RPM to Hertz: Running Speed and 1x Line on a Spectrum

Reading a Running Speed as a Line on the Spectrum

A vibration spectrum means nothing until you know how fast the machine was turning when it was captured. Every diagnosis on a condition monitoring route — imbalance, misalignment, looseness, a spalled bearing race — starts by locating the running speed on the frequency axis and then asking what sits above it. Analysers plot hertz; nameplates, tachometers and work orders talk in revolutions per minute. This page bridges the two so the first peak can be pinned down before anything else is interpreted.

Conversion factor: divide by 60 — 1 RPM = 0.016666667 Hz, because a minute holds sixty seconds. A fan measured at 1 485 RPM therefore puts its 1× line at 24.75 Hz, with 2× at 49.5 Hz and 3× at 74.25 Hz.

What the Analyst Is Actually Looking For

The 1× Line Is the Machine's Signature

One peak per revolution is the fundamental every rotor produces. Its height tracks how much rotating force is off centre, and its position on the axis is fixed entirely by shaft speed, so the same fault slides along the spectrum when the machine is run faster.

Orders Are Multiples of That One Number

Talking in orders rather than hertz makes a report portable: 2× means the same thing on a 1 200 RPM blower as on a 3 600 RPM pump, even though one peak lands at 40 Hz and the other at 120 Hz.

Rolling-Element Tones Fall Between the Orders

Race and ball defect rates come out at awkward multiples such as 4.1× or 6.2× of shaft speed. That non-integer position is exactly what separates them from mechanical harmonics, and it only becomes visible once shaft speed is known in hertz.

Speed Drift Smears the Whole Picture

A belt-driven or inverter-fed machine whose speed wanders a couple of percent during a long average spreads its own peaks into blurred humps. Recording the speed at the moment of capture is part of the measurement, not a formality.

Setting Up a Route Point Before You Collect

These are the four things worth doing with a shaft speed before the accelerometer is even placed on the bearing housing.

Type in the speed you actually measured

Use the strobe or laser tacho reading rather than the nameplate figure, since a loaded machine or a belt drive rarely turns at the catalogue value. The field starts at 1 and updates as you type, so correcting 1480 to 1485 takes a single keystroke.

Note the 1× position and its first harmonics

Double and triple the result mentally so you know where 2× and 3× should appear. Those three positions are where cursor placement begins on almost every spectrum you will ever open.

Turn an order span into an Fmax setting

Decide how many orders the measurement has to cover — ten for general condition, forty or more when rolling-element faults matter — and multiply the hertz figure by that count. The collector wants that product in hertz, which is why the conversion has to happen before the setup screen is filled in.

Take a peak back the other way

When a suspicious peak is already on screen in hertz, the swap arrows reverse the pair so its equivalent shaft speed appears directly; typing into the second box works just as well, since both accept input. The copy button hands over the digits alone, which is what a report table or a spreadsheet column wants.

Where the Orders Fall on a Machine at Speed

The speeds below cover most of what a plant route contains: four-, six- and eight-pole driven equipment plus the two-pole pumps and compressors. Reading across gives the first two cursor positions and a sensible upper limit for a bearing-oriented measurement.

Measured shaft speed	1× running speed	2× harmonic	Fmax covering 40 orders
600 RPM	10 Hz	20 Hz	400 Hz
900 RPM	15 Hz	30 Hz	600 Hz
1 200 RPM	20 Hz	40 Hz	800 Hz
1 500 RPM	25 Hz	50 Hz	1 000 Hz
1 800 RPM	30 Hz	60 Hz	1 200 Hz
3 000 RPM	50 Hz	100 Hz	2 000 Hz
3 600 RPM	60 Hz	120 Hz	2 400 Hz

Two of those rows deserve a warning label. At 3 000 RPM the 1× line sits on 50 Hz and at 3 600 RPM it sits on 60 Hz, exactly where electrical pickup from the supply is most likely to leak into a signal. When a machine at those speeds shows an unexplained peak there, prove it is mechanical by nudging the speed and watching whether the peak travels with the shaft or stays put.

Tacho Reading Converted at the Machine

Speeds get retyped and corrected constantly on a route; both boxes recalculate on every keystroke, so a revised strobe reading never means starting over.

Cursor Frequency Back into Shaft Terms

Reversing the direction turns a hertz value read off a cursor into the shaft speed that would generate it, which is how a stray peak gets attributed to the right rotor in a train.

Values Ready for the Analyser Setup Screen

Copying returns digits with no unit and no thousands spacing, so an Fmax or a running-speed entry can be pasted into collector software without editing.

Coast-Down Speeds Checked in Sequence

Stepping through a list of speeds logged during a run-up shows how far the 1× line travels across the axis, which is what makes a resonance crossing recognisable.

Questions from the Condition Monitoring Route

What exactly is the 1× line, and why do 2× and 3× appear beside it?

The 1× line is the peak produced once per revolution by whatever rotating force is not perfectly centred or perfectly straight. Because that force repeats with the shaft, it lands precisely at shaft speed expressed in hertz. Harmonics turn up when the once-per-revolution disturbance stops being a smooth sine — a coupling that binds twice per turn, a rotor rubbing over part of its orbit, a loose foot that lifts and drops. Any clipped or distorted waveform must contain multiples of its own repetition rate, so the harsher the distortion, the taller the harmonic family marching up the axis.

Should a measurement be set in orders or in a fixed hertz span?

For a machine that holds a constant speed, a fixed hertz span is simpler and lets successive readings be overlaid directly for trending. As soon as speed varies — an inverter-fed drive, a coast-down, a turbine run-up — a fixed span smears every rotational peak, because the line drifts through several analysis bins during the capture. Order tracking fixes this by taking a tachometer pulse and sampling in step with the shaft instead of in step with a clock, so 1× stays nailed to the same position whatever the speed does. The trade-off is that anything genuinely fixed in hertz, such as a structural resonance, becomes the thing that smears instead.

Why do bearing defect frequencies land between the orders rather than on them?

Because a rolling-element cage does not turn at shaft speed. It creeps round at roughly 0.38 to 0.42 of the shaft rate, set by ball diameter, pitch diameter and contact angle, and every defect rate derives from that. Outer-race passing typically works out near 4× to 4.8× and inner-race passing near 5.2× to 6.5× on a common eight-element bearing, with neither ever a whole number. A handy sanity check is that the outer- and inner-race rates add up to the element count multiplied by shaft speed. That fractional placement is a gift for diagnosis: a peak sitting at 4.1× cannot be blamed on looseness or a coupling, so it points straight at the bearing.

How are Fmax and the number of lines chosen for a route point?

Fmax comes from the highest order that has to be visible. Ten orders is enough for a general health check; forty to seventy is normal where bearing tones and their harmonics must be resolved. On a 1 500 RPM machine that means 250 Hz and 1 000 Hz respectively. Lines of resolution then decide how finely the span is divided: 1 600 lines across 1 000 Hz gives 0.625 Hz per bin, fine enough to keep a bearing tone clear of a nearby harmonic. Resolution costs time, though — each average takes lines divided by Fmax, or 1.6 seconds here, and a six-average reading at every point on a long route adds up fast.

How does the harmonic pattern separate imbalance from misalignment?

Imbalance is close to pure: a tall 1× line in the radial directions, little above it, similar amplitude horizontally and vertically, and an axial reading that stays low on a between-bearings rotor. Misalignment pushes energy upwards and outwards — 2× grows until it rivals or beats 1×, a 3× component often joins in, and the axial direction comes alive because the coupling is pulling the shafts along their length as well as sideways. Phase settles any remaining doubt: readings taken across the coupling shift by roughly 180 degrees when the shafts are misaligned, whereas an unbalanced rotor keeps them broadly in step.

RPM to Hertz