THE SHELLNUT: TECHNICAL
Skewed placements in regular constriction cracks
Whilst it is reasonably clear that the three-dimensional design of the Shellnut is advantageous in cracks having side flare, it is less obvious that the Shellnut will generally provide more secure placements than conventional nuts even in what appear at first sight to be near-textbook convergent cracks. Yet experience over many years use suggests this is the case. As part of developing the DOWNBEAT Website, research was undertaken to investigate if this apparent experience was really true, and attributable to any unique feature of the Shellnut. What has emerged are findings of fundamental importance to wedge nut design, and for the reader with an interest in such matters, a summary of the findings is included below.
A fall protection wedge nut placed in the mouth of a constriction crack will be aligned, as near as possible, with the anticipated direction of loading, which usually (but not necessarily) will be close to the vertical fall path. With such a crack, when of apparent near-textbook proportions, the climber might reasonably expect a plane-sided nut of matching taper, or a curved nut of varying taper, to provide a solid nut placement. However, this is not always the case, because there is another Important factor influencing the quality of the placement, namely, the orientation of the crack’s principal axis within the rock face. If this axis is not aligned with the placement direction, (and understandably there is often the wrong presumption by the climber that it is), then the placement direction will be skewed relative to the axis of the crack, and a two-dimensional nut, be it plane-sided or curved, will not be a good fit, because with increasing amount of skew, rock-to-nut contact will progressively decrease to become mere point contact on opposed nut corners, resulting in an unstable placement. The outcome for the climber is a placement that ‘on the face of it’ is expected to be secure, perplexingly turns out otherwise. (See footnote).
By contrast, the three-dimensionally effective Shellnut automatically accommodates the above described skew condition, to the extent that the climber is freed from conscious awareness of its presence, because provided the nut is being pulled into a generally convergent part of a crack the placement will be sound, irrespective of skew.
Shellnut attachment to sling
In the mid 1980’s development of the Shellnut, threaded soft-steel wire was used for the nut-to-sling attachment, the single-wire ends being joined by bundling together within the nut’s hollow side. Though functional, the wire joint was unsightly, and as part of a recent review, alternative means of attachment were considered, the first being to sew the sling directly to the Shellnut with strong thread. Whilst this improved the connection appearance, without detriment to placement/removal facility, it was found in strength testing that the sewing failed at a load well below that subsequently attained by the Shellnut and sling, sewing failure being observed in the strength tests to result from the sling’s natural tendency (and ability) to straighten under high tension away from its no-load placement/removal profile on the Shellnut. Since it was considered unacceptable to permit sewing attachment failures at loads which were in the region of those developed in a big fall, (though well below the Shellnut/sling rated strength capability), and it was considered retrogressive to introduce design changes compromising the latter, some means of resilient attachment appeared to offer the best solution, giving optimum usage handling, but also elastically accommodating the sling’s flexural straightening up to rated load, before recovering, unscathed, to the unloaded state. These concepts were designed and incorporated into the tested and proven Shellnut/sling attachment illustrated in Shellnut “In Detail” Webpage photos
For further information on skewed placements please read linked technical article the not-so-simple wedge jamming crack