Undercut (technique)

Profile of the tooth of an involute toothed gear with undercut

Undercut is the unwanted cutting away of a part of the tooth flank that occurs during the manufacture of gears with a small number of teeth by hobbing or hobbing . When producing gears by gear shaping , undercut can also occur; however, the conditions there are different.

If the number of teeth on a gear falls below the limit number of teeth, the hob or the shaping tool at the tooth base cut away a piece of the involute that is necessary for the continuous rolling of the mating gear . The tooth is "undermined".

The presence of an undercut is determined by drawing or arithmetic whether the point of transition from the tooth root curve to the involute is above the useful root circle. The useful root circle is the lowest point on the tooth flank that the mating gear touches with the tooth tip.

The theoretical limit number of teeth is 17 teeth for normal gears, but the undercut is of practical importance with normal gears only below 14 teeth.

To avoid the undercut and the associated weakening of the tooth root, a positive profile shift is selected for the gear . The used part of the involute is shifted outwards and the tooth base becomes wider. The undercut can also be avoided by increasing the helix angle of the toothing.

See also

• Involute gearing

literature

• Heinz Linke: Spur gear teeth . Carl Hanser Verlag, Munich 1996, ISBN 3-446-18785-5 .