Page 53 - The Complete Rigger’s Apprentice

bushing
bearing race

reinforcing
strap

aluminum
cheek
Figure 2-14. A Schaefer block with N.T.E. bushing
and thrust-load ball bearings..

Bearings The simplest axle bearing, called a
“bushing” or “plain bearing,” is essentially a rein-
forced hole. It is closely matched to axle size to assure
the broadest distribution of load on the pin, and thus
less friction and wear, as in Figure 2-14. Today’s
Figure 2-13. A Schaefer block. slickest (3 to 5 percent), most expensive bushings
are made from an epoxy-coated carbon fiber–rein-
back to blocks, get the lightest ones that are con- forced blend of Nomex and Teflon (N.T.E.). This
sistent with your safety factor, your quality-control exotic mix is durable, stable under load, and actually
analysis, and your wallet. gets slicker with use, as bits of it are smeared onto
the axle. A nylon bushing is too soft and sticky for all
The Heart of the Block but the lightest loads. A fiberglass bushing is harder,
The sheave, that little grooved wheel, that axle-trans- but not nearly as hard or as distortion-resistant as
fixed puck, is the heart of the block. It exists to direct N.T.E. Delrin, an acetal resin, makes a good, inex-
and share the load on a line. But every time a line pensive, medium-duty bushing. And bushings made
runs over it, friction siphons off some of the force from oil-impregnated bronze are another good, low-
that the line is trying to deliver. The amount of fric- tech option. They’re not as slick as the plastics (5
tion at design load level can vary from 2 to 10 per- percent), but hold their shape well under static loads.
cent per sheave, depending on bearing efficiency. For
a 100-pound load, this translates into 8 to 40 pounds More Elaborate Bearings Ball, pin, or roller bear-
of extra force you must exert to overcome friction ings reduce friction by reducing the surface area of
in a typical four-sheave mainsheet. Technology has contact with the axle and/or block sides, and by roll-
proven most useful in the effort to minimize friction. ing instead of sliding under the load (Figures 2-12
Sheave friction is generated at four bearing and 2-13). Bronze roller bearings, still available on
points: where the rope passes over the grooved edge; some traditional blocks, are simple and bulletproof
the two sides of the sheave where they rub against enough for prolonged deep-water use, yet slick
the side of the block, and where the hub of the enough (3 to 5 percent) for the performance-minded.
sheave bears against the block’s axle. Given a fair But these days you are more likely to find roller bear-
lead and an adequate-diameter sheave (see below), ings made of Torlon, a hard, slick, durable plastic.
it is the axle bearing that usually generates the most Maintenance aside: to keep your plastic ball bear-
friction (Table 2, page 34). ings running free, rinse them occasionally with fresh

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