When asking questions about a drive, a lot of the time people take a diameter measurement to give to us. On a synchronous sprocket, this can be a little deceptive. There are three main diameters that are important, and this may not be obvious to the person taking the measurement.
First, if at all possible, always get a tooth count (or the part number). Tooth count is exact, and is the best way to get you the correct answer. If a diameter measurement is the only option, there are a few things that you need to know to get the best result.
Pitch Diameter (PD): This is a theoretical diameter, and cannot easily be measured by hand. However, this can be calculated. The formula is:
(#teeth*pitch)/pi. Pitch diameter is slightly larger than outer diameter (OD). The diameter listed in the catalog is the PD, so if you are measuring the sprocket trying to identify the tooth count, the PD number should be slightly bigger than your OD.
OD: Outer diameter is measured on the tooth surface. This is the measurement we need if you can’t provide a tooth count. Be careful when measuring this, as there is not always the tip of a tooth 180 degrees apart from one another, and you could get a measurement that is a little smaller than your actual sprocket.
Flange Diameter: This is the one that gets people into trouble. The flange is the largest diameter of the sprocket; it’s the part that keeps the belt from running off of the edge of the sprocket. Not all sprockets have flanges, and sometimes the same flange is used for a couple of different sprocket sizes. This means that if you provide us a flange diameter, we have to guess as to what sprocket you may actually be using. Avoid measuring the flange if at all possible, but if you must, make sure that you tell us that it is a flange that you are measuring, and not the OD.