KEVOS® MAKEOS · Power Transmission
Variable-Speed Belt Drive (Speed Variation, Power Rating, Rim Speed) — RMA IP-25 Sections
For a variable-speed belt drive with one or two variable-pitch sheaves, what speed variation can it give, what power does the belt carry at the smallest working pitch diameter, and is the rim speed of the variable sheave within my limit?
Variable-speed belt drive by the ANSI/RMA IP-25 rules the handbook gives: speed variation PDmax ÷ PDmin for a single variable sheave or (DR × DN) ÷ (dr × dn) for a compound drive with both sheaves variable, the belt power rating HP = dp r [K1 (dp r)^−0.09 − K2 ÷ dp − K3 (dp r)²] + Kr r (1 − 1 ÷ KSR) for the twelve standard cross sections (1422V to 4836V) at the small pitch diameter, corrected by the length and arc of contact factors, the number of belts for the design power, and the rim speed π Do × rpm of the variable sheave.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Cross section | choice | Variable-speed belt cross section: top width in sixteenths of an inch, then the groove angle. |
| Smallest working pitch diameter of the small sheave dp | mm | Smallest pitch diameter the small sheave works at, where the rating is lowest. |
| Speed of the faster shaft | rpm | Speed of the faster shaft. |
| Correction factors from the handbook tables | ||
| Speed ratio factor KSR (Table 34) | Speed ratio factor from the handbook table for the speed ratio of the drive. | |
| Length correction factor (Table 35) | Length correction factor from the handbook table. | |
| Arc of contact correction factor (Table 36) | Arc of contact correction factor from the handbook table. | |
| 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 on the drive; variable-speed drives usually use one. |
| Speed variation | ||
| Drive arrangement | choice | One variable sheave with a fixed companion sheave, or both sheaves variable. |
| Largest pitch diameter of the (driver) variable sheave | mm | Largest working pitch diameter of the variable (driver) sheave. |
| Smallest pitch diameter of the (driver) variable sheave | mm | Smallest working pitch diameter of the variable (driver) sheave. |
| Largest pitch diameter of the driven variable sheave (compound) | mm | Largest working pitch diameter of the driven sheave in a compound drive. |
| Smallest pitch diameter of the driven variable sheave (compound) | mm | Smallest working pitch diameter of the driven sheave in a compound drive. |
| Rim speed | ||
| Outside diameter of the variable sheave | mm | Outside diameter of the variable sheave, for its rim speed. |
| Speed of the variable sheave | rpm | Speed of the variable sheave. |
| Largest rim speed the sheave is made for | m/s | Largest rim speed the sheave material and design allow, from the sheave maker. |
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 variable-speed belt to rate.
- Units and format
- Choose one of 12 options from the list: 1422V; 1922V; 2322V; 1926V; 2926V; 3226V; 2530V; 3230V; 4430V; 4036V; 4436V; 4836V.
- Example
- The form opens with “2926V” 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
- The first two digits give the top width in sixteenths of an inch, the next two the groove angle.
What this means: Smallest working pitch diameter of the small sh…
- What it is
- Smallest working pitch diameter of the small sheave.
- 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 150 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The rating per belt is lowest here.
- Get it right
- Rate the belt where it runs on the smallest diameter.
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 factor KSR (Table 34)
- What it is
- Speed ratio factor for the variable-speed 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 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds the speed ratio term to the rating.
- Get it right
- Read it from the handbook table for the speed ratio of the drive.
What this means: Length correction factor (Table 35)
- 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 36)
- 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 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 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 3 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 20 belts. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 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.
Speed variation
What this means: Drive arrangement
- What it is
- Whether one sheave or both sheaves change pitch diameter.
- Units and format
- Choose one of 2 options from the list: One variable sheave; Both sheaves variable (compound).
- Example
- The form opens with “One variable sheave” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Changes the speed variation formula.
- Get it right
- Compound drives with both sheaves variable reach larger speed ranges.
What this means: Largest pitch diameter of the (driver) variable…
- What it is
- Largest working pitch diameter of the variable (driver) sheave.
- 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 200 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises the speed variation.
- Get it right
- From the sheave maker’s data for the belt section.
What this means: Smallest pitch diameter of the (driver) variabl…
- What it is
- Smallest working pitch diameter of the variable (driver) sheave.
- 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 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Lowers the speed variation when larger.
- Get it right
- From the sheave maker’s data for the belt section.
What this means: Largest pitch diameter of the driven variable s…
- What it is
- Largest working pitch diameter of the driven sheave in a compound drive.
- 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 200 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises the speed variation.
- Get it right
- Used only when both sheaves are variable.
What this means: Smallest pitch diameter of the driven variable…
- What it is
- Smallest working pitch diameter of the driven sheave in a compound drive.
- 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 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Lowers the speed variation when larger.
- Get it right
- Used only when both sheaves are variable.
Rim speed
What this means: Outside diameter of the variable sheave
- What it is
- Outside diameter of the variable sheave, for its rim speed.
- 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 210 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Rim speed rises in proportion.
- Get it right
- Measure across the flanges of the variable sheave.
What this means: Speed of the variable sheave
- What it is
- Rotational speed of the variable 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
- Rim speed rises in proportion.
- Get it right
- Usually the motor speed when the variable sheave is on the motor.
What this means: Largest rim speed the sheave is made for
- What it is
- Largest rim speed the variable sheave is designed for.
- Units and format
- Enter a number in m/s: metres per second. The form accepts 0.000001 m/s to 500 m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 35 m/s 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 the rim speed is higher.
- Get it right
- Take it from the sheave maker; it depends on the material and design.
Method and formulas
- Speed variation = PDmax ÷ PDmin (one variable sheave) or (DR × DN) ÷ (dr × dn) (compound).
- HP = dp r [K1 (dp r)^−0.09 − K2 ÷ dp − K3 (dp r)²] + Kr r (1 − 1 ÷ KSR) with the constants printed for each section (dp in inches, r = rpm ÷ 1000).
- Corrected rating = HP × length factor × arc factor; belts = design power ÷ corrected rating, rounded up.
- Rim speed = π Do × rpm.
- Example: the drive gives a 2 : 1 speed variation and one belt carries 4.7102 kW at its smallest pitch diameter.
Limitations
- The pitch diameter range a section can attain is limited (Table 33 gives 2av for each section).
- Change the speed ratio only while the drive runs.
- For 4430V the source prints 7.854 in the bracket and 9.818 in the speed ratio term; the tool follows the print.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find the speed range a variable-pitch belt drive can give
- Check the belt rating at the smallest working pitch diameter
- Check the rim speed of a variable sheave
Who it is for
- Machine designers
- Maintenance engineers
- Drive and conveyor designers
What you get
- Speed variation of the drive
- Basic rating per belt at the smallest pitch diameter
- Rating after length and arc corrections and the belts needed
- Rim speed of the variable sheave
Interpreting the result
- Rate the belt at its smallest working pitch diameter, where the rating is lowest.
- Single variable sheaves seldom exceed 3 : 1; compound drives reach about 10 : 1.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2518–2525. Topic reference; no source tables or recommended settings are reproduced.
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