A measurement of the distance across a set number of teeth, taken with flat-faced disc anvils that touch opposite involute flanks. Because the anvils contact the involutes tangent to the base cylinder, the reading does not depend on runout and converts directly to tooth thickness. The number of teeth spanned is chosen from tooth count and pressure angle so the anvils land near mid-flank.
Span measurement over five teeth is right on the low limit, so lash will be on the loose side, that's fine for this box.
Also heard as
- base tangent length
- span
- Wildhaber measurement
- W
Related terms
Tooth thickness
How thick a gear tooth is along the pitch circle, the dimension that is thinned to create backlash.
Measurement over pins
Measuring across precision pins or balls placed in opposite tooth spaces to check tooth thickness.
Base circle
The circle from which the involute tooth profile is unwound, smaller than the pitch circle.