LIFE BELTS and HARNESSES— their strength lies in stitching
features
EQUIPMENT
A number of considerations must be taken into account when testing life-saving equipment, not the least of which are the materials and methods of manufacturing.
Without any officially set criteria for the safety and use of fire service life belts and harnesses, the onus of ensuring the equipment’s top performance rests with the firefighters themselves.
TESTING PROCEDURES
The tensile strength of webbing, rope, and harnesses is the minimum force in pounds of the materials’ resistance to rupture and longitudinal stress. Such a strength test, done preferably by an independent testing laboratory, is made on a standard testing machine. Strengths should be certified by manufacturers, and independent laboratory proof of such strengths should be requested by purchasers.
Webbing for harness construction is available in various strengths, widths, and colors. A well-constructed harness made of webbing with a tensile strength of 6,000 pounds may withstand a test load of 8,000 pounds when the load is evenly distributed to all areas of the harness. However, with poorer construction and stitching, the harness constructed of 6,000-pound tensile strength webbing may withstand a load of only 4,000 pounds.
When making a strength test of a harness, all areas of the harness (stitching, leg or butt straps, and hardware) must be equally subjected to the test loads and tested as it would be worn during actual usage.
The three tests of rappelling equipment used in the New York City Fire Department prior to purchase are the static, dynamic, and performance tests.
Using a suitable manikin during the static test, the equipment is pulled until breakage occurs. Distortion of hardware, breakage of stitching, and other preliminary failures are observed and the points of failure are recorded. The static test provides assurance that materials used in the construction of equipment are according to specifications.
Following a satisfactory static test, dynamic strength test are made on a second sample of the same type and lot of equipment. This test proves the capability and safety of the equipment during the most severe and adverse conditions. With the use of a manikin, a 500-pound weight, and a lanyard providing a two-foot slack, the weight is made to fall free and the harness or belt absorbs the dynamic load. The equipment must survive a minimum of five such drops of the weight without failure of any area including stitching. All test specimens are never worn afterward by anyone for any reason.
The performance test requires firefighters to use the equipment as they would during actual usage. Rappels, lowerings, and pickups are made under controlled safety conditions for the purpose of discovering inherent problems or discomfort. Areas of discomfort must be evaluated by several persons to determine the degree of discomfort and possible side effects. The performance test assures the practicability and comfort of the equipment.
STITCHING STRENGTHS
In the manufacturing of life belts and harnesses, various methods of sewing may be found. One method is to apply a large number of random stitches over a seam. Because of the quantity of thread used, a person may believe that the equipment is strong and therefore safe to use. Do not use equipment that has been stitched in this manner, and, if found, it should be destroyed before serious injury occurs.
Seam and stitch strengths depend on the following characteristics:
- stitch type (the lock stitch is the only stitch recommended in the manufacture of safety equipment)
- thread strength
- stitches per inch
- thread tension
- pattern type (double “W”; boxed cross)
- seam efficiency of the material
As can be seen in figure 1, the lock stitch is formed with two threads. Most important, it cannot become unraveled. The needle thread (top thread) must always be size 6; any smaller would diminish the thread strength, any larger would cause the thickness of the needle to damage material fibers as it completes a stitch. The bobbin thread (bottom thread) should be either size 6 or size 5 (one thread size smaller). As shown in figure 1, the top thread must be pulled upward so that the interlaced threads are midway between the surfaces of the materials being sewn.
Stitching of safety equipment must have between four and seven stitches per inch. Where there are more than seven stitches per inch, the desired elasticity of the stitching will be adversely affected and the possibility of severed material fibers may occur. Where there are less than four stitches per inch, the threads are susceptible to snags and breakage.
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The durability of the stitching greatly depends on the thread tension during sewing. To minimize abrasion and wear, thread loops must be level with the flat surface of the material. Too much tension can produce weakened thread, uneven stitching, and damaged material. Too little tension will cause raised and skipped stitches.
The end or finish of all stitching must be backstitched at least one-half inch. Stitching must not be less than onefourth inch from the edge of the material being sewn.
The patterns most commonly used for safety equipment are the double “W” (figure 2) and the boxed cross designs (figure 3). The double “W” pattern is widely accepted as stronger and more preferable than the boxed cross pattern.
The boxed cross pattern provides more than adequate strength for use in safety equipment. Where lateral strength as well as longitudinal strength is desired, the boxed cross may be more preferable than the double “W”.
The use of rivets to reinforce sewn areas has been questioned. Rivets increase the strength of these areas, providing that reinforcing burrs and washers are incorporated in the process. Rather than first providing holes for the inserting of rivets, the rivets must be inserted directly into the material so that material threads are not broken. Where hardware is to be attached, sufficient material must be allowed to provide a stitched pattern of at least three inches in length, (figure 4).
When an overlapped area is stressed, as shown by the shaded arrows in figure 4, the material provided for the overlap slides in the direction of the white arrow. It is this movement that causes the threads to break, beginning at point “A” and progressing toward the “D” ring. When rivets are used, the tendency of the overlapped material to slide is halted until the rivets pull out. The overall result is increased strength of the stitched area.
A variation of the boxed cross pattern (see figure 5) will also provide approximately 140 stitches to the given area. Test results show that this variation is equally as strong as the double “W” pattern, if not less conducive to webbing failure. For best results, a minimum of 120 stitches and a maximum of 150 stitches should be used in a seam measuring 3 1/2 X 1 3/4 inches.
For the most efficient seams, the elasticity of the stitching should be greater than the elasticity of the material being sewn. In this way, the material will support its share of the dynamic loading that can be experienced when using life belts and harnesses.
Threads used in seam strength tests were:
Seam strength tests
A strip of webbing measuring 1 3/4 X 18 inches was used for each test. To attach the D rings, a four-inch overlap of the ends of webbing was made and the length of each stitched pattern was three inches. The tensile strength of the webbing was 9,000 pounds.
Test 1
Size 6 thread was used for the top and bottom threads in a comparison of strengths of the three point double “W” and boxed cross patterns (see figure 6).
- At 3,000 pounds, bottom stitching of the double “W” pattern started breaking.
- At 3,500 pounds, bottom stitching of the boxed cross pattern started breaking.
- At 4,000 pounds, bottom stitching of both patterns broke progressively from the ends of the overlap toward the D rings.
- At 4,500 pounds, the stitching of the boxed cross pattern failed (see figure 7).
- The top stitching of both patterns was undamaged.
Test 2
In a comparison of thread strengths, the same patterns were used but with size 6 thread for the top threads and size 3 thread for the bottom threads.
- At 1,800 pounds, the bottom stitching of the boxed cross pattern started breaking.
- At 2,400 pounds, the bottom stitching of the boxed cross pattern failed completely.
- At 2,500 pounds, the size 3 thread of the double “W” pattern started breaking.
- At 3,100 pounds, there was complete failure of the size 3 thread of the double “W” pattern (see figure 8).
From these tests it can be said that the three-point double “W” pattern is slightly stronger than the boxed cross pattern. Also, stitching will achieve higher breaking strengths when used in a double “W” pattern than when used in a boxed cross pattern.
It was recommended in the March 1984 issue of FIRE ENGINEERING that the minimum strength of a harness or life belt be 5,000 pounds. Stitching that begins to fail at 1,800 to 2,500 pounds may not provide even 4,000 pounds of equipment strength. To achieve higher strengths, visible stitch failure should not occur until approximately 3,000 pounds during static tests.
Test 3
Since threads break due to the sliding effect of the overlapped webbing, the following test was conducted. Opposing double “W” patterns were used. Unlike the three previous tests where the overlapped webbing received the bottom thread, the overlapped webbing now received size 6 top thread. The main body of webbing received size 5 bottom thread.
- At 3,000 pounds, the size 6 thread started breaking in both patterns.
- At 4,000 pounds, the size 6 thread started breaking rapidly in both patterns.
- At 4,600 pounds, there was comiplete failure of the thread in one of the patterns.
- Both top and bottom threads had breakage.
From this test it can be said that the sliding effect of the overlapped webbing causes breakage of thread on the overlapped side of the seam, even when the thread is stronger than its interlaced thread.
Test 4
In this test, opposing boxed cross patterns, sewn with size 6 top thread and size 5 bottom thread were used. Rivets were positioned as shown in figures 4 and 9.
- At 3,500 pounds, stitching started to break at the ends of the overlapped seams.
- At 5,500 pounds, stitching had broken only as far as the first rivet in both seams.
- At 5,900 pounds, there was sudden and complete failure of one seam.
Although rivets increased seam strength in the testing, rivets will adversely affect a seam of peak efficiency. The present state of the art methods of stitching may require the installation of rivets to provide breaking strengths that comply with some specifications. In the establishment of standards for fire service rappelling equipment, the use of rivets may be permitted for use only in strengthening the attachment of hook handles and accessories.
Test 5
In this test, a comparison of strength was made of the boxed cross patterns shown in figures 3 (standard pattern) and 5 (modified pattern). Size 6 thread was used for top threads and size 5 thread was used for the bottom threads.
- At 4,000 pounds, the bottom stitching of the modified pattern, which is presently used in the fire service, started breaking.
- At 4,500 through 5,000 pounds, the bottom stitching continued to break progressively toward the “D” ring.
- At 5,400 pounds, there was complete failure of the presently used pattern. There weren’t any broken stitches in the modified pattern (see figure 12).
From this test it may be said that the combination of size 5 bottom thread and size 6 top thread developed a higher seam strength than the seams having size 6 top and bottom threads. This is consistent in achieving seam efficiency where the elasticity of stitching should be greater than the material being sewn.
Test 6
Where the highest level of seam efficiency is reached, the material should break before the stitching fails. In this test, a comparison of strengths of the four-point double “W” pattern and the modified boxed cross pattern was made. Size 6 thread was used for the top threads and size 5 thread was used for the bottom threads.
- Stitching did not begin to break until the force of 5,000 pounds was reached.
- There was very little stitch failure until the webbing exploded at 7,300 pounds pull. Stitching at the seam edge of the four-point double “W” pattern apparently caused the webbing fibers to separate. There wasn’t any appreciable damage of threads in the modified boxed cross pattern.
As the tests show, seam efficiency can be attained through the use of proper thread size and tenacity in conjunction with the type of pattern selected. Webbing should be spiral weave construction, having a tensile strength of at least 6,000 pounds. Thread color must contrast to that of the webbing to facilitate thread inspection.
Stitching can be applied either manually, with the sewing machine operator controlling the material as it’s fed past the needle, or the direction of needle movement can be controlled by a robot mechanism. The modified boxed cross pattern can be applied manually, as that particular pattern is prone to needle breakage and missed stitching.
A sewing machine with a robot mechanism to apply four point double “W” patterns is very efficient, although it costs the manufacturer several thousand dollars for such equipment. When stitched automatically, the stitching pattern is uniform throughout the seam area. When applied manually, the uniformity of stitching and pattern design will vary with the sewing machine operator.
With increased demands being placed on our rescue safety equipment, we can’t afford to overlook any area of either the manufacturing or training in the use of life belts and harnesses. Though certification of equipment strength and workmanship may be obtained when dealing with reputable manufacturers, a more thorough equipment check is our own responsibility. The more we know about equipment the more quality control we can expect from our suppliers.