A poorly planned vibration route produces numbers you cannot compare month to month, so early bearing and unbalance faults slip through. This article shows you how to pick measurement points, sensor directions and route intervals so each reading tracks the same physical location under comparable conditions. After reading, you can build or audit a route for a pump, fan or motor train and justify each point on the list.
Why point selection decides the value of the whole route
A vibration sensor reads the structure between the bearing and the mounting surface. If you move the sensor 50 mm along the casing, the amplitude at 1× running speed can change noticeably, and bearing fault frequencies can drop into the noise. Consistency matters more than the absolute best spot: the same point, the same stud or magnet, the same direction, every visit.
ISO 17359 sets the general framework for condition monitoring and ISO 13373 covers vibration condition evaluation procedures. Both expect documented measurement locations. In practice that means a point ID list with a sketch: bearing positions numbered drive-end (DE) and non-drive-end (NDE), plus any points on gearboxes, belts or foundations that carry a decision.
Rules that hold on site:
- Measure at every rolling element bearing that carries load, on the bearing housing itself, not on a cover or guard.
- For sleeve-bearing machines above roughly 300 kW, prefer shaft relative measurements with proximity probes per ISO 21940-21 conventions; casing readings can miss shaft motion.
- Add points where the fault mechanism lives: gearbox casing near each mesh, belt spans for sheave issues, motor feet for looseness.
Which directions do you actually need
Three directions per bearing cover the common failure modes: horizontal (H), vertical (V) and axial (A). Horizontal stiffness is usually lowest, so unbalance and looseness show first in H. Vertical readings help separate foundation problems. Axial readings catch thrust-related faults, misalignment and bent shaft behavior.
Budget-driven shortcuts:
- Motors and pumps on rolling bearings: H and A on each bearing; add V where the H/V ratio is unusual or the foundation is soft.
- Fans with belt drives: add a radial point on each sheave side to catch sheave eccentricity and belt frequency content (see the Belt & Gear Mesh Frequencies tool for the expected orders).
- Gearboxes: radial at each bearing plus axial on the casing near the mesh.
Mount the sensor perpendicular to the surface, within about 10 degrees. A tilted magnet reading mixes directions and corrupts trend comparisons. Stud mounting gives the widest usable frequency range; a two-pole magnet on a painted surface can lose reliable response above roughly 1–2 kHz, which hides early bearing tones. Check the mounting resonance by striking the sensor lightly and confirming the ring frequency sits well above your highest analysis band.
How often should you measure
The interval must be shorter than the time a defect needs to grow from detectable to damaging. That growth time depends on machine speed, load, bearing type and lubrication, so no single number fits all machines. ISO 17359 recommends starting from criticality and adjusting with experience.
A workable starting scheme:
- Critical, unspared machines: continuous monitoring or daily readings.
- General purpose pumps, fans, motors: weekly to monthly.
- Standby and low-duty equipment: quarterly, plus a reading after every restart or repair.
Shorten the interval when any of these appear: amplitude crosses an alarm band, a new bearing tone appears, or a repair changed the rotor or foundation. Lengthen it only after a machine has stayed in the same condition band for several consecutive visits.
Setting alarm limits with ISO 10816-3 and ISO 20816-3
ISO 10816-3 (now largely superseded by ISO 20816-3) gives vibration severity zones for industrial machines with power above 15 kW, grouped by foundation type: rigid or flexible. The zones are expressed as RMS velocity in mm/s in the 10 Hz to 1,000 Hz band, measured on bearing housings. Zone A is new machine condition, B is unrestricted long-term operation, C is restricted operation, D is damage risk.
Use the standard's zone boundaries as initial alarm limits, then refine with your own baseline. A practical rule: alarm when the reading exceeds twice the machine's own baseline RMS velocity, provided it also exceeds the zone C boundary. Trend in velocity for general machinery; use acceleration (g RMS or gE, envelope) for rolling bearing fault detection, and displacement for low-speed machines below roughly 120 rpm where velocity underestimates severity.
The ISO 10816-3 Vibration Severity Evaluator applies the zone boundaries to your readings directly, and the Vibration Unit Converter converts between displacement, velocity and acceleration at a given frequency.
Site procedure for a new route
Follow this checklist on the machine train:
- Draw the machine train and number every bearing: 1 DE motor, 2 NDE motor, 3 DE pump, 4 NDE pump, and so on.
- Mark H, V and A at each bearing per the rules above; record the exact sensor spot with a center punch or permanent marker.
- Record nameplate data: speed, power, bearing types and numbers. Compute bearing fault frequencies with the Bearing Fault Frequency Calculator and store them in the route so the analyst sees expected defect orders.
- Choose the measurement quantity per point: velocity RMS for severity, acceleration or envelope for bearings, displacement for low speed.
- Set the Fmax per point: 10× running speed for general machinery, higher (2–5 kHz and above) where bearing or gear mesh tones matter.
- Take a baseline set with the machine at stable, normal operating load. Note process conditions (flow, pressure, temperature) with every reading, including the baseline.
- Set alarm and trip limits from ISO 10816-3/20816-3 zones and the baseline doubling rule.
- Fix the route order and sensor type in the data collector so every future visit repeats steps 2 and 6 identically.
- Review the route after the first three visits; delete points that never carry information, add points where trends are ambiguous.
Linking route findings to balancing and alignment actions
A route finds the symptom; the follow-up needs the right tool. A dominant 1× peak with steady phase points to unbalance; confirm the tolerance against ISO 21940-11 and plan a field correction with the Single-Plane Balancing Calculator or the Two-Plane Balancing Calculator depending on the aspect ratio of the rotor. A strong 2× with high axial vibration suggests misalignment; check against the Shaft Alignment Tolerances before scheduling laser alignment. When a natural frequency sits near running speed and amplitude varies with speed, verify with the Resonance & Critical Speed Simulator before ordering balance weights.
When to call for an on-site measurement
Route readings tell you that something changed; they do not always tell you what to do. Call for on-site dynamic measurement when amplitude exceeds the trip zone, when phase or spectrum shape changes between two consecutive visits, when a machine cannot reach full speed without vibration, or when a repair must be planned around a balance or alignment correction. REI Balance performs field balancing and diagnostic measurements on site; describe the machine and the readings through the contact page and we will propose the measurement plan.
