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Unit 4 — Electrical Fundamentals
Section 3 — Working With Motors

3.6 — Pulleys and Belts

Belt drives connect motors to HVAC/R equipment such as fans, blowers, and pumps. This lesson covers pulley selection and installation, alignment and tension procedures, belt types and their characteristics, and field methods for measuring and selecting belt lengths.

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3.6.1 — Pulley Selection and Installation

Motor pulley (sheave) selection determines the speed ratio between the motor and the driven equipment. Selecting the correct pulley sizes is the first step in designing or servicing any belt drive.

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Speed Ratio Formula

Driven Speed (RPM) = Motor Speed (RPM) × (Motor Pulley Diameter ÷ Driven Pulley Diameter)

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Worked example

A 1 750 RPM motor with a 3″ (76 mm) pulley driving a 12″ (305 mm) pulley:
Driven Speed = 1 750 × (3 ÷ 12) = 437.5 RPM

Selection Considerations

Pulley Installation Procedure

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Installation Steps

  • 1 — Shaft Preparation: Clean shaft thoroughly, remove burrs, and confirm keyway is properly cut
  • 2 — Key Installation: Install key in shaft keyway, ensuring proper fit without looseness or forced fit
  • 3 — Pulley Mounting: QD bushing — install bushing in pulley, tighten bolts evenly in alternating pattern; Setscrew — position pulley, align setscrew over key, tighten firmly
  • 4 — Positioning: Position motor pulley flush with end of shaft or per manufacturer recommendation, typically within 1.6–3.2 mm (1/16–1/8″) of shaft end
  • 5 — Tightening: Use proper torque — typically 14–20 N·m (10–15 ft·lb) for small pulleys, higher for larger units

3.6.2 — Pulley Alignment and Belt Tension

Misalignment and improper belt tension are the two leading causes of premature belt failure and excessive bearing wear. Both must be checked and set correctly at installation and verified during preventive maintenance.

Pulley Alignment

Pulleys must be aligned in two planes simultaneously. Parallel alignment ensures pulley shafts are parallel to each other; angular alignment ensures pulley grooves lie in the same plane.

📏 Straightedge Method
  1. Loosen motor mounting bolts to allow adjustment
  2. Place a straightedge across the face of both pulleys
  3. Verify the straightedge contacts both pulleys evenly across their full faces
  4. Adjust motor position side-to-side to achieve alignment
  5. Verify from both sides of the drive
  6. Confirm straightedge is parallel to both shafts
🔗 String Method
  1. Wrap non-elastic string around both pulleys
  2. Adjust until string tension is equal on both sides
  3. Measure distance from string to each pulley face on both sides
  4. Adjust motor position until all four measurements are equal
Laser alignment tools project a beam parallel to one pulley and measure deviation at the second. Use for critical applications where straightedge precision is insufficient.
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Alignment Tolerances

  • Offset (parallel misalignment): Within 5 mm per metre of center distance (1/16″ per foot)
  • Angular misalignment: Within 0.5 degrees
  • Visual check: Belt should run centered in both pulleys without riding on groove edges

Belt Tension

⬇️ Too Loose
  • Belt slips under load
  • Excessive heat and rapid wear
  • Squealing noise under load
  • Reduced power transmission efficiency
⬆️ Too Tight
  • Excessive bearing load and shaft deflection
  • Premature bearing failure
  • Belt fatigue and reduced belt life
  • Increased motor current draw
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Deflection Method — Most Common Field Method

  • Measure the span length (center-to-center distance between pulleys)
  • Target deflection: typically 1.5 mm per 100 mm of span (1/64″ per inch)
  • Apply moderate thumb pressure perpendicular to the belt at mid-span
  • Typical result: 13–25 mm (1/2–1″) deflection for a 610 mm (24″) span
  • Adjust motor position to achieve specified deflection, then re-torque mounting bolts

Strand tension gauges measure actual belt tension in N or lb for more accurate results. Sonic tension meters analyze vibration frequency when the belt is plucked — most precise, requires specialized equipment.

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Tension Adjustment Procedure

  • Loosen motor mounting bolts
  • Move motor away from driven equipment using adjusting screws or jacking bolts to increase tension
  • Verify pulley alignment is maintained throughout adjustment
  • Check belt tension using chosen method
  • Tighten mounting bolts evenly while holding motor position
  • Recheck tension and alignment after final tightening
  • Run motor briefly and recheck — new belts require re-tensioning after initial run-in

3.6.3 — Types of Belts

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V-Belts
Most Common in HVAC/R
Classical V-Belts

Trapezoidal cross-section wedges into matching pulley grooves, providing grip through wedging action. Good efficiency (95–97%), absorbs shock, and tolerates slight misalignment.

Section Top Width Thickness
2L, 3L, 4L, 5L6–16 mm (1/4″–5/8″)Fractional HP
A13 mm (1/2″)8 mm (5/16″)
B16 mm (5/8″)11 mm (7/16″)

Typical HVAC life: 2 000–3 000 hours. Require periodic tension adjustment.

Narrow V-Belts (3V, 5V, 8V)

Narrower, deeper profile providing greater power transmission in a smaller space. Used in compact drives and higher horsepower applications.

Cogged V-Belts

Notches on the inner surface increase flexibility, reduce bending resistance, and improve heat dissipation.

  • Run cooler than standard V-belts
  • Up to 2% higher efficiency
  • Better performance on small-diameter pulleys
  • Longer life in demanding applications
  • Direct replacement for standard V-belts — no pulley change required
Typical uses: Supply Air Fans Return Air Fans Condenser Fans Blower Assemblies
⚙️ Synchronous (Timing) Belts

Teeth mesh with grooved timing pulleys for positive drive — no slippage. Require precise alignment and special pulleys.

  • Exact speed ratio maintained — no slip
  • High efficiency (98–99%)
  • No initial tension required (tension for tracking only)
  • Quiet operation; no lubrication needed
  • More expensive; require timing pulleys

Used in direct-drive fans, precision equipment, small compressors, positive displacement blowers.

➡️ Flat Belts

Modern flat belts use synthetic materials (polyurethane, polyester) with reinforcement. Less common in HVAC/R but used in specialty applications.

  • Efficient at high speeds
  • Very quiet operation; low maintenance
  • Require flat or crowned pulleys for tracking
  • Limited power transmission compared to V-belts
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Belt Selection for Replacement

  • Match belt type: Use the same type as original unless intentionally upgrading
  • Correct length: Use the original belt part number or measure accurately
  • Quality grade: Consider premium or cogged belts for critical or high-cycle applications
  • Matched sets: In multi-belt drives, replace all belts simultaneously with a matched set from the same manufacturing lot
  • Environment: Select materials rated for operating temperature, oil exposure, or other environmental factors

3.6.4 — Belt Length Measurement

Correct belt length is essential for proper tension and fit. There are four methods for determining belt length — use the simplest method that gives accurate results.

1️⃣ Read the Belt Number

The easiest and most reliable method. Read the belt number printed on the belt surface:

  • A42 — Section A, 42″ inside circumference
  • 4L320 — 4L section, 32.0″ outside circumference

When upgrading to cogged or premium belts, use the same base number — cogged versions share the same length designation.

2️⃣ Measure Existing Belt

Use when markings are illegible:

  1. Remove belt from pulleys
  2. Lay flat without stretching
  3. Measure inside circumference with a flexible tape
  4. Note cross-section (measure top width and thickness)
  5. Match to standard belt number from manufacturer catalog
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Do not measure a worn or stretched belt — elongation gives inaccurate readings.
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3️⃣ Calculate from Pulley Diameters and Center Distance

Use when designing a new drive or when the belt is unavailable:

L = 2C + 1.57(D + d) + (D − d)² ÷ (4C)

  • L = Belt pitch length (inside circumference)
  • C = Center distance between pulley shafts
  • D = Large pulley pitch diameter
  • d = Small pulley pitch diameter

Note: Pitch diameter is larger than groove diameter by an amount that varies with belt cross-section. Consult manufacturer data for pitch diameter specifications.

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4️⃣ String Method — Field Measurement

  • Position pulleys at the desired center distance
  • Wrap a non-elastic string around both pulleys in the belt path
  • Mark the string where the ends meet, then measure total length — this gives approximate outside circumference
  • Convert to inside circumference by subtracting twice the belt thickness
  • Select the nearest standard belt size; if between sizes, choose the next larger and adjust center distance
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Common Standard Belt Lengths (Outside Circumference)

Section Available Lengths
3L 25.0″ to 40.0″ (250, 270, 290, 310, 320, 340, 360, 380, 400)
4L 22.0″ to 60.0″ (220–500 in 20-unit steps, 550, 600)
A-section 26″, 28″, 30″, 32″, 35″, 38″, 42″, 46″, 48″, 53″, 55″, 60″, 68″, 75″, 80″, 90″, 96″, 105″, 112″, 120″

If the calculated length falls between standard sizes, select the next larger size and adjust center distance to achieve correct tension.

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