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Home » How-To » Pulley & Belt Calculator

Pulley & Belt Calculator

Interested in using our S3M Timing Pulleys or Belts?

This useful tool will tell you the center distance between pulleys, or if you want a certain center distance, it will tell you what belt size you need based on what pulleys you have chosen!

The dropdowns show all of the FingerTech pulley and belt sizes for easy selection.
If you are using sizes we don't sell, simply type the dimensions into the text boxes and ignore the dropdowns. (The calculator even works for different pitch!)
See below for an example and definitions!

Pulley Information
Pulley 1 Pitch Diameter mm Variable
Pulley 2 Pitch Diameter mm     Variable
Pulley Ratio to 1 Variable
Belt Information
Center to Center Distance mm    in Set
Belt Length mm total Set
Speed Information
Pulley 1 RPM Set
Pulley 2 RPM Set
Minimum Belt Wrap Angle Degrees

Example 1: You know the distance available between the pulleys and want to find the belt size.
Say your robot has one drive motor per side but you want to make it four-wheel-drive by belt driving the two other wheels.
To keep weight down you have chosen to use 15T pulleys. Your front wheels are 5" from your back wheels.

What to do:
Select the 15T pulleys from both pulley dropdowns. (The pitch diameter automatically updates.)
Type "5" into the Center to Center inch distance. (The mm distance updates to 127mm. It doubles as a handy unit converter!)
Make sure the Pulley Ratio is checked as Variable, and the Center Distance is checked as Set.

Click the CALCULATE button and watch as the Belt Length dropdown selects the appropriate size of belt (100 Tooth) and tells you that there will be 1mm of belt slack.

Example 2: You know the pulley ratio required and want to find the distance between pulleys.
You have a weapon motor that spins at 14000rpm but you would prefer your titanium blade to spin at 7000rpm.
You have around 3" between the weapon motor and blade shaft (axle) but it can change a bit.

What to do:
Select 15T and 30T from the pulley dropdowns. Type "3" into the Center to Center inch distance. Type "14000" into the Pulley 1 RPM.
Make sure the Pulley Ratio is checked as Variable, and the Center Distance and Pulley 1 RPM are checked as Set.

Click CALCULATE and the Pulley Ratio will show you "0.5 to 1", meaning your Pulley 2 will be turning at half the speed of Pulley 1. The Belt Length dropdown will show a 74 Tooth belt with 1.4mm of slack.
To get your exact Center Distance, you now select the Belt Length as Set, and update the Belt Length by changing the dropdown to something else then back to 74 Tooth. (Otherwise the 1.4mm of slack will be used in the calculation!)

Click CALCULATE again to see the perfect Center Distance of 3.028". The Pulley 2 RPM shows you that your blade will spin at 7000RPM.
The Minimum Belt Wrap Angle tells you that the smaller pulley will be using 47% (169/360degrees) of its teeth (in this case, 7 teeth).

Pitch Diameter: Coincides with the circle made by the reinforcing cord of the timing belt.
Pulley Ratio: (Pulley 1 Pitch Diameter / Pulley 2 Pitch Diameter) OR (Pulley 1 RPM / Pulley 2 RPM)
Center Distance: Distance between the center of rotation of Pulley 1 and Pulley 2.
Belt Length: If you were to cut a belt and lay it flat, it would be this long. A 3mm pitch belt has 1 tooth every 3mm, so a 100T belt will be 300mm long.
Belt slack: A perfectly tensioned belt will have no slack. If you have a set Center Distance you will probably have some slack (but not much thanks to our many belt sizes!). Under 1mm is good for a robot. Above starts getting sloppy so you will want a tensioner, or maybe go up in pulley size.
Minimum Wrap Angle: This tells you how much of the pulley is being gripped by the belt. This refers to the smaller pulley because the larger pulley always has more than 180 degrees.

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