On a large machine carried by fluid-film bearings, the part you most want to watch is the shaft itself, because that is where the clearances are tight and where contact with the bearing does the damage. ISO 7919 is the standard family that governs how relative shaft vibration is measured and evaluated, using proximity probes that watch the shaft directly rather than sensors bolted to the casing. It expresses the result as a peak-to-peak displacement in micrometers and grades it into evaluation zones. This is the shaft-side counterpart to the casing-vibration limits, and turbomachinery acceptance usually leans on both.
ISO 7919 shaft vibration in one line: ISO 7919 is the international standard family covering the measurement and evaluation of relative shaft vibration, meaning the vibration of the rotating shaft measured relative to its bearing housing by proximity probes and expressed as peak-to-peak displacement. It differs from ISO 10816 and ISO 20816 casing criteria, which measure vibration on non-rotating parts, and turbomachinery acceptance typically applies both shaft and bearing-housing limits together. It is now consolidated under the ISO 20816 umbrella.
ISO 7919 is built around a specific measurement: how much the shaft moves relative to the bearing that carries it. That motion is captured by proximity probes, non-contact sensors mounted in or near the bearing that sense the gap to the shaft surface many thousands of times per second. Because the probe is fixed to the housing and looks at the shaft, what it reports is the relative displacement between the two, which is exactly the quantity that governs whether the shaft is getting close to rubbing the bearing. The measured quantity is displacement, reported in micrometers.
The standard expresses shaft vibration as peak-to-peak displacement, the full swing of the shaft from one extreme of its motion to the other, rather than as an RMS or a velocity. Peak-to-peak is the natural choice here because the concern is physical: the largest excursion of the shaft is what closes the clearance to the bearing, so the worst-case swing is the number that matters for avoiding contact. This is a different measured quantity and a different philosophy from the velocity-based casing limits, and mixing them up is a common source of confusion.
Like the casing standards, ISO 7919 grades the measured displacement into evaluation zones with the familiar A through D meaning, where A is new-machine condition and D is capable of causing damage. The boundaries depend on the type and size of the machine, and for shaft vibration they are related to the running clearances in the bearings, since a displacement that is trivial in a large loose bearing could be dangerous in a tight one. This is why the shaft limits are set with the specific machine in mind rather than as a single universal number.
The reason turbomachinery is evaluated on both shaft and casing vibration is that the two measurements answer different questions. A proximity probe watching the shaft sees the shaft's motion inside the bearing directly, which is exactly what you want on a machine with a heavy rotor in fluid-film bearings, where the casing is massive and may barely move even when the shaft is orbiting hard. A casing or bearing-housing sensor, by contrast, sees how much the structure is shaking, which is what matters for the health of the housing, the supports, and the foundation. Neither measurement fully substitutes for the other.
On a large turbine or compressor, a fault can show up strongly in one measurement while looking mild in the other. A rotor problem may drive large shaft displacement inside a heavy, quiet casing, so the shaft probes catch it while the casing reading stays calm. A structural or support problem may shake the housing without the shaft moving unusually relative to it. Applying both acceptance criteria means the machine is judged on how the rotor behaves and on how the structure behaves, which is why turbomachinery protection systems commonly carry both proximity probes on the shaft and sensors on the housing.
The two families were long documented separately, ISO 7919 for shaft vibration and ISO 10816 for casing vibration, but both are now consolidated under the ISO 20816 umbrella that provides a common framework for the two kinds of measurement. The consolidation does not change the fact that they measure different things in different units; it simply brings the guidance for both under one coordinated standard so an engineer can apply shaft and housing acceptance criteria consistently. Reading a shaft limit in micrometers of displacement and a housing limit in millimeters per second of velocity are still two distinct evaluations, just now framed together.
Proximity-probe shaft vibration is almost always tied into a machinery protection system, because a shaft that closes its clearance to the bearing can do serious damage very quickly. The ISO 7919 zone boundaries feed naturally into that system's alarm and trip logic, with the boundary out of acceptable long-term operation used as an alert and the boundary into the damage-capable zone used as a danger or trip level. Because the limits are expressed as peak-to-peak displacement tied to the specific machine, they can be configured once and then enforced continuously.
Bringing that shaft data into a SCADA history alongside the protection function adds the trending and context that a bare trip system does not provide. Merobix pulls the proximity-probe displacement readings into a browser-accessible history where each shaft measurement is stored over time and compared against its zone limits, so a slow growth in peak-to-peak displacement is visible as a trend long before it reaches an alarm. Keeping the shaft data next to the casing vibration in the same history is what lets an operator see both evaluations for a machine side by side.
The value of the continuous record shows most clearly when both measurements are watched together over time. A rising shaft displacement with a quiet casing points the analysis toward the rotor, while a rising casing reading with steady shaft displacement points it toward the structure, and having both trends in one place makes that comparison immediate. For remote turbomachinery where a specialist visit is slow, historizing shaft and housing vibration together turns the ISO 7919 shaft limits from a purely protective trip point into an early-warning input that supports planning a repair before the machine has to be stopped.
ISO 7919 governs relative shaft vibration measured by proximity probes and expressed as peak-to-peak displacement, while ISO 10816 governs vibration measured on non-rotating parts such as bearing housings and casings, typically expressed as velocity. They measure different things in different units and answer different questions, which is why turbomachinery is often evaluated against both. Both are now brought together under the ISO 20816 umbrella.
Peak-to-peak displacement captures the full swing of the shaft from one extreme of its motion to the other, and that largest excursion is what closes the clearance to the bearing. Because the concern on a fluid-film bearing is the shaft getting close enough to rub, the worst-case swing is the physically meaningful number rather than an averaged value. That is why shaft limits are given in micrometers peak-to-peak rather than in velocity.
Not necessarily. On a large machine with a heavy casing, the shaft can orbit substantially inside its bearing while the massive casing barely moves, so the shaft probes see a problem the casing sensors miss. The reverse can also happen, where a structural issue shakes the housing without the shaft moving unusually relative to it. This is exactly why both measurements are used, since one can be alarming while the other looks calm.
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