A cement screw conveyor startup torque specification must cover the hardest credible restart, not just normal running. Cement left in the casing can settle; material can fall back on an incline; and a screw directly under a silo may remain under head load. Breakaway torque can therefore exceed the torque inferred from steady motor current.
VOGO’s project-configured tubular cement conveying equipment is selected from material, layout, duty and drive inputs. Published capacity or motor references do not promise that a particular drive will restart a filled machine.
Classify the Inlet Before Calculating Torque
A screw receiving limited flow from an upstream valve or feeder is a control-fed conveyor. A screw whose inlet remains covered under a hopper, bin or silo controls withdrawal and is a screw feeder. That case adds head load and inlet shear; ordinary conveyor tables cannot be applied directly.
The cement silo discharge screw guide explains increasing pitch, tapered outside diameter, mass-flow interfaces and why the screw alone does not cure arching or rat-holing.
Cement Screw Conveyor Startup Torque Load Cases
| Load case | Main resistance | Required evidence |
|---|---|---|
| Empty start | Bearings, seals, reducer and rubbing | Supplier friction calculation or measured no-load current |
| Normal loaded start | Trough loading and material resistance | CEMA-based supplier calculation |
| Settled restart | Static resistance after dwell | Defined dwell and material condition |
| Inclined fallback | Material accumulated toward the inlet | Angle and controlled-stop case |
| Flood-loaded feeder | Head load and inlet shear | Feeder calculation, not a conveyor table |
| Restriction | Rising fill before trip | Safe torque and trip delay |
The KWS horsepower method separates empty friction from material power and accounts for drive efficiency. It does not alone prove breakaway torque for settled cement or feeder duty.
Convert Power to Shaft Torque
Torque (N·m) = 9550 × shaft power (kW) ÷ speed (rpm)
If a restart calculation requires 7.5 kW at 45 rpm, the shaft torque is 1,592 N·m. This is an example, not VOGO data. Verify reducer continuous and peak output torque, service factor, motor torque, VFD overload, coupling, key, shaft, screw pipe, coupling bolts, starts per hour and protective settings.
SEW-EURODRIVE project-planning guidance treats motor torque, gear-unit load and application factors as separate checks. Use the manual for the proposed reducer.
Check Low-Speed Control and Protection
A VFD can shape acceleration but cannot create unlimited torque. Confirm overload duty, minimum speed, motor cooling and start frequency. The selected Siemens SINAMICS instructions govern parameterization and protection.
A defensible sequence proves the discharge path, starts downstream equipment, ramps the screw, proves minimum speed, and only then permits upstream feed. Stop feed on zero speed, sustained overcurrent or downstream trip. Do not use repeated reversing as routine blockage clearing.
Loaded-Restart Acceptance Test
Record material condition, inlet classification, conveyor fill, dwell, ramp, peak current, achieved speed, restart time, temperatures, trip response, leakage, rubbing and vibration. Feeder testing must include the storage system and isolation method.
Opening the conveyor while energized is not an acceptance test. OSHA’s hazardous-energy standard requires isolation and verification before servicing. The inclination guide supplies the fallback and gravity-load checks.
Distinguish Breakaway, Acceleration and Running Torque
Breakaway torque acts at zero or near-zero speed and must overcome static resistance. Acceleration torque adds the inertia of the motor, reducer, screw and material. Running torque follows after speed and material flow stabilize. A single peak-current number cannot distinguish these conditions unless speed and time are logged with it.
Use a time-based trace during commissioning. Mark the instant of start command, brake or gate release, first detected shaft motion, minimum-speed proof and steady operation. A high current before shaft motion points toward breakaway resistance. A long high-current ramp after motion begins can indicate excessive acceleration demand, unstable filling or an unsuitable VFD ramp. A later rise after feed starts points toward material or downstream loading.
Define the Stop That Creates the Restart
The governing restart depends on how the previous stop occurred:
- A planned stop should remove upstream feed and empty the conveyor for a validated time.
- A downstream trip may leave the casing partly or fully loaded.
- Loss of power can stop every machine simultaneously and defeat the normal sequence.
- A feeder isolation gate may close slowly or trap material under head.
- An inclined conveyor can retain material even after an emptying run.
Document the worst credible stop. If plant controls cannot guarantee a clean stop during power loss, the drive calculation must not assume an empty restart. Consider an uninterruptible control supply only as a controls measure; it cannot keep an unpowered drive turning.
Set Protective Torque From the Weakest Approved Component
Create a torque-limit table for the reducer, coupling, key, drive shaft, screw pipe, coupling bolts and any backstop. Convert the VFD current or electronic torque limit to screw-shaft torque using the motor torque, reducer ratio and efficiency. The trip must occur before the weakest component’s approved short-time limit, with allowance for measurement uncertainty and control delay.
A zero-speed switch should prove actual screw movement, not merely motor rotation. Mounting a target upstream of a failed coupling can produce a false “running” signal. Where practicable, sense the driven screw or use two independent indications appropriate to the risk assessment.
Procurement Questions for Loaded Restart
Ask the supplier to state:
- calculated continuous and loaded-start shaft torque;
- assumed material density, dwell, fill and inlet head;
- motor torque and inverter overload at minimum speed;
- reducer continuous, peak and thermal ratings;
- maximum allowed starts and jogs per hour;
- protective current/torque and time settings;
- whether reverse jogging is permitted;
- the acceptance test and excluded conditions.
If any item is “by others,” identify the responsible party. A motor supplier may confirm motor torque without confirming screw-shaft strength, while the conveyor supplier may assume a drive package supplied elsewhere. The interface must be closed before purchase.
What to Send With the Drive Enquiry
Provide a one-line diagram, screw speed, motor supply, material density range, moisture, inclination, control-fed or flood-loaded inlet, normal fill, worst stop, dwell before restart, starts per hour and required recovery time. State whether a VFD, brake, backstop or isolation gate is included and who supplies it.
Ask for a torque-speed curve or tabulated motor and reducer capability at the screw shaft. The response should name the governing component and protective setting, not merely confirm “adequate torque.”
Revalidate After Process Changes
A higher silo level, different isolation gate, steeper replacement layout, new cement source or longer stop dwell can invalidate the original restart case. Review torque whenever these conditions change, even if nominal tonnes per hour stays constant.
Trend peak current and time-to-speed. A gradual increase can reveal seal drag, bearing wear or material buildup before the drive reaches its trip threshold.
Conclusion
Specify cement screw conveyor startup torque from a documented worst-case restart: material, fill, inclination, dwell, inlet head and downstream state. Convert shaft power to torque, then verify every drive component and protection setting. If the inlet is flood loaded, use a feeder calculation. Send the supplier the load register and acceptance test for confirmation.