KEVOS® MAKEOS · Power Transmission
Narrow V-Belt Power Rating (3VX, 5VX, 5V, 8V) and Number of Belts
For a narrow V-belt drive (3VX, 5VX, 5V or 8V), what power does one belt carry at my small sheave diameter and speed, what is the arc of contact, and how many belts does the drive need for the design power?
Power rating of narrow V-belts by the ANSI/RMA IP-22 formula the handbook gives: HP = dp r [K1 − K2 ÷ dp − K3 (dp r)² − K4 log (dp r)] + KSR r per belt, with dp the small sheave pitch diameter in inches, r the faster shaft speed in thousands of rpm, the cross-section constants K1 to K4 and the speed ratio correction KSR; the rating times the length and arc of contact correction factors, the arc of contact 2 cos⁻¹ ((D − d) ÷ 2C), the belt speed, and the number of belts for the design power (power × service factor).
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Cross section | choice | Narrow V-belt cross section. |
| 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 correction KSR (Table 4) | Speed ratio correction read from the handbook table for the cross section and the speed ratio shown (0 for a ratio of 1.00). | |
| Length correction factor | Length correction factor from the handbook table for the belt length. | |
| 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 narrow V-belt to rate.
- Units and format
- Choose one of 4 options from the list: 3VX; 5VX; 5V; 8V.
- Example
- The form opens with “5V” 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
- Use the handbook selection chart for the design power and speed of the faster shaft.
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 180 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 355 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 800 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 1750 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 correction KSR (Table 4)
- What it is
- Speed ratio correction added to the narrow V-belt rating.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.083 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds KSR × r to the rating per belt.
- Get it right
- Read it from the handbook table for the section and the speed ratio shown; 0 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 15 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 2 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
- dp and D in inches; r = rpm ÷ 1000; HP = dp r [K1 − K2 ÷ dp − K3 (dp r)² − K4 log₁₀(dp r)] + KSR r.
- K1, K2, K3, K4: 3VX 1.1691, 1.5295, 1.5229 × 10⁻⁴, 0.15960; 5VX 3.3038, 7.7810, 3.6432 × 10⁻⁴, 0.43343; 5V 3.3140, 10.123, 5.8758 × 10⁻⁴, 0.46527; 8V 8.6628, 49.323, 1.5804 × 10⁻³, 1.1669.
- Corrected rating = HP × length factor × arc factor (1 hp = 0.7457 kW).
- Arc of contact = 2 cos⁻¹ ((D − d) ÷ 2C); belts = design power ÷ corrected rating, rounded up.
- Example: one 5V belt carries 12.91 kW, so the drive needs 2 belts for 18 kW.
Limitations
- The constants are for ANSI/RMA narrow 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.
- The rating formula falls off at high belt speeds; 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 narrow V-belts a drive needs
- Check an existing drive against the belt ratings
- Compare cross sections and sheave sizes for a 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
- The design power is the power to transmit times the service factor.
- Larger small sheaves raise the rating per belt and reduce the number of belts.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2492–2494. 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 Narrow V-Belt Power Rating (3VX, 5VX, 5V, 8V) and Number of Belts calculator