Vibration assessment
Introductory disambiguation
Vibration assessment aims to ensure reliable and safe operation of a machine, based on evaluation of the machine operating state by means of vibration measurements. Local diagnostics/analysis of machine components is outside the scope of this documentation. Solutions for diagnostic condition monitoring of components such as roller bearings and gear units are described separately below.
References to common standards
A number of standards exist for assessing machine vibrations, including the following:
- ISO 5348, Mechanical vibration and shock - Mechanical mounting of accelerometers
- ISO 10816, Mechanical vibration - Evaluation of machine vibration by measurements on non-rotating parts (previously VDI Guideline 2056). This standard consists of several parts.
- ISO 10816-3 refers to industrial machines with a rated capacity of more than 15 kW and rated speeds between 120 rpm and 15000 rpm, measured on site.
- ISO 10816-7 refers to centrifugal pumps for industrial application
- ISO 10816-21 refers to wind turbines with horizontal axis and gearbox
- ISO 7919, Mechanical vibration - Evaluation of machine vibration by measurements on rotating shafts. This standard consists of several parts.
- ISO 7919-3 refers to coupled industrial machines
- ISO 7919-2 refers to stationary steam turbines and generators with a capacity of more than 50 MW an nominal operating speeds of 1500 min-1, 1800 min-1, 3000 min-1 and 3600 min-1
- ISO 20816-1, Mechanical vibration - Measurement and evaluation of machine vibration. Consolidation of ISO 7919-1 and ISO 10816-1.
Evaluation of machine vibrations based on DIN ISO 10816-3
The scope of this standard includes steam turbines up to 50 MW, electric motors and fans. Because the scope is quite wide, the standard is explained in more detail below. The standard aims to classify the machine state in four different classes by means of vibration data for acceptance measurements and operational monitoring.
Assessment criteria according to the standard are the RMS value of the vibration velocity and the RMS value of the vibration displacement. Usually it is sufficient to measure the vibration velocity. The additional evaluation of the vibration displacement is recommended if low frequency components are encountered. If both vibration parameters are logged and analyzed, the poorer of the two determined classes is applied.
The frequency range of the vibrations to be captured depends on the machine speed:
- 10 Hz to 1000 Hz for speeds of more than 600 rpm
- 2 Hz to 1000 Hz for speeds of less than 600 rpm
Suitable measuring points are characterized by the fact that they reflect the dynamic forces of the machine as purely as possible. For example, locations where local resonances occur are not suitable. Suitable locations tend to be bearing stands and bearing covers; measurements are usually carried out in two orthogonal directions.
The classification also takes into account the machine substructure, subdivided into rigid and elastic substructures. If the lowest natural frequency of the whole system consisting of machine and substructure is at least 25% higher than the main exciting frequency (generally the rotational frequency), the substructure can be regarded as rigid, otherwise as elastic. This evaluation should be carried out separately for each measuring direction (two orthogonal directions, see above).
DIN ISO 10816-3:2009 describes four evaluation zones (A, B, C, D), with limit values as listed in the following table.
Machine group | 1 | 2 | |||
Installation | rigid | elastic | rigid | elastic | |
RMS value of the vibration velocity in mm/s | 11,00 .. ∞ | D | D | D | D |
7.10 .. 11.00 | D | C | D | D | |
4.50 .. 7.10 | C | B | D | C | |
3,50 .. 4,50 | B | B | C | B | |
2.80 .. 3.50 | B | A | C | B | |
2.30 .. 2.80 | B | A | B | B | |
1.40 .. 2.30 | A | A | B | A | |
0.00 .. 1.40 | A | A | A | A | |
Machine group | 1 | 2 | |||
Installation | rigid | elastic | rigid | elastic | |
RMS value of the vibration displacement in µm | 140 .. ∞ | D | D | D | D |
113 .. 140 | D | C | D | D | |
90 .. 113 | D | C | D | C | |
71 .. 90 | C | B | D | C | |
57 .. 71 | C | B | C | B | |
45 .. 57 | B | B | C | B | |
37 .. 45 | B | A | B | B | |
29 .. 37 | B | A | B | A | |
22 .. 29 | A | A | B | A | |
0 .. 22 | A | A | A | A | |
Zone A | The vibrations of recently commissioned machines tend to be in this zone. |
Zone B | Machines with vibrations in this zone are usually regarded as suitable for continuous operation without restrictions. |
Zone C | Machines with vibrations in this zone are usually regarded as unsuitable for continuous operation. The machine may generally be operated in this state for a limited period, until a suitable opportunity for remedial measures arises. |
Zone D | Vibration values in this zone are usually regarded as dangerous in the sense that damage to the machine may occur. |
Machine group 1 | Large machines with a rated output of 300 kW to 50 MW and electrical machines with a shaft height of more than 315 mm |
Machine group 2 | Medium-sized machines with a rated output of 15 kW to 300 kW and electrical machines with a shaft height between 160 mm and 315 mm |
The described vibration assessment according to ISO 10816-3 is implemented in three different examples. See: