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Some Belt Tensioning Thoughts
We had a really great conversation on the HVAC School Facebook Group about some belt tension best practices and it turns out that even a lot of really smart and experienced techs are not aware of all the factors related to belt tensioning.
Myth #1 is that amperage is used to set belt tension. Now don't get me wrong, checking amperage before and after changing belt tension is an excellent practice to ensure you are not binding the bearings from over tension, it does not tell you whether or not the belt is at optimum tension.
I think Browning summarizes it best in this statement from their Browning tool box technician app
Ideal tension is the lowest tension at which the belt will not slip under peak load conditions
Getting a belt too tight shortens the life of the belt and bearings and can cause high amperage. Leaving a belt too loose will shorten the belt life and result in loss of airflow and noise.
Many techs confuse the sheave adjustment, designed to alter the pulley ratio and the airflow with the belt tension adjustment. These are not the same thing and serve separate purposes.
The adjustable sheave allows the pulley faces to adjust closer or further from one another, resulting in a belt that rides closer to the hub when looser (halves further apart) or closer to the edge when tighter (halves further separated) THIS ADJUSTMENT IS FOR FAN SPEED ONLY NOT TENSIONING
A properly tensioned belt should not slip significantly when starting, it should not be noisy and it should not bounce around. If you tighten the belt check the amps before and after and the motor should not overamp.
The correct tension method is to get the belt close to the correct tension by feel with a deflection of 1/64 of an inch for every 1″ of distance between the two pulley centers. You can then use an app or a chart like THIS ONE to find the proper force to generate this deflection.
You would then use a belt deflection tool like the one shown above to test the deflection force required and adjust accordingly. The video below demonstrates this.
I like what Jeremy Smith stated in the group “Belt tension has less impact on motor amperage than pitch diameter of the sheave and how that affects total airflow. Use the Emerson tool and the app (or paper chart if you're all stone age) Record tension and other data (sheave diameter, center to center length, rpm and proper tension) on the blower housing.”
Check those belts during commissioning, maintenance and service and change them as needed.