Cooling Tower Fan Sheaves

Cooling tower fan sheaves are pulley components used in the belt drive system of many cooling towers, especially forced draft and induced draft types. They play a key role in transmitting power from the motor to the fan shaft by means of V-belts. Here’s a breakdown of what they are, why they matter, and how they’re managed:

🔧 What Is a Cooling Tower Fan Sheave?

A fan sheave is a grooved wheel mounted on the fan shaft. It works in tandem with a motor sheave (attached to the motor) and is connected by V-belts. The motor turns the motor sheave, which turns the fan sheave via the belts — spinning the fan and pushing or pulling air through the tower.

⚙️ Key Components of a Sheave System

Fan Sheave: Mounted to the fan shaft; rotates the fan.
Motor Sheave: Attached to the electric motor.
V-Belts: Multiple belts that transmit torque.
Bushings (e.g., QD or Taper-Lock): Used to mount the sheave securely to the shaft.
Keyways and Set Screws: Help secure the sheave and prevent slippage.

📏 Importance of Sheave Sizing

Sheave diameter directly affects fan speed (RPM):

A larger fan sheave or smaller motor sheave slows the fan.
A smaller fan sheave or larger motor sheave increases fan RPM.

Incorrect sheave sizing can:

Overload motors
Cause insufficient airflow
Increase wear and energy usage

Formula:

$RPM\text{Fan RPM} = \left( \frac{\text{Motor Sheave Diameter}}{\text{Fan Sheave Diameter}} \right) \times \text{Motor RPM}$

⚠️ Common Issues with Cooling Tower Sheaves

Worn Grooves – Causes belt slippage, noise, and reduced efficiency.
Misalignment – Leads to premature belt wear or sheave failure.
Rust or Corrosion – Especially in outdoor, humid, or chemical-exposed environments.
Loose Fit or Damaged Bushing – Can cause vibration and shaft damage.

🔍 Maintenance Tips

Check belt tension and alignment monthly
Inspect sheave grooves for wear — a groove gauge can help
Replace worn belts and sheaves in sets to ensure even load distribution
Lubricate bushings as needed (depending on type)
Use anti-corrosion coatings or stainless steel for harsh environments

🏗️ Materials & Types

Cast Iron: Most common, durable, affordable
Aluminum: Lightweight but less common for high-torque
Steel or Stainless Steel: Used in corrosive or industrial-grade applications

Types:

Fixed Diameter Sheaves
Adjustable (Variable Pitch) Sheaves – Useful for fine-tuning fan speed during commissioning or seasonal adjustment

✅ Sheave Manufacturers Common in Cooling Towers

Browning
Dodge
TB Wood’s
Martin Sprocket & Gear
Lovejoy

Sheave Sizing Calculator

Here’s a Cooling Tower Fan Sheave Sizing Calculator you can use manually or in Excel. It helps you determine the correct fan sheave diameter based on your desired fan speed.

⚙️ Sheave Sizing Formula

$RPM)\text{Fan Sheave Diameter} = \left( \frac{\text{Motor Sheave Diameter} \times \text{Motor RPM}}{\text{Fan RPM}} \right)$

✅ Inputs You Need:

Parameter	Description	Example
Motor RPM	Motor’s rated speed (usually 1750 or 3600)	1750 RPM
Motor Sheave Diameter	Diameter of the motor pulley	6.0 inches
Desired Fan RPM	Target fan speed (check tower specs)	875 RPM

📊 Sample Calculation

Given:

Motor RPM = 1750
Motor Sheave = 6 inches
Desired Fan RPM = 875

Then:

$inches\text{Fan Sheave} = \left( \frac{6 \times 1750}{875} \right) = 12 \text{ inches}$

So, you’d need a 12-inch fan sheave to get 875 RPM from a 1750 RPM motor using a 6-inch motor sheave.

📋 Excel Version (Formula)

You can paste this into a spreadsheet:

A	B
Motor RPM	`1750`
Motor Sheave Dia	`6`
Target Fan RPM	`875`
Fan Sheave Dia	`=B2*B1/B3`

That cell will calculate the fan sheave diameter dynamically.

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