Sprocket tooth dimensions

[Philrock]

I’m detailing an ANSI #40 sprocket. The machinist will profile it, so the teeth must be fully dimensioned.

The diagram and formulae in Machinery’s Handbook call for the sprocket seat radius to be slightly larger than the chain roller radius. The diagram is built around the center of the seat radius.

Interpretation 1: The center of the seat radius is actually located at the pitch radius. The diagram in Machinery’s Handbook says the distance between seat radius centers is equal to the pitch, which seems to confirm this interpretation, but this leaves the roller not seated in the bottom of the seat. In the case of no. 40 chain, the roller radius is .0023" less than the seat radius.

Interpretation 2: The bottom of the seat radius is located at (pitch radius – roller radius). My old Diamond Chain catalog has pretty much the same diagram as the one in Machinery’s Handbook, but does not say the distance between seat radius centers is equal to the pitch. Furthermore, the Diamond catalog says the caliper diameter for even no. of teeth is the pitch diameter – roller diameter, which seems to confirm this interpretation.

Which interpretation is right?

 

[dvd]

I vote for interpretation #1. The roller runs up or down the tooth face to accommodate changes in chain pitch length. If you didn't have space under the roller then chain length changes (in particular shortening)due to temperature change wouldn't be possible. The space is there to allow for this as well as to run dirty chain. As chain lengthens due to stretch, wear or thermal growth the chain no longer runs at the nominal pitch diameter.