Why velocity is the severity measure
Vibration can be expressed three ways, and they emphasise different frequency ranges. Displacement dominates at low frequency, acceleration at high frequency, and velocity sits in the middle — which is why ISO 10816 judges machine severity on broadband velocity RMS over roughly 10 to 1000 Hz. It weights the common fault mechanisms fairly evenly:
Because frequency appears in both relations, the same velocity means very different displacements at different speeds. At 3,000 rpm, 2.8 mm/s RMS corresponds to about 25 µm peak-to-peak; at 600 rpm the same velocity means roughly 126 µm — five times the movement.
The four zones
- Zone A — typical of a newly commissioned machine in good condition.
- Zone B — acceptable for unrestricted long-term operation.
- Zone C — unsatisfactory for continuous running. The machine may keep going until a suitable repair opportunity, but it is on borrowed time.
- Zone D — severe enough to cause damage. Investigate now.
Worked example
A 45 kW pump on a rigid foundation reads 2.8 mm/s RMS at 3,000 rpm:
- 45 kW puts it in class II, where the B/C limit is 2.8 mm/s
- 2.8 mm/s sits exactly at the top of zone B — still acceptable, with no margin left
- Running frequency = 3,000 ÷ 60 = 50 Hz
- Displacement ≈ 25 µm peak-to-peak, acceleration ≈ 0.13 g peak
The same reading on a small 5 kW machine would be class I, where the C/D limit is 4.5 mm/s but the B/C limit is only 1.8 mm/s — putting it in zone C and requiring attention. Machine class changes the verdict entirely.
Field notes
- Trend beats absolute value. A machine that has doubled from 0.8 to 1.6 mm/s is deteriorating even though both figures sit in zone A. ISO 10816 itself notes that a change of 25% of the B/C limit is significant regardless of the zone.
- Measure consistently. Same point, same direction, same mounting, same load and speed. A magnetic mount on a painted surface reads differently from a stud-mounted accelerometer, and comparing the two produces phantom trends.
- Broadband severity tells you something is wrong, not what. Diagnosis needs a spectrum: 1× running speed suggests imbalance, 2× suggests misalignment, and high-frequency energy suggests bearings.
- These limits apply to non-rotating parts — bearing housings and casings. Shaft-relative measurement with proximity probes follows ISO 7919 and different limits entirely.
- Class is about the machine and its support, not just power. A large machine on a flexible foundation tolerates more vibration than the same machine mounted rigidly.
Frequently asked questions
What is an acceptable vibration level in mm/s?
It depends on machine class. For a medium machine of 15 to 75 kW on a rigid foundation, up to 1.12 mm/s RMS is zone A for new machines, up to 2.8 mm/s is zone B for unrestricted long-term operation, and above 7.1 mm/s is zone D where damage is expected.
What do ISO 10816 zones A, B, C and D mean?
Zone A is typical of newly commissioned machines. Zone B is acceptable for unrestricted long-term operation. Zone C is unsatisfactory for continuous running — the machine may run until a repair opportunity. Zone D indicates vibration severe enough to cause damage.
Why is vibration measured in velocity rather than displacement?
Velocity correlates well with the destructive energy of vibration across the 10 to 1000 Hz band where most machine faults appear. Displacement dominates at low frequency and acceleration at high frequency, so velocity gives the most even weighting across the range of interest.
How do I convert vibration velocity to displacement?
For sinusoidal motion, displacement peak-to-peak in micrometres equals velocity RMS in mm/s times 1000, times √2, times 2, divided by 2πf. The frequency term means the same velocity implies very different displacements at different speeds.
Representative ISO 10816-3 evaluation zones, for reference and education. Machine acceptance criteria should follow the applicable part of the standard and the manufacturer's limits. See our disclaimer.