Page 54 - The Complete Rigger’s Apprentice

water, and then apply one drop of “One Drop,” from and stability under load. Nylon, for instance, is
the McLube company, to the bearing race. very cheap and light but can distort under heavy
Racing gear, and even the light-air gear on a load. Worse, it swells when wet, increasing sidewall
cruiser, demands the absolute minimum of weight friction. Delrin sheaves are more stable and nearly
as well as friction for maximum ease, speed, and as cheap, making a good medium-duty sheave.
smoothness of adjustment. Sheavologists have come Bronze, stainless, and aluminum sheaves are heavy
up with some exquisite variations on bearing themes and expensive but will stand up to extremes.
for these applications. Nowadays most use bearings
made from Torlon. You’ll find blocks with roller Sheave Shape A consideration peculiar to modern
bearings on the axle, and ball bearing races on the blocks is that of sheave score profile (Figure 2-15).
sidewalls, for thrust loads. Resistance under moder- It used to be that all sheaves had a semicircular
ate loads can get down around 2 percent. Schaefer’s profile, and this shape is still appropriate for three-
Circuit Sheave uses an NTE bushing for the axle strand and standard braided lines.
and keeps the thrust-load ball bearings, making for With wire-to-rope halyards, the long-accepted
a simpler, less expensive sheave with comparable standard was a semicircular profile with a notch cut
friction characteristics. out of the bottom. The semicircle was for the rope,
Plastic ball, needle, and roller bearings are the notch for the wire. Because serious loads only
extremely light and easy to maintain (just keep come onto these halyards after the rope part is clear
them away from solvents and grit, and rinse them of the sheave, the little notch doesn’t tear up the
regularly with fresh water), but they will distort fibers. All-wire halyards simply used a very skinny
under heavy, static loads. This is most often a prob- sheave with a semicircular profile. But for all-wire
lem with halyard sheaves for mainsails and jibs, a or part-wire halyards the modern V-groove sheave
good spot for metal roller bearings or perhaps an supports the wire much more completely, reducing
NTE bushing if weight is an anorexic issue. fatigue and the onset of “meathooks,” sharp stubs of
The “Air Block,” pioneered by Harken, is the broken wire yarns. And it’s also easy on fiber rope.
biggest innovation in rigging since the invention Most sheave-related problems occur when sail-
of sheaves. If I’m exaggerating, it’s only slightly. In ors change halyard materials or diameters without
this type of block, the axle hole has been expanded changing sheaves. Running an all-rope halyard
beyond all belief, leaving a big hole in the middle. over an old-style wire-and-rope sheave with a sharp
There is no axle at all; the sheave rests on a bed of edged notch is asking for trouble (when the sail is
bearings, which in turn rest on a rim. Because this fully hoisted, the rope will chafe away in the notch).
rim is so wide, there are a lot more bearings under A different problem can occur if you do replace a
the load than with a standard block, so the load is sheave to match halyard type, but the new sheave
distributed over a greater surface. This means that doesn’t fit snugly in the block or mast mortise. The
a higher load can be sustained with less friction, halyard can chafe on entrance and exit points or the
and with less distortion to plastic bearings. Many top of the mortise, or can jump out of the sheave
other manufacturers now make similar blocks, and groove and jam between the sheave and the mor-
competition is driving the development of simpler, tise wall, an eventuality that can be prevented with
cheaper designs. The improvement is so great, even “keeper bars” (Figure 2-16).
without taking weight savings into account, this has
become the standard block type. Sheave Size Related to the question of sheave
score profile is sheave size relative to rope diame-
Sheave Material Loads on the sheave itself are not ter; different materials are more or less susceptible
so concentrated as those on the bearing, but sheave to fatigue, and increased sheave diameter reduces
material can be important relative to cost, weight, fatigue by reducing the sharpness of the bend the

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