Fan drive arrangements are defined by the Air Movement and Control Association International (AMCA). (We address drive arrangements in detail in a separate presentation.) When comparing and contrasting direct drive to belt drive fans, I think we should address efficiency, safety, reliability, maintenance, and motor replacement independently. As we talk about belt drive fans in this blog post, we will assume that the manufacturer is using high quality heat, oil, and static resistant V-belt drives.
Efficiency
In a direct drive fan, by definition, the impeller is directly connected to the motor shaft. There are no power transmission losses&#;the energy that the electric motor develops is transmitted directly to the impeller, which passes into the airflow and pressure development.
On a belt drive fan, there is a power transmission loss associated with the drive from the motor to the fan shaft. When properly selected and installed, the belt drive losses can vary from 20% on small fractional motor powered fans to 3 to 4% on large motor driven fans. To keep it simple, a good rule of thumb would be to assume about a 5% drive loss for motors 5 hp to 100 hp.
Safety
Unless the motor is in the airstream of the fan, it must have access to ambient air circulation for motor cooling. Moving parts such as belts, pulleys, and shaft coolers must be guarded in accordance with OSHA and/or other local requirements. Motor replacement on direct drive fans requires disassembly of the fan. It can be a safety issue because disassembly will cause personnel contact with airstream components.
Reliability
When properly selected, installed, aligned, and tensioned, belts should provide many years of quality service. On both direct drive and belt drive fans, the motor has two bearings. Belt drive fans have two additional bearings. The two bearings on direct drive fan motors which are 7½ horsepower and larger are typically regreaseable.
Direct drive fans should be designed for acceptable bearing life. This is done by the fan designer and manufacturer to make sure that the axial, radial, and thrust loads are fully/properly considered in the design.
Maintenance
Direct drive fans with relubricable motor bearings must be lubricated as per the fan manufacturer's recommendations in terms of grease type and frequency. Belt drive fans have the two motor bearings (similar to direct drive fans) plus the two shaft bearings. Those motor and shaft bearings need to be lubricated as per the fan manufacturer's recommendations.
The belt alignment and tension need to be maintained on belt drive fans. Typically, this is not a huge task&#;it's important to understand that belts from the factory will stretch within the first several days of operation. The belts are aligned and tensioned properly by the fan manufacturer. When the fan is put into service, it should be allowed to run from 2 to 4 weeks, depending on the amount of time the fan is in operation daily. Then, the fan needs to be locked out and the belt tension needs to be adjusted (because they have stretched).
While the belts are tensioned, the alignment should also be checked. The belts do not continue to stretch after the run-in period. Once the belts are retensioned after the initial 2 to 4 week run-in, they should give a long lifetime of quality service.
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While lubricating the motor and shaft bearings, the maintenance people should observe the belts for slack tension or deterioration, which might have been caused by poor tension or misalignment. (This becomes evident due to the formation of residual belt dust.)
Motor Replacement
By definition, a direct drive fan's impeller is attached to the motor shaft. The impeller has to be removed from the motor shaft before the motor can be changed out. This requires disassembly of the fan, which means airstream contact.
There are inherent critical access and time issues with changing the motor on a direct drive fan. Some potential implications and questions are: Is there enough room to perform disassembly? Where is the disassembly going to be performed? Will disassembly occur on the roof or inside the building? Do the fans have to be removed from the system/building?
Belt drive fan motor replacement is typically easier. Of course, you must be able to access the fan. Then you must remove the guards, relieve the belt tension, remove the motor and belts, and check tension and alignment. Unlike direct drive fans, though, you do not have to remove the fan from the system, there is no airstream contact, and there is no disassembly of the fan.
In either case, it is advisable to check the fan's balance after the motors have been replaced.
Couple driven fans or fans with the electric motor attached to a coupling are addressed in a separate presentation. MK Plastics offers a complete range of direct drive and belt drive fans for your needs. To learn more, contact MK Plastics today.
MKPlastics005-018 Transcription
Shayla: Thank you for joining us today for the MK Plastics Podcast. I'm talking today with Vice President of US Sales and Marketing, Keith Lins, and Keith, I'm wondering today, can you&#;from an application standpoint&#;could you contrast belt drive to direct drive fans?
Keith: Well, fan drive arrangements are defined by the Air Movement and Control Association International and drive arrangements in detail are addressed in a separate presentation. But when comparing and contrasting direct drive to belt drive fans, I think we should address the issues independently and those issues would be efficiency, safety, reliability, maintenance, and motor replacement. When we are talking about belt drive fans, we will assume that the manufacturer is using high quality heat, oil, and static resistant V-belt drives.
Shayla: Okay.
Keith: So let's take the first point, which is efficiency. In a direct drive fan by definition, the impeller is directly connected to the motor shaft. There are no power transmission losses. The energy that the electric motor develops is transmitted directly to the impeller, which passes into the airflow and pressure development. On a belt drive fan, there is a power transmission loss associated with the drive from the fan shaft&#;from the motor shaft to the fan shaft. When properly selected and installed, the belt drive losses can vary from 20 percent on small fractional motor powered fans to 3 to 4 percent on large motor driven fans. A rule of thumb for fans above 7 and a half horsepower, just to keep it easy would be to assume about a 5 percent drive loss for motors, say, 5 horsepower and above to 100 horsepower. Let's talk about safety for a minute. In either case, and unless the motor is in the airstream in the fan, the motor must have access to ambient air circulation for motor cooling. Moving parts such as belts, pulleys, shaft coolers, must be guarded in accordance with OSHA or other local requirements. Motor replacement on direct drive fans requires a disassembly of the fan. That can be a safety issue because disassembly of the fan will cause contact&#;personnel contact with airstream components, which will be addressed a little bit later in the presentation. Let's talk about reliability. Belts, when properly selected, installed, aligned, and tensioned, should provide many years of quality service. Belt drive fans have two extra bearings on the fan. The motors have two bearings whether the fan is direct drive or belt drive. A belt driven fan adds two extra bearings to the system. Direct drive fans have motor bearings only and on motors typically 7 and a half horsepower and larger, those motor bearings are regreaseable so direct drive fans should be designed for acceptable bearing life and this is done by the fan designer and fan manufacturer to make sure that the axial, radial, and thrust loads are fully and properly considered in the design to give adequate, acceptable bearing life. Let's talk about maintenance. Direct drive fans with relubricable motor bearings must be lubricated per the fan manufacturer's recommendation on terms of grease type and frequency. Belt drive fans have the motor bearings like direct drive fans plus they have two shaft bearings and the motor and shaft bearings need to be lubricated per the fan manufacturer's recommendations. The belt alignment and tension need to be maintianed on the belt drive fans. Now, typically this is not a huge task because people need to understand that belts from the factory will stretch within the first several days of operation. So, the belts are aligned and tensioned properly by the fan manufacturer. When the fan is put into service, that fan should be allowed to run 2 to 4 weeks depending on the amount of time the fan is in operation daily and then that fan needs to be locked out and the belt tension needs to be tightened, adjusted, because those belts have stretched. While the belts are tensioned, the alignment should be checked. Now, the belts do not continue to stretch after the run-in period so once those belts are retensioned after the run-in period, that set of belts should give you quite a long time of quality service. When lubricating the motor and shaft bearings, the maintenance people should observe the belts for slack tension or for deterioration which might be caused by poor tension or misalignment and that evidences itself in the form of residual belt dust. Fifth point is motor replacement. This is a biggie. Direct drive fans by definition, the impeller is attached to the motor shaft so the impeller has to be removed from the motor shaft before the motor can be changed out. This requires disassembly of the fan, which means airstream contact. It also implies do we have room to do this? Where are we going to do this? Are we going to do this on the roof or inside the building or do the fans have to be removed from the system or removed from the building? So there are critical access and time elements issues to changing the motor on a direct drive fan. Belt drive fan motor replacement is typically easier. You have to be able to access the fan, of course. Then remove the guards, relieve the belt tension, remove the motor, remove and reinstall the motor shiv on the new motor, reinstall the new motor and belts, check tension and alignment. You have no removal of the fan from the system, no airstream contact, and no disassembly of the fan. In either case, it is advisable to check the fan balance after the motors have been replaced. Couple driven fans or fans with the electric motor attached to a coupling are addressed in a separate presentation. MK Plastics offers a complete range of direct drive and belt drive fans for your needs.
Shayla: If you have any questions about anything Keith has talked about today, you can reach MK Plastics at 888-278-.
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