Synchronous belts vs. V-belts

If your air handlers meet the criteria, you can improve your HVAC efficiency.

By Brent Oman

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Although most of the air handlers in HVAC systems use V-belt drives for power transmission, synchronous belts offer advantages that, over time, can mean real savings.

The main advantage of synchronous belts is energy efficiency. Conversion to a synchronous belt drive is an easy, cost effective way to reduce air handling unit operating costs.  For example, if electrical costs are $0.12 per kilowatt-hour, the annual savings for a 50-HP motor running 24 hours per day would exceed $2,000. Estimate total annual energy savings by multiplying the savings per motor by the number of similar motors in a plant and adding the savings of the motors of different horsepower. There are software packages that calculate both annual energy savings and payback period of synchronous belts.

Slip vs. grip

The high efficiency of synchronous belts derives from their construction. Since V-belts have thicker cross-sections than synchronous belts, they require more energy to bend around sheaves. The wedging action of V-belts creates a dependence on friction and generates more heat than a synchronous belt tooth in sprocket grooves. Poorly maintained V-belts slip generating more heat and energy loss. When properly maintained, V-belt drive efficiency can run as high as 95 to 98% at the time of installation. During operation, however, V-belt efficiency deteriorates as much as five percent. The efficiency of a poorly maintained V-belt may fall an additional 10%.

Synchronous belts rely on tooth grip and do not slip and retain an energy efficiency of around 98% over the life of the belt.

Few plants maintain HVAC V-belt drives at optimum belt tension. Failure to properly retension V-belts results in belt slip. Synchronous belts with their high modulus, low stretch tensile cords need little or no retensioning. Less attention from maintenance personnel translates to additional savings.

While synchronous belt drives are a natural choice for HVAC, ensure an individual air handler is a good candidate for conversion. The structures of many air handling units are not sufficiently rigid. Synchronous belts are sensitive to fluctuations in the sheave center-to-center distance that inadequate brackets causes.

What to look for

With the drive locked out, check structural rigidity by pushing the two belt spans toward each other. Look for relative movement in the structure, not the belt. If either the motor or center distances move, the drive structure is insufficient for simple conversion.

The structure needs reinforcement to obtain maximum performance from a synchronous belt drive.

Consider also the start-up load when evaluating drives for potential conversion. The fan inertia produces start-up loads as high as 150 to 200% of the normal operating load. While V-belts may slip under excessive load, synchronous belts transmit the full start-up load. That phenomenon can collapse the drive center distance if the structure is not sufficiently strong. If the center-to-center distance reduces sufficiently, the synchronous belt may ratchet (jump teeth), potentially damaging both the belt and the motor or fan.

The combination of high start-up loads and weak structure is of concern on a system that frequently cycles on and off. Drives that run continuously only experience start-up load intermittently so they are not as sensitive.

If the structure appears to be weak, a high start-up load further degrades the performance of synchronous belts. If the start-up amperage is 1.5 to two times the steady state amperage, inspect the structure to ensure it is robust enough to prevent center distance collapse at start-up.

Air handlers that have a soft start and those driven by an AC inverter are ideal candidates for conversion to synchronous belts. Since the start-up loads are low and applied gradually, an unreinforced structure that might otherwise be too weak for a synchronous belt drive is now likely to be a good candidate for conversion. If you observe no unusual vibrations in the V-belt span, you usually can use a synchronous belt drive without reinforcing the structure of the air handler.

Troubleshooting noise

The ducting connected to most HVAC drives can amplify otherwise insignificant noises. Noise also may result from undersized, poorly lubricated, worn, or misaligned bearings; rotating components creating air movement; or a structure that flexes under load to cause belt misalignment and increased tooth interference.

Synchronous belts, like any other power transmission drive system, have certain noise levels. Synchronous belts generate noise caused by the slight interference as the belt teeth enter and exit the sprocket grooves. Since the belt noise increases with interference increases, accurate tensioning and alignment reduces the tendency of the drive to make noise. Use the following guidelines as an aid in designing and selecting quieter synchronous belt drives.

  • Noise generally increases with belt speed and decreases with decreasing sprocket diameter.
  • Increased dynamic belt tension tends to increase noise. Increasing sprocket diameters decreases belt tension. Achieve a balance between belt speeds and tensions.
  • Noise increases with increasing belt width.
  • Installed sound insulation in the belt guard or enclosure to further reduce the drive's noise if belt noise is still excessive after following the other guidelines.

Drive alignment

Synchronous belts are sensitive to misalignment. Using them on drives that depend on inherent misalignment leads to inconsistent belt wear and premature tensile failure caused by unequal tensile member loading. The high modulus tensile members in synchronous belts provide length stability over the life of the belt. Consequently, misalignment causes unequal load distribution across the width of the belt top. In a misaligned drive, only a small portion of the belt top width carries the load, resulting in reduced performance.

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