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Cause Of Type 17 Riser Failures Identified
John Sherman of the Jump Shack has just completed an extensive study which has culminated in a conclusive explanation of the various types of failure modes of Type 17, 3-Ring risers. Sherman, parachute engineer and designer of the Racer line of harness/container systems, revealed the findings of his study at the International Parachute Symposium and Riggers' Conference in Orlando, Florida, in March, 1993.
The study included but was not limited to pull testing risers manufactured by various companies. Math models were also created and analyzed using sophisticated computer programs.
A critical aspect of Shermans' findings is that the failures have less to do with the strength of the webbing itself and more to do with mechanical advantage or disadvantage of the lever and pulley systems of the 3-Ring release. Two problems are actually occurring. The most evident, the failure of the webbing just above the middle ring, is caused by the guillotining of this webbing during load onset. The second failure mode, the most dangerous, is that improper ring geometry causes the "lever" to become inefficient resulting in the yellow cable being pulled through the "0" grommet. "We can easily predict either scenario with simple observation of the assembled ring set and by taking a few simple measurements," says Sherman.
The most important result of the study is that the ultimate strength of mini-ring 3-Ring system has been identified. No matter what type of webbing is used, the mini 3-Ring is limited to about 3600 lbs. Either the yellow cable pulls through the grommet, or the rings themselves fail when the riser is exposed to 3600 lbs. In light of the above limitations, Type 17 webbing, with a strength of 2500 lbs., is an excellent match with the mini-rings. This combination will produce a riser with a total strength of 3600 lbs. if all conditions of design are met.
Normal parachute openings generate shocks in the range of 2-4 G's. With a suspended load of 200 lbs., this equates to 400-800 lbs. A shock load greater than this is considered aberrant. Such loads can be identified by inspection of the middle ring where it contacts the harness ring. During aberrant loads "Brinelling" or denting of the middle ring occurs. Risers which have failed in the field and been returned show considerable evidence (dents)
of repeated hard openings. If severe denting is encountered, the equipment should not be used until the cause for these hard openings is identified and resolved.
This problem, which we now define as an opening shock problem not a riser failure problem, originally begged for solution by making a stronger riser. The industry originally responded to this problem by trying to increase the strength of the riser.
The implications of simply beefing up (strengthening) the riser system is that the shock load or energy will be passed on down to the harness, making harness stitching more vulnerable. There has been a harness failure with type 8 risers in Germany resulting in the jumper falling out of the harness. Inspection of the failed harness revealed no defects in the harness webbing, the harness thread, or the manufacturing process. While the design of the harness may be inadequate (it is not designed to "FAIL SAFE"), it is in fact a design which is in common use. With this consideration in mind, the Jump Shack urges all manufacturers and governing safety bodies to consider the potential carnage which might have and still can occur without the "fuse link" function of the Type 17 Riser. This riser has acted as a critical fusible link in the opening force chain. Some type 17 riser failures have no doubt prevented what would have been harness failures. It is believed that some of the opening shocks generated were in excess of 15 G's which severely injured the jumper. Now with the riser beefed up or replaced with what is thought to be stronger risers, will we get more harness failures on harnesses which are not designed to be "FAIL SAFE?"
Thicker webbing or reinforcement of existing webbing may adversely affect the geometry of the rings. Although thickening or rolling the webbing will prevent the scissoring/ guillotine action between the rings, tolerances must be closely held so as not to interfere with proper function of the release system.
After seeing the failure of the yellow cable, additional tests were performed to identify the local forces required to pull the cable through the grommet. That force when compared to the load on the riser when the failure occurred will give the total mechanical advantage of that particular set of rings. Results of calculations done using these dimensional relationships compare favorably to direct relationships referenced above.
Our tests show no failures of the webbing at the grommet hole location. The grommet hole location has one third of the webbing removed from the center of the webbing to insert the grommet. On Jump Shack manufactured risers, this area, is comprised of 2 thicknesses of type 17 (2500 lbs. each) and two layers of 1" type 4 (1000 lbs. each). If the simple math of adding the strength of the layers and subtracting the strength portion of the removed area is applied we get a predicted potential strength of 4600lbs. Our design also includes a special hole preparation method and reinforcement stitching which apparently work.