HVAC System

Synchronous belts vs. V-belts

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

By Brent Oman

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.

Synchronous belt drive misalignment should not exceed 1/4 angular degree or 1/16-inch per foot of center-to-center distance. Checked misalignment with a straightedge between the driver to driven and from driven to driver to take into account the effect of parallel and angular misalignment.

Drive misalignment also causes belt tracking problems. Although some belt tracking is normal and won't affect performance, optimum operation of the drive can only be achieved when the belt is contacting one flange in the system. When the belt contacts flanges on opposite sides of the sprockets, the result can be undesirable parallel misalignment.

Improper installation of the bushing can result in the bushing and sprocket assembly being cocked on the shaft. This leads to angular misalignment and also increases the possibility of vibration. It is important to follow the installation instructions included with the bushing.

Belt tension

Proper belt tension is important to optimum belt performance and longevity. The two extremes of improper tensioning are under- and over-tensioning.

Under-tensioned belts prematurely wear the belt teeth and possibly even ratchet (jump teeth) under heavy start up loads, shock loads, or structural flexing. Over-tensioned belts wear in the land area between the belt teeth which leads to premature belt failure. Over-tensioning also can damage bearings, shafts, and other drive components.

Both under-tensioning and over-tensioning result in shortened belt life. It is important to use the proper initial static tension values when installing the belt. Find these values in design catalogs or consult your local belt representative.

Other considerations

Air handling units are unique in the dramatically effects a small change in RPM at the driven shaft causes. The air flow and motor current are both related to the fan speed. Since one of the major advantages of synchronous belts is improved energy efficiency, it is important that the synchronous belt drive design achieve the proper driven speed.

Base the synchronous belt drive speed ratio on a tachometer-measured driven shaft RPM and not on one calculated from the speed ratio of the existing V-belt drives. An exception would be if a slipping V-belt providing insufficient air flow. A properly designed synchronous belt drive provide consistent air flow at an energy cost consistent with the fan speed. A small change in the fan speed causes a much larger change in the actual horsepower and amperage requirements. The following equation shows the general relationship:


HP1/HP2 = (RPM1/RPM2)3

where: HP1 = initial horsepower
HP2 = horsepower at the new fan speed
RPM1 = initial fan speed
RPM2 = new fan speed

As an example, consider increasing the fan speed from 1,100 to 1,125 RPM using a 25-HP motor. In this case,

(25/HP2) = (1,100/1,125)3
HP2 = 25 X (1,125/1,100)3
25 X (1.0227)3
25 X 1.07
26.7 HP

In this example, the fan speed only increased 2.3% but the horsepower requirement increased by 7%. The message is that you should select drive components and fan speed with great care.

A synchronous belt drive should not necessarily be chosen on the basis of the diameters of the existing V-belt sheaves. The actual fan speed was slower than the theoretical V-belt sheave speed ratio would indicate because the V-belt drive slips. Where possible, measure the speed of the fan shaft using a mechanical contact tachometer or a strobe tachometer.

It is easy to reduce the overall size of a belt drive when converting to synchronous belts.

While this can be an advantage if shaft length is limited, take care when selecting sprocket diameters. Belt pull and the resulting bearing load is directly proportional to the diameter of the sheaves or sprockets in the drive. The larger the diameter, the lower the belt pull.

If there is any concern over the rigidity of the structure, make sure that the synchronous sprockets are the same size or larger than the V-belt sheave diameters. This keeps the belt pull roughly equal to that of the existing V-belt drive while minimizing the possibility of structural deformation.

To prevent possible damage to the motor bearings, be sure that the motor sprocket diameters meet the National Electric Manufacturers Association recommended minimums. Remember that a small difference in diameter makes a large difference in belt pull. For example, replacing a 10-inch V-belt driver sheave with an eight-inch sprocket yields a 20% reduction in diameter and a corresponding 20% increase in belt pull.

While a two-inch diameter reduction may not seem like much, the 20% increase in belt pull potentially could be significant to both the structural rigidity and the bearing performance.

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