Fabric Club Of BD: Oct 27, 2010

SHEDDING

Labels: Weaving Manufacturing Technology | at 10/27/2010 11:42:00 PM

Object of Shedding Mechanism

A shedding mechanism separates the warp threads into two layers or divisions to form a tunnel known as “shed”. The shed provides room for passage of the shuttle. A shed may be formed by means of tappets, cams, etc.

Types of Tappet Shedding Mechanism
      Generally there are two types of shedding:
      1. Negative shedding
      2. Positive shedding
      In plain looms, tappets are used to for m sheds.

Negative Tappet Shedding

In a tappet shedding mechanism, if the tappet controls only one movement, either an upward or downward movement of the heald shafts, then the shedding is known as “negative tappet shedding”. The heald shafts are returned by some external devices like springs, dead weights, rollers, etc.

Positive Tappet Shedding

In a tappet shedding mechanism, if the tappet controls both upward and downward movements of the heald shafts, then the shedding is known as positive tappet shedding.

Examples :

1. Jamieson’s tappet

2. Barrel tappet and

3. Ordinary tappet

Tappets and Cams

These are irregular metallic pieces used to produce an up-and-down motion in followers and levers. The up-and-down motion is obtained by giving rotary motion to these pieces. If the follower and lever are required to get a continuous up-and-down movement, a cam or wiper is used. If the follower and lever are required to produce up-and-down movement with regular intervals of rest, tappets are used. Figure 1 shows a pair of tappets and a cam. There are specific portions in tappets that correspond to “dwell” periods, i.e. regular intervals of rest for the major parts involved in the motion.

A pair of tappets Cam

Figure 1: Tappets and Cam

BASICS OF WEAVING-1

Labels: Weaving Manufacturing Technology | at 10/27/2010 06:01:00 PM

Introduction

The process of producing a fabric by interlacing warp and weft threads is known as weaving. The machine used for weaving is known as weaving machine or loom. Weaving is an art that has been practiced for thousands of years. The earliest application of weaving dates back to the egyptian civilization. Over the years, both the process as well as the machine have undergone phenomenal changes. As of today, there is a wide range of looms being used, right from the simplest handloom to the most sophisticated loom.

In this rang, the most widely prevelant loom, especially with reference to India, is the ubiquitous “plain power loom”. In this and in the chapters that follow, the various mechanisms associated with the plain power loom are discussed in elaborate detail.

Basic Mechanisms in a Plain Power Loom

In order to interlace wrap and weft threads to produce a fabric, the following mechanisms are necessary on any type of loom:

Primary mechanisms
Secondary mechanisms
Auxilliary mechanisms

Primary Mechanisms

These are fundamental or essential mechanisms. Without these mechanisms, it is practically impossible to produce a fabric. It is for this reason that these mechanisms are called ‘primary’ mechanisms. The primary mechanisms are three in number.

Shedding mechanism
Picking mechanism
Beat-up mechanism

Shedding mechanism

The shedding mechanism separates the warp threads into two layers or divisions to form a tunnel known as ‘shed’

Picking mechanism

The picking mechanism passes weft thread from one selvedge of the fabric to the other through the shed by means of a shuttle, a projectile, a rapier, a needle, an air-jet or a water-jet. The inserted weft thread is known as “pick”.

Beat-up mechanism

The beat-up mechanism beats or pushes the newly inserted length of weft thread (pick) into the already woven fabric at a point known as “fell of the cloth”. These three mechanisms namely shedding, picking and then beat-up are done in sequence.

Secondary Mechanisms

These mechanisms are next in importance to the primary mechanisms. If weaving is to be continuous, these mechanisms are essential. So they are called the ‘secondary’ mechanisms. They are:

Take-up motion
Let-off motion.

Take-up motion

The take-up motion withdraws the cloth from the weaving area at a constant rate so as to give the required pick-spacing (in picks/inch or picks/cm) and then winds it on to a cloth roller.

* 5 Wheel take up motion.
* 6Wheel take up motion.
* 7 Wheel take up motion.

Let-off motion.

The let-off motion delivers the warp to the weaving area at the required rate and at constant tension by unwinding it from the weaver’s beam. The secondary motions are carried out simultaneously.

* Positive Let-off motion
* Negative Motion.

Auxilliary Mechanisms

To get high productivity and good quality of fabric, additional mechanisms, called auxilliary mechanisms, are added to a plain power loom. The auxilliary mechanisms are useful but not absolutely essential. This is why they are called the ‘auxilliary’ mechanisms. These are listed below.

Warp protector mechanism
Weft stop motion
Temples
Brake
Warp stop motion (Predominantly found in automatic looms)

Warp protector mechanism

The warp protector mechanism will stop the loom if the shuttle gets trapped between the top and bottom layers of the shed. It thus prevents excessive damage to the warp threads, reed wires and shuttle.

Weft stop motion

The object of the weft stop motion is to stop the loom when a weft thread breaks or gets exhausted. This motion helps to avoid cracks in a fabric.

Temples

The function of the temples is to grip the cloth and hold it at the same width as the warp in the reed, before it is taken up.

Brake

The brake stops the loom immediately whenever required. The weaver uses it to stop the loom to repair broken ends and picks.

Warp stop motion

The object of the warp stop motion is to stop the loom immediately when a warp thread breaks during the weaving process.

History of Textiles & Tertiary Motions on loom

Labels: Weaving Manufacturing Technology | at 10/27/2010 01:01:00 PM

The weaving is a process of formation of fabric with interlacement of two or more sets of yarns using a stable machine called loom. Human beings have started using the woven fabrics since the drawn of history. If we exclude the stone-age period, we may conveniently say the history of civilization is also, to some extent, the history of weaving. Aitken says there is evidence that the Egyptians made woven fabrics over 6000 years ago. Though primitive civilizations used coarser threads to make fabrics which were crude and coarse, there are references of fine fabrics made from filament of silk in China. Silk was one of the most important products in China 4000 years ago.

Earlier version of power-loom was run by two men. Fortunately steam power was available by 1765. Soon power-loom were driven by steam and most of the wooden parts were replaced with iron. After the steam engine and cast iron in early 1800, great attention was paid to increasing productivity of the machine. To help achieve the increase in productivity, William Radeliffe patented a dressing frame in 1803 for sizing and drying the warp threads prior to winding on to a weavers beam. Fast development in the loom took place and by 1821 there were over 50,000 looms in operation in some 32 mills in the north of England. In just over 10 years from that date, the number had increased to some 1, 00,000 and the basic loom had almost developed to the machine we know today. Also between 1819 and 1842 the average speed of the power-loom had increased from 60 to 140 picks per minute with the rise on productivity, as a result England became world's richest industrial power.

Traditional looms then were stopped every few minutes in order to replace the empty weft pirns or cop in the shuttle and this limited the number of looms, a weaver could operate to about four. James Northrop, an English man who immigrated to America and worked for the Draper Corporation, completed an automatic weft transfer system which replaced the weft pirn in the shuttle without slowing or stopping the loom in 1889. This mechanism enabled the weaver to tend 16 looms. The Northrop Automatic looms quickly came to use in America, so that by 1930, 90% of the American looms were automatic compared with only 5% in Britain.

Similar developments took place elsewhere also, Ruti, a major loom maker of Switzerland manufactured automatic bobbin changing Northrop loom in 1898. In Japan also, Toyoda, Sakamoto, Tsudakoma, etc also developed shuttle looms with automatic weft transfer. After World War II, more productivity and efficiency were essential to overcome increasing labor costs in Western countries. It was also realized that more productivity is the key to reducing manufacturing costs of the loom. All attempts were concentrated to studying various factors affecting speed of the loom and the loom with higher speed were made available.

Chronological background of auxiliary motions in brief:-

· The automatic loom stopping system was invented by R. Miller in England in 1796. The loom was automatically stopped when a short pick occurred.

· In 1894 Northrop devised a means for automatic weft replenishment without stopping the loom which was called automatic loom.

· In 1786 Dr. Cartwright foresaw the need of a warp stop motion. And the warp stop motion was invented by other parties afterward. But no satisfactory and commercially successful method of stopping the loom on the breakage of a warp thread has been developed until 1900.

· In early 1900’s, the warp stop motion was successfully introduced in Northrop looms.

· The first shuttle change motion enabling weft threads of different colors to be inserted was constructed by J. P. Reid and T. Johnson in 1835.

· The protector motion was first introduced in a loom manufactured by Richard Roberts in 1830.

· The first self acting loom temple was invented by Ira Draper and was introduced in Northrop Draper Loom.

There are three types of weaving loom motions. These are Primary, Secondary & Tertiary motion. Primary & Secondary are the basic motion of any loom. But there are various types of tertiary motion which is generally depend on the loom type. For produce the good quality fabric loom this tertiary motion is very important. All the loom stop motion (warp stop, weft stop, warp protector etc) & others important auxiliary mechanism (temple, break, selvedge etc) includes in tertiary motion.

The invention of various shedding, picking, beating, let-off and take up mechanism help to increase productivity undoubtedly. But they all depend upon the tertiary mechanisms. Besides the innovation of different tertiary mechanisms make the task easier. Quality control is even better than the earlier. The productivity increases as well as the efficiency and accuracy.

The importance of tertiary motion is given below;

· To minimize warp breakage fabric fault.

· To produce the better quality fabric.

· To minimize the filling yarn breakage.

· To protect the reed & warp.

· To maintain the warp & weft tension.

· To remove the weft mixing & replenishment fault.

· To increase production efficiency & accuracy.

Advantages of the auxiliary mechanism are given in below;

· A drop wire assembly, one wire for warp yarn, to stop the machine when a warp end is slack or broken.

· A tension sensing and compensating whip roll assembly to maintain tension in warp sheet.

· A mechanism to stop the machine when a filling yarn breaks.