4.1 Seam engineering, the determination of the best stitch type, seam configuration, and thread type which should be used for a particular assembly, requires a thorough knowledge of many variables. The improper selection of any one component can result in failure of the sewn junction, and ultimately failure of the product manufactured.

4.2 General Characteristics-The characteristics of a properly constructed sewn seam are strength, elasticity, durability, security, and appearance. These characteristics must be balanced with the properties of the material to be joined to form the optimum sewn seam. The end use of the item will govern the relative importance of these characteristics. The selection of the seam type and stitch type should be based upon these considerations.

4.2.1 Strength-The seam efficiency of the sewn seam should be as high as possible. This will produce sewn seam strength with a balanced construction that will withstand the forces encountered in the use of the sewn item. The elements affecting the strength of a sewn seam are:

4.2.1.1 Fabric type and strength,

4.2.1.2 Seam type,

4.2.1.3 Stitch type,

4.2.1.4 Stitch density (spi),

4.2.1.5 Thread tension, and

4.2.1.6 Thread strength.

4.2.2 Elasticity-The elasticity of a sewn seam should be slightly greater than that of the material which it joins. This will enable the material to support its share of the forces encountered for the intended end use of the sewn item. The elasticity of a sewn seam depends upon:

4.2.2.1 Fabric type and strength,

4.2.2.2 Seam type,

4.2.2.3 Stitch type,

4.2.2.4 Stitch density (spi),

4.2.2.5 Thread tension, and

4.2.2.6 Thread elasticity.

4.2.3 Durability-The durability of a sewn seam depends largely upon its strength relative to the elasticity of the seam and the elasticity of the material. However, in less elastic, tightly woven or dense materials, there is a tendency for the plies to "work" or slide on each other. To form a durable sewn seam in these types of fabrics, the thread size must be carefully chosen. The stitch density also needs to be carefully determined for the material so as not to cause excess tension which will unbalance the elasticity and cause puckering. It is also important to minimize abrasion and wear by contact with outside agencies to promote durability.

4.2.4 Security-The security of a sewn seam depends chiefly upon the stitch type, spi, and its susceptibility to become unraveled. The stitch must be well set to the material to prevent snagging that can cause rupture of the thread and unraveling of certain stitch types.

4.2.5 Appearance-The appearance of a sewn seam generally is governed by the proper relationship between the size and type of thread, the stitch density, and the texture and weight of the fabric.

4.2.5.1 In addition to these general characteristics, the technique and skill of the sewing machine operators also govern the appearance of the sewn seams. Some of the factors which will adversely affect the appearance are shown in Table 1.