Belt Pass Frequency Calculator

Article

1) What belt pass frequency means

Belt pass frequency (BPF) is how often a marked point on the belt completes one full loop. It is a useful “signature” for belt-driven machines because it links the drive geometry to repeating impacts, tracking wander, and modulation seen in vibration or sound. It helps align spectra with inspection notes and reduces misidentification during troubleshooting.

2) Typical ranges you may see

In many industrial drives, belt speeds fall around 5–30 m/s, while belt loop lengths often range from 1–10 m. That combination commonly produces BPF values from roughly 0.5–30 Hz (30–1800 CPM). Small, fast drives can exceed this range, especially with short belts.

3) Why length accuracy matters

BPF is inversely proportional to belt length, so a 3% length error creates about a 3% frequency error. For timing belts, using pitch length improves repeatability. For V-belts, measure the installed path if possible, or use catalog effective length to keep comparisons consistent.

4) Using pulley RPM and diameter

If belt speed is unknown, the calculator converts pulley RPM into belt speed using the pitch diameter. For example, a 150 mm driver pulley at 1200 RPM gives a speed near 9.42 m/s before slip. This is ideal when you have tachometer data but no line-speed spec.

5) Accounting for slip and load

Slip reduces belt speed, shifting BPF downward. Well-tensioned timing belts can be near 0% slip, while V-belt slip is often 0–5% depending on load and tension. A 2% slip changes frequency by 2%, which is enough to move a peak between FFT bins.

6) Harmonics and sidebands

BPF rarely appears as a single tone. Mis-tracking, joints, or belt ribs can create harmonics at 2×, 3×, and higher. If there is pulley eccentricity or belt tension variation, you may also see sidebands around BPF spaced by shaft RPM or belt-span natural frequencies.

7) Diagnostics you can support

Use BPF to separate belt-related peaks from bearing or gear-mesh frequencies. Rising BPF harmonics can indicate a damaged belt section, glazing, or contamination. A changing BPF over time may point to tension loss, increasing slip, or a drive ratio change after maintenance.

8) Reporting and trending tips

Record both Hz and CPM so different teams can compare results. Trend the same measurement location, sensor orientation, and machine load. Export the harmonics table for route-based monitoring, and re-run the calculation whenever belt length, pulley diameter, or tension settings change.

FAQs

1) Is belt pass frequency the same as pulley RPM?

No. Pulley RPM is rotations per minute. BPF is belt loops per second (Hz) or per minute (CPM). BPF depends on belt speed and total belt length, not just shaft speed.

2) Should I use pitch diameter or outside diameter?

Use pitch diameter whenever possible. Pitch diameter better represents where the belt transmits motion, so speed and BPF estimates are closer to reality than using outside diameter, especially for V-belts.

3) What slip value should I enter?

If you do not know slip, start with 0% for timing belts and 1–3% for V-belts under steady load. If measured BPF is lower than expected, increase slip until it matches observations.

4) Can I calculate BPF if I only know motor RPM?

Yes. If you know the pulley ratio, convert motor RPM to driver pulley RPM. Then enter pulley RPM, pitch diameter, and belt length to estimate belt speed and BPF.

5) Why do I see several peaks near the BPF value?

Multiple peaks can occur due to harmonics, belt splice effects, belt ribs, or modulation from pulley eccentricity. Load changes can also smear peaks. Checking 1× to 10× harmonics helps confirm belt-related patterns.

6) What unit should I prefer, Hz or CPM?

Use Hz for engineering calculations and vibration standards, and CPM if your plant practices use cycles per minute. This calculator provides both, so you can paste results into reports without manual conversion.

7) How often should I recompute BPF?

Recompute after belt replacement, tension adjustments, pulley changes, or speed control updates. For condition monitoring, also recompute if slip is suspected to have increased because that shifts BPF and its harmonics.