Units of measurement

Absolute units

All these parameters may be measured in either metric or Im­perial units. The latter are still used in the USA and hence are commonly available in this country because of the wide avail­ability of American instrumentation. Those commonly used are shown in Table 15.2. Reference may also be made to Chapter 22 Units and Conversions, for further guidance.

 Property SI = Metric Imperial — US Displacement Jim — 0.001mm Thous — mils = 0.001 in Velocity Mm/s In/s Acceleration M/s2 G’s (1g = 32.17ft/s2) Frequency Hz = cyc/sec Cyc/min
 Table 15.2 Units used in vibration measurement

Decibels and logarithmic scales

Frequency is almost invariably plotted logarithmically to keep the scale length down to a reasonable size. It results in the lower frequency part being expanded whilst the high frequency part is compressed. A constant percentage resolution is ob­tained over the whole chart.

In like manner logarithmic scales may be used for plotting vibra­tion velocities and accelerations. As the absolute values can vary enormously, and to enable vibration levels to be easily compared, decibel scales are often used. From our knowledge of noise levels it is appreciated that the decibel (dB) is the ratio of one level with respect to a reference level. It therefore has no dimensions. To obtain absolute values the reference level must be known.

It is an unfortunate fact that there are two commonly used sets of reference levels — marine/defence and those recommended in ISO 1683. These are set out in Table 15.3.

For the same absolute values ISO levels will therefore be 20dB higher than marine/defence levels. In the fan industry it is be­lieved that the latter are almost universal perhaps because the values for a fan’s vibration closely align with the figures ob­tained for the Noise Power Level in dBW ref 1012 watt. We all get a little worried using values above 120 AdB!

RPM

№ (cpm)

 Judgement R10 ■50 VDl 2056 ISO 2372 ■ 170- BS 4675 -5 :10 100-! 160- -10 50- 150- *5 100- Unsatisfactory D -5 140- -1 10: J-0.5 130- -0.5 50- Just J » 5- A. Satisfactory 1 C 120- R0.1 • ■ Jc RO. l ■0.05 Tz — 3 Satisfactory ‘b E • 110- S’ 1 •■5= 1 : *0.05 § 3 ‘:1 § O ; 90- R0.01 : 0.005 1 O’ Ul 5 "Q.01 80- -0.5 *0.001 -■ 0.1- : 0.005 70­ 60- •0,0005 5- Good A 0.05- R 0.0001 RO.001 50- *0.00005 0.01- ; 0.0005 :0.1 40- R 0.00001 0005- -0.000005 1-
 Urn mils

 In/s

 100-

 Figure 15.3 Machine vibration nomogram for converting absolute parameters into decibel values

Ance, dominates all others, it can be applied to more complex wave forms, without undue error.

The nomogram in Figure 15.3 is a simple way of carrying out these conversions.

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