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How To Calculate Pulley Size From RPM

Pulleys 101: A Brief Overview

Properly sizing pulleys is important for a range of industries: aerospace, agriculture, manufacturing, pharmaceutical, automotive, and robotics, just to name a few. The breadth of industry use for pulleys is primarily because pulleys are commonly used for connecting pumps to engines or motors via drive belts.

The pulley you’ll need to size is almost positively one of two styles: 1) fixed bore style or 2) tapered bushing style. The pulley is likely either made out of cast iron or aluminum.

how to calculate pulley size from rpm

Why It’s Important to Size Pulleys for Each Use

Sizing pulleys correctly is critical to maintaining their functionality and proper operation. Correctly sizing pulleys for each application also ensures that you maintain the right RPM, or revolutions per minute. RPM speed is the determining factor for the pump output flow rate. Pump outflow rate may be measured in liters or gallons per minute, for example.

It’s very important to make sure you have the correct pulley sizing of motors and engines relative to the pump. You should also calculate the RPM of your motor or engine (drive unit). Without the correct pump RPM, your pump will not perform as well as it was designed to. For instance, with the wrong RPM calculation, your pump may be revolving, or turning, slower than it is able to, resulting in decreased performance. And on the other hand, with the wrong RPM calculation, a pump could be turning too quickly, resulting in excessive valve wear, elastomer failure, and other untimely mechanical failures.

Beginning with the incorrect RPM will cause you to size equipment improperly, as well.  Ultimately, the valuable life of your equipment will be shortened, and your equipment’s output flow rates will be reduced, leading to overall lower levels of efficiency.  In essence, making sure you know how to calculate the RPM of pulleys correctly will lead to far greater cost savings and productivity gains than you might have suspected. 

How to Size Pulleys Using RPM (Revolutions Per Minute)

Find the Ratio of the Required Pulley

The simplest way to find the ratio of the required pulley is to divide the driven component (pump) by RPM. For example, if the pump RPM is 1070 for full output, while the motor is 1750 RPM, the ratio of the required pulley would be (pump RPM) divided by the motor RPM, or:

1070 / 1750 = 0.611

Going by our above example, your pulley ratio must be 0.611 for your pump to function correctly.  Now say, for example, you have a 4-inch pulley on the motor. You would use the ratio we discovered in our last step (0.611) to figure out what size pulley you would need on your pump. With a ratio of 0.611, and a 4-inch pulley on your motor, you would need a 6.55-inch pulley on your pump.  Broken down into a math problem, this looks like:

4 inches / 0.611 = 6.55 inches

Now let's say we're working with the same pump from the above example, but a gas engine drives this pump. We would take the pump RPM of 1070 and divide it by the engine RMP, let's say, 3400. Here's what that equation would look like:

1070 / 3400 = 0.315

Let's say the drive pulley on the engine in the above problem is also a 4-inch pulley. To calculate the size of the pump pulley, you would need to divide the size of the drive pulley on the engine by the ratio we just calculated in the above problem. This results in the following equation:

4 inches /. 0315 = 12.7 inches

In order for your pump to run at the correct RPM of 1070, your pump pulley should be exactly 12.70 inches in diameter. 

If you are working with pulleys, it's important to understand how these numbers are calculated. In some cases, however, it can be helpful to use this handy calculator. When using the calculator, if you know "any 3 values (Pulley sizes or RPM) and need to calculate the 4th, enter the 3 known values and hit Calculate to find the missing value. For example, if Pulley 1 is 6" diameter, and spins at 1000 RPM, and you need to find Pulley 2 size to spin it at 500 RPM, enter Pulley 1 = 6, Pulley 1 RPM = 1000, Pulley 2 RPM = 500, and hit Calculate to find Pulley 2 diameter."

How to Calculate Multiple Pulley Sets by RPM

The above calculator is also helpful if you need to calculate multiple pulley sets by RPM. From the website,

"To calculate multiple pulley sets, where the first driven (large) pulley shaft drives the second driver (small) pulley, and so on, enter first (small) driver pulley RPM and pulley set diameters and centers separated by commas. The RPM of the first driven (large) pulley is transferred to the second set small (driver) pulley, and so on. The sets of pulleys will be drawn at the bottom of the page with each set RPM reductions and total RPM reduction through all entered sets."

Still Need Some Expert Help with Pulleys?

As you have already learned, determining which pulleys are the right ones to use is critical. Failure to use the correct sized pulleys can cause you to drive your pumps incorrectly. Finding that you could use some help from a specialist with determining which pulley size is right for your equipment? Contact an expert custom pulley manufacturer that aims to provide high-quality products in efficient and economical ways.

Illinois Pulley & Gear (IPG) manufactures a large variety of high-quality timing pulley stock and timing belt pulleys. Made on-demand, IPG is able to produce virtually an unlimited type of product. All IPG pulley stock is precision machined from bar stock at their Schaumburg facility. They do not use extrusions or imported pulley stock. Pick your material, tooth profile, and any number of teeth, and IPG will produce it.