Bearing Analysis · DWG LS-018

Bearing fault frequency calculator (BPFO, BPFI, BSF, FTF)

Vibration defect frequency reference for 81,000+ bearings across 32 manufacturers. FTF, BSF, BPFO and BPFI are dimensionless multipliers of shaft turning speed — enter a shaft RPM to convert them to Hz / CPM. When a bearing is active in your calculation string, its designation is looked up and loaded automatically.

Maker data (bearing geometry)Last reviewed

Source: SKF Spectrum Analysis (PDF) · Power-MI: Frequencies of a gear assembly

Multipliers follow the standard kinematic bearing equations (pure rolling, stationary outer ring); see Method below for the equations, terms and assumptions.

Validation
  • Fafnir 6309 at 1,800 RPM gives BPFO 91.11, BPFI 148.89, BSF 58.65 and FTF 11.40 Hz, matching the walkthrough. Recomputing from the implied geometry (8 balls, d/D 0.2408, θ 0°) reproduces all four multipliers to within 0.1%.

How calculators are checked: editorial standards.

Spectrum patterns: FTF — Cage BSF — Rolling Element BPFO — Outer Race BPFI — Inner Race Machine faults & more → Drawn at your shaft speed in the Freq Calculator
Enter a search to filter.
BRG ID ↕ Manufacturer ↕ Bearing Type / Model ↕ # B/R ↕ FTF ↕ BSF ↕ BPFO ↕ BPFI ↕ Lube dims Use in Calc

Defect Frequency Calculator

Enter bearing defect frequency multipliers and shaft speed to calculate fault frequencies.

Display units: cycles per second

Tip: Click "Calc →" on any row in the Search page to auto-fill these fields.

Spectrum Patterns at Your Shaft Speed

FFT spectrum — synthetic example

      Time waveform

      Shapes are illustrative, drawn from the multipliers and RPM above — not measured data. Severity is relative amplitude on a fixed scale. Confirm with trend, phase and your own analyzer.

      Gear Mesh Frequencies & Polar Waveform

      GMF = teeth × shaft RPM ÷ 60. Enter both tooth counts and the pinion speed; every gear-pair frequency is shown in Hz, CPM and orders.

      Orders relative to

      Condition polar chart: Spectrum Patterns above → Gear mesh → Polar tab. fHT = GMF × Na ÷ (TP × TG), where Na (assembly phase factor) is the product of the common prime factors of the two tooth counts. APF = GMF ÷ Na.

      Sources: GMF and sidebands at each gear’s running speed, 3× GMF wear check: SKF, Spectrum Analysis (PDF), section 10 Gears. Hunting tooth, assembly phases Na and APF: Power-MI, Frequencies of a gear assembly and Crystal Instruments, An Ear for Gears. Eccentricity, wear and misalignment (2× GMF) patterns: Power-MI, Troubleshooting gears.

      Common Electric Motor Bearings

      Curated list of common SKF 6200/6300 series deep groove ball bearings used in electric motors. Frequency multipliers shown.

      BRG IDManufModelSeries # B/RFTFBSFBPFOBPFILube dimsUse in Calc

      SKF Bearing Number Conversion

      AFBMA inch-designation to SKF metric number cross-reference for electric motor bearings.

      Bearing CategorySeriesAFBMA NumberSKF Number

      How to Use This Reference

      Understanding Defect Frequency Multipliers

      The values in this database (FTF, BSF, BPFO, BPFI) are dimensionless multipliers of shaft turning speed. To get actual fault frequencies, multiply by shaft speed:

      Frequency (Hz) = Multiplier × RPM ÷ 60
      Frequency (CPM) = Multiplier × RPM

      Example: Shaft at 1,800 RPM, BPFO = 5.43
      BPFO = 5.43 × (1800 ÷ 60) = 162.9 Hz  or  9,774 CPM

      Defect Frequency Definitions

      FTFFundamental Train Frequency — cage defect. Low-amplitude, typically subharmonic. Cage wear or lubrication issues.
      BSFBall Spin Frequency — rolling element (ball or roller) defect. Spalling on rolling elements. Often modulated by FTF.
      BPFOBall Pass Frequency Outer race — outer race defect. Most common fault mode. Stationary race in most applications → consistent amplitude.
      BPFIBall Pass Frequency Inner race — inner race defect. Rotating race → amplitude modulated at 1× RPM. Sidebands at 1× RPM around BPFI peaks.

      Manufacturer Codes

      BAN — Bantam — Torrington Bantam division, now Timken BAR — Barden — Schaeffler Group BCA — BCA Bearings — Federal-Mogul brand BOW — Bower — Federal-Mogul brand COO — Cooper — Cooper Split Roller Bearings, now Timken CSI — CSI (unidentified code) DGE — Dodge — now RBC Bearings FAF — Fafnir — now Timken FAG — Schaeffler (FAG) — Schaeffler Group INA — Schaeffler (INA) — Schaeffler Group KAY — Kaydon — now SKF KBC — KBC Bearings KOY — Koyo — JTEKT LBT — Link-Belt MCG — McGill — now Regal Rexnord MES — Messinger — Messinger Bearings MHP — MHP (unidentified code) MRC — MRC — now SKF NDH — New Departure Hyatt — former General Motors bearing division NIC — Nice — Nice Ball Bearings, now RBC Bearings NSK — NSK NTN — NTN REX — Rexnord — now Regal Rexnord RHP — RHP — now NSK ROL — Rollway — Rollway Bearing ROT — Rotek — slewing rings SEA — Sealmaster — now Regal Rexnord SKF — SKF SNR — NTN-SNR — NTN Group STR — STR (unidentified code) TMK — Timken TOR — Torrington — now Timken

      Quick Workflow for Field Analysis

      1. Look up bearing model on the Search Database tab
      2. Click "Calc →" to load FTF/BSF/BPFO/BPFI into the calculator
      3. Enter shaft RPM in the Freq Calculator tab
      4. Compare output frequencies to your vibration spectrum peaks
      5. Look for harmonics and sidebands to confirm fault mode
      Method, worked example & FAQ

      How this calculator works

      The database stores four multipliers for each bearing: FTF (cage), BSF (ball or roller spin), BPFO (outer race) and BPFI (inner race). Each one is a multiple of shaft turning speed. It depends on the bearing's internal geometry, so it is fixed for a given bearing and applies at any speed.

      The calculator multiplies each one by shaft speed: frequency (Hz) = multiplier × RPM ÷ 60 and frequency (CPM) = multiplier × RPM. Orders shows the multiplier itself. Results include the 1× shaft reference and a harmonics table from 1× to 5× of each fault frequency. The spectrum patterns panel draws illustrative shapes at your shaft speed. They are not measured data.

      Search the database by model number, manufacturer or number of balls/rollers, then click Calc → to load a bearing's multipliers into the Freq Calculator tab. You can also type the multipliers by hand. The Motor Bearings tab lists common 6200/6300-series motor bearings. The SKF Conversion tab cross-references AFBMA inch designations to SKF numbers.

      Method: the kinematic equations behind the multipliers

      The page's code does not compute the multipliers from bearing geometry: it reads each bearing's stored FTF, BSF, BPFO and BPFI and multiplies them by shaft speed. The stored values follow the standard kinematic equations for a rotating inner ring and a stationary outer ring:

      • BPFO = (N/2)·fr·(1 − (d/D)·cosθ)
      • BPFI = (N/2)·fr·(1 + (d/D)·cosθ)
      • BSF = (D/2d)·fr·(1 − ((d/D)·cosθ)²)
      • FTF = (fr/2)·(1 − (d/D)·cosθ)

      The page shows BSF, the single ball spin frequency, not 2×BSF. Some references, including SKF's, list the ball defect frequency as 2×BSF because a damaged ball strikes both races once per spin. The 2× column of the harmonics table gives that value. The stored values are orders (multiples of shaft speed, fr = 1). Hz and CPM are those orders × RPM ÷ 60 and × RPM.

      TermMeaning
      NNumber of balls or rollers (one row)
      dBall or roller diameter
      DPitch diameter (the circle through the rolling-element centres)
      θContact angle (0° for a deep-groove ball bearing under radial load)
      frShaft speed: RPM ÷ 60 in Hz, or 1 when working in orders

      Assumptions: pure rolling with no slip, a stationary outer ring and rotating inner ring, and rigid geometry (no load- or clearance-driven change in contact angle). Real bearings slip a little, so measured frequencies differ from the calculated ones, typically by about 1–2% (Randall & Antoni, Rolling element bearing diagnostics — A tutorial, MSSP 25, 2011). The outer-race, ball and cage tones usually come in slightly below the calculated values. Source for the equations: SKF, Spectrum Analysis (PDF).

      Inputs explained

      • Shaft speed (RPM): defaults to 1,800. It is pre-filled when your calculation string carries a speed.
      • FTF, BSF, BPFO, BPFI: dimensionless multipliers, loaded with Calc → or typed by hand. A blank multiplier is skipped.
      • Display units: Hz (cycles per second), CPM (cycles per minute) or Orders (multiples of shaft speed).
      • Bearing label: optional, for your reference.

      Worked example

      Search the database for an SKF 6205 (9 balls) and run it at 1,780 RPM. The record's multipliers are FTF 0.398, BSF 2.357, BPFO 3.585 and BPFI 5.415. Shaft speed is 1,780 ÷ 60 = 29.67 Hz.

      FrequencyHzCPM2× (Hz)
      FTF (cage)11.8170823.61
      BSF (rolling element)69.924,195139.85
      BPFO (outer race)106.366,381212.71
      BPFI (inner race)160.659,639321.29
      1× shaft29.671,780—

      All values are the calculator's output for this record. The 2× column comes from the harmonics table.

      Worked example 2: Fafnir 6309 at 1,800 RPM (the walkthrough video)

      The database record for the Fafnir 6309 has N = 8 balls and multipliers FTF 0.380, BSF 1.955, BPFO 3.037 and BPFI 4.963. At 1,800 RPM, fr = 30 Hz.

      FrequencyOrdersHz
      BPFO (outer race)3.03791.11
      BPFI (inner race)4.963148.89
      BSF (rolling element)1.95558.65
      FTF (cage)0.38011.40

      The record stores no ball or pitch diameter. The geometry is implied by the multipliers: with θ = 0°, BPFO = 4·(1 − d/D) = 3.037 gives d/D = 0.2408. Putting that ratio back into the equations gives BPFI 4.963, FTF 0.3796 (11.39 Hz) and BSF 1.9565 (58.70 Hz). All four match the stored values to within rounding (0.1% or better).

      Frequently asked questions

      How do I calculate BPFO and BPFI?

      Multiply the bearing's BPFO or BPFI multiplier by shaft speed. In hertz, frequency = multiplier × RPM ÷ 60, and in CPM, frequency = multiplier × RPM. For example, an SKF 6205 has a BPFO of 3.585, so at 1,780 RPM its outer-race frequency is 106.36 Hz (6,381 CPM).

      What do FTF, BSF, BPFO and BPFI mean?

      FTF is the fundamental train (cage) frequency and BSF is the ball or roller spin frequency. BPFO is the ball pass frequency of the outer race, and BPFI is the ball pass frequency of the inner race. Each is a bearing defect frequency expressed as a multiple of shaft speed.

      Can I look up bearing fault frequencies by bearing number?

      Yes. The Search Database tab covers 81,000+ bearings from 32 manufacturers. Search by model number, manufacturer or number of rolling elements, then click Calc → to load the multipliers. Design variants can differ (an SKF 6205 has 9 balls and a 6205E has 8), so pick the exact record.

      Why don't my spectrum peaks land exactly on the calculated frequencies?

      Use the actual running speed rather than the nameplate speed. Bearing defect frequencies assume pure rolling, so measured tones often sit slightly off the calculated values. Confirm a fault with harmonics, sidebands and trends.