Quick answer
When a newly installed fan V‑belt loses tension shortly after the first start‑up, the most common causes are normal belt stretch during the initial run‑in, an incorrect tensioning technique, pulley or sheave mis‑alignment, worn or damaged sheave grooves, loose motor or base hardware, use of the wrong belt section or length, or a mismatched belt set. Verify tension with the belt‑manufacturer’s prescribed method after the recommended run‑in period, then re‑inspect pulley condition and alignment before applying any additional tightening.
Problem / job
Fan drive belts that appear loose, emit squeal, or deliver reduced torque within a few operating hours after replacement are a recurring maintenance issue in cement grinding and kiln‑fan applications.
Evidence
Collect belt part numbers and matched‑set identifiers, record the initial tension reading, note the elapsed run‑hours before the first drop, measure pulley centre‑line alignment, inspect sheave groove depth, and check motor‑base bolt torque and any movement of tensioning brackets.
Decision
Separate the expected belt‑stretch phenomenon from underlying problems such as incorrect belt selection, worn sheaves, mis‑alignment, loose mounting hardware, or over‑/under‑tensioning before deciding on corrective action.
Seven‑step diagnostic workflow
- Confirm belt specification and matched set. Retrieve the OEM drawing or supplier data sheet for the fan drive. Verify that the installed belt matches the required cross‑section, length, and that it belongs to a matched set (if the drive uses multiple belts). If the part number differs, replace the belt with the exact OEM‑approved item before proceeding.
- Review the tensioning method and run‑in requirement. Most manufacturers prescribe a specific tensioning technique—often a deflection measurement, a target frequency, or a calibrated tension gauge reading. Confirm that the crew used the correct method and that a post‑run‑in re‑check was scheduled. If the run‑in period was omitted, plan a controlled re‑tension after the belt has settled for the recommended hours.
- Inspect sheave grooves for wear or damage. Remove the belt and use a groove‑depth gauge or a visual inspection tool to assess the condition of each sheave. Look for rounded edges, glazing, or material loss that allows the belt to sit deeper than designed. Worn grooves reduce the effective wedging angle and accelerate tension loss. Replace any sheave that fails to meet the OEM groove‑profile tolerance.
- Check pulley and sheave alignment. With the drive stopped, lock out and tag out the fan drive according to plant LOTO procedures. Use a dial indicator, laser alignment kit, or straight‑edge method to verify that the centre lines of the motor and fan shafts are parallel within the tolerance specified by the equipment manufacturer. Mis‑alignment creates lateral forces that pull the belt out of its optimal position, leading to premature tension loss.
- Verify motor‑base and tensioner hardware integrity. After the drive has been isolated, inspect all mounting bolts, slide‑rail pins, and tensioner adjustment screws for movement or loosening that may have occurred during the first start‑up. Torque bolts to the OEM‑specified values and re‑secure any sliding brackets. A shifting base can change the effective belt length and cause the tension to drop.
- Identify contamination sources. Examine the belt path for oil, water, cement dust, or other foreign material that can reduce friction and cause slip. Clean the belt and sheave surfaces with a solvent approved for the belt material, then re‑install the belt with a fresh, dry wipe. Persistent contamination may indicate a leaking bearing, oil‑sealed motor, or inadequate sealing of the fan housing.
- Retension to the approved target and validate. Using the manufacturer‑approved tension measurement (deflection, tension gauge, or frequency), apply the final tension. Record the reading, then operate the fan under a representative load for the run‑in period. After the specified hours, repeat the tension measurement. Acceptance is achieved when the reading remains within the documented range and no abnormal heat, slip, or vibration is observed.
Diagnostic matrix
| Evidence | Likely direction | Next check |
|---|---|---|
| Tension drops once then stabilizes after retension | Normal belt stretch during run‑in | Schedule the manufacturer‑specified re‑check after the run‑in period |
| Belts sit deeper in sheave grooves than when new | Worn or damaged sheave, or wrong belt cross‑section | Measure groove depth, compare to OEM profile, verify belt section |
| Motor or base mounting bolts have moved | Loose mounting hardware or sliding tensioner | Torque all mounting bolts to spec, inspect tensioner slides |
| One belt in a multi‑belt set carries a different load | Mismatched belt length or set, or uneven sheave wear | Confirm matched‑set numbers, re‑measure each sheave groove |
| Excessive belt temperature, squeal, or visible slip | Insufficient tension, mis‑alignment, or contamination | Re‑measure tension, verify alignment, clean belt path |
| Visible oil or water on belt surface | External contamination source | Locate leak, improve sealing, replace contaminated belt if needed |
Acceptance criteria
Declare the fan drive acceptable when all of the following conditions are met:
- The installed belt matches the OEM‑specified part number, cross‑section, length, and belongs to the correct matched set.
- Sheave grooves are within the tolerance limits defined by the equipment manufacturer and show no glazing, rounding, or excessive wear.
- Motor and fan shaft centre lines are parallel within the alignment tolerance documented for the specific fan model.
- All motor‑base bolts, tensioner adjusters, and slide‑rail pins are torqued to the OEM values and show no movement after a representative operating period.
- Measured belt tension (deflection, gauge load, or frequency) remains within the manufacturer‑approved range after the prescribed run‑in hours.
- No abnormal belt temperature rise, audible squeal, or visual slip is observed at the normal fan load.
- The drive operates without any LOTO‑related safety incidents, and all isolation points remain secured throughout the verification run.
Common mistakes and repeat‑failure traps
- Skipping the run‑in re‑check. Applying the final tension only once and assuming it will hold ignores the natural stretch that occurs during the first few hours of operation.
- Using an incorrect belt length or cross‑section. Even a small deviation can change the effective tension and cause premature loosening.
- Neglecting sheave inspection. Worn grooves are a frequent hidden cause; installing a new belt on a damaged sheave will not solve the problem.
- Relying on visual “tightness” alone. Subjective feel is not a substitute for the manufacturer‑specified measurement method.
- Over‑tightening to compensate for perceived slack. Excessive tension can overload bearings, increase belt wear, and generate heat.
- Failing to lock out power and mechanical energy. Performing adjustments without proper LOTO exposes personnel to sudden belt engagement or motor start‑up.
- Ignoring contamination. Oil, water, or cement dust on the belt or sheave reduces friction and accelerates tension loss; cleaning must be part of every re‑tension cycle.
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