Vibration of Machinery - Part 2
Consider an electric motor operating at 3,600 rpm, in one year, it rotates 1,892,000,000 times. What if your technician says he has some vibration reading on the cover plate of DE/NDE #bearings? Be careful!!!
This is worth noting that vibrations can't be eliminated at all. They are an indication of how smooth energy flow is happening between different parts of a machine. It is desirable to have a machine "as smooth as baby skin". A management program of #dairyprocessing plant machinery #vibration can reveal important insights to a #plantmanager. #Maintenance managers can take important decisions about machinery maintenance to restore its health. So, let's discuss fundamental characteristics in vibration measurement and how these are implemented in maintenance programs.
Machinery vibrational movement is classified into three dimensions i.e. Axial, Tangential, and Radial. This is illustrated in picture below: ?
Period
The period of the vibration is the time required to complete one oscillation. That is the total time required for the mass to move from the rest position to the upper limit, back through the rest position to the lower limit, and return to the rest position. It has units of seconds per cycle.
?Frequency
Frequency is the repetition rate of vibration per unit of time. The frequency, f, has units of cycles/second, or hertz (Hz), and is the reciprocal of the period in seconds. In rotating machinery applications, we are often interested in expressing the frequency in cycles per minute, or cpm, so that the frequency can be directly compared to the rotative speed of the machine, measured in revolutions per minute, or rpm.
This allows examination of the vibration frequency in terms of multiples of the rotational speed. The two-times rotational speed is known as the second harmonic (2X), and the three-times rotational speed is the third harmonic (3X). The rotational speed is also known as the fundamental frequency.
Phase?
Phase is the angle of vibration issue. Phase is used to dynamic balance and identifies resonance (critical speed) problems. It is not normally used in routine measurements.
Displacement
Displacement describes the position of an object from its resting position. It is movement of rotating part w.r.t center point. This applies to stationary structures or very low-speed equipment. Acceptance criteria are also speed dependent e.g. 25 micrometer is excellent at 1800 rpm but it is very dangerous at 30,000 rpm.
?Velocity
Velocity describes how rapidly the object is changing position with time. The velocity of a vibrating object is continually changing. At the upper and lower limits, the object stops and reverses its direction of travel, thus its velocity at these two points is zero. While passing through the neutral or position of rest, the velocity is at its maximum. Since the velocity is continually changing with respect to time, the peak or maximum velocity is always measured and commonly expressed in inches-per-second peak or millimeters per second (mm/s) peak. Rather than using peak velocity root mean square velocity (rms) is normally used in industrial practice.
Acceleration
Acceleration is the rate of change of velocity with respect to time. Typically, vibration acceleration is measured in terms of G’s or the number of times the normal force due to gravity i.e. 9.8m/s.
?These concepts can be explained in the below picture by an example of pendulum oscillation.
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Vibration Severity Guidelines
Vibration measurements are generally governed by ISO TC 108. American #standards are governed by ANSI. Here are general guidelines for velocity measurement method; most widely used in industrial practice and displacement method. ISO standards like ISO 20816 series are worth reading in this context.
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In part 3 of this series of articles; most common vibration categories with underlying root causes will be discussed.
For Part 1, follow the link below
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