RPM to Frequency
Converting shaft speed to hertz - the anchor for every vibration analysis.
Why this matters
Where it occurs
Vibration analysts convert machine RPM into hertz to mark the 1x, 2x and 3x reference lines on a spectrum.
Why calculate it
A vibration spectrum is plotted in hertz, but machines are specified in RPM - the conversion anchors every fault signature.
What decision it supports
Is that peak at running speed (unbalance), at twice running speed (misalignment) or at a bearing frequency?
What happens if it is wrong
A 60x conversion error mislabels every fault signature - a 2x misalignment peak gets read as a 120 Hz mystery.
The concept
Shaft speed in hertz is RPM divided by 60: a 1800 RPM motor spins at 30 Hz.
The 1x reference is that frequency; 2x and 3x are its multiples. Unbalance lives at 1x, misalignment adds a strong 2x.
Bearing fault frequencies (BPFO, BPFI, ball spin) are calculated from geometry and usually do not fall on shaft harmonics - which is exactly how the detective tells them apart.
Formula
Shaft Frequency
f = RPM / 60
- f = Rotation frequency (hertz (Hz))
- RPM = Shaft speed (revolutions per minute)
Worked example
Problem: A motor runs at 1800 RPM. What is its 1x shaft frequency?
- Use f = RPM / 60.
- Substitute: f = 1800 / 60.
- Calculate: f = 30 Hz.
30 Hz - the 1x line that anchors the whole spectrum reading.
Quick solve: practice it
A two-pole motor runs at 2970 RPM. What is its 1x frequency?
Quick-solve habit: calculate, then ask yourself - does this answer make sense in real units?
Games that use this mathematics
Apply the calculation inside a workplace simulation.