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A pellet mill knocking sound is usually a mechanical fault, but material impact, feed pulsation, a loose component, or resonance can produce a similar result. These must be ruled out before the knock is treated as an internal mechanical fault. A knock is an impact, not a continuous noise. Something is making contact that should not make contact, or something that should be tight has come loose.

The urgency is higher than for most other noise faults. A knocking sound from a rotating assembly usually means metal is striking metal. Continuing to run a machine with a metallic knock can damage the die, the rollers, the coupling, or the main shaft within a short time, and the damage is usually not repairable by adjustment.

This article covers how to locate and classify the knock, how knock and vibration differ, how to tell a mechanical knock from a process noise, which components produce a knock, how coupling type changes what you hear, which checks to run before stopping, when to stop rather than monitor, what damage to inspect after a stop, and what information to send a supplier if the source cannot be found on site.

How to Locate and Classify the Knock

A knock should be located and classified before the machine is stopped, because it may not reproduce after cooldown. Record the location, the repetition pattern, and the load behaviour.

Measurement item	What to record	Why it matters
Location	Die chamber, coupling, gearbox, main shaft bearing, belt or pulley, motor, frame	Narrows the component
Repetition	Once per revolution, multiple per revolution, irregular	Separates single damage from multiple damage or a foreign object
Load behaviour	Worse under load, unchanged, disappears under load	Points to coupling, gear, or a loose component
Baseline	A recording of normal running noise	Confirms whether the knock is new
Vibration at bearing housings	Direction and amplitude	Confirms whether the knock is accompanied by vibration

A mechanic’s stethoscope or a listening rod can localise the knock to a specific housing. A non-contact vibration meter can confirm whether the knock is accompanied by vibration at a bearing. A sound recording made during normal running is the most useful baseline. The acceptable noise level and vibration level for the machine come from the machine manual and the supplier documentation for the specific model, not from a general figure.

Knock and Vibration Are Different Signals

Knock and vibration often appear together, but they are different signals and are measured differently.

• Knock is an impact. It is heard as a discrete sound and repeated at a rate that depends on the fault.
• Vibration is continuous motion. It is measured at the bearing housing or the frame in velocity or acceleration.
• Resonance can make a small impact produce a loud knock at a specific operating condition, without a progressive mechanical fault.

A knock without measurable vibration at the frame is often a loose component or a coupling insert, not a bearing fault. A knock with measurable vibration at a bearing housing is more likely to be a bearing or gear fault. The two should be recorded together.

Coupling Type Changes What You Hear

Coupling type affects the knock pattern and the inspection method. The machine manual and the coupling supplier documentation take precedence over this section.

Coupling type	Knock pattern	What to inspect
Flexible insert coupling	Regular knock, worse under load, often once per revolution	Insert wear, insert hardness, bolt tightness
Gear coupling	Regular knock that changes with load, may be multiple per revolution	Tooth wear, lubrication, backlash
Disc or membrane coupling	Regular knock, often metallic, may be worse at certain speeds	Disc or membrane condition, bolt tightness, misalignment

A knock from a flexible insert coupling is usually the insert, not the coupling body. A knock from a gear coupling is usually backlash or tooth wear. A knock from a disc coupling is usually a damaged disc or a loose bolt. The repair is different for each type.

What Produces a Knocking Sound

Coupling

Why it knocks: A worn coupling insert, a loose coupling bolt, or a misaligned coupling produces a knock each time the torque reverses or the coupling takes up backlash. The knock is regular, follows rotation speed, and is usually worse under load.

What to check: Coupling condition against the machine manual. Insert wear. Bolt tightness. Alignment against the alignment tolerance in the machine manual and the coupling supplier documentation.

What to do: Replace the coupling insert or coupling. Realign to the manual tolerance. Do not continue running with a knocking coupling. The impact load is transmitted through the shaft to the bearings and the gearbox, and it accelerates fatigue damage in those components. The extent of the damage depends on impact frequency, load magnitude, and component material.

Gearbox

Why it knocks: A chipped or broken gear tooth produces a knock once per revolution of that gear. A damaged bearing in the gearbox produces a knock at a rate that depends on the number of rolling elements. The knock changes with load and is often accompanied by a temperature rise.

What to check: Oil condition for metal particles. Gearbox temperature. Noise pattern against rotation speed and load.

What to do: Stop the machine. Replace oil if it is contaminated. If the knock persists with correct oil, internal inspection is warranted. Do not continue running a gearbox with a knocking sound, because gear tooth damage progresses quickly.

Die Chamber and Rollers

Why it knocks: A foreign object that is too large to pass through the die moves inside the die chamber and strikes the rollers or the die. A damaged roller or a damaged die surface produces a knock each time the damaged area contacts material. A roller that is not rotating freely produces an irregular knock as it drags.

What to check: Die chamber contents with the machine stopped and locked out. Roller free rotation. Roller surface condition. Die hole condition.

What to do: Remove the foreign object before restart. Replace a damaged roller or die. Do not continue running with a foreign object in the die chamber, because it damages the die and rollers as it moves.

Main Shaft Bearing

Why it knocks: A bearing with a damaged raceway or a damaged rolling element produces a knock or a series of knocks as the damaged area passes the load zone. The knock is often accompanied by vibration and a temperature rise.

What to check: Bearing temperature against the machine manual. Bearing noise. Vibration at the bearing housing.

What to do: Stop the machine and inspect. Do not continue running with a knocking main shaft bearing, because bearing failure usually damages the shaft and the surrounding components.

Belt and Pulley

Why it knocks: A damaged belt, a worn pulley, or a loose pulley produces a knock at the rate of the belt joint or the pulley damage. The knock is usually worse under load and may be accompanied by belt slip noise.

What to check: Belt condition and tension. Pulley condition and tightness.

What to do: Replace the belt or pulley. Adjust tension and alignment to the manual specification.

Motor

Why it knocks: Motor bearing damage produces a knock that follows rotation speed. Electrical faults do not usually produce a knock, but a loose motor mounting can.

What to check: Motor bearing noise with the motor isolated. Motor mounting bolt tightness.

What to do: Stop the machine and lock out power. Replace the motor bearing according to the motor supplier procedure, or tighten the mounting bolts to the manual specification.

Frame and Foundation

Why it knocks: A loose component, a loose anchor bolt, or a foundation that has moved produces a knock that is often irregular and worse at specific speeds. This is often mistaken for an internal mechanical fault.

What to check: Anchor bolt tightness against the manual torque specification. Frame condition. Foundation condition for cracks, settlement, or movement.

What to do: Tighten bolts to the manual specification. Repair or replace a damaged foundation. Do not continue running on a loose foundation, because the knock accelerates wear on every other component.

Mechanical Knock or Process Noise

A mechanical knock has a regular pattern that follows rotation speed. A process noise has a pattern that follows material feed or changes with the material. The two are not treated the same way.

What you observe	Likely source	First action
Regular knock that follows rotation	Mechanical fault	Stop the machine and inspect
Irregular knock that follows material feed	Foreign object or material problem	Stop the machine and inspect the die chamber
Knock that appears only at one speed	Loose component, resonance, or a bearing defect that only shows at that speed	Check mounting and foundation, then bearing condition
Knock that appears only under load	Coupling, gear, or a loose component	Inspect coupling, gearbox, and mountings
Knock with vibration and temperature rise	Advanced bearing or gear damage	Stop immediately

A mechanical knock should not be treated as a process noise, even if the machine continues to produce pellets. The knock indicates an impact that is damaging the components each time it occurs. A knock that clears on its own usually means the loose component has moved to a different position, but it can also mean that a material blockage has temporarily removed the impact condition. It is not evidence that the fault has gone.

Immediate Checks Before Stopping

These checks can be made with the machine running and narrow the cause before a stop decision.

Check	What to look for	What it tells you
Knock location	Which area is loudest	Narrows the component
Knock pattern	Once per revolution, multiple per revolution, or irregular	Separates single damage from multiple damage or foreign object
Knock behaviour under load	Worse, unchanged, or disappears under load	Points to coupling, gear, or loose component
Noise compared with the baseline	New or increasing compared with the normal-run recording	Confirms the knock is a fault
Vibration at bearing housings	New or increasing	Points to bearing or coupling damage
Motor current	Above, within, or below the manual range	Separates load change from mechanical fault
Temperature readings	New or rising trend	Points to advanced bearing or gear damage
Anchor bolts and frame	Visible movement or looseness	Points to foundation problem

If the knock is new compared with the recorded baseline, regular, and follows rotation speed, stop the machine. If the knock follows material feed, stop and inspect the die chamber for a foreign object.

When to Stop Rather Than Monitor

Do not keep running to see whether the knock clears. Stop the machine when any of the following applies. Limits must come from the machine manual.

Condition	Action
Metallic knock from the die chamber	Stop immediately. Inspect die chamber, rollers, and die holes.
Knock with new vibration or temperature rise	Stop immediately. Inspect bearings, coupling, and gearbox.
Knock from the coupling area, worse under load	Stop immediately. Inspect coupling and alignment.
Knock from the gearbox	Stop immediately. Inspect oil and gearbox internals.
Knock with motor current above the manual range	Reduce feed rate first. If knock and current do not fall, stop.
Knock from a loose frame or foundation	Stop and inspect before restart.

Cooldown and Inspection Safety

Components that are at fault may be at a temperature that causes burns, and rotating parts must be stationary before inspection.

Check area	Safety point before inspection
Die chamber	Allow the die chamber to cool before opening; lock out power
Roller assembly	Confirm the rotor is stationary and locked out before checking roller rotation by hand
Coupling and gearbox	Confirm the machine is locked out before inspection
Bearings	Allow housings to cool before touching; use a temperature reading first
Foundation and anchor bolts	Confirm the machine is locked out before checking bolts

Damage Inspection After a Knock Event

A knock can leave damage that is not visible from the control panel. Inspect the following after a stop and cooldown.

Component	What to check
Die	Cracks, chipped hole edges, uneven wear, or embedded foreign material
Rollers	Flat spots, chipped edges, uneven wear, or a roller that does not rotate freely
Roller bearings	Free rotation, noise, lubricant condition
Main shaft bearing	Temperature after cooldown, noise, lubricant condition
Coupling	Insert wear, bolt tightness, misalignment, cracks
Gearbox oil and internals	Metal particles, discoloration, gear tooth condition through the inspection cover
Belt and pulleys	Glazing, cracking, wear, or looseness
Motor	Bearing noise, mounting bolt tightness
Frame and foundation	Loose bolts, cracks, or movement

If damage is present in the die or rollers, follow the manufacturer procedure. If the hole geometry has changed, treat the die as damaged and replace it. Wear limits and acceptance criteria must come from the machine manual and the supplier documentation.

Restart After a Knock Event

Restarting is not the same as fixing. Confirm the following before returning to normal feed rate. Limits must come from the machine manual.

Acceptance item	What to confirm
Knock	No knock at reduced or normal feed rate
Vibration	No new or increasing vibration compared with the baseline
Noise level	At or below the recorded normal-run level
Motor current	Stable and within the manual range
Bearing and gearbox temperature	Stable and within the manual range
Coupling and alignment	Within the manual tolerance
Die and roller condition	No damage, no embedded material
Output and pellet quality	Consistent with the historical baseline

If the knock returns at reduced feed rate, stop and check in this order before increasing feed:

1. Knock location and pattern against the tables above.
2. Die chamber contents, roller condition, and die hole condition.
3. Coupling condition, insert wear, and alignment.
4. Gearbox oil condition and temperature.
5. Bearing temperature and noise.
6. Anchor bolts, frame, and foundation.

If all six checks are within the manual specification and the knock still occurs at reduced feed rate, do not increase feed. Stop the machine, record the conditions, and contact the supplier. Continuing to run at this point adds impact damage without correcting the cause.

Preventing Knock Faults

A knock fault is usually a maintenance condition that builds over time. The checks below are aimed at catching the condition before it produces a knock.

Check	Suggested frequency	Purpose
Knock and noise at each area	Each shift	Detect a new knock before it becomes a fault
Noise and vibration against the baseline	Each shift, or per the manual	Confirm whether the knock and vibration have increased objectively
Coupling condition and alignment	Per the manual	Prevent coupling wear and misalignment
Gearbox oil condition	Per the manual	Detect gear and bearing damage before a knock
Roller free rotation	Weekly, with the machine locked out	Detect roller bearing or seizure risk
Roller clearance and surface	Weekly, or after roller replacement	Detect uneven wear and clearance drift
Bearing temperature and lubrication	Each shift, or per the manual	Detect bearing problems before failure
Belt tension and pulley condition	Weekly, or per the manual	Prevent belt and pulley knock
Anchor bolts, frame, and foundation	Per the manual	Prevent loosening and foundation movement

General maintenance intervals should follow the machine manufacturer manual.

Spare Parts to Hold for Knock Events

Knock events can require replacement parts rather than adjustment. Exact part numbers and recommended stock levels must come from the machine manual and the supplier spare parts list.

Spare part	Why it is needed after a knock event
Coupling and coupling insert	Wear or impact damage from the event
Roller bearings	Impact and load exposure during the event
Main shaft bearings	Impact and load exposure during the event
Belt and pulleys	Wear or impact damage from the event
Anchor bolts	Loosening or damage from the event
Die and rollers	Damage from foreign objects or metal-to-metal contact
Gearbox oil and seals	Contamination and seal exposure during the event

Whether the die, rollers, coupling, or bearings need replacement depends on hole entrance diameter change, roller surface condition, coupling wear, bearing temperature and noise, and pellet density decline against the historical baseline. The limits must come from the machine manual and the supplier documentation, not from a general figure.

When a Knock Is a Configuration Problem

If a knock recurs despite die and roller condition, coupling, gearbox, bearings, belt, mounting, and foundation all being confirmed within range, the machine configuration may not match the material or the operating pattern. Confirmation must come from the machine manual, a pelletizing test, and the supplier documentation.

This is more common when a machine selected for one material is now running a denser or more abrasive material, or when a machine selected for intermittent operation is now running continuous production.

In that case, operating adjustments will reduce how often the knock occurs but will not eliminate it. The options are to change the die and roller configuration, or to change the machine mounting and foundation design. A pelletizing test with the actual material is the most reliable way to confirm which option is suitable before making a change.

If the knock cannot be located on site, send the material type, moisture, particle size, die specification, roller condition, feed rate, motor current at normal operation, knock location, knock pattern, knock behaviour under load, noise level against the recorded baseline, vibration measurement point and direction, coupling type, and the conditions under which the knock appears for a configuration review.

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Maintenance parameters, torque specifications, alignment tolerances, vibration limits, and acceptance criteria in this article must be confirmed against the machine manual, the control cabinet setting, and the die, roller, coupling, and drive supplier documentation. This article does not provide universal values for these items.