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MAINTENANCE FOR THE C-O-TWO TRANSFER UNIT | Up
Aircrew Survival Equipmentman 2 - Aviation theories and other practices | Next
Figure 9-1.—The lockstitch |
CHAPTER 9
SEWING MACHINES
Learning Objective: Upon completion of this chapter, you will be able to
identify and maintain the different types of sewing machines used in the process
of repairing or fabricating survival equipment.
Sewing machines are like any other tool you
use. If you don’t have the correct one, the task
is harder or impossible to complete. The same
applies to sewing machines. You need the right
machine for the job; whether it be lightweight,
medium weight, or heavyweight, there is a
machine designed to perform each task. You work
with various types of sewing machines in the
process of repairing or fabricating items in the
shop. You need to have all the knowledge and skill
YOU can possibly acquire about these machines to
fulfill your duty as a PR. If you don’t know how
to operate and maintain the sewing machines, they
will stand idle, not operate properly, or not work
at all. When you have a job to do, you need
equipment that is operational. Without the proper
knowledge of sewing machines, you will not have
the confidence to perform necessary sewing
machine repairs.
Before you can learn to operate and maintain
a sewing machine, you must learn the language
of the sewing trade. Through your supervisor and
this text, you should become familiar with this
language. It is very important that you form a
habit of referring to the parts of a sewing machine
by their proper names. It would be difficult to
communicate with other PRs and impossible to
pass a rating exam if you do not know the proper
names of the different parts of a sewing machine.
Take time to study the illustrations in this chapter
that show the important sewing machine parts and
their names.
Sewing machines are classified as two types—
OSCILLATING and ROTARY. Both types are
operated by electric motors and are fitted with
rheostats and special clutch arrangements that
enable the operator to control the speed.
When it comes to classifying sewing machines
into oscillating and rotary, the important part is
the rotary hook and oscillating shuttle. This is the
device that is out of sight in the base of the
machine, but does the very important job of
forming each stitch after the needle has passed
thread through the fabric.
Oscillating sewing machines have a sewing
hook that rocks back and forth through half of
one revolution to complete one stitch.
Rotary sewing machines have a hook that
makes two complete revolutions to complete one
stitch.
The type of stitch commonly used and made
by sewing machines in repair work is the
lockstitch. The lockstitch makes use of two
separate threads. One comes from the spool down
through the eye of the needle, the other from the
bobbin. In making the lockstitch, these two
threads must become interlocked, as shown in
figure 9-1.
The thread passing through the eye of the
needle is pushed down through the material being
sewn. As the needle travels downward to the
material, a spring pulls tension on the needle
thread to keep it taut to prevent any slack that
might tangle the thread around the needle.
After the needle reaches its lowest position and
starts its upward movement, the process shown
in figure 9-1 begins. A small loop of thread forms
alongside the needle beneath the throat plate. The
sewing hook catches this loop and carries it
around the bobbin, which floats in its track in the
bobbin case (view B of figure 9-1). By locking the
loop of needle thread around the bobbin thread,
the sewing hook forms the stitch.
As the needle completes its upward movement,
the thread tension disks hold the needle thread
firmly. The thread take-up lever, rising quickly,
pulls on the loop that has been formed, and thus
tightens the stitch. When the thread take-up lever
9-1
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