understanding bearing vibration frequencies

8/10/2019 Understanding Bearing Vibration Frequencies

1/3

September 2003

1

www.easa.com

By Dave Felten

Mechanical Field Service Department

L&S Electric, Inc.

Schofield, Wisconsin

Editors Note: The following article was sub-mitted for publication by Paul Gullickson ofL&S Electric, Inc. and a member of the Techni-cal Services Committee.

***Welcome to the age of predictive maintenance

technologies. More and more of our customers areusing tools such as vibration analysis to assess thehealth of their rotating equipment.

Many of our customers are using this technol-

ogy to assess new and rebuilt rotating equipmentonce its installed and running.

This serves two main purposes:1. It demonstrates the quality of the newly

acquired/repaired equipment (taking theburden off the supplier/service center shouldthe equipment vibrate once its installed).

2. It provides a baseline for trending.

Unfortunately, these initial vibration readingscan be pushed into an alarm status by manycustomer-related issues such as poor couplingalignment and/or machine installation. This iswhy its so important for todays repair facility

to provide the customer with baseline vibra-tion data gathered during its final test run,providing evidence that the rotating equipmentran within general vibration guidelines beforebeing shipped.

Vibration frequency analysis can expose manymechanical and electrical problems in an electricmotor. The purpose of this article is to discuss oneof these: Bearing Defects.

Rolling Element Bearing DefectsDefects in general rolling element bearings can

be generated by fatigue, wear, poor installation, im-proper lubrication and occasionally manufacturing

faults in the bearing components shown below.Being the typical bearing found in electric mo-

tors, the rolling element (or anti-friction) bearingis made up of the following components as illus-trated in Figure 1:1. Outer Race

2. Inner Race

3. Cage

4. Rolling Element (ball, roller or tapered roller)

Figure 1 also shows the Pitch Diameter (Pd)which is the span between the centers of two op-

posite rolling elements.

rolling elementcage

outer race

balll roller tapered roller

Rolling Elements

Pd

inner race

Figure 1. Rolling Element BearingComponents

When a bearing spins, any irregularity in theraceway surfaces or in the roundness of the rollingelements excites periodic frequencies called fun-

damental defect frequencies.These are:

1. FTF Fundamental Train Frequency (fre-quency of the cage)

2. BSF Ball Spin Frequency (circular fre-quency of each rolling element as it spins)

3. BPFO Ball Pass Frequency of the Outer race(frequency created when all the rollingelements roll across a defect in the outer race)

4. BPFI Ball Pass Frequency of the Inner race(frequency created when all the rollingelements roll across a defect in the inner race)

Fundamental defect frequencies depend uponthe bearing geometry and shaft speed. Once youidentify the type of bearing installed, you can ei-ther calculate the defect frequencies yourself orrequest the defect multipliers from the manufac-turer. (Providers of vibration analysis softwareoften incorporate a database that contains thesemultipliers from various bearing manufacturers.)

Understanding Bearing Vibration Frequencies

Continued on Page 2
http://www.easa.com/http://www.easa.com/


2/3

EASA CURRENTS

2

www.easa.com

Example 1:If you have the defect multipliers at your dis-

posal, the process of calculating the defectfrequency is as follows:

1. Look up the bearing number that is exhibitingthe suspect vibration frequency on a table likethe one below:

Bearing # of FTF BSF BPFO BPFIID Rolling

Elements

9436 19 .434 3.648 8.247 10.753

9437 19 .434 3.648 8.247 10.753

9442 22 .443 4.191 9.740 12.260

2. Multiply this number by the shaft speed matedwith this bearing and you have the defectfrequency that would be generated by a defecton the element in question. See Figure 2.

9.740 x 351 rpm shaft speed = 3419 cpm

Example of an Outer Race Defect

Route Spectrum 10-Dec-02 10:21:50

OVERALL= .1482 V-DG PK = .1484LOAD = 100.0 FPM = 870. (351. RPM)

0 8000 16000 24000

0

0.02

0.04

0.06

0.08

0.10

Frequency in CPM

Freq:Ordr:Spec:

3435.0 9.794 .02555

A= BPFO: 3419

A A A A A A A

PK

VelocityinIn/Sec

Figure 2. Outer Race Bearing Defect

If the frequency and harmonics (multiples) ofit are present on the vibration spectra, you mostprobably have an outer race bearing defect. Itcould be a spall on the raceway, electrical fluting,false brinelling acquired during bearing storage orequipment transport, etc.

For those interested in calculatingthedefect frequencies, the formulas are

listed below. Today, it is difficult to get

these parameters. Typically the manu-facturer will simply supply one with the

multipliers.

Youll need to find the followingkey parameters:

1. Rolling Element Diameter2. Pitch Diameter3. Number of Rolling Elements4. Contact Angle

5. Speed

Formula 1: FTF =S/2* [1 (

Bd/Pd* cos!)]

Formula 2: BSF =

Pd

/2Bd* S * [1 (

Bd

/Pd*cos!

)

2

]

Formula 3: BPFO =Nb

/2*S * [1 (Bd

/Pd* cos!)]

Formula 4: BPFI =Nb

/2* S * [1 + (Bd

/Pd* cos!)]

Where:

FTF = Fundamental Train Frequency (Hz)

BSF = Ball Spin Frequency (Hz)

BPFO = Ball Pass Frequency of Outer Race (Hz)

BPFI = Ball Pass Frequency of Inner Race (Hz)

Nb = number of rolling elements

S = speed (revolutions per second)

Bd = ball diameter (in or mm)Pd = pitch diameter (in or mm)

! = contact angle (degrees)

Be advised that there will be occasions whenthe calculated defect frequencies dont exactlymatch the bearing defect frequencies that appearin the vibration spectra.

Typically this is due to higher than normalthrust loads which cause the bearing to run at adifferent contact angle. These abnormal thrustloads can be caused by sources such as mis-

alignment.Also, not all bearing manufacturers use the

same number of rolling elements in a particularbearing size.

The most common bearing problem is the outerrace defect in the load zone; inner race faults arethe next most common. It is very rare to see a faultat the bearings ball spin frequency or BSF.

Understanding Bearing Vibration. . . Continued From Page 1

Continued on Page 1


3/3

September 2003

3

www.easa.com

Action Step:The presence of any of thesefour fundamental fault frequencies shouldresult in the repair technician replacing thebearing and ensuring the housing fits and shaft

journals are within tolerance.Finally, its worth discussing the presence of

mechanical looseness, which manifests itself asharmonics of 1x running speed, on a new or re-built bearing housing or journal. This indicationof looseness could be coming from poor basemounting or one of the following:1. Loose housing-to-outer race fits

2. Loose journal-to-inner race fits

3. Excessive internal bearing clearance

Sleeve Bearing DefectsSleeve bearings do not make use of rolling ele-

ments; rather, the shaft rides on a layer oflubricating oil inside the bearing bore. The lubri-cant is either sealed inside the bearing, gravity fedto the bearing or pumped in (pressure fed).

Sleeve bearings which have excessive wear/clearance exhibit a vibration spectrum similar tothe one in Figure 3. Notice the series of runningspeed harmonics (up to 10 or 20). Wiped sleevebearings often show much higher vertical ampli-tudes than horizontal. A higher axial reading onone end than the other provides further indication,with the higher vibration level on the end with thedamaged bearing.

Bad Bearing Fits Between Housing and Outer Race

Route Spectrum 16-Oct-02 10:37:20

OVERALL= .3307 V-DG PK = .3280 LOAD = 100.0 RPM = 1783. (29.71 Hz)

0 20000 40000 60000 800000

0.05

0.10

0.15

0.20

Frequency in CPM

Freq:Ordr:Spec:

1800.0 1.010 .02434

PK

VelocityinIn/Sec

Figure 3. Looseness from Wear/Clearance Problems

In contrast, mechanical looseness caused byloose mounting bolts or cracks in the frame struc-

ture or bearing pedestal typically look like thespectrum in Figure 4.

Loose Mounting

Route Spectrum 24-May-02 09:35:33

OVERALL= 1.10 V-DG PK = 1.10 LOAD = 100.0 RPM = 3495. (58.25 Hz)

0 20 40 60 80 100 120 140

0

0.1

0.2

0.3

0.4

Frequency in kCPM

PK

VelocityinIn/Sec

Freq:Ordr:Spec:

3.450 .987 1.033

Figure 4. Structural Looseness

Excessive looseness can also causesubharmonic multiples at exactly 1/2 or 1/3 x rpm(.5x, 1.5x, 2.5x, etc.).

SummaryThe final vibration test in the service center

helps ensure the customer is receiving a top-notch repair. Youll rest easier knowing that yourrepair work passed general vibration guidelinesand your sales team will love selling this added

value to your customers.By providing documented vibration spectra

with a rotating equipment repair service, the cus-tomer is less likely to immediately call yourservice center when the newly repaired equipmentis reinstalled and vibrates. Theyll be more proneto verify things within his control such as themounting and coupling alignment.

Understanding Bearing Vibration. . . Continued From Page 2

The final vibration test in theservice center helps ensure the

customer is receiving a top-notch repair. Youll rest easierknowing that your repair workpassed general vibrationguidelines and your sales teamwill love selling this addedvalue to your customers.

understanding bearing vibration frequencies

Documents