=1. Sheaves= are grooved iron or steel wheels used to carry or guide
a rope. The general method of mounting them on a frame for hoisting
light loads is shown in Fig. 1. The journal boxes are so constructed as
to be easily taken apart for inspection or repair. For hoisting heavy
loads, the timbers must be braced, as is explained under the heading
Head-Frames in this Section. Sheaves are of two styles--those composed
entirely of cast-iron and those with cast-iron hubs and rims and
wrought-iron or soft-steel arms or spokes.
[Illustration: FIG. 1]
=2.= The =cast-iron sheave=, Fig. 2, has arms with a cross-section, as
shown at _a b_, and with the flanges of the arms tapering from the hub
to the rim; that is, _d_ is greater than _c_ and _f_ is greater than
_e_. The bottom of the groove _g_ in the rim should be a circular arc,
whose radius is a little larger than that of the rope used over the
sheave, to allow for the angling of the rope due to its fleeting on the
drum. The flanges _h_ are made quite deep to prevent the rope jumping
off.
This sheave is cheaper than a combined cast-iron and wrought-iron or
steel sheave, and for many purposes it is entirely satisfactory. Its
great weight is an objection, because it adds to the weight on the
journals and also offers considerable resistance to being set in motion
and stopped.
[Illustration: FIG. 2]
If a sheave is merely used to carry the rope or to deflect it only a
little, the contact and pressure between the rope and the sheave is
small; consequently, the power of the rope to turn the sheave will be
slight. In such a case, when the rope starts or stops quickly, as it
usually does in modern hoisting plants, the heavier the sheave the more
will it lag behind the rope and the greater will be the wear on the
rope due to slipping.
=3.= The sheave with a cast-iron hub and rim and wrought-iron or
soft-steel spokes, Fig. 3, is an excellent and extensively used
sheave, especially the larger diameters. The spokes are screwed
into the hub and rim and are carried to the right and to the left of
the hub alternately, as shown in Fig. 3 (_b_), so as to take hold of
the opposite ends of the hub, thereby giving stiffness to the sheave
against any side stress.
[Illustration: FIG. 3]
With a sheave having cast-iron arms, the load from the rope is
transmitted to the shaft by a compressive stress through the arms
directly under the load; that is, if a rope runs over the sheave, Fig.
2, putting a load on it from _j_ to _k_, this load will be transmitted
as a compressive stress through the arms _l_ and _m_ to the hub and
the shaft. Of course, a part of this load is carried around the rim to
the lower arms and is supported by them in tension, but these lower
arms are not considered in designing the sheave because cast-iron is
of comparatively little value in tension, whereas it is of great value
in compression. In the case of the sheave with wrought-iron arms, or
spokes, Fig. 3, the load is transmitted around the rim to the side
opposite its point of application and is carried from there to the hub
and shaft by the tension of the spokes; in fact, from the method of
construction, the spokes in this sheave act only by tension. The sheave
is strong and rigid, and much lighter than a cast-iron sheave of the
same strength, so that there is less wear between it and the rope due
to any slipping action when it is started or stopped.
[Illustration: FIG. 4]
[Illustration: FIG. 5]
=4.= Sometimes, the spokes, instead of being radial as in Fig. 3,
are made tangent at the center of the wheel, Fig. 4, to an imaginary
circle, which is about 2 inches in diameter for a 10-foot sheave.
Alternate pairs of spokes are made tangent to the opposite sides of
the circle, so that they pull against each other, and this makes the
sheave rigid in both directions. That is, spoke _A_ is tangent to the
right side of the tangent circle and _A′_ to the left side, while spoke
_B_ is tangent to the right side of the circle and _B′_ to the left
side. The pair _B B′_ is joined to one end of the hub, while the pair
_A A′_ is joined to the other end, thus giving lateral stiffness to the
sheave. This arranges the spokes in groups of four, so that the total
number must be some multiple of four. The tangential direction of the
spokes is often necessary in very large sheaves carrying heavy loads,
because with such a sheave it requires considerable force to turn the
shaft in its bearings, and while radial spokes act only as long levers
in turning the shaft, with tangential spokes there is also a direct
pull to do it.
=5. Wood-Lined Sheaves.=--The rims of all sheaves are made either
solid or with wooden lining, as shown in section in Fig. 5. One flange
_a_ of the rim is a separate piece that is held on by bolts _b_. The
wooden lining is in the form of blocks placed with the grain of the
wood running radially and held securely by clamping together the two
flanges with bolts, as shown. With such a sheave, there is much less
wear on the rope than there is with one that has a plain cast-iron rim.
The wear of the sheave proper is also avoided, because as the blocks
wear down they are taken out and replaced by new ones.
=6. Diameter of Sheave.=--The size of a sheave about which a rope
bends is determined generally by the size of the rope to be used, as
explained under Wire Ropes in _Hoisting_, Part 2; but, if the rope is
simply to be supported in a straight line, the space available for
setting the sheave and its cost and weight usually determine the size
used. The minimum allowable diameter of sheave should not be used
unless it is necessary to do so, for the larger the sheave the less
will be the wear of the rope due to the bending, and the longer the
life of the rope, but the cost of the sheave, which increases with the
size, puts a limit in the other direction.
=7. Rollers and Carrying Sheaves.=--Wooden or iron rollers are
sometimes used for rope carriers or guides, instead of light sheaves,
when the rope has merely to be supported and there is no bending of
the rope, excepting the slight amount due to the sagging between the
rollers. The diameter of the rollers is of little importance in such
cases so far as the rope is concerned. If they are for use on a slope
to keep the rope from dragging on the ground, they must be small,
because the cars must run over them, and mine cars are usually made
low because of restricted headroom in the mine. Rollers and carrying
sheaves are fully described and illustrated in _Haulage_.
If a hoisting rope changes its course from a straight line, even if
the deflection is only a small amount, a roller is not advisable and a
sheave should be used, if possible.