Chapter 2
JOINT EFFICIENCY
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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