Know how to design flat belt drives pulleys
17 Jun, 2021

Flat rubber belts came into existence at the beginning of the century as a replacement for leather belts. With the introduction of V-belts, fewer machines had the convenience of using flat belts, and their production became a matter of supplying the replacement parts essentially. However, recent developments brought in the flat belt drive pulleys helped overcome the previous drawbacks of high tension and tracking. The new designs and advancements in the materials helped transmit both low and high power and cost-efficient at speeds that usually exceed other designs.

Steps to Follow While Designing Flat Belt Drive Pulleys

Finding Pulleys’ Diameters

The first step involves analyzing the diameters of the driving and driven pulleys of the flat belt drive pulleys. If you do not have the diameters of the driving and driven pulleys, you can find it out with the help of the following equation:

{N_1/N_2}=(D/d)= Velocity ratio where, 

N1 is the speed of the driving pulley in rpm 

N2 is the speed of the driven pulley in rpm

D is the diameter of the larger driven pulley in m

d is the diameter of the smaller driving pulley in m

Here, we assumed that the driven pulley is more prominent in diameter than the driving pulley 

After finding the pulleys’ diameters, do not forget to change them to the nearest standard value and fix the same as the permanent value.

Additional read: Flat Belt Pulleys Guide | A brief yet effective flat belt pulleys guide

Finding the Speeds of the Driving and Driven Pulleys

If you do not know the speed of any pulley, you might have the velocity ratio or the pulley diameters. You can use the same formula mentioned above for determining the rate. However, you can use the procedure only when the percentage slip is zero. If you have a percentage slip for each pulley, use the following formula to determine the pulley’s unknown speed and diameter. 

{N1/ N2}= (D/d) (1-{S1+ S2}/100) = Velocity ratio where, 

S1 is the percentage slip between the driving pulley and the belt 

S2 is the percentage slip between the driven pulley and the belt 

If you have the thickness of the belt (t), you need to use the following equation: 

{N1/ N2}= {D+t}/{d+t}(1-{S1+ S2}/100)= Velocity ratio

Finding Design Power in kW

You can find the design power in KW using the following equation: 

Design power= {{Rated KW*Load correction factor}/ {Arc of contact factor* smaller pulley diameter factor}}

However, you might need to correct factors.

Determining the Correction Factors

  1. Load Correction Factor

The following table will help in determining the load correction factor. 

Type of LoadApplicationsLoad Correction Factor
Normal LoadWhen you know the maximum load1.0
Steady LoadUsed in light duty fans, centrifugal pumps, printing machinery, textile machinery, screens, alligators, evaporators and light machinery1.2
Intermittent LoadsUsed in heavy machine tools, heavy duty fans and blowers, air compressor, reciprocating pumps, elevators, mill machinery, line shafts, paper mill and saw mill machinery1.3
Shock LoadsUsed in hammers, grinders, crushing machines, disintegrators, vacuum pumps, rolling mills, tube mills, ball mills, automated machinery and stamp presses1.5
  • Arc of Contact Factor

For determining the arc of contact factor, first, you need to find the arc of contact using the following equation:

Arc of contact= 180⁰- {{D-d}/C}*60⁰ where,

D is the diameter of the larger pulley

d determines the diameter of the smaller pulley

C is the distance between the two pulleys from the center

After determining the arc of contact, please change it to the nearest standard value and its corresponding correction factor.

  • Smaller Pulley Diameter Factor

The table below represents the smaller pulley diameter factor

Smaller Pulley Diameter Factor (in mm)Smaller Pulley Diameter Factor
Up to 1000.5
Above 7501.0

Finding Belt Velocity

You can find the maximum velocity of the belt by using the following equation:

V={{pi dN1}/6} m/s where,

d is the diameter of the smaller pulley in m

N1 is the speed of the smaller pulley in rpm

Finding the Belt’s Width

You can determine the width of the belt using the formula:

b= {{Design power}/ {number of piles* load rating}} mm

Do not forget to change the width to the nearest value and fix the permanent value.


Shree Shakti Pulleys emerged as one of the prominent manufacturers of different types of pulleys. The engineering excellence of the professionals offers a wide range of industrial pulleys.

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