Brother b831 Service Manual page 81

Trouble
Excessive puckering
Upper thread
cutting misses
Lower thread
cutting misses
Cause
Check
Remedy
Upper thread tension is too
Upper thread tension
Set the upper thread ten
high.
sion as weak as possible
Lower thread tension is too
high.
Lower thread tension
Set the lower thread ten
sion as weak as possible.
Thread take-up spring is
too strong.
Thread take-up spring ten
sion.
Set the thread take-up
spring tension as weak as
possible.
Thread take-up spring
Thread take-up spring
Set the thread take-up
spring stroke as small as
possible.
stroke is too large.
stroke
Presser foot pressure is
Presser foot pressure.
Increase presser foot
too weak.
pressure.
Sewing speed is too fast
(motor rotation is too fast).
Decrease sewing speed
(replace the motor pulley
with a smaller one).
When using a machine for
med.-thick material, thread
may tighten excessively
depending on the thread
and material.
Replace with a slotless
presser foot (for med -
thick materials).
Improper thread trimmer
Adjust thread trimmer
timing.
timing.
Damaged or bent movable
knife tip..
Replace the movable knife.
Movable knife tip and
rotary hook stop gap is too
large
Set the movable knife tip
and rotary hook stop gap
to 0 05 ~ 0 2 mm.
Improper tension release
adjustment
Improper needle and
rotary hook timing.
Movable knife stroke is
small (insufficient movable
knife extension).
Lower thread loop spread
ing does not match the
rotary hook.
With the tension release
disengaged, adjust the
tension release lever and
presser bar lift gap to 1.5
mm.
Bobbin case opener gap.
Adjust the rotary hook and
bobbin case opener gap to
0.2 mm.
Adjust the rotary hook
point and needle gap to
0.05 mm.
Needle and rotary hook
point gap.
Needle plate and rotary
hook gap.
Adjust the rotary hook and
needle plate gap to approx.
1.3 mm.
Needle bar lift stroke and
height.
Adjust the needle bar lift
stroke and height.
Lower thread loop spread
ing.
Install the slider assembly
so It extends out to the
needle side.
Use normal lower thread
loop spreading (loop
spreading varies with old
and new rotary hook
models).
Movable knife tip is too
low.
Movable knife position.
Check the rotary hook
height.
—
Rotary hook is too high.
Rotary hook height.
Check the rotary hook
height.
Damaged or bent movable
knife tip.
Replace the movable knife.
Movable knife stroke is too
small.
Press the slider assembly
out to the needle side.
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