⚠️ Disclaimer: These tasks are guidelines only. They do not include lockout/tagout (LOTO), energy isolation, or other safety requirements. Review and verify suitability for your specific equipment and application. Add all required safety procedures per your company's policies and regulatory requirements before use. You are responsible for the safe and appropriate execution of all maintenance activities.
Mixed flow fans fail the way most industrial equipment fails — gradually, then all at once, with plenty of warning signs that nobody recorded. The impeller picks up buildup one layer at a time until the imbalance is measurable. The bearings run hot for months before anyone checks. The belt stretches incrementally until it slips. None of this is sudden. All of it is preventable.
This checklist covers standard preventive maintenance for mixed flow fans used in industrial HVAC, exhaust, and process ventilation applications.
For the broader failure modes and PM strategy behind any fan type, start with Industrial Fan and Blower Preventive Maintenance: Why Most Programs Miss the Failures That Matter.
How to Use This Checklist
Record specific findings — not checkmarks. "Bearing temp: 156°F" is a finding. "Checked" is not. If the impeller has visible buildup, write down where it is, how thick, and what material. If the belt shows glazing on two ribs, write that down. Trend this data over time and the equipment tells you what's changing. Ignore it and you're just burning PM hours for compliance credit.
A bad finding looks like this: OK. A good finding looks like this: Inlet bell — light rust scale on lower 30% of bell, no structural damage, cleaned in place.
Visual Inspection Tasks
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect fan housing, inlet bell, and discharge for dents, corrosion, cracks, or debris accumulation. | Every PM | MEC |
| Check impeller visually for blade erosion, corrosion, cracking, or buildup. Note any imbalance indicators such as excessive vibration. | Every PM | MEC |
| Inspect motor exterior for overheating, discoloration, or physical damage. Verify motor ventilation openings are clear. | Every PM | MEC |
| Inspect inlet screen or guard for damage, blockage, or corrosion. Clean and repair as needed. | Monthly | MEC |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Inspect fan housing, inlet bell, and discharge for dents, corrosion, cracks, loose panels, or accumulated debris. Compare to prior PM condition. Document any deterioration. | Every PM | MEC |
| Inspect impeller blades for erosion, pitting, cracking, corrosion, or uneven buildup. Any material buildup on blades causes imbalance — document and schedule cleaning if needed. | Every PM | MEC |
| Inspect motor exterior for overheating signs, discoloration, cracked housing, or physical damage. Verify cooling openings are unobstructed. Record any findings. | Every PM | MEC |
| Inspect inlet screen or guard for blockage, corrosion, or damage. Clean thoroughly. Blocked inlet significantly reduces airflow and increases motor load. | Monthly | MEC |
Operational Checks
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Listen for abnormal sounds while fan is running — rattling, grinding, squealing, or thumping. Investigate and document any unusual noise. | Every PM | MEC |
| Verify fan rotates in the correct direction on startup. Confirm airflow is in the intended direction. | Every PM | MEC |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Listen and feel for abnormal operating sounds while fan is running — rattling, grinding, squealing, rumbling, or intermittent thumping. Vibration felt through housing may indicate imbalance or bearing failure. | Every PM | MEC |
| Verify fan rotates in the correct direction on startup and produces airflow in the intended direction. Incorrect rotation is a common result of motor wiring changes or VFD replacement. | Every PM | MEC |
Mechanical Inspection
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Check all accessible fasteners — housing bolts, motor mount bolts, and access panel hardware — for looseness. Retighten as needed. | Monthly | MEC |
| Inspect drive belt(s) or coupling for wear, cracking, glazing, or misalignment (if belt or flexible coupling driven). Replace worn belts; adjust tension to spec. | Monthly | MEC |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Check all accessible structural fasteners — housing bolts, motor mounting bolts, support frame hardware, and access panel fasteners. Retorque to spec where loose. | Monthly | MEC |
| Inspect belt drive: check belt tension, wear, cracking, glazing, and fraying (if belt driven). Check sheave/pulley alignment. Adjust tension per manufacturer spec; replace worn belts. Log belt size and condition. | Monthly | MEC |
| Inspect flexible coupling elements for cracking, wear, or rubber/elastomer degradation (if direct-coupled). Verify coupling alignment is within tolerance. | Monthly | MEC |
| Verify fan shaft for axial and radial play. Excessive movement indicates bearing wear. Check and record shaft runout at the hub if accessible. | Quarterly | MEC |
| Verify motor mounting hardware — base bolts, foot pads, and vibration isolators — are tight, undamaged, and correctly positioned. Retorque mounting bolts to spec. | Semi-Annually | MEC |
| Check fan shaft alignment to motor (direct-coupled installations) using a dial indicator or laser alignment tool. Misalignment accelerates bearing wear and increases vibration. Record offset and angularity readings. | Annually | MEC |
| Perform a full motor-off inspection of impeller: check blade attachment points, hub, and set screws for integrity. Verify impeller is secure on shaft — check key, keyway, and locking hardware. | Annually | MEC |
Lubrication
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Lubricate fan shaft bearings per manufacturer specifications — correct grease type and quantity. Do not over-grease. | Quarterly | MEC |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Clean fan housing interior and impeller of accumulated dust, lint, or process deposits. Even small buildup on one blade causes measurable imbalance. Use appropriate PPE for the material type. | Quarterly | MEC |
| Lubricate fan shaft bearings per manufacturer specifications — correct grease type, quantity, and interval. Run fan briefly after greasing to purge excess grease from re-lube type housings. Log lubricant type and amount. | Quarterly | MEC |
| Inspect fan shaft bearings for elevated temperature using an IR gun or contact thermometer. Housing temperature should not exceed ambient + 40°C or manufacturer limit. Investigate any hot bearings immediately. | Quarterly | MEC |
Cleaning
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Clean interior of fan housing and impeller of dust, lint, or process buildup that could cause imbalance or reduce airflow. | Quarterly | MEC |
Note: The Reference Checklist combines impeller cleaning with bearing inspection under Lubrication above (MFF-010 through MFF-012).
Electrical Inspection
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect all accessible electrical connections at the motor — check for looseness, corrosion, or heat damage. Do not operate with damaged wiring. | Semi-Annually | ELE |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Measure and record motor operating current on all leads using a clamp meter. Compare to nameplate FLA and prior readings. Values trending upward may indicate impeller buildup, restriction, or bearing degradation. | Quarterly | ELE |
| Inspect all motor electrical connections at the terminal box for looseness, corrosion, discoloration, or damaged insulation. Retorque to spec. Apply anti-oxidant compound to aluminum terminations if applicable. | Semi-Annually | ELE |
| Perform an insulation resistance (megger) test on the motor at 500V or 1000V DC per motor rating. Record results in MΩ. Values below 1 MΩ require immediate follow-up. Trend results over time. | Semi-Annually | ELE |
| Review and trend all recorded PM data — motor current, bearing temps, insulation resistance, vibration observations, and defect history. Identify deteriorating trends and adjust PM scope or frequency as warranted. | Annually | ELE |
Failure Modes This Checklist Targets
Impeller Imbalance from Buildup Material deposits on fan blades are almost never uniform. One blade picks up more than another. The mass difference creates a rotating imbalance that accelerates bearing wear, loosens fasteners, and eventually damages the housing. Even light buildup matters — catch it before it gets structural.
Bearing Failure Fan shaft bearings run continuously under combined radial and axial loading. Under-lubrication accelerates wear; over-lubrication generates heat and degrades the grease. Either path ends the same way. Trending bearing temperature and catching abnormal noise early is the difference between a planned bearing replacement and an unplanned shutdown.
Belt Wear and Slippage Belt-driven mixed flow fans are routine until the belt isn't. Glazing from slippage, cracking from age, and tension loss from stretch are all gradual — and all catchable on a monthly inspection if someone is actually looking at the belt instead of checking a box.
Incorrect Rotation Direction A fan running backwards moves significantly less air than design intent and loads the motor differently than expected. This happens after motor wiring changes, VFD replacement, or any time the motor is swapped. It takes ten seconds to verify. Most programs skip it entirely.
Motor Electrical Degradation Loose terminal connections generate heat. Heat degrades insulation. Trending motor current over time reveals loading changes long before a failure is imminent. A current reading trending up over three consecutive PMs is a finding — not a data point to ignore.
Housing and Structural Loosening Vibration loosens fasteners over time. A housing bolt that was tight six months ago may not be now. Fan vibration amplifies when fasteners are loose — which loosens more fasteners. Catching one loose bolt on a monthly check prevents the kind of secondary damage that turns a $15 part into a costly housing repair.
Download This Checklist
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