Jun 25, 2009

Metric ACME Lead Screw - A Common Engineering Error Most People Make for Trapezoidal Metric Threads

I have seen many people are using the term Metric ACME Screw. In fact there is no such screw called ACME metric screw, instead its called Trapezoidal metric threads. The major difference between an unified ACME thread and a Trapezoidal metric threads is the included thread angle. Trapezoidal metric threads has an included angle of 30° and for unified ACME thread its 29°.

ACME and Trapezoidal Metric Lead Screw Threads

There are two variations for Trapezoidal metric threads one is ’ISO Trapezoidal metric threads (ISO 2904-1977)’ and the other is DIN 103 Trapezoidal metric threads.

Tr 10x2-7e, Is a sample designation for a Trapezoidal metric thread. Where Tr stands for Trapezoidal Metric Thread, 10 stands for nominal diameter in mm, 2 stands for pitch in mm and 7e stands for threads tolerance class symbol. 

The major difference between ISO and DIN screw threads are

ISO Trapezoidal Metric Threads (ISO 2904-1977)

  • ISO standard applies a variety of pitches for a particular diameter.
  • In ISO 2904-1977, the same clearance is applied to both the major diameter and minor diameter.

DIN 103 Trapezoidal Metric Threads

  • The DIN 103 standard applies a particular pitch for a particular diameter of thread.
  • DIN 103 the clearance in the minor diameter is two or three times greater than clearance in the major diameter.

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