Sheave
Sizing
**Please
read all of the sizing specifications below in order to
provide the correct dimensions for each different size
sheave.** |
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| Materials |
I
use Black Acetal (also known as Delrin) to make my sheaves.
It is the most durable, cost effective material for the application
that
I have found. I press in bearing bronze bushings to take the pin
load and wear. Bearing bronze is oil impregnated cast bronze. It
is self
lubricating and does not corrode like straight bronze or brass. I
use minimum 1/8” wall bushings for strength and longevity.
Bushings are standard sizes to facilitate future replacement.
Sheaves are grooved
for specific line size. (5/16”, 3/8”, etc.)
Sheaves can be deceptively simple looking to measure. Please read all of the
following very carefully. When measuring for your new sheaves, take your time.
Be sure to measure and examine each sheave box, be it a block, a masthead, boomend
or whatever. Do not assume any measurement without checking it. |
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| Sheave
Boxes |
Many sheave boxes on blocks, mastheads or boomends are not
cut true or square, resulting in the sheave binding at some
point. This is very common, even on otherwise very nice looking
mastheads. Also common is for the pin seats (the hole through
which the pin passes) to be drilled out of square, producing
a pin angle other than 90 degrees. Remember, there are two
planes in which a pin can be out of square. One plane is easily
visible when looking through the box, the other plane is perpendicular
to the first, and not so easily seen. Do not assume your sheave
box to be square or the pin angle as it should be. Careful
inspection and accurate measuring is necessary to ensure a
good spin to your sheaves. |
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| Sheave
Diameter |
Sheave diameters are best expressed in inches and fractions
of inches. Line clearance in the box is a function of sheave
diameter and groove depth.
Groove depth is usually about 1/3 to 1/2 the rope diameter. Measure and record
your groove depth and related clearances to prevent binding. On new sheaves,
1/3 the rope diameter is enough for bearing and control. A full rope diameter
between outer sheave edges and upper or inner face of sheave box is a good rule
of thumb to provide adequate clearance. |
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| Wire
to Rope conversion |
Often
I make new sheaves when wire halyards
are switched out for low stretch rope. The width of
those old wire sheaves, and thus the sheave box, is often less
than is necessary for clearance for the desired new halyard
diameter. The rule of thumb (above) goes into action here because
the old wire needed little or no clearance beyond the outer
sheave edges. One more consideration, we don’t want the
new halyard to ride on the surface of the mast as it enters
or exits the sheave, so we want the new sheave diameter large
enough to prevent this, but small enough to preclude binding
at the top of the box. Sometimes all of these are not obtainable,
and some recutting of the box is necessary, or some compromise
of these factors must be made. |
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| Sheave
Width |
Sheave
widths measured in thousandths are sometimes necessary
on refits, though usually I get measurements in fractions.
Sheave width can be critical in many situations. Optimum
working total side clearance in a clean, square box is generally
around .062” (1/16”) Sheave width and pin angle
are directly related. The more out of square the pin angle,
the narrower
the sheave must be to prevent binding. If there is doubt
about squareness, cut a round piece of wood the desired thickness
with the pin hole drilled square through it (hole saws work
great) and try it out.
When making new blocks, or designing any sheave box, I use these dimensions:
For
3/8” rope, use a 1/2” sheave, with a 9/16” slot
For ½” rope, use a 5/8” sheave, with an 11/16” slot
(etc.)
Remember to leave plenty of room for varnish inside the
slot on wooden blocks. |
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| Pins |
Pin diameters are best measured in thousandths. This accuracy
is needed to ensure a good spin. Some pins appear to be a standard
fractional size, but are actually slightly oversize or metric
size. It can be worth the effort to obtain a set of calipers
or a micrometer to check exact pin size if in doubt. Much better
than getting your new sheaves and discovering they will not
spin on your pins. I cannot guarantee a proper fit without
your pin measurements in thousandths of an inch, or fitting
your pins to bushings at my shop.
Often pins are gnarled up at the ends, so a little judicious filing is necessary
to get the bushings to slide onto the pin easily.
Pin diameters are generally not changed because changing the hole diameter in
the sheave box is usually not easy, necessary or desirable.
Pins are available in stainless steel or silicon bronze. Titanium also makes
a good pin material, especially aloft where weight is critical, though it is
spendy stuff. I usually drill the pins to take 1/8” cotter pins, supplied
in bronze.
Pin Measurements:
- Pin diameter
- Overall length of pin
- Grip Length (measured between cotter
pin holes, not center to center)
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| Sheet
Rollers |
Sheet
rollers are measured just like sheaves, with careful attention to
the groove depth and diameter. I often rebuild complete sheet rollers
cars. |
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**After
reading and understanding all of the above, you are ready to provide
the dimensions for each different sized sheave.**
You
are now ready to proceed to the request
for quote (RFQ) form. |
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