1 Scope
This document provides guidance on the selection of suitable vibration standards for specific machine types and thus the selection of appropriate vibration measurement and evaluation methods. GB/T6075, GB/T11348 and other standards relevant to machine vibration are outlined.
This document gives an overview of the relevant standards and summarises their scope of application. It also gives the theoretical basis of analysis for determining whether to carry out vibration measurements on non-rotating parts, rotating shafts or both, for those machines where no previous experience exists. As there may be characteristics of a particular machine that lead to a different choice of the most appropriate measurement method, this document is not intended to replace the practical experience that manufacturers or users already have with a particular machine.
The aim of this document is not to equip the reader with all the technical details of the criteria required to carry out measurement and evaluation work on a particular machine, but to guide the reader through the selection of the appropriate criteria, which are the ones that give the necessary details. Then, with suitable training, the reader will be able to carry out the measurement or evaluation work.
2 Normative references
The following documents constitute essential provisions of this document through normative references in the text. Among them, note the date of the reference document, only the date corresponds to the version applicable to this document; do not note only the lake reference document, its latest version (including all the change orders) applicable to this document.
GB/T 2298 Glossary of mechanical vibration, shock and condition monitoring (GB/T 2298-2010, ISO 2041 :2009, IDT)
GB/T 6444 Glossary of mechanical vibration balance (GB/T 6444-2008, ISO 1925:2001 ,IDT)
3 Terms and definitions
The terms defined in GB/T 6444 and GB/T 2298 and the following terms and definitions apply to this document.
3.1
Shaft absolute vibration
Shaft vibration measured from absolute coordinates.
3.2
Shaft relative vibration
Shaft vibration measured on a sensor support (e.g. bearing housing).
3.3
Pedestal vibration
Vibration of the bearing support structure.
3.4
dynamic stiffness of bearing
The stiffness of bearing components taking into account the effects of damping and mass.
4 Evaluation of machine vibrations
4.1 General provisions
GB/T 6075 provides guidelines for the measurement and evaluation of vibration in different types of machines, based on measurements of non-rotating parts of machines, and GB/T 11348 provides guidelines for the measurement and evaluation of vibration in different types of machines, based on measurements of rotating axes of machines.
4.2 Machine vibration standards and guidelines
There are four types of machines that can be classified according to their vibration measurement and evaluation needs.
a) Reciprocating machines with rotating and reciprocating parts, such as diesel engines and certain types of compressors and pumps. Low frequency vibration measurements are usually carried out on the machine's
Low frequency vibration measurements, typically in the range of 2 Hz to 1 000 Hz, are carried out on the main structure of the machine.
b) Rotating machines with rigid characteristic rotors, such as certain types of electric motors, single-stage pumps and low speed pumps. The vibration of the main structure (e.g. the bearing cap or housing) is usually measured and the vibration value represents the excitation force of the rotor due to unbalance, thermal bending, friction and other excitation sources.
c) Rotating machines with flexible rotor characteristics, such as large steam and gas turbine generators, multistage pumps and compressors. The machines can be in different vibration modes as they accelerate through one or more critical speeds to operating speed. In such machines, the measured vibration values of the structural components may not be fully indicative of the rotor vibration. For example, even if the measured vibration values on the bearing cap are not significant, the flexible rotor can produce large displacements, leading to machine failure. Therefore, only direct measurement of shaft vibrations is permitted.
d) Rotating machines with quasi-rigid rotors, such as some steam turbine rotors, axial flow compressors and fans. These machines contain a special type of flexible rotor, where the vibration values measured on the bearing cap are used to assess shaft vibration.
Chapter 8 details an analytical method that is used to determine the vibration measurement method (or standard) that works best on a particular machine, based on the physical and structural characteristics of the machine.
4.3 Classification of machine vibration intensity
In the classification of machine vibration intensity, the motion variable (displacement, velocity or acceleration) used depends on the applicable standard, frequency range and other factors. When classifying machine vibrations in the range 10 Hz to 1 000 Hz, the speed of vibration is usually chosen since it is a direct measure of machine vibration intensity. For lower and higher frequency vibrations, the preferred measurement quantities are displacement and acceleration respectively.
For simple harmonic motion, the kinematic variables can be chosen as peak or root mean square (r.m.s.) values. However, for machines with complex motion, the use of these two variables produces significantly different results, mainly due to special waveforms (e.g. pulse-like waves or high-frequency harmonics), so the choice of peak or r.m.s. value requires a different trade-off. For rotating machines in the speed range 600 r/min to 12 000 r/min, the RMS values of the vibration velocity are in the best agreement with the vibration intensity. Vibration intensity is originally defined as the maximum value of the broadband root mean square of the velocity measure in the frequency range 10 Hz to 1 000 Hz. Therefore, the relevant standards for this machine type (e.g. GB/T 6075.2) define specified points for the assessment of the structure.
Note. Vibration intensity is now the general term for the maximum vibration value measured on non-rotating parts of a machine, whether displacement, velocity or acceleration (see GB/T 13824).
4.4 Measurement methods and instruments
The cited standards include, among others, methods for measuring shaft-to-housing relative signals, absolute measurements and inertial applications.
5 Measurement on non-rotating parts
5.1 GB/T 6075.1 gives general guidelines for the measurement and evaluation of machine vibration on non-rotating parts. This is part 1 of a series of standards which provide separate guidelines for each generic category of machine. These guidelines are based on vibration measurements and vibration variations and are relevant for operational monitoring and acceptance testing.
6 Measurement on rotating parts
6.1 GB/T 11348.1 provides specific guidance on the measurement of vibration on rotating shafts of machines. Such machines often have flexible rotor-shaft systems, and by measuring these rotating parts, changes in vibration state can be detected more clearly and sensitively. In addition, machines with relatively rigid and/or heavy housings compared to the rotor mass are often the typical preferred type of machine for shaft vibration measurements.
Industrial turbines, gas turbines and turbocompressors, for example, have several vibration modes in the operating speed range where shaft measurements provide a better view of the response due to unbalance, misalignment, thermal bending, friction and bearing unloading.
7 Relevant standards
7.1 GB/T 6072.5 covers the basic requirements and definitions for torsional vibration of shaft systems driven by reciprocating internal combustion engines (RICs). GB/T 6072,5 covers units driven by internal combustion engines for land, locomotive traction and marine use, but excludes units used to drive road and earth moving machinery, agricultural tractors, industrial tractors, cars, trucks and aircraft.
8 Analytical guidance for the selection of suitable vibration criteria for specific machines
8.1 General
The analysis method described below can be used for the selection of vibration measurement methods. The method is based on the dynamics of the machine and is characterised by the ratio of the dynamic stiffness of the base to the dynamic stiffness of the bearing, α (see Appendices B and C for additional information). Practical examples of stiffness values are given in Appendix D. The choice of vibration measurement method can be based on the value of α in Figure 1.
Appendix A (informative) lists mechanical vibration criteria by application area
Appendix B (informative) Bearing dynamics
Appendix C (informative) Dynamic stiffness of bases
Appendix D (informative) Examples of typical values of bearing and base dynamic stiffness
Appendix E (informative) Composite dynamic stiffness of bearing components and bases
References
1 Scope
2 Normative references
3 Terms and definitions
4 Evaluation of machine vibrations
5 Measurement on non-rotating parts
6 Measurement on rotating parts
7 Relevant standards
8 Analytical guidance for the selection of suitable vibration criteria for specific machines
Appendix A (informative) lists mechanical vibration criteria by application area
Appendix B (informative) Bearing dynamics
Appendix C (informative) Dynamic stiffness of bases
Appendix D (informative) Examples of typical values of bearing and base dynamic stiffness
Appendix E (informative) Composite dynamic stiffness of bearing components and bases
References
