Changing belt driven fan speeds

One of the duties normally assigned to the TAB firm on a TAB project is to make final adjustments to fan speeds to achieve design air flow. Doing so, professionally, will result in the system performing as close to design as the installed equipment will provide.

By Leonard Maiani, NEBB February 17, 2016

One of the duties normally assigned to the test and balance (TAB) firm on a TAB project is to make final adjustments to fan speeds to achieve design air flow. Doing so, professionally, will result in the system performing as close to design as the installed equipment will provide.

NOTE: This discussion is for fan speed change only and does not account for other unit components such as cooling coils, etc. These applications may require additional consideration when speeding up or slowing down a fan.

Considerations when changing a fan’s speed:

1. Motor Horsepower

2. Manufacturer’s documented maximum speed

3. Fan and motor sheave sizes

4. Number and type of belts

5. Motor wire size

6. Starter/disconnect size

7. Overload size

Reducing Fan Speed

Slowing the fan speed down is the safest, while speeding the fan up involves a bit of research to assure that all components are operating safely.

When slowing a fan down, items 1, 2, and 4 through 7 do not play into your decision as to how much to slow it down. Items 3 & 4, however, do require some thought. The motor or drive sheave, when changed, should not be so small that there is insufficient belt contact to drive the fan without slippage. When the fan sheave is changed, care should be taken to not exceed the dimensions of the cabinet or belt guard and should not come into contact with the drive or motor sheave. Belts should be selected to provide for future adjustment of tension as they wear in. The number of belts should NOT be reduced due to slowing the fan down. The entire drive package is sized to meet the specified percentage above the nameplate motor horsepower (e.g. 150% of motor horsepower). The belt tension should be adjusted to the manufacturer’s recommended tension to ensure the longevity of the drive package. All belt manufacturers recommend re-adjustment of tension shortly after installation of the new belts (e.g. 24 hours of operation). This information should be passed on via notes in the TAB report or conveyed to the maintenance staff at some point.

Where the drive package consists of more than one belt, the belts should be ordered as a matched set. Some belt manufacturers do not require this. In any case, the actual belt lengths must match.

Increasing Fan Speed

When speeding a fan up, all seven items, in the list above, come into play. Brake horsepower increases as the cube of the change in fan speed. In other words, a 10% increase in fan speed will result in a 33% increase in brake horsepower. The TAB firm must never exceed the nameplate horsepower when speeding a fan up. The TAB firm must research the fan manufacturer’s data to ascertain the maximum operating RPM of the fan and must never exceed this RPM or damage to the fan may occur.

If it determined that an increase in speed (without exceeding the maximum RPM) would require a change in motor horsepower, then a complete check of the electrical service to the motor would be required to ascertain that the components and wiring are adequate or need to be upgraded to meet the requirements of a new, larger, motor. Also, when increasing the motor horsepower, the drive package (number and size of belts) needs to be reviewed to meet the project specification. If there is no specified requirement, the drive package should be sized between 130% and 150% of the new motor nameplate. At this point, the belt guard may need to be modified as well.

If additional belts are required to accommodate the higher motor nameplate horsepower, it will be the TAB firm’s responsibility to see that the specification is met. If the drive package is provided by the vendor or contractor, any discrepancies in what they provided should be reported in the TAB report.

When all is said and done, the fan speed should deliver design air flow with the motor running at or below nameplate brake horsepower and somewhat below the manufacturer’s rated maximum RPM. 

General Comments

1. Do not roll the belts off and on the sheaves while they are under tension. Relieve the tension prior to belt removal or replacement.

2. Use Pitch Diameter (PD) when calculating sheave diameters and belt lengths. (Outside diameters vary between manufacturers.)

3. Double check sheave bore sizes before ordering replacement sheaves. If the sheave or bushings are not marked, it is best to use a caliper to check shaft diameters. The chamfer on the end of the shaft could easily skew a reading with a tape measure.

4. If belts squeal or slip on startup when belts are properly tensioned, note that this is occurring in the TAB report and recommend a different drive package to resolve problem. This is not as critical on 24/7 projects where fans run continuously, such as hospitals, but where systems are on a timed schedule or sequenced on and off, this could dramatically shorten the life of the belts and drive sheave.

5. Recommend against the use of belt dressings. This a stopgap at best. Belt dressing dries the belt and shortens life expectancy.

6. Whenever a fan is shipped with a companion sheave on the fan shaft, it should be replaced with a companion sheave. A standard driven sheave will put undue stress on the outer belts of a multi-belt drive.

7. Verify that all set screws are tight and taper bushing bolts are tightened evenly and to the proper torque. (Over torqueing can make for a long day when one or more bolts snaps off in the sheave.)

8. When installing a sheave with a taper bushing, verify that the sheave runs true with no wobble. Uneven tightening of the bolts can cause it to run with a wobble.

9. Final sheave alignment is critical to the life of the drive components. Use a straight edge or string to verify proper alignment. (Banded, gear-belt, and micro V belt drives require detailed attention to alignment and tension. Follow Manufacturer’s instructions.)

10. Over-tensioning of belts, to eliminate noise or slippage can result in premature failure of either the fan or motor bearings or both. Verify that the proper belts and tension are as they should be.

11. Change out belts like for like. Notched belts do not have the same horsepower rating as standard sheathed belts.

12. Banded belts should only be installed on new sheaves. If the sheaves are worn, they may cut through the banding.

13. Report any anomalies or discrepancies observed during and after the speed change such as; increased noise, excess vibration, throbbing noise, oil canning of duct work, deforming of duct work, etc. 

14. Do not leave fan motors overloaded under any circumstances unless directed IN WRITING by the specifying agency.

Safety Concerns

1. Observe Lock-out/Tag-out protocol.

2. Wear snug fitting clothing and short sleeves when possible to avoid clothing being caught in the drive package when in operation.

3. If you must wear a tie, make it a clip-on or modify a standard tie with a Velcro release at the back of your neck.

4. Corral the ponytail. If you have long hair, keep it tucked into a cap or some other restraint.

5. Do not put your fingers on the underside of the belts when installing the belts. Rolling belts on under tension really exacerbates this hazard.

6. Keep body parts out of the line of fire if one or more belts break or otherwise jump off the sheave on startup.

Leonard Maiani, NEBB Technical Director (Former/Retired). This article originally appeared on NEBB. NEBB is a CFE Media content partner. Edited by Ksenia Avrakhova, production coordinator, CFE Media,