JUKI MF-7900 Series Engineer's Manual page 189

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine
Trouble
3. Needle breakage
4. Worn out needle point
Cause (1)
3-1) Needle entry
3-2) Spreader
3-3) nterference of looper with
scooping movement of
needle
3-4) Needle guard
3-5) Needle thickness No.
3-6) Thread tension
3-7) Feed dog height, Needle
height
3-8) UT59
Thread hook
4-1) Needle guard
4-2) Interference with looper
4-3) UT59
Thread hook
Cause (2)
1-A) Needle hits the throat plate or presser
foot because the needle entry is not
correctly positioned for the plate or
foot.
2-A) Too little clearance between spreader
and needle.
3-A) Needle hits looper resulting in needle
breakage.
4-A) Too strong contact against needle
guard or needle point hits needle
guard due to incorrect position.
5-A) Too thin needle for fabric used.
6-A) Needle thread tension is too strong.
7-A) Feed dog is too high or needle height
is too low resulting in needle break-
age due to needle swerve.
8-A) Needle hits thread hook.
1-A) Incorrect height or incorrect position,
front and rear
2-A) Moving amount, front and rear is ill
balanced.
3-A) Needle hits thread hook.
Check and corrective measures
Align the needle entry to the needle.
Refer to the standard adjustment figures.
Make adjustment so that looper does not hit
the looper. For the correction of the contact at
looper's back, adjust the moving amount, front
and rear.
Refer to the standard adjustment figures.
Use thicker needle.
Reduce the needle thread tension.
Refer to the standard adjustment figures.
Check to be sure that air is supplied.
Check to be sure that thread hook is located in
the standby position.
Check to see the height of needle guard and
clearance between needle guard and needle.
Adjust moving amount, front and rear and cor-
rect the incorrect position of looper's back and
the contact when it returns.
Refer to "12.-(9) Setting the raveling preven-
tion parameter".