Power transmitted by a belt drive

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) · v
• F₁ = P/v + F₂
• F₂ = F₁ − P/v
• v = P / (F₁ − F₂)

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. 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.