Chapter 2


Joint efficiency is the percentage of the measurement of strength when applied to the junction or fabrication of two or more materials. An example is the cross seam in a canopy gore where two panels of fabric are joined. The strength of the seam needs to be greater than the strength of the fabric. To achieve this, there are several factors that need to be considered in the design. These include the following:

• Fabric—The weight and weave of the fabric affects the type of junction used.
• Thread type—This is affected by the weight of the fabric. Generally, the lighter the fabric, the smaller the thread used. Accordingly, a smaller needle is used in order not to damage the weave of the fabric.
• Stitch type—This is determined by the type of seam needed for the design. For the French fell seam normally used in joining the panels of a canopy, the 301 straight stitch is used.
• Stitches per inch—This has a direct correlation to the size of the thread used and the stitch type. There is a fine balance between the security of the seam and overstitching. Too many stitches per inch will dramatically affect the strength of the seam by perforating the material. The number of rows of stitching also affects this. While more rows generally increase the strength of the seam, too many perforate the material as well.
• Thread tension—As lighter fabric and thread are used, the thread tension balance becomes more important.
• Reinforcing—The addition of reinforcing through the use of tapes, cords, etc., adds to the strength of the seam. However, their use may also reduce the elasticity of the seam at the same time.

Some of the previous factors also can affect heavier materials such as tapes and webbings. In working with webbings in harness design, most construction methods have tended to overbuild the junctions. This has been done primarily because the materials have readily accepted heavier threads and stitch patterns.

An area that needs to be addressed is that of restitching webbing. Until recently, there was not much study done to determine how much strength is lost in this process. G.S. Dunker, a parachute engineer, conducted a study that evaluated the variables introduced when restitching webbing junctions. Some of these variables included the following:

• The treatment or conditioning of the webbing. Condition R webbing has a resin treatment to make it stiffer as opposed to condition U or untreated webbing.
• The size and condition of the needle used in the sewing. Larger needles make larger holes. A blunt needle or one whose point is damaged will do more damage to the webbing and weaken it.
• The size of the thread used.
• The stitch pattern used and length. A W-W pattern is stronger than a box X pattern.
• The number of times the webbing is re-sewn.

All of these affect the ultimate strength of the webbing junction or stitch pattern. Taking into consideration the above variables, tests were conducted in measuring the strength of a simple lap junction. The results showed that there was little difference between the un-sewn sample and the first re-sewn test. The greatest difference was between the first and second re-sewn sample, with approximately an eight percent decrease in the strength of the junction. There was minimal drop in strength in succeeding tests. Initial conclusions were that if the disassembly process was done carefully with little damage done to the webbing, the re-sewing process had minimal effect on the ultimate strength of the re-sewn junction. The study is titled “The Prediction of Junction Efficiency for Parachute Harnesses: Experimentations in Breaking Strengths and Wear Characteristics,” Arizona State University, May 2001.

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