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Pulse Jet Pointers
Vol. 5, No. 1

Sheave Shenanigans

When selecting a V- belt drive for a fan, the required information for any drive supplier are shaft sizes, center distance, RPM of driver and driven shafts, motor HP/frame size, and minimum service factor desired. The drive supplier usually cranks up his computer which generates three or more possible selections, all of which he may or may not share with you. He will certainly price the one best and least expensive selection.

If you ask for variable pitch drive, he should ask what RPM range you are after, and if the service factor you desire is to be computed at the minimum, maximum, or some intermediate speed. But he won't! Instead, he will proceed as follows:

1) Put the variable pitch sheave on the smaller diameter. Fan shaft slower than the motor speed, then it will be the motor sheave. Fan shaft faster, then it will be the fan sheave. This of course limits the variation, probably to about 10% either side of the center point.

2) Next he will shade your request toward the more equal sheave selections, so if you have a fan running faster than motor speed, he will make nearly all adjustments toward a slower speed than desired. If you have a fan running slower than motor speed, he will make nearly all adjustments to a faster speed than desired.

3) Now comes the triple whammy - he will figure the service factor at a minimum speed or maximum speed as suits the situation. Whatever gives the

more equal sheave sizes yields the most belt wrap and therefore the highest service factor. Say you ask for a 1.2 service factor. He picks "F" for "fast" in his computer program (that is if you have asked for a slow-down drive) and the computer picks a drive that will yield 1.2 or better with the adjustable sheave cranked up to the maximum RPM. When this same drive is cranked down to the lowest speed, the service factor can drop below 1.0!

Customer (conservative guy that he is) starts up his fan at the slowest speed and snap-snap-snap, all the belts break. He gets real confused at this point because you tell him to speed the darn thing up to maximum and he won't have any trouble. How can this be? The reason is that the motor, upon starting, whips up the drive at maximum torque, regardless of the eventual break-horsepower, which at a slow RPM can be very low. The drive gets its test on the start of the motor. Once it is running, it could actually survive on a 1.0 service factor. It is the torque on the start up that tests the metal of any V-belt drive, and for the same reason, service factors are always figured on motor horsepower, not on fan break-horsepower.

Also note, variable speed drives are never easy to adjust, especially when the horsepower exceeds 25. Figure two men, two hours to adjust a 25 HP V-belt drive, if they have done it before! Well, this was not directly about dust collection, but I thought you should have the information.

Bruce Beckert


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