KEVOS® MAKEOS · Power Transmission
V-Ribbed Belt Power Rating per Rib (J, L, M) and Number of Ribs
For a V-ribbed (poly-V) belt drive with J, L or M section, what power does one rib carry at my small sheave diameter and speed, and how many ribs does the belt need for the design power?
Power rating of V-ribbed belts by the ANSI/RMA IP-26 formulas the handbook gives, per rib: HP = dp r [K1 ÷ (dp r)^0.09 − K2 ÷ dp − K3 (dp r)²] + K2 r (1 − 1 ÷ KSR) for J, L and M sections, 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 ribs 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 | V-ribbed 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 factor KSR (Table 30) | Speed ratio factor read from the handbook table for the speed ratio shown. | |
| Length correction factor (Table 28) | Length correction factor from the handbook table for the belt length and section. | |
| Arc of contact correction factor (Table 29) | Arc of contact correction factor from the handbook table for (D − d) ÷ C. | |
| Duty | ||
| Power to transmit | kW | Power the drive must transmit. |
| Service factor | Service factor for the driver and driven machine. | |
| Ribs on the belt you will fit | ribs | Number of ribs on the belt you will fit. |
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 V-ribbed belt to rate.
- Units and format
- Choose one of 3 options from the list: J; L; M.
- Example
- The form opens with “L” 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 75 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 150 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 300 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 2900 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 30)
- What it is
- Speed ratio factor for the V-ribbed 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 1.1202 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 per rib.
- Get it right
- Read it from the handbook table for the speed ratio shown.
What this means: Length correction factor (Table 28)
- 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 (Table 29)
- 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 0.92 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 (D − d) ÷ C, which 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 5.5 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 1.3 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: Ribs on the belt you will fit
- What it is
- Number of ribs on the belt you intend to fit.
- Units and format
- Enter a number in ribs: ribs. The form accepts 1 ribs to 100 ribs. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 ribs 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 ribs are fitted than needed.
- Get it right
- Standard rib counts depend on the section.
Method and formulas
- HP per rib = dp r [K1 ÷ (dp r)^0.09 − K2 ÷ dp − K3 (dp r)²] + K2 r (1 − 1 ÷ KSR), dp in inches, r = rpm ÷ 1000.
- K1, K2, K3: J 0.1240, 0.08663, 0.2318 × 10⁻⁴; L 0.5761, 0.8987, 1.018 × 10⁻⁴; M 1.975, 6.597, 3.922 × 10⁻⁴.
- Corrected rating = HP × length factor × arc factor; ribs = design power ÷ corrected rating per rib, rounded up.
- Example: one L rib carries 1.1496 kW, so the belt needs 7 ribs for 7.15 kW.
Limitations
- H and K sections are for specialised uses and are not covered by the formulas.
- V-ribbed belts run at higher tension than V-belts; check the shaft and bearing loads.
- 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 the number of ribs a V-ribbed belt needs
- Check a poly-V drive on small sheaves
- Compare J, L and M sections
Who it is for
- Machine designers
- Maintenance engineers
- Drive and conveyor designers
What you get
- Speed ratio, (D − d) ÷ C, belt speed and arc of contact
- Basic rating per rib
- Rating per rib after length and arc corrections
- Design power and the number of ribs needed
Interpreting the result
- The arc of contact factor is read for (D − d) ÷ C, which the tool shows.
- The design power is the power to transmit times the service factor.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2513–2518. Topic reference; no source tables or recommended settings are reproduced.
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Open the V-Ribbed Belt Power Rating per Rib (J, L, M) and Number of Ribs calculator