⚙️ Shaft Size Calculator
Calculate Minimum Shaft Diameter
How to Use This Calculator
Enter Power
Input the power transmitted by the shaft in horsepower (HP).
Enter RPM
Enter the rotational speed of the shaft in revolutions per minute (RPM).
Select Material
Choose the shaft material from the dropdown. This affects the allowable shear stress.
Calculate
Click calculate to get the minimum required shaft diameter and torque.
Formula
T = (HP × 63025) / RPM
d = ((16 × T) / (π × τ))^(1/3)
Where:
- T = Torque (lb-in)
- HP = Power (horsepower)
- RPM = Rotational speed (rev/min)
- d = Shaft diameter (inches)
- τ = Allowable shear stress (PSI)
- 63025 = Conversion constant
Example Calculation
If you have:
- Power: 10 HP
- RPM: 1800
- Material: Steel (8000 PSI)
Calculation:
T = (10 × 63025) / 1800 = 350.14 lb-in
d = ((16 × 350.14) / (π × 8000))^(1/3)
d = (5602.24 / 25132.74)^(1/3) = 0.615 inches
About Shaft Size Calculator
The Shaft Size Calculator is an essential tool for mechanical engineers and designers working with rotating machinery. It calculates the minimum shaft diameter required to safely transmit power based on torque and material properties. Proper shaft sizing prevents failure due to shear stress and ensures reliable operation of mechanical systems.
When to Use This Calculator
- Machine Design: Determine minimum shaft diameter for new equipment
- Motor Coupling: Size shafts connecting motors to driven equipment
- Power Transmission: Calculate shaft sizes for belt, chain, and gear drives
- Safety Analysis: Verify existing shafts can handle required loads
- Material Selection: Compare shaft sizes for different materials
Why Use Our Calculator?
- ✅ Instant Results: Get shaft diameter calculations immediately
- ✅ Easy to Use: Simple interface requiring only power, RPM, and material
- ✅ Multiple Units: Results in both inches and millimeters
- ✅ 100% Free: No registration or payment required
- ✅ Accurate: Uses standard mechanical engineering formulas
- ✅ Material Options: Select from common shaft materials
Common Applications
Motor Shafts: Calculate minimum shaft diameter for electric motors connecting to pumps, fans, compressors, and other driven equipment.
Machine Tools: Size shafts for lathes, mills, drill presses, and other workshop machinery transmitting power from motors to spindles.
Industrial Equipment: Design shafts for conveyors, mixers, crushers, and manufacturing machinery.
Automotive Applications: Calculate shaft sizes for driveshafts, half-shafts, and accessory drives in vehicles.
Tips for Accurate Results
- Add 10-20% safety factor to calculated diameter for real-world applications
- Consider keyways, splines, and stress concentrations - these reduce effective strength
- For variable loads, use maximum torque conditions
- Account for bending moments if shaft is not pure torsion (use combined stress analysis)
- Standard shaft sizes should be selected from available diameters (often in 1/8" or metric increments)
Frequently Asked Questions
Why do I need a safety factor?
Safety factors account for variations in material properties, manufacturing tolerances, unexpected loads, and stress concentrations from keyways, threads, or splines. A 10-20% increase in diameter provides a reasonable safety margin.
What if my shaft has bending loads?
This calculator assumes pure torsion. If your shaft experiences bending (from pulleys, gears, or other side loads), you need combined stress analysis. The required diameter will be larger when bending is present.
How do keyways affect shaft strength?
Keyways reduce shaft strength by creating stress concentrations. A common rule is to increase shaft diameter by 25% when keyways are present, or use the calculated diameter as the diameter at the keyway bottom.
Can I use this for hollow shafts?
No, this calculator is for solid circular shafts. Hollow shafts require different formulas that account for inner and outer diameters. The polar moment of inertia formula changes for hollow shafts.
What are typical shear stress values?
For steel shafts, typical allowable shear stress is 6,000-12,000 PSI depending on material grade and application. This calculator uses conservative values: Steel 8,000 PSI, Stainless 7,000 PSI, Aluminum 3,000 PSI, Bronze 6,000 PSI.