As outlined in our previous report on lifelines, high modulus polyethylene (HMPE) fiber rope has revolutionized sailboat rigging during the past decade (see Fiber lifeline Protection Plan, PS September 2015 online), however sailors arent the only ones benefitting from the introduction of this super-strong, low-stretch cordage. Virtually any application that once employed a stranded wire-rope-ranging from tow cables to hoists-is now also served by HMPE. The widespread availability of generic brands of this super-strong, low-stretch cordage made us curious. How do these non-marine brand products compare to known marine brands? And what types are best suited for the various sailboat applications?
One of the interesting conclusions from our testing was the surprising strength among all of the test fasteners when loaded in the shear. Even old fasteners, as old as 20 years or more, can continue to offer good service in small spars if maintained reasonably well and engineered with an adequate safety margin.
During the first round of mechanical terminal testing, testers were surprised to see that wire failures were occurring at loads that were at least 10-percent lower than expected. The swaged-end fittings had been applied by a rotary swage machine, which is considered to be a very reliable way to form swaged terminals.
The most common error with poured sockets is insufficient cleaning of the wire, and/or using the wrong solvent to clean it. Even dry, stainless wire, when new, can have a residue of lubrican't left over from the extrusion process. If this lubrican't is not removed, it can compromise the adhesion of the wire yarns to the filler material in the socket.
All of the mechanical terminals we tested are based on the same assembly concept: The rigger opens the cover yarns, and then slides a wedge over the core yarns, near the end of the wire. The rigger closes the cover yarns over the wedge, and then, screws together the two outer parts of the terminal over the wedge, compressing the material, and thus generating friction. Simple, but like any assembly, it can be done wrong. Here are a few things to watch out for when assembling a mechanical terminal.
Swageless wire-rope terminals have long been a favorite piece of rigging kit among all kinds of sailors. These terminals are inspectable, reusable, and can be assembled with simple hand tools. But for all of their acknowledged advantages, data is scarce about their mechanical efficiency. How much, if any, do they weaken the wire rope they are attached to?
Standing rigging has been evolving ever since natural fiber lines were lashed, knotted, or spliced to wooden spars, and led down to crudely fashioned, iron deck hardware. The challenge has always been the transition point where the straight-line pull (tension) changes direction, creating a stress riser in the rope, wire, or rod. Today, the materials have changed, but the challenge remains the same.