The 3-Ring System – Retain and Release Sub-System
The 3-Ring Riser Release System is sub tended by a retain and release sub-system complementing the load reduction task performed by the interaction of the rings. This sub-system consists of two retaining cables and housings. These components perform as a “Push / Pull” or “Boden Cable” retaining and releasing system.
The 3-Ring release cable is routed through the housing that is encased in the “yoke” of a harness and container system. The release cable retains the fabric loop, which emanates from under the risers’ confluence wrap; this is routed through a grommet mounted in an “eye” terminal, attached to the end of the housing. Withdrawal of the cutaway cable releases the fabric loop. The grommeted “eye” attached to the end of the housing, serves to keep the housing attached to the loop. This means that as the cable is pulled from the fabric loop, the loop, depending on the amount of friction between the cable and loop, will travel with the cable until a countering force is met. The edge of the grommet should provide such a force.
If the housing is compressible, as most 3-Ring housings are, the compression resistance of the housing must be greater than the force required to remove the cable from the loop. If it is not, the housing will compress until the imbalance of forces is reversed. As the compression increases, the resistive force increases; however, because the cable is not moving relative to the loop, the amount of cable pulled out of the other end of the housing increases and if combined to a second release point, this second point could release asymmetrically.
The metal housings most commonly used in the industry today are both compressible and extendible. They extend about 7. 5% and compress about 10%. This means that the long housing commonly used on the left side, which is about 40 inches in length depending on rig make, will compress 4 inches and stretch about 3 inches. This indicates a delta of 7 inches.
The short side housing is about 10 inches, again depending on rig make, yoke size, etc. This indicates a compression of 1 inch and a stretch of ¾” with a delta of 1.75″. If you add the deltas of both sides you get 8.75 inches. The worst case scenario for non-symmetrical cutaway would be equal to this delta of 8.75″. While
this worst case scenario is not highly likely, some significant amount of differential may be problematic . At what point does this occur?
Most manufacturers specify a 7-inch cable extension from the end of the housing. The end of the housing is approximately ½” from the center of the grommet. If we stack up all of the realistic numbers, we can calculate the maximum cutaway differential that could be anticipated on today’s systems. Suppose that the left side release cable grips the loop and locks up, 6.5″ from the end of the left side cable. The left side housing compresses 4 inches, and 4 inches of cable is pulled out of the right side housing, leaving 2.5 inches engaged. Even if the right side is extended .75 inches it still has 1.75 inches engaged. Therefore is apparent that a single sided cutaway is far less likely using metal housings.
For non-metal or tape/webbing housings all bets are off. Systems incorporating “Soft Housings” are subject to fail under extreme asymmetrical loads due to near infinite compressibility, and their failure to provide a “push.” This may result in the off chance of one side releasing, leaving the other side locked up with a now exacerbated load. This is apparently what happened to U.S. Team Leader, Ernie Butler in a near fatal accident that left him partially paralyzed.
This examination reveals the inherent asymmetry in even properly built 3-Ring systems. The system could be improved through the use of extendible, non-compressible housings. Jump Shack has located a supplier for this type of housing and is in the process of incorporating them “across the board” on our complete product line. We hope to have this augmentation completed by January 1, 1998. Additionally, the “slickness” of the cable and the friction to the loop is of major consideration. Current practices require lubrication of the yellow Lolon cable (Lolon is the brand name of the familiar Nylon-based product), on a weekly or monthly basis. To further address the problem, Jump Shack has developed a red-colored, pure FEP Teflon coated cable, which does not require lubrication. This cable reduces cable pull effort by 50% over unlubricated Lolon.
Our erudite opinion is that soft housings are simply “not an option.”
Parachute Labs, Inc. d.b.a. Jump Shack