Gearbox for Screw Conveyor: Selection Guide with Torque Examples
A screw conveyor almost always needs a right-angle, high-ratio drive with a hollow output that sits on the conveyor's drive shaft — which is why the gearbox for screw conveyor duty is normally a shaft-mounted helical-worm or helical-bevel unit rather than a foot-mounted one. The selection itself comes down to four numbers: screw diameter, conveying length, material bulk density and the required output speed. This guide works through them in the order an engineer actually uses them, and ends with a worked torque example you can copy.
What Really Decides the Gearbox for a Screw Conveyor
Four inputs fix the drive, and only four:
- Screw diameter (150 mm to 600 mm is the usual band), which sets the torque arm the reducer must resist
- Conveying length and incline, which set the friction and lift component of the power
- Bulk density and abrasiveness of the material, which set the starting torque and the service factor
- Required output speed, typically 20–60 rpm for a standard-pitch screw
Everything else — frame size, ratio, mounting style, bore diameter — falls out of these four. Buyers who start from "we need a 5.5 kW unit" almost always end up with the wrong frame, because motor power tells you nothing about the torque the screw needs at 25 rpm.
Torque, Speed and the Only Formula You Need
The relationship between power, speed and torque is fixed:
T (N·m) = 9550 × P (kW) / n (rpm)
and the ratio comes straight from the speeds:
i = motor speed ÷ required output speed
So for a 4 kW motor running a screw at 30 rpm, the output torque is 9550 × 4 / 30 ≈ 1273 N·m, and with a four-pole motor at 1450 rpm the ratio is 1450 / 30 ≈ 48, rounded to the nearest available step (i = 50 in most ranges).
Two things to note. First, the calculation uses the required torque, not the motor's full-load torque — a screw conveyor selected on the motor's rated torque will be oversized and expensive. Second, the ratio always comes from speeds alone; the frame size comes from torque and service factor.
| Screw diameter | Typical output speed | Typical motor power | Ratio (4-pole motor) |
|---|---|---|---|
| 150 mm | 45–60 rpm | 1.5–2.2 kW | 25–32 |
| 200 mm | 35–50 rpm | 2.2–4 kW | 30–40 |
| 250 mm | 30–45 rpm | 4–5.5 kW | 32–50 |
| 300 mm | 25–40 rpm | 5.5–7.5 kW | 36–58 |
| 400 mm | 20–32 rpm | 11–15 kW | 45–72 |
| 500–600 mm | 15–25 rpm | 18.5–30 kW | 58–96 |
Treat that table as an order-of-magnitude check for early budgeting, not as a selection. The real duty point always needs the material data.
Starting Torque and Service Factor
A screw conveyor that starts full of material needs far more torque than one that starts empty, and the gearbox must survive that peak. The service factor is the multiplier applied to the running torque to cover it.
| Duty condition | Typical service factor | Example |
|---|---|---|
| Uniform, light material, starts empty | 1.0 – 1.25 | Dry sand, plastic granules, starts empty |
| Moderate shock, starts partly loaded | 1.25 – 1.5 | Grain, cement powder, regular starts |
| Heavy shock, starts fully loaded | 1.5 – 2.0 | Wet sand, sludge, clay, frequent starts |
| Very heavy, jam-prone | 2.0 + | Sticky or fibrous material, blocking risk |
Once the service factor is applied, the frame size is chosen so the reducer's rated output torque (with the service factor already included in its rating table) exceeds the calculated torque. Sizing on running torque alone is the single most common mistake in screw conveyor drives, and it shows up as a gearbox that runs hot and fails a bearing within the first year.
Shaft Mounted or Foot Mounted?
Most screw conveyors use a shaft-mounted reducer with a hollow output and a torque arm, because it removes the coupling and the floor space. The alternative — a foot-mounted reducer with a solid output and a coupling — is chosen when the conveyor shaft is short or the reducer must be anchored.
| Criterion | Shaft mounted (hollow output) | Foot mounted (solid output) |
|---|---|---|
| Coupling | Not needed | Needed, plus alignment |
| Floor space | None — hangs on the shaft | Base frame required |
| Reaction | Torque arm or reaction rod | Foundation bolts |
| Axial thrust from screw | Must not be carried by the reducer | Taken by a separate thrust bearing |
| Best when | Feeders, conveyors, elevators | Short shafts, retrofit into existing frames |
The axial thrust point matters more than the table suggests. A screw conveyor generates a substantial axial force along its shaft, and that force must be taken by a dedicated thrust bearing at the drive end — never by the reducer's own output bearing. A unit sized correctly for torque will still fail early if the screw thrust is fed into it. Ask for the reaction drawing and check where the thrust goes before you approve the layout.
For reference, a hollow-shaft unit such as the SA47 hollow shaft worm gearbox is the typical answer for small and mid-size feeders, while larger conveyors move up to bigger hollow-output frames.
Ratio Limits and Why You Cannot Always Have 100:1
Screw conveyors like slow shafts, and 20 rpm on a 1450 rpm motor means a ratio of about 72. That is reachable, but it pushes the reducer into territory where efficiency and heat become the limiting factors.
- Helical-worm units reach 100:1 in one body, but efficiency falls as the ratio rises and the gearbox runs hotter. Keep an eye on the thermal rating, not just the torque rating.
- Helical-bevel or helical units are far more efficient, but a single stage only gives about 5:1 to 8:1, so a ratio of 70 needs two or three stages.
- If the screw is inclined or the material is heavy, check the holdback requirement. An inclined screw can run backwards under the weight of the charge when the motor stops, and the reducer's self-locking tendency is not a substitute for a holdback device or a brake motor.
A practical rule: below about i = 60, a helical-worm unit is usually the cheapest answer; above that, compare it against a two-stage helical unit on total running cost rather than on purchase price.
Heat, Sealing and Dust
Screw conveyors are dirty, often hot, and often outdoors, and the reducer has to live with all three.
- Thermal rating — the ability to shed heat matters more than the mechanical rating on continuous duty. A reducer that transmits the torque but overheats will destroy its oil and its seals.
- Sealing — NAK oil seals are standard in most industrial ranges, but for dusty or wash-down areas specify FKM (FPM) seals and a protected breather.
- Mounting position — the oil-level mark differs for each mounting position (M1 to M6). Fitting a unit in a position it was not filled for starves the bearings or overfills the sump.
- Lubrication — GB L-CKC220~460 or an equivalent such as Shell Omala 220~460 covers most duty; the first oil change after run-in is not optional.
Worked Example: 300 mm Screw, Wet Sand
Take a 300 mm screw conveyor, 12 m long, horizontal, handling wet sand at roughly 1.4 t/m³, driven at 30 rpm by a four-pole motor at 1450 rpm.
- Ratio: 1450 ÷ 30 = 48.3 → select i = 50.
- Torque allowance: assume the conveyor needs about 3.5 kW at the screw shaft, giving 9550 × 3.5 / 30 ≈ 1114 N·m running torque.
- Service factor: wet sand starts partly loaded and can block, so use 1.5 → design torque 1671 N·m.
- Frame: pick the smallest hollow-output frame whose service-factor-inclusive rating exceeds 1671 N·m — in the SA range that lands around the 77 mm to 97 mm frames.
- Thrust: fit a thrust bearing at the drive end and confirm the torque arm can take the reaction in a direction that suits the site.
That is the whole method. If you have the material data but not the conveyor's power figure, run the duty point through the WGT Motor & Gearbox Online Selection System — it takes motor power, ratio and mounting on one screen and returns a model number you can compare against a quotation.
FAQ
What type of gearbox is used for a screw conveyor?
Most screw conveyors use a shaft-mounted right-angle reducer with a hollow output and a torque arm — typically a helical-worm or helical-bevel unit. Foot-mounted reducers with a coupling are used where the conveyor shaft is short or an existing base frame must be reused.
How do I calculate the torque for a screw conveyor drive?
Torque in N·m equals 9550 times the shaft power in kW divided by the output speed in rpm. Apply the service factor for the duty (1.0–1.25 for light uniform duty, up to 2.0 for heavy shock or fully loaded starts) and select a frame whose rated output torque covers the result.
Should the gearbox take the screw conveyor's axial thrust?
No. The thrust generated by the screw must be carried by a dedicated thrust bearing at the drive end. Feeding that axial force into the reducer's output bearing is one of the most common causes of early gearbox failure on screw conveyors.
What output speed should a screw conveyor run at?
Most standard-pitch screw conveyors run between 20 and 60 rpm, with larger diameters and heavier materials at the lower end. The speed is limited by the material's flow characteristics, not by the gearbox, but it is the figure that sets the reduction ratio.
Do I need a holdback on an inclined screw conveyor?
Yes, if the incline is steep enough that the charge can run back when the motor stops. Self-locking in a worm reducer is a useful tendency but not a guaranteed safety device; fit a holdback or a brake motor where run-back is a hazard or a safety concern.
Get Your Screw Conveyor Drive Sized
Send us the screw diameter, conveying length and incline, material bulk density, required output speed and the duty pattern (starts per hour, loaded or empty). We will come back with the frame size, the ratio, the service factor we applied and a dimension drawing that shows where the thrust bearing goes — usually within one working day, with EXW and CIF figures side by side.
