Capture the as-found condition before disturbing it

Lock out and verify zero energy before opening guards or touching the screw. Photograph the coupling, bolt heads, nuts, shaft ends and any displaced flighting. Record whether the failure occurred during starting, steady running, restart under load or immediately after a previous repair. Save both halves of every fractured fastener. Note polished areas, fretting, elongated holes, witness marks, bent washers and any evidence that the bolt was loose before it fractured.

Collect operating evidence from the same event: motor current trend, drive speed, upstream feed rate, downstream availability, level or blockage alarms and operator observations. A fracture during a loaded restart points to a different mechanism from a bolt that gradually loosens over several shifts. Do not infer torque or material grade from appearance alone; verify the drawing, bill of materials or OEM manual.

Seven-step diagnostic workflow

1. Confirm the failure mode

Determine whether the bolt is actually sheared, pulled in tension, loosened and battered, or whether the coupling holes have worn until the joint hammers. A clean fresh fracture after a jam is different from a polished fracture surface accompanied by fretting. If the bolt is missing, inspect the coupling holes and surrounding guards for secondary impact evidence before assuming a simple shear event.

2. Check for plugging and abnormal material load

Inspect the inlet, trough, discharge and downstream equipment. Hardened cement, wet buildup, foreign material or a blocked discharge can create torque far above normal conveying duty. Compare the event with motor-current history. Remove the process restriction before mechanical adjustment; otherwise a new bolt can fail for exactly the same reason.

3. Turn and inspect the screw train safely

With the equipment isolated and using the approved maintenance method, check whether the shaft train moves consistently through its rotation. Look for local tight spots rather than judging only total resistance. Inspect flight-to-trough and flight-to-cover contact marks. A bent shaft, distorted trough, displaced hanger or damaged flight can produce a once-per-revolution load pulse that repeatedly attacks the coupling.

4. Inspect hanger and end bearings

Check each support for free movement, contamination, abnormal heat history, lubrication condition and mounting security. A partially seized hanger bearing may still rotate by hand after cooling yet generate high running torque. Look for displaced bearing housings and evidence that the shaft has been forced away from its natural centerline.

5. Verify shaft and coupling alignment

Clean mating surfaces and check shaft-end alignment, coupling engagement and visible runout using the plant’s normal precision-maintenance practice. Do not pull a misaligned shaft into position by tightening coupling bolts. Correct trough supports, hangers, shaft condition or coupling fit first. If the coupling holes are oval, cracked or heavily fretted, replacing only the fastener does not restore the joint.

6. Verify the fastener specification and assembly

Confirm bolt diameter, length, thread engagement, property class or grade, nut and washer arrangement, and tightening method against the conveyor manufacturer’s documentation. Never upgrade to a stronger bolt merely because the previous one broke: if the original fastener is intentionally selected to protect more expensive components, an unapproved substitution can transfer damage to the shaft, gearbox or screw.

7. Separate drive shock from conveyor resistance

Inspect the gearbox, drive coupling, backstop where fitted, motor controls and start sequence for evidence of shock loading. Review whether the conveyor is routinely stopped full and restarted under load. If mechanical resistance is normal but failures cluster at starts, investigate process sequencing and drive behavior rather than repeatedly changing mechanical alignment.

Evidence-to-decision table

Technical checks that prevent false conclusions

Use trends, not isolated numbers. Motor current is useful only when compared at similar feed and operating conditions. Temperature evidence should be compared between equivalent bearings and against established plant baselines. Alignment acceptance and fastener tightening values must come from the specific conveyor or component documentation; this guide intentionally does not invent universal tolerances because screw diameter, shaft arrangement, coupling design and service duty vary.

Also distinguish cause from consequence. A loose coupling can damage holes, but an overloaded conveyor can first loosen the joint. A bent flight can result from a foreign object and later become the recurring source of torque pulsation. Preserve the sequence of evidence so the maintenance report explains what happened first.

Verification, acceptance and return to service

Before removing lockout, verify that guards are restored, tools and loose material are removed, coupling hardware matches the approved specification, supports are secure and the screw has no unexplained tight spot under the approved maintenance check. Confirm the discharge path is available.

Perform the site-approved test sequence. Start unloaded where the process permits, observe vibration, noise and current behavior, then introduce material progressively. Watch the repaired coupling and adjacent supports from a safe position. Acceptance requires stable operation without abnormal contact, repeated fastener movement, unexplained current spikes or bearing distress. Reinspect the joint after the plant’s prescribed commissioning interval and document the as-left condition.

Common mistakes and repeat-failure traps

• Installing another bolt without preserving the fracture evidence.
• Using a higher-strength fastener without engineering or OEM approval.
• Pulling misaligned shafts together with the coupling bolts.
• Ignoring a blocked discharge because the conveyor turns after material is disturbed.
• Checking only the nearest bearing while a downstream hanger is binding.
• Restarting full without reviewing why material remained in the trough.
• Declaring success after a no-load test without controlled loaded verification.

Reference discipline

Use the conveyor OEM manual, coupling drawing, fastener bill of materials, motor and gearbox documentation, plant lockout procedure and historical condition-monitoring records as the controlling references. Where those documents specify limits, tightening methods or inspection intervals, they supersede generic guidance.