Single-Needle Lockstitch Machine

Overview

A single-needle lockstitch machine is the workhorse of garment and general sewing. It joins two layers of cloth with a 301 lockstitch, the seam that looks the same on both faces and holds because two separate threads interlock inside the fabric. One thread comes from a spool above the machine and runs down through the Upper Thread Path to the needle. The other sits on a small Bobbin under the throat plate. Every stitch ties one thread to the other at a knot buried in the middle of the cloth.

The machine is built around a cast iron Head Casting and Bed. The overhanging Arm Casting carries the needle bar and take-up, the flat Bed Casting houses the hook and the Feed Mechanism, and the Main Shaft and Drive turns both ends in time. A Throat Plate sits between the upper and lower mechanisms with a hole for the needle and slots for the feed dog teeth.

How a lockstitch forms

The stitch forms in a fixed sequence repeated every revolution of the main shaft. The Needle Bar drives the Needle down through both layers of cloth, carrying the upper thread to its lowest point. As the needle starts to rise, the thread on the back of the needle, behind the scarf, lags and bulges out into a small loop. Timing is set so the point of the Rotary Hook arrives at exactly this moment and catches that loop.

The hook turns at twice the speed of the main shaft, so it sweeps the loop a full way around the Bobbin Case. As the loop travels, it opens wide and passes completely over the bobbin sitting inside the case. A Position Finger holds the case from spinning so the loop can clear it. Once the loop has gone all the way around, the upper thread has been wrapped once around the lower thread coming off the bobbin.

At that point the Take-up Lever swings up fast. It pulls the slack of the upper thread back out of the hook and draws the interlock up tight, setting the knot in the middle of the two cloth layers. While this happens the Feed Mechanism has advanced the cloth by one stitch length, so the next needle entry lands ahead of the last. The cycle then repeats. Stitch balance depends on the upper Tension Assembly and the leaf spring on the bobbin case pulling against each other so the knot sits centered, not on either face.

Upper thread path

The upper thread leaves the cone on the Thread Stand, passes through several Thread Guide eyelets, and enters the Tension Assembly. There two Tension Disc discs pinch the thread under spring load set by the Tension Nut. A Check Spring takes up slack just before the needle pierces the cloth, which keeps the loop clean for the hook to catch. The thread then runs through the Take-up Lever and down to the Needle, held by the Needle Clamp. When the operator lifts the foot, a Tension Release Pin opens the discs so the work pulls free without dragging.

Feed and presser

Cloth moves by drop feed. The toothed Feed Dog rides on the Feed Bar, which is driven through a four motion path by the Feed Lift Cam and the Feed Drive Eccentric on the lower shaft. The dog rises through the throat plate, grips the cloth, carries it back one stitch length, drops below the plate, and returns to start again. It only feeds while the needle is clear of the cloth. Pressing down on the work from above is the Presser Foot on its Presser Bar, loaded by the Presser Spring. The Lift Lever or the Knee Lifter raises it to load and turn the work.

Stitch length and reverse

Feed travel is adjustable. The Stitch Dial changes the throw of the feed eccentric through the Feed Regulator Link, which sets stitch length anywhere from zero to about 5 mm. Pressing the Reverse Lever flips the feed direction so the machine sews backward, which is how the operator backtacks the start and end of a seam to lock it.

Drive, lubrication, and control

Power comes from a Servo Motor mounted under the table and coupled close to the Main Shaft. The shaft runs the needle bar crank and take-up, and a Timing Belt ties it to the Hook Shaft in a fixed 1:2 ratio so the hook always meets the loop on time. At up to 5000 stitches a minute the hook race and bearings need constant oil, so the Lubrication System uses an Oil Pump and felt Oil Wick feeds from a sump, with Oil Seal parts keeping oil off the cloth. The Controls read the Foot Pedal for speed and a Synchronizer for needle angle, so the Control Box can stop the needle up or down on command and trigger trimming.