V-Belt Mechanisms – Design and Calculations

As you know the v-belt mechanism is used for power transmissions between different shafts or between power generators and shafts. Also, with a proper design and equipment selection for the v-belt mechanism, we can make a power transmission with a torque converter. Here are the steps to design a v-belt mechanism for types of machinery.

Steps to Design V-Belt Mechanism

V-belt mechanisms in engines.
V-belts are common construction elements on internal combustion engines.

Step 1: Define the rated power for driving the electric motor or shaft;

In the v-belt mechanism, there is always a prime mover that gives the motion to the system. This moving energy would come from an electric motor or a moving shaft. So, you need to define or know the rated power of this moving energy.

Step 2: Determine the ‘design power’ from rated power information;

And then, if you have the power information of the driving motor or shaft, you need to multiply it with a unitless number called ‘service factor’ to obtain ‘design power’. We determine the service factor according to some parameters;

  • Driven machinery type with designed mechanism,
  • Driving motor or mechanism types such AC or DC electric motors,
  • We will use hours that the designed mechanism.

Also, here you can see an example of a V-belt mechanism factor determination table according to the parameters stated above;

Service factor selection for v-belt mechanisms.
Table of service factor selection for V-belts(SRC: Machine Elements In Mechanical Design).

Furthermore, with this service factor, you need to multiply the rated power of the driving mechanism to obtain ‘design power’.

Step 3: Standard V-belt mechanism selection;

And also, you need to select a standard v-belt mechanism from catalogs provided by v-belt mechanism producers. We prepare these catalogs generally according to changing design power and rpm of the input(smaller) sheave.

We introduce an example catalog in which you can select your v-belt mechanism. There are dozens of catalogs are available from v-belt producers.

Example of standard V-Belt selection
Example of standard V-Belt selection table(SRC: Machine Elements In Mechanical Design).

And then select the standard V-belt mechanism according to the design power and speed of the faster shaft.

Step 4: Determine the diameters of sheaves according to input and output speeds;

According to these input and output sheave speeds in terms of rpm, you need to determine the sheave diameter. So, we must know the belt speed. If your belt speed is known;

V-belt mechanism speed formula

You can determine the input sheave diameter from the formula above. Input Speed per Output Speed gives the belt ratio. By using this ratio, you can find D2 which is the diameter of the second output sheave.

Step 5: Select standard sheave sizes from catalogs;

You can find the standard driving sheave size from sheave catalogs, which is close to the found diameter. Deviations around 1-2% are no problem.

Make the same thing for driven sheave, and compare the output powers. Deviations of 1-2% are not important.

Step 6: Determine the power rate of the selected v-belt mechanism;

The power rate of a belt means the power that a belt can carry. This is obtained by using special graphs that you can find the power rate with the information of small sheave pitch diameter and rpm of the small sheave.

We gave an example of power rates for 3V belts below;

Rated power example table.
Rated power table example((SRC: Machine Elements In Mechanical Design).

As you can see above, you need to specify your pitch diameter at the x-axis of the graph and find the related rpm interval. You can find the power rate on the y-axis.

Step 7: Specify a proper center distance(C) between sheaves;

In general, the proper range for C is given as;

Power range.

According to the sheave diameters that you specified before, specify a proper C inside this range.

Step 8: Compute the required belt length and determine the standard belt length;

In the belt catalogs that you selected your v-belt mechanism from them, you can find the standard v-belt mechanism lengths.

You can compute your required belt length from the calculator available in the given link easily.

Belt length examples.
Example of belt length standards((SRC: Machine Elements In Mechanical Design).

For example, you found the length requirement of the 5V v-belt mechanism must be 101,5 in. The nearest value for this is 100 as you see above table.

Step 9: Re-calculate the center distance of sheaves with this standard v-belt mechanism length;

With the information on belt length(L) that you found above, you can re-calculate the center distance which must be inside the specified range.

Click on the given link to open the center distance(C) calculator available in Mechanicalland.

Step 10: Calculate wrapping angles of the v-belt mechanism on big and small sheaves;

The wrapping angle information is very important to find the required correction factors to calculate the number of belts required in the v-belt system. You can calculate the wrap angles of sheaves by using the calculator available in Mechanicalland.

Step 11: Determine the correction factors;

We can obtain different kinds of correction factors to multiply them with the rated power of the selected standard v-belt mechanism.

Wrap angle correction factor for v-belt mechanisms.
Wrap angle correction graph example(SRC: Machine Elements In Mechanical Design).

For example, you can specify a correction factor with the information on the angle of the wrap above.

Belt length correction factor.
Belt length correction factor graph example(SRC: Machine Elements In Mechanical Design).

Also, you can determine the belt length correction factor-like the above example. Use the standard belt length(L) that you specified.

Step 12: Find the corrected power rate and number of v-belt mechanisms required;

We found the power rate. of the v-belt in step 6. Multiply the correction factors that you found in step 11 with the power rate to find the corrected power rate of the selected standard v-belt.

Divide the design power with the corrected power rate of the v-belt mechanism. For example, if you found 2.71, you must use 3 of the selected standard v-belt in the system.

Important Points to Remember in the Design of V-Belt Mechanism System

  • In the installation phase of the v-belt system, we must provide an initial tension that we recommend by the v-belt mechanism manufacturer.
  • The temperature of the environment that the v-belt mechanism is used must be between the recommended temperatures.
  • Wrap angle of small sheave must be bigger than 120 degrees because the problems may occur(slipping) in-service time below 120 degrees.
  • In the installment of the v-belt mechanism, center distances must be decreased to attach the v-belts easily.
  • If the belt speed is less than 1000ft/min, you need to use other kinds of constructions such as gear systems, chain systems, etc.
  • The alignment of sheaves is an important thing for the proper working of the v-belt mechanism. Sheaves must be in the same direction and parallel.

This is the general information about the v-belt system design according to the environmental parameters.

Span Length Calculations for V-Belt Mechanisms

Calculator Of Span Length Of V-Belts

The span length is defined by the placements of shafts in machinery systems. But in the V-belt mechanism, you need to calculate this value to obtain the required span length of used v-belts in machinery. This is the formula of the span length in V-belts;

Span lenght in V-belt mechanisms.

As you see that, you need to have the information about the diameters of used sheaves in the v-belt design. ‘D2’ is the diameter of the bigger sheave of the belt mechanism and ‘D1’ is the smaller sheave. ‘C’ is another parameter called ‘center distance’ that you can calculate by using a calculator available in Mechanicalland.

The Angle of Contact Calculation of V-Belt Mechanisms

Angle Of Contact Calculator For V-Belt Sheaves

What is the Angle of Contact for V-Belt Sheaves?

In V-Belts, the angle of contact is a very important value. It states that the physical contact with the belt and the sheave is an angular value. For example, 150 degrees of angle of contact means that the belt sheave contact with the v-belt in 150 degrees around its perimeter.

180 degrees of angle of contact means that the drive ratio is 1, and there is no power or torque conversion between shafts. The diameters of the sheaves are equal. The formulae that the angle of contacts are calculated;

Angle of contact.

As you can see above, the first and the second angle of contact for each sheave in V-belts are calculated as θ1 and θ2. The input values of ‘D1’ and ‘D2’ are the diameters of sheaves. ‘D2’ must be the bigger sheave.

The ‘C’ value is calculated via another formula. You can calculate the ‘C’ by using this calculator available in Mechanicalland.

Center Distance and Pitch Length Calculator for V-Belt Mechanisms

Center Distance(C) And Pitch Length(L) Calculator

What are the Center Distance and Pitch Length Values of V-Belt Mechanisms?

V-belts are frequently used machinery parts to transmit power and torque between shafts that are placed in different locations. In the design process of V-Belts, you need to make very detailed calculations to obtain the most suitable belt system for a piece of machinery. Center distance and pitch length values are needed to be calculated for further design calculations of these mechanisms.


In the above illustration of general V-belts, you can see the center distance(C) and pitch length(L) values above. The formulae of these values are;

Center distance formula.

This is the center distance formula that you can calculate by using, D1 and D2 pitch diameters that are illustrated above. Here B is calculated as;


To calculate pitch length, you need to have the center distance(C) and ‘B’ values to calculate the pitch length(L);

Pitch length formula.

As you can see, all the values are calculated via these formulae. You can use our calculator to calculate these values easily.


In your calculations take special care of your units. The use of proper sets of units is very important.

Do not forget to leave your comments and questions about the v-belt mechanism design below. Your precious feedbacks are very important to us.

Take a look at the other engineering calculators available in Mechanicalland!

Source Of Graphs And Tables: Robert L. Mott – Machine Elements For Mechanical Design


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