Calculation
P = (F1 − F2) · v
Only the difference between tight- and slack-side tension does work; it is multiplied by belt speed.
P = (F1−F2)·v
Belt Drive Power Calculator
Only the difference between tight- and slack-side tension does work; it is multiplied by belt speed.
MINTSI
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Result
Select a target and calculate.
Understand the inputs
- Transmitted power P — The mechanical power actually transmitted by the belt. Computed as a result from the difference of the two strand tensions times belt speed.
- Tight-side tension F1 — The tight-side tension – the force in the taut, pulling belt strand (the side coming from the driving pulley). Source: a belt-tension meter, or from the capstan (Euler–Eytelwein) equation (see the belt-tension-ratio calculator).
- Slack-side tension F2 — The slack-side tension – the force in the looser, less taut belt strand.
- Belt speed v — The belt's linear travel speed at the pulley rim. Computed as a result from π·d·n/60, or measured directly.
Example
F1 = 256.6 N, F2 = 100 N and v = 25 m/s give P ≈ 3.92 kW.
Assumptions and limits
Steady-state operation; belt mass, centrifugal effects and efficiency losses are excluded.
Understand Belt Drive Power Calculator
This calculator finds the missing quantity among transmitted power P, tight-side tension F1, slack-side tension F2 and belt speed v from the other two. It shows why only the difference between the two belt-strand tensions actually does work — not the full tight-side tension.
What does this quantity describe?
The mechanical power transmitted by a belt drive is the product of the tension difference between the tight and slack strands and belt speed: P = (F1 − F2)·v. Slack-side tension F2 merely provides preload and does not itself contribute to useful power.
When starting up a belt-driven conveyor or compressor, tight-side tension F1 rises well above slack-side tension F2 while the belt already runs at nearly constant speed. Manufacturer guidance typically has V-belts running at about 20 to 25 m/s, rarely above 30 m/s — at higher speeds, centrifugal effects and belt wear increase disproportionately.
Formula and variables
P = (F1 − F2) · v
P = (F1 − F2) · vF1 = P/v + F2F2 = F1 − P/vv = P / (F1 − F2)
| Symbol / input | Meaning |
|---|---|
| Transmitted power P | The mechanical power actually transmitted by the belt. Computed as a result from the difference of the two strand tensions times belt speed. |
| Tight-side tension F1 | The tight-side tension – the force in the taut, pulling belt strand (the side coming from the driving pulley). Source: a belt-tension meter, or from the capstan (Euler–Eytelwein) equation (see the belt-tension-ratio calculator). |
| Slack-side tension F2 | The slack-side tension – the force in the looser, less taut belt strand. |
| Belt speed v | The belt's linear travel speed at the pulley rim. Computed as a result from π·d·n/60, or measured directly. |
Choose the inputs correctly
Enter two of the four quantities. F1 and F2 must belong to the same operating point; v is the belt's actual travel speed, not a pulley's rotational speed.
How to use the calculator
Select the target quantity and enter the other known values with units. If needed, first find belt speed using the existing tangential-speed calculator from pulley diameter and rotational speed.
Worked example
Given F1 = 256.6 N, F2 = 100 N and v = 25 m/s, substitution gives P = (256.6 − 100) · 25 ≈ 3,916 W ≈ 3.92 kW.
Understand the result and units
Transmitted power of 3.92 kW shows the mechanical power actually usable at the output for this tension difference and belt speed. Required motor power additionally needs the belt drive's efficiency factored in.
Forces in newtons (N), speed in m/s, power in watts (W) or kilowatts (kW).
Typical applications
The formula is used to size belt drives for compressors, fans, pumps and conveyors, and to back-calculate transmittable power for existing drives.
Assumptions, limits and common mistakes
The formula assumes steady-state operation at constant belt speed. How large a tension difference can be reached without slip is limited by the capstan equation through friction coefficient and wrap angle – see the belt tension ratio calculator. At high speeds, centrifugal force reduces the belt's contact pressure on the pulley and therefore the actually transmittable tension difference; belt mass and efficiency losses are not included here.
Common mistake: Do not substitute the full tight-side tension F1 for the tension difference (F1 − F2) in the power formula — only the difference between the two strand tensions does work. Also, don't confuse belt speed with a pulley's rotational speed; the two are linked through pulley diameter but are different quantities.
Frequently asked questions
Why does only the tension difference count, not the full tight-side tension?
Because slack-side tension F2 is already present as preload and performs no additional work. Only the portion beyond preload (F1 − F2) actually transmits power.
How fast do typical V-belts run?
Usually around 20 to 25 m/s, rarely exceeding 30 m/s in practice, since centrifugal effects and wear increase markedly at higher speeds.
How does this formula relate to the capstan equation?
The maximum tension difference (F1 − F2) a belt can transmit without slipping is bounded by the capstan equation; the power calculated here must not exceed that limit.