CHAPTER 7 | Drives

Cooling tower fans can be directly coupled to the motor, or, connected via gear boxes or V-belts.

Direct drives are limited to small diameter fans; Otherwise, tip speeds would be too great- even with 900rpm motors. Direct drives are, therefore, generally found on small cooling towers; Or, medium size towers with numerous small fans.

V-belts are used on the full range of factory assembled towers- from less than 10 tons to over 1,000 tons. They allow the manufacturer to easily tailor the speed of fans through the judicious selection of sheave (pulley) diameters. Fixed pitch prop fans can be used. [Similar to centrifugal fans- their fixed geometry pretty much demands V-belt drives.] V-belt drives do not require precise alignment and work fine after the rigors of shipment and rigging.

Belts and sheaves come from numerous manufacturers. Many offer proprietary designs where the belt and sheave have special shapes peculiar to that manufacturer. I am only familiar with those that are ‘conventional’. This is to say, manufactured to standard, industry wide tolerances.

To keep from having different sheaves for each fan shaft diameter, they are made with the center hole machined on a taper. A bushing with the correct taper and bore size adapts the sheave to the fan shaft. This tapered attachment method insures concentricity of the shaft and sheave and locks the sheave securely. Disassembly is made easy by extra threaded holes that turn the bushing into a ‘puller’.

Bushings are described by letters- defining the taper geometry- and the bore size/shaft diameter. For example... SK x 1-11/16”.

Like sheaves, bushings can be proprietary or conventional; They can’t be mixed.

The glossary has ‘B’ section belt, sheave, and bushing data. Anyone working with these items regularly should secure a catalog from a manufacturer such as Woods or Electron.

Gear boxes are used on just about all large field erected cooling towers and many factory assembled towers- all of the prop fan, draw-thru type. Almost all are right angle gear drives with the input shaft horizontal and the output vertically ‘up’. The designer selects the ratio closest to his/her requirement based on the fan being used, noise levels and cost. Limited ratios are available and the fan blades must be ‘pitched’ to the proper angle to achieve the desired performance for the specific project- typically not a problem since large fans are built on site anyway.

Cooling tower manufacturers seem to subscribe to high quality standards for their gears but it would not hurt to specify as follows (borrowed from the Ceramic Cooling Tower Company):

“Speed reducers shall be right angled gear drive type specifically designed for cooling tower service. Gears shall be spiral bevel or helical type designed in accordance with the Cooling Tower Institute and American Gear Manufacturer’s Association Standards with a minimum service factor of 2.0 based on motor horsepower. The gear shall be suitable for both forward and reverse operation. Oil fill and vent lines shall be extended outside the fan stack and provided with an oil level sight glass to facilitate routine inspection and maintenance.”

Fan motors can connect directly to the gear box or through a drive shaft. Either way, motor and gear alignment are critical and mountings must be solid. Various manufacturers make flexible couplings specifically for difficult cooling tower duty. It is in the cooling tower manufacturer’s best interest that these items work properly and be trouble free; The cooling tower specifier should not concern his/her self unduly with materials except to specify couplings that use no rubber if ozone water treatment will be present.