In machines with oscillaUng
loop
takers,
such
as
central
bobbin
or
beak
shuttle mochines, the loop forming
part,
in moving back and forth,
passes
the needle
thread
loop around the bobbin
cose
which is carried either In
a
center
or
off-center
position.
|
With the double-revolution horizontal rotary hook, the type of loop
taker
which is generally used in high
speed
sewing machines
end
also in tlie
PFAFF
433,
the
needle
thread
loop
is passed
arourid
the stationary
bobbin
case at every other revolution of the
hook.
'
In
rotary
hook machines which have no mechanical opener, the needle thread,
hoving
passed
around
the
bobbin
case,
has
to
turnl
the
bobbin
case
slightly
in
order
to
make
an
opening
through
which
it
can
fiass.
The
friction
between
hook
race
and
bobbin
case
trjcreases
in
proportion
io
the
sewing
speed.
By
the
same
token,
the
pressure
of
the
position
finger
against
the
position
slot
inc/"easas
as
the
sewing
speed
goes
up.
As
a
result,
the
needle
thread
has
to
overcome
a
stronger
resistance
when
passing between position finger and edge of slot and may break. The
only
remedy
to
this
condition
is
to
ease
the
threap
tension.
The
drawback of this remedy lies in the fact that the needle thread
tension,
in
most cases, has to be eased to an extent
which
will
make the proper
setting of stitches impossible when the machine is operated at o reduced
speed.
To
eliminate
these
disadvantages,
PFAFF
higji
speed
sewing
machines
are organized with a mechanical bobbin case opener whose finger,
which
is
carried
on
the
opener
shaft,
assumes
the
function
of
slightly
rotating
the bobbin case at the right time and contrary to the direction of rotation
of the
sewing
hook.
In
this
case the
needle
thread can pass
freely
between
the
position
finger
and the
position
slot.
The
advantages of
this
device
for
high
speed
sewers
are
manifold.
First of all, as the needle thread is not subjected to additional or fluctuant
strain,
the
upper
tension
need
not be
changed,
regardless
of the
tsewing
speed. For this reason, the thread tensions may .be so set os to ensure
an even setting of stitches and prevent
puckering
of the material at
aniy
speed and also
in
sheer
fabrics.
Secondly,
sincd
the danger of thread
breaking has been greatly eliminated, it is possible to use threads of
ci
low
tensile
strength
even
for
high
speed operations.
Thirdly,
the
machine
will
sew
flimsy
fabrics at top speed
even
when
the bearing surfaces of the
hook
are
not
completely
smooth
yet.
|
All
of the above
advantages
which are inherent in the mechanical bobbin
case opener have lead to its incorporation not
only
in
the
light
but also
in the heavy styles of the
PFAFF
433.
|
Timing
of the
mechanical
opener
should
be performed
only
by o
mechanic
as
instructed
in
Section
29.
17. Tilting
the
Machine
Back
To
facilitate the
removal
of packed
lint
between
sewing
hook
and feed
dog,
strip
the
knee
lifter
and
tilt the machine
bad
16
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