Calculating Output Speed Using Pulley Diameters and Input Speed

14 Comments

1. Lakshan Weerasiri on October 17, 2017 at 11:34 am

   I would like to know how that above mentioning formula is derived

   
   
   
2. Michael Keohane on October 25, 2017 at 5:34 pm

   Laksan,

   These formulas are in a variety of engineering texts/reference guides. My typical source is the “Marks Standard Handbook for Mechanical Engineers.”

   
   
   
3. Andrew Smith on March 19, 2018 at 6:27 pm

   The input speed of a transmission with a ratio fo 3.5:1 is 3600 RPM. What is the expected output speed? I couldn’t figure it out.

   
   
   
4. Michael Keohane on March 28, 2018 at 8:22 pm

   Andrew, A 3.5:1 gear ration means that the smaller gear needs to rotate 3.5 times to make the larger gear rotate once.

   So for 3600 RPM input it would be 3600/3.5 = 1028.57

   Regards,

   Michael

   
   
   
5. morty on June 21, 2018 at 10:21 pm

   I have a question. I have a air compressorand the pump is required to run 1050rpm. My motor runs 1375rpm and I have 6in pulley on motor and pump came with a 14in. I’m thinking I’m way under 1050rpm. More like 600rpm. Am I on the right path? I need to get at least 1000rpm thx

   
   
   
6. Jesse on September 9, 2018 at 7:36 pm

   Good lord just do this (6.5) / (11.5) = _____- x rpm of motor.. How simple…..

   
   
   
7. Karim on September 18, 2018 at 11:48 am

   If don’t know both input or output RPM and diameter of both pulley are known
   Then how to find speed

   
   
   
8. Michael Keohane on September 19, 2018 at 6:53 pm

   Karim,

   Hopefully you can see one or both of the shafts. If so you can use a strobe or a tach to get the speed of at least one of the shafts and then do the math as appropriate. If you can get to both shafts then use the strobe/tach and get the actual speeds on each and then compute the ratio.

   
   
   
9. Michael Keohane on September 19, 2018 at 6:57 pm

   Jesse,

   I understand and thanks for the comment!The reason I wrote it that way is because we get questions with any combination of unknowns. Wasn’t trying to confuse folks but maybe I did!

   Thanks for reading and we would love it if you would like to submit a post.

   
   
   
10. Lou Bona on November 26, 2018 at 11:09 am

    I have a 9″ pulley on furnace blower and 3 1/2″ on motor. 9″ pulleys are no longer available and I can only get an 8″ pulley. What combination of pulleys can I use to have same fan speed?

    
    
    
11. Michael Keohane on November 27, 2018 at 11:47 am

    Lou,
    Your current ratio is 3.5/9 = .388 so any combination of pulleys that provides that ratio will maintain the speed. If you use an 8″ pulley on the fan than a 3″ pulley on the motor will be close. 3/8 = .375 ratio.

    If the motor is 1800 rpm your current fan speed is 1800(.388) = 698 rpm. A 3″ to 8″ pulley would give you 1800(.375) = 675

    
    
    
12. Arun on December 6, 2018 at 5:24 am

    If my pump rotating at 100 rpm per min,,, how can i reduce the rpm to 25rpm per minute

    
    
    
13. vaibhav on June 27, 2019 at 5:46 am

    if we have input rpm and output rpm so how do we calculate diameter of pulley for output and input

    
    
    
14. William Leamer on May 28, 2020 at 3:51 pm

    I have a 2.25 motor pulley and 3.25 fan pulley motor is single phase 3600 rpm what is the fan rpm?