Pulley Speed Ratio: Larger Vs Smaller Pulleys

When speaking of rotational speeds of motor-driven pulleys, you’re probably asking these types of questions: 

• What’s the difference between big and small pulleys in terms of speed? 
• Does a bigger drive pulley increase speed? 
• Does a bigger drive pulley increase torque? 
• How do I make my driven pulley spin faster or slower? 
• How do I change the (rotational) speed of my pulley? 
• How do I increase my pulley speed? 
• Will a smaller pulley spin faster or slower? 
• Which pulley will turn faster or slower? 

We’ll attempt to answer them given these assumptions: 

• You won’t be replacing your electric motor. Of course, you can change the speed of the pulleys by using a motor with a different speed rating, but we’re assuming you don’t want to do this. 
• You’ll be using the same drive pulley (the one attached to the motor shaft). Some call this the driver pulley. 
• You’re only interested in changing the speed of your driven pulley (the one driven by the belt). There can be more than one driven pulley (and therefore, more than one belt) in a more complex system.

Diameter ratio of drive and driven pulleys 

Without getting too technical, you just change your driven pulley to one with a bigger or smaller diameter relative to your driver pulley. 

• The bigger your driven pulley (vs the driver), the slower it will turn but the more torque it will produce. 
• The smaller your driven pulley (vs the driver), the faster it will turn but the less torque it will produce.  

Put simply: 

• To speed up a driven pulley (and achieve less torque) it should be smaller than the drive pulley. 
• To slow down a driven pulley (and achieve more torque) it should be larger than the drive pulley. 

Here’s how you can calculate the exact values: 

Values: 

• D1: Diameter of the drive(r) pulley that is attached to the motor shaft 
• V1: Velocity/Speed of the drive(r) pulley (We'll just use 1000 rpm in the table below for simplicity.) 
• D2: Diameter of the driven pulley 
• V2: Velocity/Speed of the driven pulley 

To answer how fast your driven pulley (V2) will be: 

• V2 = V1 (D1 / D2) 

To answer the size of your driven pulley (D2): 

• D2 = D1 (V1 / V2) 

Here are some pre-calculated values* for your convenience: 

*The values of D1 and D2 must have the same unit (ie both in inches, or both in millimeters). Convert one to the other, if you must.

*1 inch = 25.4 millimeters 

It is worth noting that the RPM is a theoretical, calculated value, just as the rated RPM of your motor is. The exact RPM you achieve will, naturally, depend on your specific set up. 

To make your life easier, here’s a calculator courtesy of Engineering Toolbox. 

• Pulley 1 is the drive(r) pulley connected to your motor. 
• Pulley 2 is the driven pulley. 

The values for pulleys 3 and 4 are optional. Use those fields if you have more than one driven pulley, otherwise just leave them blank. 

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