Setting the proper tension on a drive belt is critical to it's proper functioning and longevity. Because drive belts are used to mechanically connect two or more rotating pulleys, the tension of the belt is directly transmitted to the pulleys, creating a sideways "load." Too much load will destroy the pulleys, while too little load will allow the belt to slip or run off the pulley. The only way to know for sure whether the belt has the proper tension is to measure the tension with a tension gauge, then compare that value to what the device manufacturer suggests as "proper tension," more officially called the "recommended deflection force in pounds."
Turn off the device or machine that incorporates the belt to be measured. Ensure that the machinery cannot be started by a third party, by locking the device or otherwise rendering it inoperable.
Ensure that the belt to be measured is correctly positioned within the pulley grooves, or correctly centered on a nongrooved pulley, and has been correctly installed.
Measure the full "span" of the belt to be measured by holding a tape measure at the center point, or axle, of one pulley, then extending the tape measure until it reaches the same point, the axle, on the other pulley. The distance between the axles is the belt span.
Hold the tension gauge in one hand. Set the body of the tension gauge so that it is calibrated to correctly represent the "span" in inches of the belt just measured. The tension gauge is adjusted by twisting it at either end like an adjustable curtain rod; the tool handle is marked so that as you screw or unscrew it, the correct span of the belt in inches is displayed on the handle of the tool.
Reset the sliding O-ring on the end of the tension gauge handle that measures the deflection force in pounds. This O-ring moves along a plunger located within the handle of the tool, much like a tire pressure gauge.
Hold the tension gauge midway between the axles of the two belt pulleys previously measured. With the tool perpendicular to the belt, apply a downward force to the belt (using the tool) in the middle portion of the belt span. This will cause the belt to "deflect" or sag. Keep applying pressure on the belt until the scale in the tools handle shows you have attained 1/64 inch of deflection.
Look at the plunger and O-ring configuration at the end of the tool's handle. The O-ring will have moved on the plunger to represent the amount of deflection force in pounds required to deflect the belt 1/64 inch. Tighten or loosen the belt until deflecting the belt 1/64 inch requires the correct amount of force according to the manufacturer's suggested belt tension.
