Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 1, Basic Information
Section 6, Vibration Tolerances
There have been several vibration guides, originating from various instrument manufacturers, National Electrical Machinery Association (NEMA), the American Petroleum Institute (API), International Standards Organization (ISO) and The Hydraulic Institute. However, because the combination of all these guides and standards is very confusing, Update has prepared a guide (not a "standard") based on its own experiences.
Update's guide is based on seismic readings measured on the machine's case or frame, as near to the bearing as practical. The English units do not directly convert to the metric units as they both have been "rounded out." Such estimated numbers are reasonable as no present guide takes into consideration the differences between large, heavy framed machines such as reciprocating compressors and light framed machines such as gas turbines, machines with rolling element bearings and those with plain bearings. Certain committees have been working on separate guidelines for vibration measured with non-contact proximity pickups. However, so far committees have not all come up with the same numbers. Therefore, the chart found on the following page is only a guide.
The guide is also based on velocity units as almost all machine speeds are over 1000 rpm and under 6000 rpm (with proportional harmonic frequencies). However, displacement units are often better at relatively low speeds. For example, consider approximately 1000 cpm as a rough guide as to when displacement units may provide a better guide. For under approximately 500 rpm, displacement should be a better guide or velocity guide units may have to be halved. For under 200 rpm, velocity units are almost useless. (See section "Considerations for Low Frequency Vibration.") Unfortunately, there may be instrument-related difficulties when using displacement units. For example, transducers used by most analysts are accelerometers. The vibration is picked up as acceleration and then converted to velocity units through a mathematical process called “integration.” This produces some “integration noise” that is usually visible at the lowest frequency end of the spectrum. For a single integration, the frequency range and amplitude of this noise is usually quite small and not too disturbing, except for very low speed machines. However, converting acceleration to displacement units requires what is called “double integration” which provides a considerably large amplitude and frequency range of integration noise. This usually either interferes with or totally obscures the bona fide peaks of vibration frequencies obtained from relatively low speed machines. There are also some situations where displacement amplitudes provide better guidelines than velocity units, regardless of speed, such as when concerned about surface finish of metal, wood, paper "barring," or waviness in rolled aluminum or steel.
This textbook contains only part of the information in our Practical Vibration Analysis seminar.