What belt speed is recommended for V-belts and flat belts?

For classical V-belts, belt speeds between 5 and 25 m/s are common, with 10–20 m/s often used in practice. Flat belts can run faster, typically 20–30 m/s. Very high speeds increase centrifugal effects and reduce power capacity.

How is belt length calculated for an open belt drive?

For an open belt drive, the approximate belt length is L = 2C + (π/2)(D1 + D2) + (D2 − D1)² / (4C), where C is the center distance and D1, D2 are pulley diameters in the same units.

What safety factor should I use in belt drive design?

For general industrial drives, a safety factor between 1.3 and 2.0 on power or allowable tension is typical, depending on load type (steady, moderate shock, heavy shock), duty cycle, and environment. Always check the belt manufacturer’s recommendations.

Belt Drive Design Calculator

Size pulleys, belt length and check power capacity for V-belt and flat-belt drives. Enter power and speeds, then iterate diameters and center distance until the design is feasible.

Mechanical Design Check Iterative

Belt Drive Design Inputs

Design mode

Required power P

Mechanical power to be transmitted.

Driver speed N₁ (rpm)

Driven speed N₂ (rpm)

For “check existing”, this is computed from diameters but you can still override.

Belt type

Coefficient of friction μ

Typical: 0.25–0.35 for dry V-belts, 0.2–0.3 for flat belts.

Allowable belt stress σ_allow (MPa)

Effective allowable working stress in belt material.

Belt width b (mm)

Belt thickness t (mm)

Pulley & Geometry

Small pulley diameter D₁ (mm)

Usually the driver. Adjust to keep belt speed reasonable.

Large pulley diameter D₂ (mm)

If left blank in “size from speeds”, it will be computed.

Center distance C (mm)

Try 1–3 times the larger pulley diameter as a starting point.

Minimum wrap on small pulley (deg)

Design target; actual wrap is computed.

Results

Kinematics

Speed ratio i = N₁/N₂: –
Computed N₂ (rpm): –
Belt speed v (m/s): –
Belt length L (mm): –
Wrap angle small pulley θ₁ (deg): –
Wrap angle large pulley θ₂ (deg): –

Forces & Power

Tight side tension T₁ (N): –
Slack side tension T₂ (N): –
Net effective tension (T₁ − T₂) (N): –
Estimated transmitted power (kW): –
Safety factor on power: –
Max working stress σ (MPa): –

How this belt drive design calculator works

This tool follows classical belt drive design principles used in mechanical power transmission handbooks and manufacturer manuals (e.g., Gates, SKF). It lets you:

Size pulley diameters from required driver and driven speeds.
Estimate belt length and wrap angles for an open belt drive.
Check belt tensions, transmitted power and safety factor based on belt width, thickness and allowable stress.

1. Speed ratio and pulley diameters

The basic speed ratio (ignoring slip) is:

\( i = \dfrac{N_1}{N_2} = \dfrac{D_2}{D_1} \)

where:

\(N_1\) = driver speed (rpm)
\(N_2\) = driven speed (rpm)
\(D_1\) = small pulley diameter (mm)
\(D_2\) = large pulley diameter (mm)

In “size from speeds” mode, the calculator uses your chosen small pulley diameter \(D_1\) and the required speed ratio to compute:

\( D_2 = i \, D_1 = \dfrac{N_1}{N_2} D_1 \)

2. Belt speed

Belt speed is computed from the small pulley:

\( v = \dfrac{\pi D_1 N_1}{60\,000} \quad [\text{m/s}] \)

Recommended ranges:

V-belts: typically 5–25 m/s (10–20 m/s is common).
Flat belts: up to 20–30 m/s, depending on construction.

3. Belt length and wrap angle (open belt)

For an open belt drive, an approximate belt length is:

\( L \approx 2C + \dfrac{\pi}{2}(D_1 + D_2) + \dfrac{(D_2 - D_1)^2}{4C} \)

where \(C\) is the center distance (same units as diameters).

The wrap angle on the smaller pulley is approximated by:

\( \theta_1 \approx \pi - 2\alpha \quad,\quad \alpha = \arcsin\left(\dfrac{D_2 - D_1}{2C}\right) \)

The calculator reports \(\theta_1\) and \(\theta_2\) in degrees and warns if the wrap on the small pulley is below your target (e.g., 150°).

4. Belt tensions and power capacity

The maximum allowable belt tension is estimated from belt cross-section and allowable stress:

\( A = b \, t \quad,\quad T_{\text{max}} = \sigma_{\text{allow}} \, A \)

where:

\(b\) = belt width (mm)
\(t\) = belt thickness (mm)
\(\sigma_{\text{allow}}\) = allowable working stress (MPa = N/mm²)

For a given friction coefficient μ and wrap angle θ (radians) on the small pulley, the tension ratio is:

\( \dfrac{T_1}{T_2} = e^{\mu \theta} \)

Assuming the tight side tension reaches \(T_{\text{max}}\), the slack side tension is:

\( T_1 = T_{\text{max}} \quad,\quad T_2 = \dfrac{T_1}{e^{\mu \theta}} \)

The effective driving force is:

\( \Delta T = T_1 - T_2 \)

and the corresponding power capacity:

\( P_{\text{cap}} = \dfrac{\Delta T \, v}{1000} \quad [\text{kW}] \)

The safety factor on power is:

\( SF_P = \dfrac{P_{\text{cap}}}{P_{\text{required}}} \)

5. Interpreting the results

SF on power ≥ 1.3–1.5: usually acceptable for steady loads.
SF on power < 1.0: belt is undersized – increase width, use multiple belts, or adjust diameters.
Wrap angle too low: increase center distance, use an idler, or change pulley diameters.
Belt speed too high: increase pulley diameters or reduce speed; check manufacturer limits.

Practical belt drive design tips

Choosing belt type

V-belts: compact, good for moderate power and shorter center distances, less sensitive to misalignment.
Flat belts: efficient at high speeds and long centers, but require good alignment and tensioning.

Recommended workflow

Enter required power and driver/driven speeds.
Pick a belt type and initial small pulley diameter.
Click “Auto-size large pulley & speeds” and check belt speed.
Adjust center distance until wrap angle and belt length are reasonable.
Click “Compute belt forces & checks” and review safety factor and stress.
Iterate belt width, thickness or use multiple belts until the design is safe.

This calculator is intended for preliminary design and educational use. For critical applications, always verify with belt manufacturer data and relevant standards.

Belt drive design – FAQ

How do I choose pulley diameters for a belt drive?

Start from the required speed ratio \(i = N_1/N_2\). Choose a practical small pulley diameter based on belt type and shaft size (e.g., 100–250 mm for many industrial V-belt drives). Then compute the large pulley as \(D_2 = i D_1\). Check belt speed and adjust diameters until belt speed and center distance are within recommended ranges.

What belt speed is recommended?

For classical V-belts, 5–25 m/s is typical, with 10–20 m/s common in practice. Flat belts can run faster, often 20–30 m/s. Very low speeds require larger belts to transmit the same power; very high speeds increase centrifugal effects and can reduce belt life. Always check manufacturer recommendations for your specific belt profile.

How much wrap angle do I need on the small pulley?

A wrap angle of at least 120° is often cited as a minimum; 150° or more is preferred for higher power or lower friction. Less wrap reduces the maximum transmissible torque because the tension ratio \(T_1/T_2 = e^{\mu \theta}\) decreases with θ. If wrap is too low, increase center distance, add an idler, or change pulley diameters.

How do I account for shock loads and service factors?

Manufacturers provide service factors based on load type (uniform, moderate shock, heavy shock), daily operating hours and environment. Multiply the nominal power by the service factor to get an equivalent design power, then size the belt drive for that higher value. Alternatively, aim for a higher safety factor on power (e.g., 1.7–2.0) for severe duty.