STITCHES PER INCH (SPI) - WHAT YOU SHOULD KNOW

TECHNICAL BULLETINFrom this formula, you can see the impact that stitches per inch, thread strength and stitch selectionhave on the strength of the seam. Generally, the more stitches per inch, the greater the seamstrength. There are rare cases where adding stitches per inch can actually damage the fabric so thatthe seam is weakened, however, this only happens on specific fabrics that can be damaged byexcessive needle penetrations.Given:- 301 Lockstitch Superimposed Seam- Thread Used Top & Bottom = T-24 Perma Core – Strength: 2.6 lbs.Estimated SeamStrengthSPI 6 8 10 1223.4 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 strengthof the seam, as long as the fabric doesn’t rupture before the thread. If the same seams were sewnwith a lower tenacity spun polyester thread, this also will effect the resulting seam strength.Given:- 301 Lockstitch Superimposed Seam- Thread Used Top & Bottom = T-27 Spun Poly – Strength: 2.2 lbs.Estimated SeamStrengthSPI 6 8 10 1220 lbs. 26 lbs. 33 lbs. 40 lbs.Some manufacturers substitute a smaller bobbin thread when sewing lockstitch seams to minimizethe number of times it takes the sewing operator to change the bobbin. Remember, however, thatthe resulting seam strength will be much less and will be determined by the strength of the bobbinthread and not the needle thread.How to Measure 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 make this measurement easier, however, you can placea ruler next to the seam and perform the same task.Figure 2. Stitch Counter Measuring SPISPI is measured by counting the number of lengths ofthread found within one inch. As you can see here,there are approximately 9 SPI sewn in this seam.Page 2 of 4