated during a long shift.
A helical-bevel gearbox generally offers higher efficiency than a comparable high-ratio worm arrangement. It can be useful for conveyors running for long hours or carrying sustained loads. Compare actual supplier data at the required ratio, rather than assuming every gearbox in one family behaves alike.
Specify installation orientation, ambient temperature and dust conditions. Confirm that oil filling, draining and inspection remain accessible after guards and nearby equipment are installed.
Quick enquiry checklist
- Material, bulk density, required capacity and screw dimensions in mm
- Required screw rpm, running torque in Nm and starting condition
- Motor kW, actual operating rpm and any variable-speed range
- Working hours, starts per hour, loaded starts and known peak loads
- Hollow-output shaft details or foot-mounting and coupling dimensions
- Installation orientation, thrust support, ambient conditions and photographs or drawings
Frequently asked questions
Can I select the gearbox from motor kW alone?
No. Motor kW must be considered with motor speed, reduction ratio, gearbox efficiency and the torque required by the conveyor. Starting loads, service factor and thermal capacity also need checking.
Is a hollow-output gearbox always better?
No. It can simplify the drive arrangement, but the conveyor shaft, torque arm, supporting structure and maintenance access must suit it. A foot-mounted unit may fit the existing machine more easily.
Should the screw conveyor use a worm or helical-bevel gearbox?
Compare both at the required torque, ratio and operating hours. A worm unit may suit the layout and duty, while a helical-bevel unit generally offers an efficiency advantage. Use the proposed units' mechanical and thermal ratings.
What if the conveyor often starts full?
Tell the supplier explicitly and provide the expected starting torque or conveyor design data. Loaded starts can govern gearbox, motor and coupling selection even when normal running power is modest.
Related: Conveyor Gearboxes · Service Factor Calculation · Hollow vs Solid Shaft Gearboxes · Contact