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Basic methods used by DREAM™ for diagnostics and prediction

  • Detects and identifies bearing defects after mounting and during operation including problems of installation.

  • Diagnoses bearing condition by one vibration measurement, no history required.

  • Makes a long-term prediction of rolling element bearing condition from one vibration measurement

  • Collects and stores the data about the bearing condition.

Basic features of DREAM and DCFIL bearing test systems

The method used is based on envelope spectrum analysis of random vibrations created by friction forces in the bearings. In a non-defective bearing, the friction force is constant in time and the exited high frequency vibration is a stationary, random process. During the rotation of a defective bearing, the friction force and the random vibration become amplitude modulated with a period, T, and depth, m, because of a periodic change in the friction coefficient or pressure on the race surface. Shock pulses are considered to be a particular case of amplitude modulated friction forces whose appearance results from a periodic change in the friction coefficient. Thus, in the case of the defects, harmonic components appear in the envelope spectrum which are absent in envelope spectrum of a non-defective bearing. The type of defect can be identified by the harmonic component frequencies, the defect depth by the difference L between the harmonic Lh and the random Lr components, or in the time function by T and m respectively (see below).

12 types of bearing defects

No. Type of defect

Main frequencies of modulation


Revolution around outer (fixed) race



Nonuniform radial tension



Misalignment of outer race



Wear of outer race



Cavities (cracks) on outer race

kfout; k=1,2,3...


Wear of inner race

kfr; k=1,2,3...

the decay of amplitudes with increase of k


Cavities (cracks) on inner race



Wear of cage and rolling elements

fc or (fr-fc)


Cavities, spallings on rolling elements



Defects of several bearing surfaces

fout + fin = zfr or
fout + fr

no fout-fr or



Slip of race

kfr k=1,2,3...

no other frequencies of modulation


Defects of lubrication


increase of high frequency vibration level

Defect Frequency Equations

- rotor frequency
- cage frequency
- rolling element rotation frequency
- rolling element pass frequency, outer race
- rolling element pass frequency, inner race
z  - number of rolling elements
drol - diameter of rolling element
dc - diameter of cage
- contact angle between balls and outer race

Defect type identification (above) and defect level characterization (below).

With one single measurement, this method can give an accurate prediction for non-failure operation of a bearing for up to 20% of the specified bearing service life, as was proven by experimental work in a 15 year period with bearings having dimensions from 20 mm to 2 m in machines with rotating speeds from 10 to 10000 RPM. The program was designed to make the process of diagnostics and prediction automatically, increasing bearing diagnostic productivity and effectiveness. The combined use of National Instruments hardware and DREAM/DCFIL software gives even an unqualified operator the following abilities:

  • to monitor and give a long-term condition prediction for rolling element bearings,

  • to supervise the quality control of bearing mounting,

  • to significantly decrease maintenance expenses.


The input data for the program are:

  • from the operator - the rotation frequency

  • from the bearing archive - bearing inner and outer race diameters, the number and diameter of the rolling elements, the number of rows and the contact angle, bearing specified service life in the diagnosed machine.


The output data for the program are:

  • the estimation of the bearing condition,

  • the types and severity (incipient,  medium,  severe) of the identified defects,

  • the predicted  service life without failure,

  • the recommendations for corrective actions.




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