Number of Teeth is the total count of the number of teeth(projections) around the whole circumference of a sprocket. For sprockets with very few teeth it is easy to simply count the number of teeth. However, for sprockets with a higher number of teeth, attempting to count the teeth may not be very practical or accurate. For our REV gears, we have taken all of the guesswork out of this process. Simply use the reference guide on page 17 by printing that page out at full scale and then match your sprocket to its correct outline on the page to identify the sprocket.
Pitch Diameter(PD) is an imaginary circle which is traced by the center of the chain pins when the sprocket rotates while meshed with a chain. The ratio of the pitch diameter between sprockets can be used to calculate the gear ratio, but more commonly and much more simply the ratio of the number of teeth is used for this calculation.
Pitch represents the amount of pitch diameter in inches per tooth. Gears with a larger pitch will have bigger teeth. Common pitches are 0.25”, known as # 25, and 0.375” (#35). The REV Robotics building system uses #25 chain.
Outside Diameter(OD) will always be larger than the pitch diameter but smaller than the chain clearance diameter. The outside diameter does not account for the additional diameter added by the chain, so it should not be used to check for assembly interference.
Chain Clearance Diameter is the outside diameter of a sprocket with chain wrapped around it. The chain clearance diameter will always be larger than the pitch diameter and the outside diameter. The chain clearance diameter should be used when checking for interference when placing sprockets very close to other structures.