Belt-Driven Large-Diameter Axial Fan Supplier: Pulley Ratios, Tip Speed and Trimming Speed on Site

NextAir Systems, Faridabad, India. Last reviewed October 2026.

A big axial fan wants to turn slowly. A standard four-pole motor turns at nearly 1,500 rpm at 50 Hz. Connect the two directly and a 1,600 mm impeller would run at a tip speed above 120 m/s, which is too fast for the noise and the stress. A belt drive puts a pulley ratio between motor and fan, so the impeller turns at the speed its design wants, whatever the motor does.

NextAir builds axial fans up to 1,600 mm with both direct and belt drive. This page shows what the pulley ratio does, and why a belt drive is also a way to adjust the fan after commissioning.

At a glance

• The speed ratio is the motor speed divided by the fan speed. A 1,480 rpm motor driving a fan at 740 rpm is a 2:1 ratio.
• A 1,600 mm impeller at 960 rpm has a tip speed of about 80.4 m/s. At 600 rpm it is 50.3 m/s, and sound power falls by about 10.2 dB while shaft power falls to 24%.
• A belt drive lets you change the fan speed by changing a pulley, trimming flow and pressure to the system as built.
• Belts and bearings need maintenance that a direct-drive fan does not.

Why belts on large fans

Direct drive is simple: the impeller sits on the motor shaft, so the fan runs at the motor’s speed. For small fans that is fine. For a large-diameter fan, motor speeds are too high. A belt drive puts the motor beside the fan, connected by pulleys, so the speed ratio is a free choice. It also keeps the motor out of the airstream, which matters when the gas is hot, wet, corrosive or dusty, and it lets the bearings and drive be placed where they can be reached.

The speed table

The table takes a 1,600 mm impeller, our largest, driven by a four-pole 50 Hz motor at about 1,480 rpm, and shows what four fan speeds mean: the pulley ratio, the tip speed, the shaft power relative to 960 rpm (the fan law, power varies with the cube of speed) and the change in sound power, which varies as 50 log₁₀ of the speed ratio.

The relationships are the fan laws, set out in what speed does to flow, power and noise, and apply when the fan runs on the same system curve. A fan that runs at 600 rpm instead of 960 delivers 62.5% of the flow, 39% of the pressure and about 24% of the power, and is about 10 dB quieter. A belt drive is how you land on the speed you want.

Trimming on site

No design predicts the system exactly. The duct is a little more or less resistive than calculated, or the load turns out different. With a belt drive you can change the pulley after commissioning and move the fan’s operating point, without touching the motor or the impeller. That is a significant advantage: a direct-drive fan stays at its design speed unless you add a variable frequency drive. Ask for a pulley set that allows some adjustment either way, and for the system effect to be considered at selection, as in system effect.

What belt drive costs you

• Maintenance. Belts stretch and wear, bearings need greasing and the drive needs guards. Plan the intervals.
• Losses. A belt transmits slightly less than all the motor’s power, and the loss rises with poor tension.
• Space and safety. The drive takes space and must be guarded.
• Alignment. Pulleys and shafts must be aligned and the belts tensioned correctly, or vibration and early failure follow. Balance and vibration limits are covered in fan vibration and ISO 14694.

Check that the pulley and belt set are rated for the power at the highest speed you might use, with a service factor. If you plan to trim the speed upwards later, the motor, the bearings and the impeller must all be good for the higher speed, so the design limit should be stated at the start. Ask the supplier for the maximum safe speed of the impeller, and what happens to the vibration limits if you approach it, because trimming up is easier on paper than in a fan already near its limit.

Direct or belt: a quick guide

Impellers are dynamically balanced on our own computerised machines, motors and variable frequency drives are bought-out items matched to each fan, and fans ship fully assembled, partially assembled or as CKD kits. The range is up to 1,600 mm, 2,400 Pa, 185,000 m³/h and 100 HP, as separate limits. For the supplier checklist see how to choose an axial fan manufacturer.

What to send us for a quotation

• Airflow and static pressure at the duty point, and the system resistance
• Gas condition: temperature, humidity, dust or fumes
• Whether the motor must be kept out of the airstream
• Noise limit at the boundary
• Material needed: mild steel, SS 304 or SS 316
• Delivery format: assembled, partially assembled or CKD

Send these to us through our contact page and we will confirm the duty is inside our range and propose the drive. If the fan replaces an existing one, tell us its pulley sizes and motor speed, because that is the quickest way to see what ratio has worked on your system. See the axial fan range, our guide to industrial axial fans and our pages for cement plants and steel plants, where large fans are common. Our stainless steel axial fans page covers corrosive duty.

Frequently Asked Questions