How to use this calculator
- Enter shaft span. Use the effective free length between supports.
- Enter tube geometry. Set outside diameter and wall thickness.
- Enter material properties. Set modulus and density for the tube material.
- Review safe speed. Use the speed factor to compare operating RPM with critical speed.
How it works
A long tube has a bending natural frequency. The calculator treats the driveshaft as a simply supported uniform tube and uses omega = pi^2/L^2 x sqrt(EI/(rho A)). It converts angular speed to RPM and applies the entered operating-speed factor.
If torque capacity or bending stress is the question instead, use the shaft torsion calculator or shaft diameter calculator.
Worked example
Verified against the live calculator
A 48 in long, 3 in OD, 0.12 in wall steel tube screens near 7,200 rpm first critical speed with the default beam model. With a 0.75 factor, suggested operating speed is about 5,400 rpm.
Frequently asked questions
What is driveshaft critical speed?
Critical speed is a bending resonance speed where a shaft can vibrate strongly. This calculator estimates the first bending critical speed of a uniform tube.
What formula does this use?
It uses the first bending natural frequency of a simply supported uniform beam: omega = pi^2/L^2 x sqrt(EI/(rho A)).
Is this enough for final driveshaft design?
No. Final design needs end constraints, tube quality, balance, joint angles, weld yokes, torque, dents and manufacturer validation.
Why does diameter matter so much?
Tube bending stiffness depends on the fourth power of diameter, so a larger tube can raise critical speed substantially.
Method & assumptions
- Uses a uniform simply supported tube approximation for first bending critical speed.
- Real limits depend on yokes, welds, balance, joint angles, tube straightness, dents and support stiffness.