Technical Articles
Torque Limiter Coupling Slipping on a Cement Screw Feeder: Blockage, Start-Up Load or Wrong Setting?
A screw feeder under a cement silo trips its Torque Limiter Coupling, someone resets it, and before the shift is over it slips again. From the control room, three different problems look identical: a real mechanical overload, a start-up torque the setting never allowed for, or a limiter that is set wrong or has drifted. Raising the setting on the wrong diagnosis just moves the failure to the gearbox, the screw shaft or a flight weld.
Five causes behind a slipping torque limiter coupling
Slipping is a symptom, not a diagnosis. On cement screw feeders the cause nearly always falls into one of five groups:
- Material blockage: cement compacts while standing, picks up moisture, or carries lumps and foreign objects into the screw.
- Flight jamming: a bent or worn flight rubs the trough, build-up closes the clearance, or a hanger bearing catches the screw.
- Bearing drag: end or hanger bearings run dry or take in dust, so running torque creeps up.
- Start-up torque: the screw was stopped full and has to break away packed material on every start.
- Setting or condition: the set point was low from commissioning, was changed during maintenance, or has dropped because the friction surfaces are worn or contaminated.
The first three are genuine loads, and the limiter is doing its job. Start-up torque is real but short-lived, so it usually calls for a change in how the feeder stops and starts rather than a higher setting. Only the last group is a fault in the protection device itself.
Read the trip: current, timing and material
Pull the motor current trend and the stop/start log for the period before each slip. Read current as a trend, not an exact torque value: at light load much of it is magnetizing current, so the shape of the curve tells you more than the number.
| Slip pattern | Current before the slip | Site findings | Likely cause |
|---|---|---|---|
| Mid-run, after a steady climb | Well above normal running level | Packed or wet material, lumps at the inlet | Material blockage |
| Every start, fine once running | Only the starting peak is high | Screw stopped full, material compacted | Start-up torque |
| Same point in each revolution | Regular spikes once per turn | Rub marks on flight or trough, bent shaft | Flight jamming |
| More frequent over time | Running baseline higher than before | Hot or noisy bearings, dry grease points | Bearing drag |
| Random, at normal current | No rise before the slip | Nothing abnormal in the screw | Setting or limiter condition |
Then look at the material. Freshly aerated cement can flush through a screw almost like a liquid, while the same product left in a silo cone settles into a dense mass, so a feeder that only slips after long stops is telling you about the material. If the drive has a soft starter or frequency converter, check the ramp and the drive's own torque limit too, because a very short ramp turns every start into a shock load.
Inspect by mechanism, not by habit
On a friction limiter, disc springs clamp friction linings between hub and flange, and slip torque depends on spring force and friction coefficient. Check lining wear and glazing, grease migrating from a nearby bearing and cement dust packed between the faces, then compare spring length or the adjusting nut with the setting marks and confirm it is locked. Repeated slipping heats and glazes the linings, which lowers friction, so one overload can start a chain of nuisance slips.
A ball or roller detent limiter disengages instead of slipping continuously. Inspect pockets and rolling elements for wear and rounded edges, which lower breakaway torque, make sure dust is not preventing clean re-engagement, and test the disengagement sensor. On a shear-pin design, repeated pin failures at normal load usually point to fatigue or worn pin bores rather than one big overload.
A cleaning or adjustment that is harmless on one design can ruin another, so work to the manufacturer's procedure for the unit you actually have. Our torque limiter type comparison and installation and maintenance guide cover each design in more detail.
Audit the setting against the screw, not the motor
A correct setting sits above the highest torque the feeder needs in normal service, including starting a loaded screw, and below the lowest torque that damages the weakest part of the drive train. That part is usually the screw shaft, a bolted joint between screw sections, the gearbox output or a flight weld, not the motor. If the two limits overlap, no setting will work and the fix has to come from the process.
Running torque comes from motor power, speed and gear ratio at the shaft where the limiter sits; a limiter on the low-speed side sees far more torque than one on the motor shaft. Allowable torque comes from the gearbox and screw makers' data, and start-up demand from the drive’s torque estimate (if available), read together with the acceleration ramp and the current trend you already have.
Set and record the value using the original manufacturer's procedure and setting marks, not by tightening until the slipping stops. If start-up is the issue, run the screw empty before planned stops, lengthen the soft-start ramp or keep the inlet gate closed during start. For the selection side, see torque limiter couplings for conveyor overload protection.
Restart checks and data for a replacement
Do not restart on a reset alone. Confirm the following first:
- The cause is found and removed: blockage cleared, foreign object out, flight repaired.
- With the drive locked out, the screw turns freely through a full revolution.
- Bearings are lubricated and quiet, and no grease has reached the limiter.
- The setting matches the recorded value, locks are in place, and worn linings or detent parts are replaced.
- The slip or disengagement sensor stops the motor.
- Start with the screw empty or the inlet gate closed, and watch current through the first run.
If the limiter needs replacing or re-specifying, have this data ready. Before we size a Torque Limiter Coupling, we check torque, speed, shaft diameter, keyway, service factor, misalignment and the working environment, and for a repeat-slip case we also want:
- Existing model or nameplate data, with photos of the installed unit and worn parts
- Protection action (slip and reset, disengage, shear pin) and sensor type
- Current set torque, motor power and speed, gear ratio and mounting position
- Shaft diameters, keyways and available space
- Fault records: slip times, current trends, stop/start log, material condition
- Allowable torque of the gearbox and screw, if known
FAQ
Can we just raise the setting to stop the slipping?
Only after you have confirmed that the current setting is below what the feeder legitimately needs, and that the new value still protects the gearbox and screw. If the slips come from a blockage or bearing drag, a higher setting turns a nuisance trip into a broken shaft.
Why does it slip only after a long shutdown?
Cement settles and de-aerates in the screw and inlet while the feeder stands, so breakaway torque at the next start is much higher than running torque. Running the screw empty before planned stops, or starting with the inlet gate closed, usually works better than changing the setting.
The limiter slips but motor current looks normal. What does that mean?
If current shows no rise before the slip, the screw load probably did not increase, so look at the limiter: set point, worn or contaminated friction faces, worn detents, a loose adjuster. Also check the trend's sampling rate, because a slow trend can miss a short spike from a foreign object.
If your feeder keeps tripping and you want a second opinion on the setting or a replacement unit, send us the data above. We machine bores and keyways to your shafts and can build a replacement from a drawing or from photos and measured dimensions of the old unit; standard models usually ship in 15–25 days, with a material certificate (mill test certificate) and dimensional inspection report supplied with the shipment on request. Send us your drawing
Need an engineering selection or replacement review?
Send power, speed, shaft diameters, keyway, installation dimensions, drawings or old-part photos. We will review the working condition before confirming the model and quotation.