BIZOMAT MAKEOS · Power Transmission
V-Belt Drive Motor Load Check (Belts, Driven Speed, Shaft Load on the Motor)
For a V-belt drive off an electric motor, how many belts does it need, does the driven speed come out right, and is the belt pull within the motor’s allowable shaft load?
Check of a V-belt or flat belt drive against its motor: the driven pulley size and speed, belt length, wrap, belt speed, effective pull, centrifugal tension, number of belts and the resultant shaft load from the tight and slack side tensions (as the V-belt drive design tool), against the grooves on the pulleys, the driven speed you need and the radial load the motor bearings are rated for at the pulley position.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Power | kW | Power transmitted. |
| Motor speed | rpm | Motor speed. |
| Motor pulley pitch diameter | mm | Pitch diameter of the motor pulley. |
| Speed ratio (motor ÷ driven speed) | Motor speed divided by driven speed (driven pulley = ratio × motor pulley). | |
| Centre distance | mm | Distance between the shaft centres. |
| Service factor | Service factor for the driven machine and running hours. | |
| Belt | choice | V-belt in a 38° groove, or a flat belt. |
| Friction coefficient | Friction coefficient between belt and pulley. | |
| Belt mass per metre | kg/m | Mass of one belt per metre. |
| Allowable tension per belt | N | Largest working tension one belt may carry. |
| Motor and duty | ||
| Grooves on the pulleys | grooves | Number of grooves on the pulleys you have. |
| Motor allowable radial load at the pulley | N | Radial load the motor maker allows at the pulley’s position on the shaft. |
| Driven speed required | rpm | Speed the driven machine needs. |
| Speed tolerance | % | How far the driven speed may differ from the speed needed. |
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: Power
- What it is
- The power the belt drive transmits from the motor to the driven machine, usually the motor rating or the driven machine’s demand.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 10000 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
- The effective pull, number of belts and shaft load rise with the power.
- Get it right
- Enter the power without the service factor; the tool applies the service factor itself.
What this means: Motor speed
- What it is
- The speed of the motor shaft carrying the small or driving pulley. With the pulley diameter it sets the belt speed.
- 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
- A higher motor speed raises belt speed, lowering the effective pull for the same power.
- Get it right
- Use the full-load speed from the nameplate, not the synchronous speed.
What this means: Motor pulley pitch diameter
- What it is
- The pitch diameter of the pulley on the motor shaft, the diameter at which the belt’s pitch line runs, from the pulley catalogue.
- 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 125 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 motor pulley raises belt speed, which lowers the effective pull for the same power.
- Get it right
- Use the pitch diameter, not the outside diameter of the pulley.
What this means: Speed ratio (motor ÷ driven speed)
- What it is
- The motor speed divided by the driven speed. The driven pulley diameter is taken as this ratio times the motor pulley diameter.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher ratio lowers the driven speed in inverse proportion and enlarges the driven pulley.
- Get it right
- Enter motor ÷ driven; a reducing drive has a ratio above 1.
What this means: Centre distance
- What it is
- The distance between the centres of the motor shaft and the driven shaft. It sets the belt length and the wrap angle on the small pulley.
- 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 500 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer centre distance increases the wrap on the small pulley, which reduces belt tension.
- Get it right
- Use the centre distance at running tension, within the adjustment range of the motor base.
What this means: Service factor
- What it is
- A multiplier on the transmitted power for the driven machine’s shock, starting load and daily running hours, from the belt maker’s tables.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 3. 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
- The effective pull, belts needed and shaft load rise in direct proportion to the factor.
- Get it right
- Take the factor from the belt maker for your driven machine and hours of running.
What this means: Belt
- What it is
- Whether the drive uses V-belts running in a groove or a flat belt on a flat pulley. A V-groove wedges the belt and raises the effective friction.
- Units and format
- Choose one of 2 options from the list: V-belt (38° groove); Flat belt.
- Example
- The form opens with “V-belt (38° groove)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Choose flat belt for a flat belt; the V-belt option applies the wedge effect of the groove angle to the friction coefficient.
What this means: Friction coefficient
- What it is
- The plain friction coefficient between the belt material and the pulley face, from the belt maker’s data. For a V-belt the tool raises it for the wedging in the groove.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher friction lowers the tension ratio needed, which lowers belt tension and shaft load.
- Get it right
- Enter the plain coefficient, not an effective V-groove value, or the wedge effect is counted twice.
What this means: Belt mass per metre
- What it is
- The mass of one metre of a single belt, from the belt maker’s data. It sets the centrifugal tension at belt speed.
- Units and format
- Enter a number in kg/m: kilograms per metre of length. The form accepts 0 kg/m to 10 kg/m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.1 kg/m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Centrifugal tension rises in direct proportion to belt mass, leaving less tension for power.
- Get it right
- Use the mass for one belt, not for the whole set.
What this means: Allowable tension per belt
- What it is
- The largest working tension one belt may carry on its tight side, from the belt maker’s data. The number of belts is chosen so no belt exceeds it.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N to 1000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher allowable tension reduces the number of belts needed.
- Get it right
- This is per belt, not for the whole set.
Motor and duty
What this means: Grooves on the pulleys
- What it is
- The number of belt grooves on the pulleys you have or propose. It is compared with the number of belts the drive needs.
- Units and format
- Enter a number in grooves: grooves. The form accepts 1 grooves to 20 grooves. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 grooves as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Count the grooves on the narrower of the two pulleys if they differ.
What this means: Motor allowable radial load at the pulley
- What it is
- The radial load the motor maker allows on the shaft at the position where the pulley sits, from the motor data sheet.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N to 10000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2500 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the allowable load at the pulley’s actual distance from the shaft shoulder; it falls as the pulley moves outward.
What this means: Driven speed required
- What it is
- The speed the driven machine needs at its shaft. The driven speed from the ratio is compared with it.
- 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 580 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the driven machine’s required shaft speed, not the motor speed.
What this means: Speed tolerance
- What it is
- How far, above or below, the driven speed may differ from the speed needed and still pass, as a share of the speed needed.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 50%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Type the percentage itself; the band applies equally above and below.
Method and formulas
- Wrap θ = π − 2 asin ((D − d) ÷ 2C); belt speed; effective pull = P × SF ÷ v; centrifugal tension m v² (as the V-belt drive design tool).
- Belts = effective pull × e^{μθ} ÷ (e^{μθ} − 1) ÷ (allowable − centrifugal) rounded up; shaft load from T₁ and T₂ and the belt angle.
- Driven speed = motor speed ÷ ratio; compared with the speed needed.
- Example: 3 belts; the motor shaft carries 1,104.2 N and the driven pulley turns 580 rpm.
Limitations
- Steady load at the pulley position given by the motor maker; a pulley mounted further out lowers the allowable load.
- Belt capacity by allowable tension; the belt maker’s power ratings govern for a final selection.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a fan or pump belt drive on a standard motor
- Change pulleys without overloading motor bearings
- Find how many belts a drive needs
Who it is for
- Maintenance engineers
- Mechanical fitters
- HVAC engineers
What you get
- Driven pulley, speed and belt length
- Belts needed and shaft load
- Against the grooves, speed and motor rating
Interpreting the result
- A larger motor pulley lowers the belt pull and the shaft load for the same power; a small pulley is the usual cause of bearing failures.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2487–2536. Topic reference; no source tables or recommended settings are reproduced.
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Open the V-Belt Drive Motor Load Check (Belts, Driven Speed, Shaft Load on the Motor) calculator