Stub Acme Screw Thread introduction
The STUB ACME thread came into being early in the 1900's and it's originally derived from the General Purpose form, the Stub Acme is generally used for applications where a coarse pitch thread of shallow depth must be used due to mechanical or metallurgical considerations.
What would a typical Stub Acme be used for? A standard 1-1/2-5 Acme is planned for a part, but the amount of metal removal would reduce the minor diameter to the point where the torque would twist the part. The solution is use a 1-1/2, 5 pitch 2 start, or 1-1/2, 5 pitch Stub Acme.
Thread height for Standard Stub Acmes is equal to 0.3 of the pitch, while the height of a Standard Acme is 0.5 of the pitch, the height of Stub Acmes is shorter.
Here is how to correctly describe a Stub Acmes: 1.4375”-12 Stub Acme-2G corresponds to 1.4375 nominal size, 12 pitch per inch Stub Acme, Class 2G. (Class 2G is the same as for the General Purpose Acme.)
Figure 1. Stub ACME thread profile
STUB ACME External thread properties:
- Thread angle: 29°
- h (Height of thread, Basic) = 0.3 x p
- t (Thread thickness) = 0.5 x p
- Frs (Root of screw) = 0.4224 x p-0.259 x (Minor diameter allowance- Pitch diameter allowance)
- lFcs (Crest of screw) = 0.4224 x p- 0.259 x Pitch diameter allowance
Table 1. Stub ACME thread dimensions