SPI (Stitches Per Inch)

The number of stitches per inch can have a direct influence on the following: 1) the seam strength; 2) the stitch appearance; and 3) the seam elasticity on stretch fabrics.

An estimated seam strength formula was developed years ago for woven seams where one piece of fabric is placed on top of another with a specific seam margin and stitched with either 301 lockstitches or 401 chain-stitches.

Obviously, the seam strength is dependent on a number of factors including:

           

·         Type & Weight of the Fabric

·         Stitch & Seam Construction

·         Stitches per Inch

·         Thread Type and Size

·         Stitch Balance (Thread Tensioning)

Formula for Estimating Seam Strength on Woven Fabrics

301 Lockstitch - Estimated Seam Strength

= SPI X Thread Strength (lbs.) X 1.5*

* 1.5 is a factor based on the average loop strength ratio of most sewing threads.

= 10 SPI X 4.0 lbs. X 1.5 = 60 lb. strength

401 Chain-stitch – Estimated Seam Strength

= SPI X Thread Strength (lbs.) X 1.7 *

* This factor is higher than a lockstitch because almost twice as much thread is consumed per inch of seam using a chain-stitch.

= 10 SPI X 4.0 lbs. X 1.7 = 68 lb. strength

From this formula, you can see the impact that stitches per inch, thread strength and stitch selection has on the strength of the seam. Generally, the more stitches per inch, the greater the seam strength. There are rare cases where adding stitches per inch can actually damage the fabric so that the seam is weakened, however, this only happens on specific fabrics that can be damaged by excessive needle penetrations.

Given:

- 301 Lockstitch Superimposed Seam

- Thread Used Top & Bottom = T-24 Parma Core – Strength: 2.6 lbs.

SPI	6	8	10	12
Estimated Seam Strength	23.4 lbs	31.2 lbs	39 lbs	46.8 lbs

As you can see from the chart above, the stitches per inch has a tremendous impact on the strength of the seam, as long as the fabric doesn’t rupture before the thread. If the same seams were sewn with a lower tenacity spun polyester thread, this also will affect the resulting seam strength.

Given:

- 301 Lockstitch Superimposed Seam

- Thread Used Top & Bottom = T-27 Spun Poly – Strength: 2.2 lbs.

SPI	6	8	10	12
Estimated Seam Strength	20 lbs	26 lbs	33 lbs	40 lbs

Some manufacturers substitute a smaller bobbin thread when sewing lockstitch seams to minimize the number of times it takes the sewing operator to change the bobbin. Remember, however, that the resulting seam strength will be much less and will be determined by the strength of the bobbin thread and not the needle thread.

Measuring the Stitch Length or Stitches Per Inch

The stitch length is measured by measuring the number of lengths of thread found within one inch. Stitch counters are available from A&E that makes this measurement easier; however, you can place a ruler next to the seam and perform the same task.

SPI is measured by counting the number of lengths of thread found within one inch. As you can see here, there are approximately 9 SPI sewn in this seam.