Setting the Right Monitoring Interval

There is no universal schedule. "Monthly" is not always right. "Quarterly" is not always wrong. The correct interval depends on one thing: how fast can a fault develop from first detectable symptom to functional failure? ISO 17359 calls this the "lead time to failure."

The rule is simple: measure at intervals shorter than half the lead time to failure. If a bearing typically takes two months from first spalling to seizure, measure at least monthly. If a fan impeller accumulates enough dust to shift vibration in three weeks, check every 10 days. The half-interval rule gives you at least two data points in the fault development window — enough to see the trend and plan action before failure.

Risk-based interval selection

ISO 17359 provides a criticality framework. Start with these intervals, then adjust based on what your data actually shows.

These are starting points. The moment you detect a change — a vibration level creeping up, a new frequency appearing in the spectrum — increase measurement frequency immediately. A machine that was "quarterly" becomes "weekly" the moment it shows a developing fault.

Continuous vs Periodic: Two Approaches, One Goal

Most plants use both. Critical assets get online systems. Everything else gets periodic routes with a portable instrument. The key is matching the approach to the criticality and fault development speed — not choosing one method for the entire plant.

Vibration Trending: What to Track and How

Collecting data without tracking changes over time is pointless. Vibration trending means comparing each reading to a baseline and to previous readings — to see whether the machine is getting better, worse, or staying the same.

Establishing a baseline

Every machine needs a reference point. Record baseline vibration under stable, documented conditions: steady speed, normal load, stable temperature. For new machines, measure after commissioning. After overhaul, allow a short run-in period (24–72 hours) before locking the baseline — vibration may shift during bedding-in as bearings seat and components settle.

Record the operating conditions with the vibration data. A vibration reading without RPM, load, and temperature context is almost useless — you can't compare a reading taken at 60% load to one taken at 100% load.

What to track: three layers

Layer 1 — Overall RMS velocity (mm/s). The simplest and fastest check. Compare to ISO 10816 zone boundaries (see table below). A single number that tells you "good, acceptable, investigate, or act now." Use this for route efficiency — it takes 30 seconds per measurement point.

Layer 2 — Key frequency components. When the overall level rises, you need to know why. Track the 1× RPM component (unbalance, looseness, buildup), the 2× RPM component (misalignment, coupling), and the high-frequency band (bearing defects). The Balanset-1A FFT spectrum shows all of these.

Layer 3 — Rate of change. The growth rate matters as much as the absolute level. A machine at 4.5 mm/s that's been stable for 12 months is different from a machine at 4.5 mm/s that was at 2.0 mm/s three weeks ago. Rapid acceleration means fast-developing fault — shorten the interval and plan action immediately. Slow linear growth supports planned maintenance at the next convenient window.

When to Rebalance: 4 Condition-Based Triggers

Balancing is not a calendar task. Do not schedule balancing "every 6 months" or "every year" without evidence. Balance when the data says so — and only when you've confirmed that unbalance is the dominant fault.

Why Vibration Returns Soon After Balancing

If vibration comes back within days or weeks after a balance job, don't rebalance again — investigate. Recurring vibration means balancing is addressing a symptom, not the root cause.

Dirty rotor. Deposits shift or flake off, destroying the balance. If you balanced a dirty impeller, the correction weights compensated for the dirt. When the dirt moves, the weights become the new imbalance source. Solution: clean to bare metal before balancing.

Thermal distortion. The rotor bows or expands unevenly when hot, shifting the mass distribution. A motor balanced cold at 20°C winding temperature may vibrate badly at 80°C. Solution: balance at operating temperature.

Loose fits. The rotor shifts on the shaft, the hub slips, or a key loosens during starts and stops. Each start changes the position slightly, so the balance changes too. Solution: fix the mechanical fit before balancing.

Resonance. Running speed near a structural natural frequency amplifies small residual imbalance. The machine appears to "need rebalancing" constantly because tiny mass changes (thermal growth, deposit shifts) get amplified. Solution: change the speed or modify the structure to move the natural frequency — see our vibration isolation guide.

Field Report: 14 Months Between Balances

A food processing plant in Central Europe had four identical 30 kW centrifugal fans on a drying line, each running at 2,920 RPM. The maintenance team was rebalancing all four every 3 months — a technician came in for a full day, balanced each fan, and left. Twelve visits per year across four fans.

We set up a monthly monitoring route using the Balanset-1A in vibrometer mode. The first three months of data showed: Fan 1 and Fan 3 were stable at 1.8–2.2 mm/s overall (Zone A/B, no action needed). Fan 2 was climbing slowly — 2.4 → 3.1 → 3.8 mm/s — with a rising 1× component indicating unbalance from product buildup on the impeller blades. Fan 4 had a strong 2× component suggesting coupling misalignment, not unbalance at all.

Result: we balanced Fan 2 (after cleaning) and aligned Fan 4's coupling. Fans 1 and 3 were left untouched. Fourteen months later, Fans 1 and 3 still don't need balancing — they're at 2.0 and 2.3 mm/s respectively.

The savings came from stopping unnecessary work. Two fans didn't need balancing at all. One needed alignment, not balancing. Only one actually had an unbalance problem. Monthly monitoring with a portable instrument cost 30 minutes per visit — the data told the team exactly which machine needed what, and when.

ISO 10816 Severity Reference

ISO 10816-3 provides vibration severity zones for industrial machines with power ratings between 15 kW and 300 kW. Use these as reference thresholds for your trending program. Your plant may set tighter limits based on experience.

These values apply to Group 2 machines (15–300 kW) on rigid foundations. For Group 1 (>300 kW) and flexible foundations, thresholds differ — consult the full standard. The key point: Zone A/B = monitor normally. Zone C = investigate and plan. Zone D = act now.

Frequently Asked Questions