Calculator inputs
Calculation history
Recent results appear here on this device.
Formula used
Belt pass frequency: BPF = v ÷ L
Pulley belt speed: v = πDN ÷ 60
Slip correction: vactual = vtheoretical(1 − s ÷ 100)
Open belt length: L = 2C + π(D+d)÷2 + (D−d)²÷4C
Crossed belt length: L = 2C + π(D+d)÷2 + (D+d)²÷4C
Belt speed measures linear travel along the belt. Belt length sets one complete circulation distance. Their quotient gives belt pass frequency.
How to use
- Select geometry, measured-length, or direct-speed calculation mode.
- Enter pulley diameters and the driver shaft speed.
- Provide centre distance or measured total belt length.
- Add slip, defects, splices, teeth, and harmonic limits.
- Enter a measured spectrum peak for comparison.
- Submit the form and review diagnostic frequency markers.
- Export results through copy, CSV, PDF, or printing.
Example data
| Input | Example | Purpose |
|---|---|---|
| Driver speed | 1,450 RPM | Sets pulley rotational speed. |
| Driver diameter | 180 mm | Calculates belt surface speed. |
| Driven diameter | 360 mm | Calculates the driven shaft ratio. |
| Centre distance | 700 mm | Estimates open belt length. |
| Slip | 2% | Corrects theoretical belt speed. |
| Defects | 1 | Marks repeating impact frequency. |
Diagnostic interpretation
Peaks near BPF can indicate repeating belt conditions. Harmonics may strengthen with impacts, looseness, or modulation. Confirm findings using inspection and operating history.
- Belt damage or localised wear
- Splice or joint problems
- Pulley misalignment or eccentricity
- Belt looseness, slip, or contamination
- Repeating tooth, lug, or cleat defects
- Multiple belts producing nearby frequency families
Frequently asked questions
What is belt pass frequency?
It is the number of complete belt circuits each second.
Is BPF the same as pulley RPM?
No. BPF depends on belt speed and total belt length.
Should pitch diameter be used?
Pitch diameter usually gives better belt-speed estimates than outside diameter.
How does slip change BPF?
Slip reduces actual belt speed and lowers belt pass frequency.
Why calculate harmonics?
Repeating impacts often create peaks at integer BPF multiples.
What does the tolerance band show?
It creates a practical search range around each expected frequency.
Can this calculator handle timing belts?
Yes. Enter total belt teeth for tooth repetition frequency.
Why compare measured frequency?
The comparison quantifies deviation between predicted and observed spectrum peaks.
Does a BPF peak prove belt damage?
No. Confirm vibration evidence through inspection and additional measurements.