Zipper faults split into two groups: the tape does not move the way it should, or the thread does not sit the way it should. Deciding which group you are in takes one scrap zip, and it prevents the most common waste, which is adjusting tension to cure a feeding problem.
Feeding or tension: split the fault first
Sew a short length on scrap and read the stitch. Stitches evenly spaced but loose, tight or looping means the fault is tension. Stitches unevenly spaced, bunched, or a tape that stalls means the fault is feeding.
Operators describe both the same way, and the two need opposite responses. Adjusting tension on a feed fault makes the seam worse and hides the real cause for another shift.
The zip does not advance
Work outwards from the tape:
- Is the presser foot fully down and the feed dogs above the plate?
- Is the stitch length set to something the feed can achieve in tape?
- Is the tape caught on the foot sole, a guide, or the needle plate slot?
- Is a puller fitted, and running faster than the feed?
- Is the feed dog worn, or sitting below the plate?
If the handwheel turns freely with the foot up and the tape still will not move with the foot down, the fault is in the feed mechanism rather than the tape.
The zip advances but skews
Skew is a guiding fault more than a feeding one: the tape edge is not held against a reference, so the work drifts.
Check that the foot guide sits against the teeth or the tape edge consistently, that the operator is not steering the tape by hand, and that the tape itself is straight. A coil wound off line will skew no matter how well the head is set, so try a second zip from the same batch before adjusting anything.
Slack and loops along the tape
Loops along a zipper seam usually mean the needle thread is not being taken up, or the bobbin is not holding. Check in this order: the thread path from cone to needle, the tension discs for a trapped fibre, the bobbin for a loose wind, and the bobbin case for a thread routed over the spring instead of under it.
One test settles it. Pull the needle thread by hand with the foot down; it should resist. If it pulls freely, the thread is not between the discs.
Stitches tight enough to distort the tape
The other extreme: the seam pulls the tape into a wave and the zip will not lie flat. Causes are upper tension too high, bobbin case tension too high, a needle too large for the tape, or a stitch length so short the tape cannot recover.
Back the upper tension off first, in small steps, sewing after each. Then lay the tape flat on the bed. A tape that curls along the seam will not be saved by pressing.
The puller and the operator’s hands
On a puller machine the puller sets the pace. If the operator also pulls the work, the tape is tensioned twice and the seam tightens as the run continues.
The check is simple. Watch the tape between the needle and the puller: it should be slack and even, not taut. A taut span means the puller is running ahead of the needle, and the seam will thin out over a long run.
Triage order that wastes the least zipper
Work in this order and use one scrap zip per check:
- Rethread the full path with the foot up.
- Change the bobbin for a known-good one.
- Sew a test and classify the fault as feed or tension.
- For a feed fault, check foot, guides and puller.
- For a tension fault, move one dial in small steps.
- If the fault survives a new bobbin, a rethread and one adjustment, stop and hand over.
The last step matters. A head that fails all three has a mechanical fault, and continuing to change zips costs more than the repair.
Where the owner-technician boundary sits
Owner and operator scope: classifying feed against tension, rethreading, swapping a bobbin, checking guides and foot position, small tension steps, and confirming the diagnosis on scrap before touching the production zip.
Technician work: feed dog height and wear, presser foot pressure linkage, puller speed relative to the needle, hook and needle timing, and any fault that survives a full rethread with a known-good bobbin. Hand over a machine whose stitch length changes by itself during a run, because that is a feed drive fault rather than a setting.
Related guides
- Zipper machine setup for foot and thread path before diagnosis.
- Zipper machine maintenance for the timing and oil round.
- Invisible zipper methods for technique faults that look mechanical.
- Feed dogs not feeding for the feed mechanism itself.
- Sewing machine tension for the upper-thread diagnosis.
- Bobbin tension for the lower-thread side.