TL;DR

A slipping belt loses fan speed and generates heat at the sheave. Check belt condition, tension, and sheave groove wear. A glazed or cracked belt must be replaced, not just re-tensioned.

What you might see

fan RPM below design speed
belt squeal or burning smell
visible shiny or glazed inner surface on the belt
belt running warm when checked by hand after shutdown

Likely causes

Tap to mark as ruled out

Belt under-tensioned from stretching over time
Glazed or worn belt with insufficient friction coefficient on the sheave groove
Sheave groove worn smooth, reducing the belt's grip surface
Wrong belt cross-section for the sheave groove profile

Required tools

Belt tension gauge or spring scale
Sheave groove gauge
Replacement belt (matching original designation)
Wrench set for motor base adjustment
LOTO kit

Safety first

Apply LOTO before opening the belt guard. A spinning sheave will pull a loose belt end or a hand into the drive.
Do not apply belt dressing spray to a slipping belt as a long-term fix. Belt dressing masks the symptom and can contaminate the sheave groove, causing the replacement belt to also slip.

Procedure

7 steps · ~30 min

1
Lock out the fan disconnect.
Warning: Never touch, inspect, or tension a belt while the fan is running. The sheave pinch point can pull fingers into the drive.
2
Open the belt guard and inspect the belt for glazing, cracks on the outer cover, fraying, or missing cogs on a cogged belt. Any of these conditions require belt replacement.[1]
3
Check the sheave grooves with a sheave groove gauge. Worn grooves (the belt bottom contacts the groove floor) mean the sheave must be replaced. A new belt on a worn sheave will slip almost immediately.[1]
4
If the belt and sheave are in good condition, check belt tension. Apply the specified deflection force at belt midspan and measure deflection. If deflection is greater than the Greenheck specification, add tension.[1]
5
Adjust tension by loosening the motor base jam nuts and rotating the tensioning screw clockwise to increase center distance. Snug the jam nuts when tension is correct.
6
Re-check tension after 2 to 4 hours of operation (new or re-tensioned belts take a short break-in period).
7
Replace the belt with an equivalent cross-section and length. Match the original belt designation exactly. Do not mix belt types on a multi-belt drive.

Sources

Greenheck CUE Centrifugal Upblast Roof Exhaust Fan Installation and Maintenance Manual, Greenheck Fan Corporation
Greenheck CUE Utility Fan installation, operation and maintenance instructions, belt tensioning and replacement procedures (general)
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