Maintenance & PM's

A loose belt can slip and overheat. Excessive tension increases bearing loads and can shorten belt and bearing life. The aim is to achieve the manufacturer’s specified tension.

This procedure applies to conventional V-belts on an adjustable motor base. Always check the instructions for the particular fan and belt.

1. Isolate the equipment

Stop the fan, isolate and lock out the electrical supply, and verify the equipment is safe before removing guards. Allow the fan to stop completely and prevent airflow from other equipment turning it. A BMS stop command alone is not isolation.

2. Inspect the belts and pulleys

Check for cracking, fraying, glazing, contamination and worn pulley grooves. Check the motor mounting and fan bearings. On multiple-belt drives, replace worn belts as a suitable matched set. Release tension before fitting belts—never force them over a pulley.

3. Check pulley alignment

The motor and fan shafts should be parallel, with corresponding pulley grooves aligned. Use a suitable straightedge or laser alignment tool. When comparing pulley faces, account for any difference between the face and groove positions.

4. Measure tension using the correct method

For many conventional V-belts, the force–deflection method uses:

Deflection distance = free belt span ÷ 64

The free span is the straight section of belt between the pulleys.

For example, a 600 mm span gives a test deflection of approximately 9.4 mm, measured at the midpoint.

Use a belt tension gauge to measure the force needed to produce that deflection. Compare it with the belt manufacturer’s specified force for the drive, including the appropriate new-belt or run-in setting.

The deflection distance alone does not confirm correct tension—the force required matters too. There is no single force setting suitable for every fan

belt.

5. Adjust the motor position

Following the mounting instructions, loosen the appropriate motor-base fasteners and use the adjustment screws to move the motor:

• Further from the fan shaft to increase tension.

• Closer to the fan shaft to reduce tension.

Adjust evenly to maintain alignment. Opening or closing a variable-pitch pulley changes fan speed; it should not be used to correct belt tension.

6. Secure and recheck

Tighten the mounting fasteners to the specified torque, then recheck alignment and tension. Check each belt on a multiple-belt drive.

7. Refit guards and test

Remove tools, refit guards and close access panels before restarting. Check for slipping, unusual noise and vibration under normal operating conditions. Have motor current checked safely against its rating. Isolate again before making further adjustments.

8. Arrange the follow-up check

Follow the manufacturer’s run-in and maintenance schedule. New conventional V-belts commonly need an early tension recheck as they settle into the grooves.

What’s your approach—do you use a force–deflection gauge or a sonic tension meter when servicing AHU fan belts?