The Acme thread form has a 29°
thread angle with a thread height half of the
pitch; the apex (or
crest) and valley (or
root) are flat. This shape is easier to machine (faster cutting, longer tool life) than a square thread. The tooth shape also has a wider base which means it is stronger (thus, the screw can carry a greater load) than a similarly sized square thread. This thread form also allows for the use of a
split nut, which can compensate for nut wear. The line of General Purpose (GP) Acme threads (ASME/ANSI B1.5-1997) are not designed to sustain external radial loads and both the nut and bolt are, ideally, independently supported (the nut by a linear guide and the screw by shaft bearings). This is due to the need to avoid "wedging" of the thread flanks when subjected to radial loads, which would contribute substantially to friction forces and thread wear. However, there is a Centralizing Acme-thread standard (also specified in ASME/ANSI B1.5-1997) which caters to applications where the threads are not radially supported, where the roots and crests of opposing threads are designed to come into contact before the flanks do under radial loads. This adds the requirement that the sum of the allowances (clearances) and tolerances on the
major diameters of nut and bolt be less than the sum of the allowances on the
pitch diameters (PD). The drawback is that for a given amount of end play (axial clearance due solely to PD clearances), closer tolerances and a cleaner work environment are necessitated in the application of a Centralizing Acme thread. Compared to square threads, disadvantages of the Acme thread form are lower efficiency due to higher friction and some radial load on the nut (angular offset from square). The thread form shown in the figure (Basic ACME thread profile) is called "basic". The actual thread heights on both the internal (nut) and external (bolt) threads differ from by
allowances (or
clearances): • A minimum root-crest clearance of (diametral) between opposing threads with 10 tpi (threads-per-inch) or fewer, and for finer pitches. (This is also true for the
minor diameters of the Centralizing Acme thread, though not its major diameters, where the allowance is made less than the PD allowance.) • A PD allowance, which makes the PD smaller than "basic" in the case of the GP and external Centralizing Acme threads, but greater in the case of the internal Centralizing Acme thread. The net effect is that the minimum thread heights are greater than "basic" for internal and external GP threads and for external Centralizing threads, and the maximum height for internal Centralizing Acme threads is shorter than "basic". The maximum diameter (within tolerance) at the crest of the external threads (called the
max. major diameter of external thread) is that of the basic thread form and equals the "nominal diameter",
D, stated in the screw's designation. The minimum diameter (within tolerance) at the crest of the internal thread (called the
min. minor diameter of internal thread) is that of the basic thread form and equals the nominal diameter minus twice the basic thread height (i.e.
D −
P). There is also a "Stub Acme" thread standard, identical in all respects to the one just described
except for the height of the basic thread being 0.3
P. ==Metric trapezoidal thread characteristics==