Doc.Ref.No.:mVMT/om/101
masibus
Issue No. :00
Model : VM908
Operator’s Manual VM-908 Vibration meter
Page 8 of 11
02/05/16
Recommended vibration levels in mm/s common findings.
The following is in part an extraction of the old standard ISO 2372 class 4, large machines on flexible
foundations, with some common findings added.
This simplified list can be used, as a first consideration, when you approach a machine newly commissioned
or after some time in operation.
Take as a good housekeeping rule to investigate the reason for any machine that vibrates above 3 mm/s
RMS. Do not leave them above 7mm/s without being assured that they will sustain long term operation
without increased wear since the machines capable of that are very few.
•
0 – 3 mm/s
Small vibrations. None or very small bearing wear. Rather low noise level.
•
3 – 7 mm/s
Noticeable vibration levels often concentrated to some specific part as well as direction of the machine.
Noticeable bearing wear. Seal problems occur in pumps etc. Increased noise level. Try to investigate the
reason. Plan action during next regular stop. Keep the machine under observation and measure at smaller
time intervals than before to detect a deterioration trend if any. Compare vibrations to other operating
variables.
•
7 – 18 mm/s
Large vibrations. Bearings running hot. Bearing wear-out cause frequent replacements. Seals wear out,
leakage of all kinds evident. Cracks in welding and concrete foundations. Screws and bolts are loosening.
High noise level. Plan action soonest. Do your best to reveal the reason. You are wearing down investments
quickly.
•
18 – mm/s
Very large vibrations and high noise levels. This is detrimental to the safe operation of the machine. Stop
operation if technically or economically possible considering the plant stop cost. No known machine will
withstand this level without internal or external damage. Reduce any further running time to an absolute
minimum.
Resonance
When working with vibrations in machine maintenance, you will soon find that resonance is a common but
rather unknown problem in modern machinery.
To understand a resonance you can compare with the string of a guitar. The string has its natural basic tune
that will ring as soon as the string is struck. The actual frequency of the tune depends on the stiffness and
the distributed mass of the string.
All machines have similar built in "tunes" with corresponding properties consisting of stiffness and mass in
the form of mechanical strings such as shafts, beams, floors and in all mechanical parts. If any natural
excitation (= alternating force) in the machine has the same or nearly the same frequency as a resonance
frequency the vibration will be amplified in this machine part, a much higher level will occur than would be
the case if the resonance would be shifted away from the excitation frequency.
