Engineering
Torque Calculator — Shaft Torque from Power and Speed
Calculate shaft torque from motor power and RPM. Free online torque calculator for dairy plant engineers sizing drives, couplings, and shafts.
Formula
(power * 9550) / speed
power
Motor Power (P) (kW)
speed
Rotational Speed (n) (RPM)
Worked Example
1
Given:
power = 7.5
speed = 40
2
Apply the formula:
(power * 9550) / speed
3
Result:1790.6 N·m1790.6 kgf·m
What is Torque?
Torque is the rotational force applied to a shaft. It is essential for sizing:
- Couplings and keyways — must handle peak torque without failure
- Gearboxes — output torque determines gearbox size
- Drive shafts — shaft diameter is designed for torque + safety factor
- Belt and chain drives — tension calculated from torque
Formula
T (N·m) = P (W) / ω (rad/s) = (P × 9550) / n
Where:
- P = power in kW
- n = rotational speed in RPM
- 9550 = conversion constant (kW × 9550 / RPM = N·m)
Torque Conversion Table
| Unit | Conversion |
|---|---|
| 1 N·m | = 0.738 ft·lbf |
| 1 N·m | = 10.2 kgf·cm |
| 1 kgf·m | = 9.81 N·m |
| 1 ft·lbf | = 1.356 N·m |
Typical Torque Values in Dairy Plants
| Equipment | Power | Speed | Approx Torque |
|---|---|---|---|
| Agitator (silo) | 7.5 kW | 40 rpm | 1,790 N·m |
| Centrifugal pump | 5.5 kW | 1,450 rpm | 36 N·m |
| Conveyor drive | 3.7 kW | 60 rpm | 590 N·m |
| Separator bowl | 15 kW | 6,000 rpm | 24 N·m |
| Butter churn | 11 kW | 25 rpm | 4,200 N·m |
Shaft Diameter from Torque (Rule of Thumb)
For solid steel shafts with moderate loading:
d (mm) ≥ (T / 5.1)^(1/3) × 10
Always apply a safety factor of ≥ 2 and verify against IS 4600 or relevant shaft design standard.