Glossary of Terms

MAJOR DIAMETER - The diameter described by a cylinder formed by the crests of the screw.

MINOR OR ROOT DIAMETER - The diameter of a cylinder formed by the roots of the threads.

PITCH DIAMETER - The theoretical diameter described dimensionally by the mean value of the major and minor diameters.

THREAD HEIGHT - Half the difference between the major and minor diameters. The basic thread height is equal to one half of the thread pitch. The basic thread height is also equal to the thread thickness at the pitch diameter.

acme screw

THREAD LEAD - The nominal translational distance produced by one turn of the thread. The lead is equal to the SCREW PITCH x THE NUMBER OF STARTS. Therefore, the lead = pitch for single start threads.

THREAD PITCH - Nominal distance between the same points on adjacent thread forms as measured parallel to the rotational axis. The pitch is equal to the screw lead divided by the number of starts.

THREADS PER INCH - Equal to the reciprocal of the pitch.

THREAD STARTS - The number of uniquely independent threads contained either on the screw or the nut.

THREAD FLANKS - The area of contact between the nut thread and the screw thread.

acme screw 2

"G" Class

  • Screw contact on both sides of nut thread
  • Side loading will cause nut thread to wedge between screw thread - casue binding

"C" Class

  • Tighter tolerances the "G" class
  • Side loading will not cause wedging, instead screw thread "bottoms out" on nut