Misalignment vibration spectrum: 1x, 2x and axial

Why misalignment matters

Shaft misalignment means the centerlines of two coupled shafts, for example a motor and a pump, don't run on one line while the machine operates. The coupling then forces the shafts to bend and the bearings to carry loads they weren't sized for. The result is extra vibration, warm bearings and couplings, worn seals and shorter bearing life. Misalignment is one of the most common faults found in rotating machinery, and one of the cheapest to fix once it is identified.

Two kinds of misalignment

• Angular misalignment: the shaft centerlines meet at an angle at the coupling. It bends the shafts and pushes them back and forth along their axis once per revolution.
• Parallel, or offset, misalignment: the centerlines are parallel but displaced. It loads the coupling sideways, twice per revolution, as each shaft pulls the other toward its own line.

Real machines usually have both at once, in the vertical and the horizontal plane. The spectrum shows which one dominates.

The spectrum signature

Here 1x is the running speed of the shaft, 2x twice it, and so on. As misalignment grows, the harmonics climb further, up to 4x and beyond, and the time waveform becomes repeatable from one revolution to the next, with one or two clear peaks per turn.

A high 2x on its own isn't proof of misalignment: looseness, a bent shaft and some coupling designs produce 2x too. Phase and axial readings are what confirm the diagnosis.

Confirming with phase

Phase compares the timing of the vibration at two points, using a tachometer reference or a two-channel analyzer. For misalignment, measure on the bearings either side of the coupling:

• Axial phase: take axial readings on the motor inboard and the pump inboard bearing. A difference of about 180° points to angular misalignment.
• Radial phase: take horizontal and vertical readings on the same two bearings. A difference of about 180° in the same direction points to parallel misalignment.
• On the same bearing: unbalance gives roughly 90° between the horizontal and vertical readings; misalignment often gives 0° or 180°.

Machines with a flexible coupling can show smaller phase differences, so compare the readings with the coupling side and the free end of each machine as well.

What looks like misalignment but isn't

• Bent shaft: high axial 1x as well, but the axial phase differs by about 180° from one end of the same shaft to the other, not across the coupling.
• Soft foot: a machine foot that doesn't sit flat distorts the frame when it is bolted down. Vibration changes when one hold-down bolt is loosened. Fix soft foot before aligning, or the alignment won't hold.
• Thermal growth: a machine aligned cold can move out of line at operating temperature. If vibration rises as the machine warms up, the cold alignment targets need an offset.
• Pipe strain: pipework that pulls on a pump casing produces the same pattern and comes back after every realignment.
• Unbalance and looseness also raise 1x and 2x. See unbalance vs misalignment vs looseness for how to separate the three.

Fixing it

Once the diagnosis is confirmed, align the machine with a laser or dial indicator system to the tolerance for its speed, and recheck at operating temperature where thermal growth is a factor. Take a new vibration reading after the job: the 2x and the axial 1x should fall back, and the result becomes the baseline for that machine.

How Misoki uses this

Prism AI checks the 1x and 2x pattern, the axial readings and, where it is measured, phase, then names the type of misalignment and how severe it is. Severity sits against ISO 20816 zones in Misoki condition monitoring, and a confirmed finding becomes a work order for the next planned stop.

Frequently asked questions