Technical Paper: How to – Vibration Analysis

Fig. 1 Analyzing vibration measurements on Omnitrend Center taken by Vibscanner 2.

It’s not a secret, that vibration on machines causes severe issues – not only on a long term perspective. Vibration, caused by any issue is the natural enemy of any rotating machine such as a pump. Due to the latest handheld high-speed data collector Vibscanner by Pruftechnik vibration of all kind on all kind of machine can be detected during the maintenance routine.

With the special software Omnitrend Center the measurement results of Vibscanner 2 (Fig. 1) can be analyzed deeply and used for an entire machine health program. But what measurements does the new high-speed data collector Vibscanner 2 deliver, how do you interpret these and how can you react on these measurements? Let us give you a quick kick-start into vibration analysis to help keep your pumps alive.

Fig. 2 The chart shows 4 zones of vibration severity ranging from good to unacceptable.
• Zone A – the vibration of newly commissioned machines would normally fall into this zone.
• Zone B – machines with vibration within this zone are normally considered acceptable.
• Zone C – machines with vibration in this zone are normally considered unsatisfactory for long term continuous operation. Machine may be operated for a limited period in this condition until a suitable opportunity arises for remedial operation. (It is advised to increase the frequency of vibration monitoring during this operating period.)
• Zone D – vibrations within this zone are normally considered to be of sufficient severity to cause damage to the machine.

Fault detection

The main objective of a vibration monitoring program is the detection of incipient machine failures. The methodologies associated with fault prediction usually involve comparing current vibration information with a vibration description of that machine or a similar machine in satisfactory operating condition. This comparison is made by two methods:

  • Comparison to industrial standards – ISO 10816-3-7
  • Comparison to a previously measured reading.

ISO 10816-3-7

The ISO 10816-3-7 is the current standard for the evaluation of “standard” rotating machine operating condition. Issued in 2009 it covers “large and medium size industrial machines with nominal power rating above 15 kW and nominal speeds between 120 rpm and 15000 rpm”. In addition pumps are added as a specific category for consideration. This range covers most rotating machines and can therefore be used as a good guide for in-situ operating condition. Variations will inevitably occur when comparing these standards to actual machine operating condition. Machines should not however be condemned because of variations in readings without first considering other potential reasons for the difference in readings.

The use of these zones shown in Figure 2 and the numerical values ascribed to them are not intended as an acceptance standard for machine manufacturers and customers but the values do help to establish alarm and warning criteria for a routine condition monitoring program. If machines are found to be operating at vibration levels consistently above the nominal values shown in the standard, investigation as to the cause should always be carried out.

 Comparison to previous readings

This method is the most widely used method of identifying changes in machine operating condition. Most commonly referred to as trending, comparison quickly shows the machine operator or manager if the machine condition has changed, and by how much, and in what period of time. Trended graphical measurement values combined with ISO alarm limits give clear visual warnings of machine condition change and with some software packages, can be used to predict likely run to failure intervals in order to schedule remedial repair. (Graphic predictions of machine trends should always be considered as a guide to likely failure and not as a definitive measurement of machine failure intervals.)

Fault mode analysis

There is a great deal of literature on diagnostic techniques employed for various types of equipment. Although we will not discuss specific diagnostic methods in this article, you will find that a great deal of information must be readily available to execute an effective vibration monitoring program. Once the vibration monitoring program flags a machine as potentially having a mechanical problem, the following questions must be answered.

  • How severe is the problem?
  • What is the problem?
  • When must the machine be taken out of service for repair?

Machinery diagnostics using vibration analysis provides information that addresses these questions. The machinery diagnostics technique viewed here is based on a technique known as “fault mode” analysis. This technique utilizes the fact that specific mechanical events, such as unbalance, misalignment, looseness, bearing defects, aerodynamic and hydraulic problems, and gearbox problems usually generate vibration frequencies in specific patterns. The frequency, amplitude and pattern of the peaks in a vibration spectrum can be a telling indication of the type of problem being experienced by the machine. The principles of “fault mode” analysis include:

  • Measurement of mechanical faults such as unbalance and misalignment generate mechanical vibration in a well defined frequency pattern.
  • Comparing the vibration levels and vibration spectra on similar types of machines will help establish the severity and cause of a vibration problem.

Fig. 3 Due to Vibscanner 2 vibration measurements can be taken within seconds – literally.

Unbalance

Vibration caused by unbalance occurs at a frequency equal to 1 rpm of the unbalanced part, and the amplitude of vibration is proportional to the amount of unbalance present. Normally, the largest amplitude will be measured in the radial (vertical or horizontal) directions.

Misalignment

Generally, misalignment can exist between shafts that are connected with a coupling, gearbox or other intermediate drives. Three types of misalignment are:

  • Angular – where the center line of the two shafts meet at an angle
  • Offset – where the shaft center lines are displaced from one another
  • A combination of angular and offset misalignment

A bent shaft looks very much like angular displacement, so its vibration characteristics are included with misalignment. Misalignment, even with flexible couplings, have two forces, axial and radial, which result in axial and radial vibration. The significant characteristics of vibration due to misalignment or a bent shaft is that it will be in both the radial and axial directions. For this reason when axial vibration is greater than one half of the highest radial measurement (horizontal or vertical), then misalignment or a bent shaft should be suspected. All misalignment conditions will produce vibration at the fundamental (1 x rpm) frequency components since they create an unbalanced condition in the machine. Misalignment will sometimes produce vibration at the second harmonic (2 x rpm).

Looseness

Mechanical looseness can be caused by loose rotating components or loose machine foundations. Mechanical looseness causes vibration at a frequency of twice the rotating speed (2 x rpm) and higher orders of the loose machine part. In most cases, vibration at the fundamental (1 x rpm) frequency will also be produced.

Bearing problems

One of the results of damage to rolling element bearings is that the natural frequencies of the bearing components are excited by the bearing defect. The resonant vibration or “ringing” occurs at frequencies between 2 kHz and 60 kHz. This vibration is most effectively measured at a level of acceleration in units of g’s Peak. Vibration is measured by the machinery monitoring system as a HFE (High Frequency Energy) measurement and gives an effective indication of the condition of rolling element bearings. Based on field experience, the shock pulse technique works well on motors and other quiet equipment. Care must be taken when using the technique on pumps and gearboxes, where flow, cavitation, and tooth meshing can produce impulses which interfere with and mask the impacts produced by bearing defects. Rotational frequencies related to the motion of the rolling elements, cage and races are also produced by mechanical degradation of the bearing. These frequencies are dependent on bearing geometry and shaft speed and can be found typically, in the 3 – 10 x rpm range and because of these reasons the “enveloping” method is the most widely adopted method of viewing specific bearing defects.

Aerodynamic and hydraulic problems

Normally associated with blade or vane machinery such as pumps or compressors, aerodynamic and hydraulic vibration is created by an unstable or unbalanced condition within the machine. In most cases this will produce a vibration at the fundamental frequency (1 x rpm) of the machine and blade pass/vane frequency components.

Gearbox problems

Gear defects or faulty gears produce low amplitude, high frequency vibration. The vibration is predominantly at gear mesh frequency. Gear mesh frequency is calculated as follows: Gear mesh frequency (GMF) = Speed of output gear x Number of teeth in output gear Example: 52 tooth gear running at 90 rpm (90/60 = 1.5 Hz). GMF = 52 x 1.5 = 78 Hz. Most gear problems exhibit vibration at the gear mesh frequency, and may be summarized as follows:

  • Unbalance – predominant at the 1 x rpm of the gear.
  • Misalignment – predominant at the 1 x rpm and 2 X rpm; may excite GMF.
  • Pitch line run out – predominant at GMF with 1 x rpm sidebands.
  • Faulty gear teeth – predominant at GMF with sidebands at 1 x rpm of faulty gear.

To assure proper machine health care throughout the whole lifetime of the machine, vibration measuring and analysis are necessary tools to reduce energy waste and maintenance costs on the long run and improve quality even on the short run. Pruftechnik technologies and services help to set your machines’ status to “running healthily”.

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