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Hello, I’m Greg Allen, Application Engineer at Greenheck. Welcome to our series of videos on Fan Maintenance.
In this video I will be demonstrating the common methods for tensioning a rooftop exhaust fan belt.
For demonstration purposes we will be using a fan with a cutaway for you to better see what we are doing.

Before beginning work on any exhaust equipment, it is important to adhere to all safety standards. Be sure to wear appropriate Personal Protection Equipment, and adhere to all local codes.
As we will have our hands exposed to rotating elements, locking out the power source is always a must.

Now that the power is removed, we can safely open up the exhaust fan.
By removing the hood we expose the belts, pulleys and sheaves and motor.

Before we get into proper belt tension, we should look at the belt to make sure it is serviceable. Meaning free of defects.
The belt will need to be replaced if it is showing signs of excessive wear, cracking, glazing or is just falling apart.
Now that we have a good belt on, we need to verify the alignment of the pulleys.
Poor alignment of the pulleys will cause premature wear, and shorten the life of the belts, pulleys and even the motor.

The motor sheave and fan pulley should be on the same plane, and not be skew.

With everything lined up, give the wheel a quick spin to check for wheel alignment and verify that there are no obstructions or failed bearings.
If the wheel is rubbing or out of alignment, please check out our other videos on center wheel.

There are several methods that can be used to check for proper belt tension.
On small units the guidance is to tension the belt just enough to prevent slippage at full speed.

For fans of larger size, measurement and calculation is required.

This diagram is taken from the units IOM, or Installation, Operation and Maintenance Manual, and gives us the instruction needed to properly tighten the belt.

There are 4 bolts that need to be loosened so that we can adjust the motor plate and apply the proper tension.

When removing a belt, the 4 bolts should also be loose so that the belt comes of easy and the belt is not stretched during installation.

The motor plate can now be moved to obtain the proper belt tension.

For smaller units we are tensioning the belt just enough to not have slippage when the fan is running.

Tighten the 4 fasteners to lock the motor in place, and safely re-apply power to check the belts tension and alignment. There should be a slight bow in the slack side of the belt when the unit is running.

For larger units, the belt tension should be adjusted such that there is 1/64” of deflection per inch of belt span.

The belt span is measured from center of shaft of the motor to center of shaft of the impellor.
We take this measurement and divide by 64.
Now we have the correct amount of deflection in the belt.
This is measured at the mid-point between the pulleys, pressing your thumb with moderate pressure.

This can be an iterative process, and the motor plate moved several times in order to get the proper belt tension.

There are several tools available on the market that can assist in this process, such as a belt tension tool.
As you can see, I like to have a long rubber band available to assist me.

Now that we have proper tension, we should bump the motor to check alignment.
It is also a good idea at this time to check the motors amp draw and compare this value to the motor’s nameplate FLA, or Full Load Amps.
If a belt is too tight, or the motor has issues, many times this manifests itself as a high current.

Thank you for your time, please check out our other videos on fan maintenance.
Also stop by Greenheck.com to look at any of the IOM’s for products we've covered today.