Rhythmic knocking once per turn — mixing arms
Symptom
A rhythmic metallic knock during mixing, once per shaft revolution.
Likely causes
- Arm or paddle bolts have lost their torque.
- A paddle is broken or missing.
- A crack at the root of an arm, where it meets the shaft clamp.
- Bolts that keep coming loose or shearing, often after a stone jam.
Most likely and cheapest to check first.
Checks
- Lock out. Check arm bolts first, by torque check or a light hammer tap.
- Look at every paddle for breakage.
- Clean the arm roots and look for cracks under any build-up.
- Check whether the arm is worn so thin that its paddle can no longer reach 3 mm.
Fix
- Retighten to torque. Use new bolts whenever a part is changed.
- Replace the broken paddle and check the gap of its neighbours.
- Replace cracked arms. Never weld a cracked cast-iron arm.
- Recheck all new bolts daily for the first three days.
Stop the equipment if
Loose bolts on an arm, or a sudden loud crash followed by rough running.
Parts you may need
A knock is an early warning
Arms are cast in spheroidal (ductile) iron or cast steel and clamped or bolted to the shaft. A knock means something has moved. Found early, it is a bolt. Found late, it is a broken arm, and sometimes a damaged shaft.
DANGERA sudden loud metallic crash means the paddles of the two shafts may have collided because the synchronisation failed. Stop immediately and check the sync shaft or coupling before restarting.
CAUTIONHit arms and paddles only with a plastic or copper hammer. A steel hammer can crack them.
⚠ Every check inside a machine starts with full lockout. Safety disclaimer