8.4 Belts, Sheaves, Alignment, and Variable Frequency Drives

Belts, Sheaves, Alignment, and Variable Frequency Drives

Drive identity and safe inspection

A belt drive transfers motor torque through a driver sheave, one or more belts, and a driven sheave on the fan shaft. Record motor and fan RPM, sheave identifiers, pitch diameters, belt section and matched-set designation, center distance, guard condition, and the manufacturer's allowable speed and power limits.

Inspect or adjust a drive only under the required shutdown and energy-control procedure. A stopped VFD display does not prove zero energy, and stored electrical or mechanical energy may remain. Replace guards before operation and keep tools, hoses, clothing, and reflective tape away from moving parts.

Look for cracked or glazed belts, rubber dust, unequal belt position in multi-groove sheaves, worn grooves, shaft or sheave runout, loose fasteners, damaged guards, and evidence of slip. A belt riding much lower in one groove can indicate groove wear or a mismatched belt. Follow the drive manufacturer's replacement requirements; multi-belt drives commonly require a matched set rather than one isolated replacement.

Pitch diameter and speed ratio

Belt-drive calculations use pitch diameter, not outside diameter. For an ideal drive without slip:

motor RPM x motor-sheave pitch diameter = fan RPM x fan-sheave pitch diameter

A 1,750 RPM motor with a 6.0-inch driver and a 14.0-inch driven sheave predicts 750 fan RPM. To predict a driven sheave for 875 fan RPM with the same motor and driver, calculate (1,750 x 6.0)/875 = 12.0 inches.

This is a geometric prediction. Actual speed differs because of motor slip, belt slip, adjustable-sheave position, and measurement uncertainty. Measure final fan RPM with an appropriate tachometer and compare it with the fan, shaft, bearing, belt, sheave, and motor limits.

On an adjustable driver sheave, moving the flanges together generally makes the belt ride at a larger effective pitch diameter and increases driven speed; moving them apart reduces effective pitch diameter and speed. The direction and adjustment procedure depend on the actual sheave design. Follow its instructions, maintain equal groove settings, secure the adjustment, realign the drive, set tension, and remeasure speed.

Alignment

Misalignment shortens belt and bearing life and can create heat, noise, and vibration. Two broad forms are:

• parallel offset, where the sheave planes are displaced while the shafts remain parallel; and
• angular misalignment, where the shaft or sheave planes are not parallel.

Use the straightedge, laser, rim, or face method specified for the drive. Measure at the required contact points and compare with the manufacturer's tolerance. Do not apply one angular or inches-per-foot limit to every sheave and belt system. Check shaft condition and sheave mounting before moving the motor merely to hide runout.

Tension

Correct tension transmits the required torque without slip while avoiding unnecessary radial load on bearings. A common force-deflection procedure specifies a test displacement, such as 1/64 inch per inch of span, and then compares the force required to reach that displacement with a manufacturer table. For a 32-inch span, that example displacement is 0.500 inch.

The displacement alone is not a tension value. The required force depends on belt section, construction, span, sheave size, number of belts, and new-versus-run-in condition. Use the actual belt and drive data or the specified sonic-tension procedure. After run-in, recheck when the manufacturer requires it.

Variable frequency drives

A variable frequency drive (VFD) changes motor speed by controlling frequency and voltage. Below base speed, and with ordinary induction-motor slip small, shaft-speed ratio is often close to frequency ratio:

RPM2/RPM1 approximately equals Hz2/Hz1

For the same fan and comparable system condition, the affinity laws then predict flow with the first power of speed, pressure with the square, and brake horsepower with the cube. If a fan develops 1.50 inches water gauge at 45 Hz, the pressure forecast at 60 Hz is 1.50 x (60/45) squared, or about 2.67 inches water gauge.

A command frequency is not proof of shaft RPM. Record commanded and displayed frequency, measured RPM, current on all phases, airflow, pressure, operating mode, and control setpoint. Controls or dampers that move during the test can invalidate a simple frequency-ratio forecast.

Operating limits

Minimum frequency is not one universal number. At low speed, a shaft-mounted motor fan may provide less cooling, a pump may lose required minimum flow, a fan may become unstable, and a pressure or airflow sensor may fall below useful range. Maximum frequency also comes from the approved motor, fan or pump, bearing, VFD, vibration, and application data; 60 Hz is not automatic permission to operate above or below every limit.

Before an authorized speed change:

1. verify that the performance deficiency and measurement boundary are real;
2. predict flow, pressure or head, and cubic power change;
3. check motor current and every mechanical and control limit;
4. change one controlled variable;
5. allow the system to stabilize; and
6. remeasure RPM, flow, pressure, phase currents, vibration, and control response.

If the required performance cannot be reached within limits, document and escalate the deficiency rather than forcing the drive beyond its approved range.