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Timing Belt Pulley Pitch Diameter & Outside Diameter Charts

The following timing belt pulley pitch diameter (P.D.) and timing belt pulley outside diameter (O.D.) charts allow the end user to quickly determine the dimensional requirements for a timing belt drive system. These charts allow the user to check for interferences before a system is designed for a given ratio. This is the second most common problem encountered when talking with an end user. Often the outside diameter of a particular timing belt pulley is not considered before a design is finalized.

Timing belt pulley pitch diameter (P.D.) has a direct relationship between the timing belt pulley and the timing belt. The reinforcing tensile cord of the timing belt forms the pitch line of the timing belt. Thus the pitch diameter of a timing belt pulley coincides with the pitch line of the timing belt meshing with it.

Pitch Diameter (P.D.) = (Pitch x Number of teeth) / ∏

Timing belt pulley outside diameter (O.D.) further builds upon the relationship between the timing belt pulley and timing belt. The outside diameter of the timing belt pulley is in contact with the tooth bottom of the timing belt while meshing with it. The distance from the timing belt tooth bottom to the tensile cord (pitch line) will determine the outside diameter of the timing belt pulley for any timing belt pitch/timing belt profile.

Outside Diameter (O.D.) = Pitch Diameter – (2 x U) Note: “U” = distance between the tensile cord and the bottom of the belt tooth 

 
Rule of Thumb
To quickly calculate the outside diameter (O.D.) in inches of any 8mm pitch timing belt pulley divide the number of teeth on the pulley by ten (10). For example an 80 tooth pulley would have an 8” outside diameter (80 teeth / 10) or a 33 tooth pulley would have a 3.3” outside diameter (33 teeth / 10).

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