May 082013

Hello there! Have you ever wondered how to solve design problems based on flat belts? This tutorial is for you. It explains flat belt design in a step-by-step manner. Read every step carefully for good understanding. For reading convenience, this tutorial has been split into four pages :)

Flat Belt Drive

Solving Design problems based on Flat Belts:

There are two ways by which you can design a flat belt drive. They are:

  1. Designing flat belt drive using manufacturer’s data
  2. Designing flat belt drive using basic equations

Among the above two methods, the former is simple, easy and practically applicable. It involves the usage of data provided by flat belt manufacturers to solve design problems. It is covered in detail in this article.

In order to design a flat belt drive using manufacturer’s data, you need to find ten different parameters. They are:

  1. Pulley Diameters (D and d)
  2. Speeds of Driving and Driven Pulleys (N1 and N2)
  3. Design Power in KW
  4. Velocity of belt (V m/s)
  5. Selection of belt
  6. Number of plies
  7. Load rating at V m/s
  8. Belt width (b)
  9. Pulley width
  10. Length of belt (L)

You can click on any one of the links above to know how to find a particular parameter.

Steps involved in Designing Flat Belt Drives:

Step 1 – Finding Pulley Diameters (D and d):

This step involves finding the diameters of the driving and driven pulleys of the belt drive. In some problems, diameters of the driving and driven pulleys may be given. If any one of them is not given, use the following equation to find it:



N1 → Speed of the driving pulley in rpm

N2 → Speed of the driven pulley in rpm

D → Diameter of the larger (driven) pulley in m

d → Diameter of the smaller (driving) pulley in m

Note: Here, we have assumed that the driven pulley is larger in diameter than the driving pulley.

After finding the pulley diameters, you must change it to the nearest standard value (greater than the one determined) and fix it as the permanent value.

A list of standard pulley diameters (in mm) is given below:

40, 45, 50, 56, 63, 71, 80, 90, 100, 112, 125, 140, 160, 180, 200, 224, 250, 280, 315, 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120, 1250, 1400,1600, 1800, 2000

Note: If you are using PSG design data book, refer page no. 7.54 (May 2012 reprint) for finding the standard pulley diameter.

Step 2 – Finding the Speeds of Driving and Driven Pulleys (N1 and N2):

In many problems, the speeds of the driving and the driven pulleys will be given. If any one speed is not given, either the velocity ratio or the pulley diameters will be given. Use the same equation from the above step to find the unknown pulley speed.


The above formula can be used in any problem where percentage slip is zero (or not given). If percentage slip is given for each pulley, use the following equation to find the unknown pulley speed or diameter.

{N_1/N_2}~=~{D/d}(1~-~{{S_1 ~+~ S_2}/100})~=~Velocity~Ratio


S1 → Percentage slip between the driving pulley and the belt

S2 → Percentage slip between belt and the driven pulley

If thickness of belt (t) is considered, use the following equation:

{N_1/N_2}~=~{D~+~t}/{d~+~t}(1~-~{{S_1 ~ + ~ S_2}/100})~=~Velocity~Ratio

See the next page for the third step.

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I am a mechanical engineer with a passion for technical stuff. I am the founder and former editor-in-chief of

  • Varadha Yamunan

    Good one :) Helped me pretty well.. Thank you

    • Surjeet S

      You are welcome… Get fresh content from by liking us on facebook and subscribing to our e-mail newsletter :)
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  • pradeep patil

    While designing belt drive set the centre height of both pulley shall be same?
    what is effect on power transmission if their is height difference?

    • Surjeet S

      The heights of the pulley centers from ground depend on the height difference between the driving and driven shafts. They may or may not be the same. If there is a height difference, the centre to centre distance (between the pulleys) increases. This will have a direct impact on the arc of contact and the design power we calculate.

      • Sarungbam Romen Singh

        Hello Surjeet,
        In the Book “Design of Machine elements” by VB Bhandhari, the arc of contact factor, Fd is multiplied with the rated power while as per the formula you’ve referred here, the said factor divides the rated power.
        [Corrected Power= {(load correction factor x rated power) x arc of contact factor}]
        Please clear me this confusion I m having with.
        I am just an Electrical Engg. Student working on the flat belt drives.
        Thanking you in anticipation.

  • Ron Verduguez

    can you give the references of the equations that you used on this design? thank you…

    • Surjeet S

      These equations can be found in any standard design databook. In India, we normally follow PSG Design Databook

      • Ron Verduguez

        thank you…

  • Dineshraj Srinivasan

    can you give design calculation for the v-belt drives ?

  • Hem Shailabh

    Hi! Can you tell me why the product of coefficient of friction and angle of wrap is minimized when designing open belt drives for certain maximum stress? Please reply asap. @Surjeet_S:disqus I am yet to start studying DME, i asked this from my knowledge of Kinematics-OM. Thanks

  • Eng Medson Naftal

    Thanks i understand well