Blower and Motor

Smaller pulley on the motor and the larger pulley on the blower fan. If you do it the other way then this happens....

Same size pulleys, same RPM.

Smaller pulley on blower blower runs faster.

Who took the motor and blower apart?

They should remember which was where and the shaft sizes are probably different too!

• Smaller shaft on the motor.
• Larger shaft on the blower.

Smart Mechanic's trick: When taking belt driven devises apart mark the pieces with the location of the part, rotation direction, location on shaft of the pulley (use a center punch lightly to mark the shaft on both sides of the pulley or a triangular file to lightly scratch the surface of the shaft) and anything else that you will need to reassemble the unit.

Take it apart in your mind before and as you take it apart. Mark it if you are not positively sure of the information above. If necessary use a pad of paper to make illustrations with dimensions of it for use when reassembling. If there is any doubt mark it in a compatible way. Ink markers, wax pencils, colored artists pencils, paint sticks, paint markers, pencils, nail polish, chalk and many others things should be considered. Just make sure the marker material is compatible with any solvents or cleaning agents.

Belt and pulleys: small pulley to big pulley speed goes down; large pulley to small pulley speed goes up; same size same speed!

Good Luck, Old Salt

If the motor and blower shafts are the same size, it would be possible to exchange the pulleys by accident. If you put the larger pulley on the motor, thus making the fan run too fast, it will probably trip the motor overload device. If so, switch the pulleys. In most cases (though not all) the motor pulley will be the smaller of the two.

The motor pulley is smaller than the blower pulley in most applications.

You really need to contact the blower manufacturer and find out the maximum allowable speed of the blower.

If you are unable to determine the manufacturer of the fan I suggest you start by selecting a pulley for the blower that is the same outside diameter as the blower cage/fan.

I would then divide the selected blower pulley diameter by a factor of 4 and purchase an adjustable pulley with a diameter of that number to fit the motor shaft.

This should give you a safe place to start testing blower performance.

I would make sure the pulleys are in alignment, set the adjustable pulley at 2/3 open, then install the drive belt and adjust to correct tension.

Then using a portable ammeter such as a clamp-on hand-held type I would start the motor and take a motor current reading.

If the motor current is within acceptable range per the motor nameplate, I would close the adjustable pulley a small increment which will increase the drive pulley diameter and increases blower speed.

Repeat the process until the reading(s) reach maximum motor current or the pulley has reached maximum adjustment or the blower has attained maximum output or satisfies application requirements.

It may be necessary to increase the motor drive pulley diameter in order to attain the blower speed desired.

(RPM₁)(PD₁) = (RPM₂)(PD₂), where PD is pulley pitch diameter.