The uses of Acme Threads usually involve special designs requiring individual
consideration.
Each tapping job becomes a minor engineering project
of its own.
The inherent exactness in application of Acme taps makes
close coordination between manufacturer and user particularly important.
An example of an Acme Thread application may be aircraft wing flap
hydraulic actuators or a simple car bumper jack.
American Standard Acme Threads are used to produce traversing motion
in valves and other power transmission mechanisms requiring high strength
and free movement over long periods of operation.
American Standard General Purpose Acme Thread: Provides clearance of
all points of contact between external and internal surfaces for
free movement. Normally these applications are indicated with a Class
of Fit symbols of “2G”, “3G”, or “4G”.
The numbers 2,3, and 4 will control the amount of backlash allowed.
Included flank angle is 29°
American Standard Centralizing Acme Threads: Clearance at pitch
and minor diameters, with limited clearance at major diameter so
that alignment of screw and nut is controlled. The class of fit
symbols here are indicated with “2C”, “3C”, and “4C”. Included
flank angle is 29°
Stub Acme are similar to General Purpose Acme but
are used where coarse pitch thread of shallow height is required.
Included flank angle is 29°
60° Stub Threads: May be substituted where axial leads do not require
the smaller flank angle. They are also for applications where
shallow thread height is required as on fitting-up bolts and nuts. Included
flank angle is 60°.
Modified Square Threads: Used as an equivalent
to Square Threads for all practical considerations. However,
they should not be used where Acme Threads can serve the same purpose.
Modified Square Threads have 10° included flank angles.
Acme thread profile characteristics pose a challenge for
internal threading because their block like profile requires
that large amounts of material must be cut and removed. Chip
load becomes a major factor and the types mentioned are designed
to control this.
Taps are furnished in normally two types for most of the above. Sets
include either one or two roughers and a finisher. Single pass
tandem taps can also be designed with the roughing section built ahead
of the finishing section on the same body.
