⚙ Pulley Torque Calculator
Estimate shaft torque, pulley rim force, output speed, and traction margin for V-belt, timing belt, flat belt, and poly-V pulley drives.
📌 Preset Pulley Drives
⚙ Drive Inputs
🎯 Torque Results
💠 Belt Material and Pulley Pair Guide
📊 Reference Tables
| Belt Pair | Typical Friction | Efficiency | Max Rim Speed | Best Use |
|---|---|---|---|---|
| Classical V-belt on cast iron | 0.32 | 95% | 6500 ft/min | General industrial drives |
| Poly-V on steel | 0.34 | 97% | 8000 ft/min | Fans and accessory drives |
| Polyurethane timing on aluminum | Positive mesh | 98% | 5000 ft/min | Indexing and servo motion |
| Leather flat belt on crowned steel | 0.28 | 93% | 5500 ft/min | Smooth line shafts |
| Small Pulley Dia. | 1750 rpm Speed | Input Torque at 5 hp | Approx. Belt Pull |
|---|---|---|---|
| 3 in | 1374 ft/min | 180 lb-in | 120 lbf |
| 4 in | 1833 ft/min | 180 lb-in | 90 lbf |
| 6 in | 2749 ft/min | 180 lb-in | 60 lbf |
| 8 in | 3665 ft/min | 180 lb-in | 45 lbf |
| Ratio | Driven Speed | Torque Multiplier | Typical Use |
|---|---|---|---|
| 1.0:1 | Same as input | 1.00x | Line shaft transfer |
| 1.5:1 | 67% of input | 1.50x | Dust collectors |
| 2.5:1 | 40% of input | 2.50x | Pumps and augers |
| 4.0:1 | 25% of input | 4.00x | Hoists and reducers |
| Driven Machine | Base Factor | Start Character | Torque Note |
|---|---|---|---|
| Fan or blower | 1.00 | Light | Lowest shock load |
| Centrifugal pump | 1.08 | Steady | Check cavitation duty |
| Reciprocating compressor | 1.18 | Pulsing | Higher peak torque |
| Hoist or winch | 1.25 | Shock | Use conservative margin |
🗂 Material and Spec Comparison Grid
Classical V-Belt
Best for moderate shock and simple pulley grooves.
Slip gives overload forgiveness, but torque accuracy is lower than timing belts.
Poly-V
High wrap contact with compact pulley diameters.
Good for fast fans where rim speed is high and torque ripple is low.
Timing Belt
Positive engagement keeps ratio exact under load.
Choose when synchronized torque transfer matters more than slip cushioning.
Flat Belt
Works well on crowned pulleys with long center distances.
Requires generous wrap and careful alignment to maintain torque.
💡 Tip Boxes
This pulley torque calculator turns horsepower, pulley diameters, wrap angle, and belt choice into practical shaft torque and belt pull values for workshop, fan, pump, and conveyor drives.
Torque is the twisting force that occur at an output shaft of a pulley system. The horsepower of the motor that the pulleys create by the gear ratio multiplies creates the torque of a system. By using a small driver pulley and a large driven pulley, the small driver pulley will multiply the torque that are delivered to the driven pulley.
However, belts dont work in the same way then gears. The belts work by using friction to transfer the power from one pulley to anothr. The friction of the belt is dependent upon the wrap angle of the belt around the pulley.
How Belts and Pulleys Work
If the wrap angle is less than 150 degree, the grip of the belt will decrease which will require an increase of belt tension. An increase for the belt tension will place additional stress upon the systems bearing and shafts. The input torque of a belt drive system can be calculated using the following formula: 63,025 times the horsepower divided by the rpm.
The value used should be the actual motor output of the motor rather than a nameplate motor rating. You can calculate the output torque by taking the input torque of the system and multiplying it by the ratio of the diameter of each of the pulleys within the system. Additionally, you should account for the efficiency of the belts.
V-belts have an efficiency of approximately 95% while timing belts has an efficiency of 98% due to the way that timing belts have a positive mesh to the other components of the system. A service factor should also be included in the calculations to account for any shock load that may impact the system. This will help to prepare the system for any sudden jolt that may occur during the operation of the machine.
Belt pull is a value that describe the tension of the belt on it’s tight side. Belt pull is a measurement that help to ensure that the longevity of the system is maintained. To calculate the belt pull, you should double the output torque of the system and divide by the pitch diameter of the driven pulley.
If the calculated belt pull for a system exceed the material limits of the belt, the belt will experience slip, heat and stretching of the belt over time. In most system with multiple belts or multiple ribs on a pulley, the load will be split so that poly V belts are often used within machines as they can provide a smooth pull to the driven component. Small pulley will increase the amount of belt force.
Consequently, small pulleys will create more higher belt forces in a machine, even at modest power levels. The way that the pulleys are arranged will impact the wrap angle of the belt and the direction in which the components of the machine will turn. Open belts will allow the wrap angle to remain as it is standard, but crossed belts will change the direction of the rotation of the pulleys while increasing the wrap angle of the belt.
A quarter turn drive will also allow for a component to be compacted while reducing the grip of the belt. Serpentine paths for the belts will allow for an even more better wrap angle for the belts but will require serpentine tensioners to ensure proper operation of the system. Finally, the type of material that is used in the belts will change the way in which the belt components interact with the machine.
For instance, neoprene V-belts will slip if the machine is overloaded, but polyurethane timing belts will maintain an exact gear ratio between the driving and driven components as the timing belts lock into place with the other components of the machine. An additional consideration of a belt drive system is the rim speed of the belts. If the rim speed of the belts is too high in relation to the capability of the belts, the belts may glaze or the cords within the belts may fatigue over time.
If the pulleys are not aligned properly or if the centers of the two shafts upon which the pulleys are mounted are not positioned correctly relative to each other, the wrap angle of the belt will decrease and the belt will slip on the driven pulley. Additionally, the shafts upon which the pulleys are mounted may be rated for the amount of torque and rpm that is placed upon them. For example, high levels of torque at low rpm may cause the key on the shaft to become fatigued.
Finally, tension should be set within the belt drive system. The tension within the belt should be set while the belt is hot and then checked again when the belt is cold.
