KEVOS® MAKEOS · Power Transmission
Classical V-Belt Power Rating (A, AX, B, BX, C, CX, D) and Number of Belts
For a classical V-belt drive (A, AX, B, BX, C, CX or D section), what power does one belt carry at my small sheave diameter and speed, and how many belts does the drive need for the design power?
Power rating of classical V-belts by the ANSI/RMA IP-20 formulas the handbook gives: HP = dp r [K1 − K2 ÷ dp − K3 (dp r)² − K4 log (dp r)] + K2 r (1 − 1 ÷ KSR) per belt for each cross section (A, AX, B, BX, C, CX, D), with dp in inches and r the faster shaft speed in thousands of rpm; corrected by the length and arc of contact factors, with the speed ratio, belt speed, arc of contact and the number of belts for the design power.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Cross section | choice | Classical V-belt cross section; X marks the cogged belts. |
| Pitch diameter of the small sheave dp | mm | Pitch diameter of the smaller sheave. |
| Pitch diameter of the large sheave Dp | mm | Pitch diameter of the larger sheave. |
| Centre distance C | mm | Distance between the shaft centres. |
| Speed of the faster shaft | rpm | Speed of the faster (smaller sheave) shaft. |
| Correction factors from the handbook tables | ||
| Speed ratio factor KSR (Table 10) | Speed ratio factor read from the handbook table for the speed ratio shown (1.0000 for a ratio of 1.00). | |
| Length correction factor | Length correction factor from the handbook table for the belt length and section. | |
| Arc of contact correction factor | Arc of contact correction factor from the handbook table for the arc shown. | |
| Duty | ||
| Power to transmit | kW | Power the drive must transmit. |
| Service factor | Service factor for the driver and driven machine. | |
| Belts you will fit | belts | Number of belts you will fit on the sheaves. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Cross section
- What it is
- Cross section of the classical V-belt to rate.
- Units and format
- Choose one of 7 options from the list: A; AX (cogged); B; BX (cogged); C; CX (cogged); D.
- Example
- The form opens with “B” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Sets the constants of the rating formula.
- Get it right
- X marks the cogged belts, which suit smaller sheaves.
What this means: Pitch diameter of the small sheave dp
- What it is
- Pitch diameter of the smaller sheave, where the belt bends most.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 140 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger small sheave raises the rating per belt.
- Get it right
- Use at least the minimum diameter recommended for the section and speed.
What this means: Pitch diameter of the large sheave Dp
- What it is
- Pitch diameter of the larger sheave of the drive.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 280 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the speed ratio and the arc of contact.
- Get it right
- Dp ÷ dp is the speed ratio used to read the speed ratio factor.
What this means: Centre distance C
- What it is
- Distance between the centres of the two shafts.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A shorter centre distance reduces the arc of contact.
- Get it right
- Measure between shaft centres with the belt at working tension.
What this means: Speed of the faster shaft
- What it is
- Speed of the faster shaft, the one with the smaller sheave.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1450 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The rating rises with speed until the belt speed limits it.
- Get it right
- Use the motor speed for a speed-reducing drive.
Correction factors from the handbook tables
What this means: Speed ratio factor KSR (Table 10)
- What it is
- Speed ratio factor for the classical V-belt rating.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 2. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.1106 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds K2 r (1 − 1 ÷ KSR) to the rating.
- Get it right
- Read it from the handbook table for the speed ratio shown; 1.0000 for a ratio of 1.00.
What this means: Length correction factor
- What it is
- Correction for belts longer or shorter than the average length.
- Units and format
- Enter a plain number with no unit. The form accepts 0.000001 to 2. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Multiplies the rating per belt.
- Get it right
- Read it from the handbook table for the belt length and section.
What this means: Arc of contact correction factor
- What it is
- Correction for an arc of contact less than 180° on the small sheave.
- Units and format
- Enter a plain number with no unit. The form accepts 0.000001 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.97 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Multiplies the rating per belt.
- Get it right
- Read it from the handbook table for the arc the tool shows.
Duty
What this means: Power to transmit
- What it is
- Power the drive must transmit to the driven machine.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7.5 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Times the service factor, sets the design power.
- Get it right
- Use the motor rating or the measured power of the driven machine.
What this means: Service factor
- What it is
- Factor for the type of driver, driven machine and hours of running.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises the design power the belts must carry.
- Get it right
- Take it from the handbook service factor tables for the belt type.
What this means: Belts you will fit
- What it is
- Number of belts you intend to fit on the sheaves.
- Units and format
- Enter a number in belts: belts. The form accepts 1 belts to 100 belts. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 belts as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Fails the drive when fewer belts are fitted than needed.
- Get it right
- Fit at least the number the tool shows as needed.
Method and formulas
- HP = dp r [K1 − K2 ÷ dp − K3 (dp r)² − K4 log₁₀(dp r)] + K2 r (1 − 1 ÷ KSR), dp in inches, r = rpm ÷ 1000.
- K1, K2, K3, K4: A 1.004, 1.652, 1.547 × 10⁻⁴, 0.2126; AX 1.462, 2.239, 2.198 × 10⁻⁴, 0.4238; B 1.769, 4.372, 3.081 × 10⁻⁴, 0.3658; BX 2.051, 3.532, 3.097 × 10⁻⁴, 0.5735; C 3.325, 12.07, 5.828 × 10⁻⁴, 0.6886; CX 3.272, 6.655, 5.298 × 10⁻⁴, 0.8637; D 7.160, 43.21, 1.384 × 10⁻³, 1.454.
- Corrected rating = HP × length factor × arc factor; belts = design power ÷ corrected rating, rounded up.
- Example: one B belt carries 4.0751 kW, so the drive needs 3 belts for 9 kW.
Limitations
- The constants are for ANSI/RMA classical V-belts; belts from a particular maker may be rated differently.
- Use at least the minimum sheave diameter recommended for the cross section and speed.
- A zero or negative rating means the sheave is too small or too fast for the section.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find how many classical V-belts a drive needs
- Compare wrapped and cogged belts of the same section
- Check an existing classical belt drive
Who it is for
- Machine designers
- Maintenance engineers
- Drive and conveyor designers
What you get
- Speed ratio, belt speed and arc of contact
- Basic rating per belt corrected for speed ratio
- Rating per belt after length and arc corrections
- Design power and the number of belts needed
Interpreting the result
- Cogged (X) belts bend more easily and carry more on small sheaves.
- The design power is the power to transmit times the service factor.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2499–2501. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Useful alongside · Library collection
Belt or chain drive: selecting, installing and maintaining flexible power transmission
In Machine Elements and Mechanical Design
What it covers: Belt and chain drives are simple until they slip, stretch or wreck bearings. How to choose between them, size a drive properly, set tension and alignment, and maintain it for long life.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Classical V-Belt Power Rating (A, AX, B, BX, C, CX, D) and Number of Belts calculator