Sewing Technology

The needle carries the needle thread through the material. The needle thread has to be interloped, intralooped or interlaced to complete the stitch. This is carried out with the help of a lower thread system. The lower thread system might perform with or without a thread depending upon the stitch class. Lower thread systems can be mainly divided into two main types i.e. for lockstitch (class 300) and for chain stitch (other stitch classes). 

For Lockstitch

For stitch class 300 also named lockstitch, technology is made by the interlacing of needle and bobbin thread and has been explained in stitches and seams. The lower thread system is composed of:

• Spool, on which the lower thread is wound. It is also known as a bobbin.
  
• Bobbin case houses the bobbin inside it.
  
• A hook, which takes the needle thread around the bobbin case to complete stitch. 
  

The hook is mounted on a shaft that rotates. For one revolution of the main shaft which drives the needle and thread take-up lever, the bobbin shaft makes two revolutions. The hook is fixed with the bobbin shaft and timed precisely with the needle so that as soon as the needle thread forms the loop, the hook takes it around the bobbin case so that both thread systems interlace with each other. Needle thread moves back and forth multiple times before they become a part of a stitch up to 40 times depending on factors like stitch length, the thickness of sewing material, etc. Comparatively, the lower thread system wound on the bobbin case is simply pulled, and its part of the stitch. Due to excessive needle thread movement they experience more abrasion and mechanical strength loss compared to bobbin thread.

The shuttle is a carrier for the under-thread. It passes completely through the enlarged needle thread loop, or guides the needle thread loop around itself. 

Straight shuttle for lockstitch

Oscillating shuttle, with XL sized bobbin (right) for lockstitch

Rotatory Hook for lockstitch

The hook guides the needle thread loop around a stationary spool case. The key part for forming a stitch is the under thread spool and its two-part capsule.

The rotary hook is mounted on the horizontally placed shaft as well as for the vertically placed bobbin shaft. Be it horizontal or vertical, the main function is the same, but the vertical bobbin shafts are used for double-needle lockstitch machines.

For chain stitches

For chain stitches, the lower thread systems used are called the loopers. they come in various forms, shapes and have different forms of motions depending upon for which stitch class they are going to be used. The main function of the looper is to:

• catch the needle thread loop
  
• hold the needle thread loop in a way that the needle is able to stitch through it on the following cycle.

The loopers are also used with looper threads like in two-thread chain stitch. These loopers not only hold the needle thread in it for the next needle penetration but also supply looper thread for interloping with needle thread.

Looper

Oscillating loopers (Left) and rotating loopers (right) for Chainstitch

Chain stitch with needle thread and looper with thread (Class 400)

Stitch formation with looper [Groz-Beckert]