Three thread forms that are often
used are the Acme thread, the square
thread, and the buttress thread. As
shown in Fig. 13.2, the Acme thread and
the square thread exhibit symmetric
leading and trailing flank angles, and
consequently equal strength in raising
and lowering. The Acme thread is inherently
stronger than the square thread
because of the larger thread width at the
root or minor diameter. The generalpurpose
Acme thread has a 14H-degree
flank angle and is manufactured in a
number of standard diameter sizes and
thread spacings, given in Table 13.1. The
buttress thread is proportionately wider
at the root than the Acme thread and is
typically loaded on the 7-degree flank
rather than the 45-degree flank. See
Refs. [13.1], [13.2], [13.3], and [13.4] for
complete details of each thread form.
Ball screws recirculate ball bearings
between the screw rod and the nut, as
shown in Fig. 13.3. The resulting rolling
friction is significantly less than the sliding
friction of the machine screw type.
Therefore less input torque and power
are needed. However, motor brakes or
screw stops are usually required to prevent
ball screws from self-lowering or
overhauling.