Of all the frequencies in a machine's vibration, one stands out as the most diagnostic: the component at exactly running speed, called 1X because it occurs once per revolution. Measured as an amplitude paired with a phase angle, 1X forms a vector that is the single most useful piece of information for diagnosing unbalance and for balancing a rotor. Isolating just this synchronous component, and tracking how its vector changes, tells an analyst things that the overall vibration level never could. This guide focuses on the running-speed component specifically: what 1X amplitude and phase are, why together they fingerprint unbalance, and how they drive balancing.
1X vibration and phase in one line: 1X vibration is the component of a machine's vibration that occurs at exactly the shaft's running speed, once per revolution, and it is described by two numbers: its amplitude and its phase angle relative to a once-per-turn reference. Together the amplitude and phase form a vector that points to where the effective heavy spot is and how large its effect is. Because unbalance produces a force once per revolution, the 1X vector is the primary fingerprint of unbalance and the quantity that balancing directly measures and corrects, distinct from the overall level, which sums all frequencies together.
1X is the running-speed, or synchronous, part of the vibration, isolated by filtering the signal down to just the frequency equal to the shaft's rotation. The full vibration signal contains many frequencies at once, but 1X keeps only the one that repeats exactly once per revolution and discards the rest. Focusing on this single component is what makes it so diagnostic, because the once-per-turn frequency is where the most common and most correctable rotor problem, unbalance, lives, and looking only at 1X removes the clutter of everything else.
The 1X measurement has two inseparable parts. The amplitude is how much running-speed vibration there is, and the phase is the angle at which the running-speed vibration peaks relative to the keyphasor once-per-turn reference. Neither number alone is complete: amplitude without phase tells you how bad but not where, and phase without amplitude tells you where but not how bad. Reported together they define a vector, with a length equal to the amplitude and a direction equal to the phase, and that vector is the working unit of running-speed diagnostics.
Thinking of 1X as a vector is what unlocks its power. A vector can be compared to another vector, subtracted from another, and tracked as it moves, which a plain amplitude cannot. This is why analysts always carry the 1X phase alongside the 1X amplitude: it lets them treat the running-speed vibration as a quantity with direction, so a change in the machine's condition shows up not just as a bigger or smaller number but as a shift in a specific direction, which is far more informative about what changed.
Unbalance is the classic 1X fault because a heavy spot on a rotor produces a centrifugal force that rotates with the shaft, pushing once per revolution in a direction that turns with rotation. That is precisely a 1X excitation, so pure unbalance shows up as strong 1X vibration with relatively little at other frequencies, and its phase points consistently at the heavy spot. When an analyst sees dominant 1X with a stable phase and little else, unbalance is the leading suspect, and the 1X vector effectively locates the heavy spot around the rotor.
The 1X phase is also what separates unbalance from other faults that can raise the running-speed amplitude. Misalignment, a bent shaft, and looseness can all contribute at 1X, but they behave differently in phase and in how they distribute energy across harmonics, so the amplitude alone would confuse them while the phase helps tell them apart. For example, the relationship between the 1X phase at different measurement points and in different directions carries information about whether the problem is unbalance, misalignment, or a bent shaft, information that is invisible if you only record amplitudes.
Tracking the 1X vector over time and across startups is one of its most valuable uses. Because it is a vector, a change in the machine can be seen as the vector moving, and the amount and direction of that movement is diagnostic. A 1X vector that shifts between two startups indicates something changed, such as a lost balance weight, a shifted rotor, or a thermal effect, and the direction of the shift hints at the cause. Watching the 1X vector run-to-run turns it into an early-warning and change-detection tool, not just a snapshot of the current state.
Balancing a rotor is fundamentally an exercise in manipulating the 1X vector, which is why 1X amplitude and phase are the quantities a balancing job measures. The analyst records the initial 1X vector, then adds a known trial weight at a known angular position and measures how the 1X vector moves in response, which reveals how the rotor responds to weight at that location. From that response the correct balancing weight and position are calculated to drive the 1X vector toward zero. Every step is measured in 1X amplitude and phase, so without the phase there is no way to balance a rotor at all.
This is the clearest reason phase, not just amplitude, must be captured. An overall level or even a 1X amplitude alone can tell you the machine is out of balance, but it cannot tell you where to put the correction weight, and putting weight in the wrong place makes the balance worse. The phase is the direction information that points to the heavy spot and lets a single, correct correction be applied. Balancing is the everyday task that most concretely demonstrates why the 1X vector, with both of its parts, is indispensable.
For remote monitoring, the 1X amplitude and phase can be derived continuously and trended by a cloud platform such as Merobix, giving operators a running record of the running-speed vector rather than just the overall level. Because the 1X vector is so sensitive to unbalance and to change, trending it lets a machine that is slowly going out of balance, or that shifts its balance between startups, be spotted early from the trend without a site visit. When the vector has drifted enough to warrant action, the same measurements feed the balancing job. Carrying 1X phase alongside amplitude on the platform, rather than the overall level alone, is what turns remote vibration monitoring from simple alarming into targeted diagnosis of the most common rotor fault.
1X vibration is the component of a machine's vibration that occurs at exactly the shaft's running speed, once per revolution, isolated by filtering the signal down to that frequency. It is called 1X because it happens one time per turn. This synchronous component is the most diagnostic single frequency because the most common and correctable rotor fault, unbalance, produces a force once per revolution and therefore shows up strongly at 1X. It is described by both an amplitude and a phase.
Amplitude tells you how much running-speed vibration there is, but phase tells you the direction, meaning where the effective heavy spot is around the rotor. Together they form a vector, and only the vector locates the problem and lets it be corrected. Balancing in particular is impossible without phase, because you must know where to add or remove weight, and adding weight in the wrong place makes the balance worse. Phase also helps distinguish unbalance from misalignment and a bent shaft, which amplitude alone cannot.
Balancing works entirely with the 1X vector. The analyst records the initial 1X amplitude and phase, adds a known trial weight at a known position, and measures how the 1X vector moves, which shows how the rotor responds to weight there. From that response the correct balancing weight and location are calculated to drive the 1X vector toward zero. Because the whole procedure is measured in 1X amplitude and phase, the phase is essential, since without a direction you cannot know where to place the correction weight.
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