_THE TRANSPORT OF MATERIAL_
The transportation of material is not altogether within the province of
a scaffolder, but it is so intimately connected—indeed, it is difficult
to say where his connection with the lifting and carrying of material
commences and finishes—that the subject is here briefly commented upon.
=Crane Engines.=—The engines of the crane are so arranged that
all motions in connection with the derrick are under the control of
the driver. The engines are double cylinder with link-motion reversing
gear. The gearing is single and double purchase for lifting; the jib
barrel is fitted with steel catch wheel and double-lock safety catch
to prevent the jib running down. The slewing gear is worked from the
crank shaft, connected to the upright shaft from bottom race or spur
wheel, and is wrought by worm and worm wheel with double-cone friction
slewing gear. This arrangement permits of slewing the crane in either
direction without reversing the engine. It might also be mentioned that
the clutch for the jib motion is hooped with malleable iron to prevent
the possibility of its bursting.
Crane engines can be worked by electrical, steam, or manual power. The
smaller cranes are now so made that either steam or manual power can be
used as required. It is of recent date that these engines have been
supplied with electrical power, and of course their use is restricted
to where this power is obtainable.
=The Crane.=—The crane consists of four parts, the mast, jib,
sleepers, and guys or stays. The mast, which rises vertically, is
connected at its base to the platform on which the engine stands;
and at the top, to the guys by a pivot which allows of rotation in a
horizontal plane. It may be of iron, balk timber of oak or pitch pine,
or in two pieces of the same, strutted and braced. The jib, which
may be built of the same materials as the mast, is fastened to the
lower end of the mast by a joint which allows of rotary movement in a
vertical plane. The steel rope or chain which supports the weight runs
from the drum placed near the engine and over the top of the mast and
jib. Wheels are placed at these points to lessen friction.
The combination of movement allowed by the pivot of the mast and joints
of the jib, enables the load to be carried to any point commanded by
the effective length of the jib, except that it cannot be placed behind
the guys. Jibs are used up to 70 feet in length. To prevent slewing
under wind pressure, jibs over 50 feet long should be fitted with wind
brakes, especially on exposed situations.
The crane will stand the greatest strain when the jib is most upright,
and, reversely, less strain as it approaches the horizontal. It is a
good rule, and one which works for safety, not to allow the top end of
the jib to reach a lower level than the top of the mast, whatever the
weight of the load carried may be.
Cranes are made suitable for derrick staging to carry a weight of 7
tons. If the boiler is attached to the rotating platform of the crane,
it helps to counterbalance the load.
Cranes, while offering the readiest means of dealing with heavy
weights, do not give the best results when used for placing material
in its final position on the building. The vibration of the engine,
the swaying of the supporting rope from the jib, and the unevenness of
lowering under the band brake, prevent that steadiness of the material
which is necessary for good fixing.
[Illustration: FIG. 105]
Fig. 105 shows a small building crane; it is worked by manual power,
and is very suitable for light work. The illustration shows the general
method of construction, but there are other patterns which give greater
power.
The crane is fitted with two hoisting ropes which are wound on the drum
at A. One rope rises while the other descends. The ropes
pass through the arms B, and when the catch C rises
against the slot, it lifts the arm up. The base of the jib to which
it is connected then rises in the sliding groove and swings inward,
carrying the load well over the platform where it is to be deposited.
When the new load begins to rise, the jib swings outward and downward,
the rope paying out as required. By this means the jibs are in use
alternately for lifting.
=Pulleys.=—The pulley (fig. 106) is a circular iron disc which
revolves freely on an axle fitted into an iron box. The circumferential
edge is grooved to receive the rope or chain which passes round it.
[Illustration: FIG. 106]
Fig. 107 is the section of a groove suitable for fibre rope driving.
The rope is gripped at its sides, thus increasing its driving power.
Fig. 108 is the section of a groove where the pulley is used as a guide
only, the rope being allowed to rest on the bottom.
Fig. 109 is the section of a groove used for wire rope. The groove is
lined on the bottom with pieces of wood or leather to give greater
friction, as the rope would be injured if it were gripped in a groove
as fig. 107.
Figs. 110 and 111 are the sections of grooves suitable for chains, the
groove receiving every other link, the alternate links lying flat.
Fig. 110 is suitable where the pulley is used as a guide only, and fig.
111 is used for driving pulleys as in Weston’s blocks.
The part elevation shown in fig. 112 is known as a sprocket wheel, and
shows the sprockets cast in the groove upon which the links catch. It
is used for driving purposes.
[Illustration: FIG. 107]
[Illustration: FIG. 108]
[Illustration: FIG. 109]
[Illustration: FIG. 110]
[Illustration: FIG. 111]
[Illustration: FIG. 112]
When the pulley cannot rise or fall, it is termed a fixed pulley,
otherwise it is considered movable.
The fixed pulley, sometimes known as a gin wheel, can only change the
direction of a force, and gives no mechanical advantage, but when used
in conjunction with a movable pulley a mechanical advantage is gained.
Fig. 113 is an illustration of the single movable pulley. The rope
is connected to the beam at A, and passes round the pulley
B and over the fixed pulley C Now, if power be
exerted on the free end of the rope sufficient to move the machine, it
will be seen that for every one inch W rises, P will
descend two inches, therefore the mechanical advantage is 2. In other
words, a workman pulling at P with a force equal to 100 lbs.,
will lift W weighing 200 lbs., but the weight will rise at
only half the speed at which P falls.
Various combinations of pulleys are possible, but the most common in
use on buildings is shown in fig. 114, which is an illustration of the
system of pulleys known as block and tackle.
[Illustration: FIG. 113]
[Illustration: FIG. 114]
The power obtained by this machine is calculated as follows:
Divide the weight by double the number of pulleys in the lower block;
the quotient is the power (in the same units as the weight) required to
balance the load.
Theoretically, there is no limit to the number of pulleys and
consequent mechanical advantage, but the friction produced, and want
of perfect flexibility in the rope, prevent any great increase in the
number.
=Differential Pulleys.=—A differential block on Weston’s principle
(fig. 115) consists of a compound pulley of two different diameters
but of one casting, and therefore rotating together. The chain is an
endless one, and passes in turn over each diameter of the pulley. One
of the loops thus formed carries a single movable pulley, while the
other loop hangs loose (see fig. 116). The power which may be applied
to the loose loop on the side which comes from the largest diameter
will cause rotation of the pulley.
The chain must be four times in length the distance through which it is
required to raise the load.
These pulleys are tested to 50 per cent. above the weight they will
have to lift in practice, and the maximum load they will carry is
stamped on the castings. The mechanical advantage derived depends upon
the difference of diameter in the compound pulley. Usually with these
machines two men are required to lift one ton.
Another common form of differential pulley is known as the worm
block, and consists of two cast-iron toothed wheels at right angles
to each other, connected by a worm thread of case-hardened, mild
steel forging. The wheel upon which the power acts is worked by an
endless chain, and the lift wheel may be fitted with a chain or wire
rope to which the load is attached. Pulleys of this kind possess in
a great degree steadiness in lifting or in lowering. This is due to
the great mechanical advantage that can be gained by their method of
construction. By these pulleys one man can lift up to 3 tons. They are
tested and stamped with the maximum safe load, as are those on Weston’s
principle. The friction between the parts is sufficient to sustain
the loads when the power is removed. The steadiness of action allows
of their use to lift stones of great weight, and lower direct into
position for fixing on the building.
[Illustration: FIG. 115]
[Illustration: FIG. 116]
=The Winch.=—A winch is a hoisting machine in which an axle is
turned by a crank handle, and a rope or chain wound round it so as to
raise a weight. It is actually a form of lever whereby a weight may be
moved through the distance required.
Fig. 117 gives a type of winch in its simplest form. The mechanical
advantage gained by its use depends upon the difference between
the radius of the driving wheel and the radius of the axle; or the
circumference of the wheel and the circumference of the axle.
If the radius of the axle were the same as the radius of the wheel, no
mechanical advantage would be gained by its use. The advantage that is
gained by the arrangement can be calculated as follows:
As the radius of the wheel is to the radius of the drum so is the
weight that can be lifted to the power applied to the handle.
_Example._
Let radius of wheel = R,
radius of drum = _r_,
power applied to handle = P,
and weight lifted = W.
(R × P)/_r_ = W.
Then if R = 12 inches,
_r_ = 4 inches,
P = 60 lbs.
(12 × 60)/4 = 720/4 = 180 lbs. weight that can be
lifted, giving a mechanical advantage of 3.
It is obvious that, owing to the restriction of size, the mechanical
advantage that can be gained by the simple machine shown on this figure
is limited. To overcome this difficulty one, two, or three pairs of
toothed wheels are introduced into the machine, being thus known as a
single, double, or treble purchase winch or crab. The difference in
the number of teeth between the pinions and wheels gives the increased
mechanical advantage that is required. The method by which to find the
mechanical advantage gained is as follows:
The pressure exerted on the handle is to the weight lifted as the
radius of the drum multiplied by the number of teeth in the pinions is
to the radius of handle multiplied by the number of teeth in the wheels.
Winches, besides lifting from the barrel, are also used in conjunction
with pulley wheels to change the direction of the force and to gain
additional mechanical advantage. If a double rope be used, considerable
time will be saved in the progress of the work.
[Illustration: FIG. 117]
Fig. 118 shows the double rope. The middle of the rope is given a few
turns round the drum, and the ends are continued over the pulleys, one
sufficiently far to reach the ground. On raising the load the higher
end of the rope descends, and is ready to carry up the next load by
the time the first has been taken off. The illustration also shows the
winch at work in connection with one movable pulley; but unless the
material is to be raised to different heights, the same system of
pulleys should be used on each rope.
[Illustration: FIG. 118]
An improved winch with an advantage over those ordinarily used has the
drum grooved in three diameters, so that, with a minimum of trouble,
a choice of mechanical advantages is gained. There is no need with
these winches to pass the rope several times round the drum, for as the
rope works in a groove, greater friction is set up; and the clutches
provided to keep the rope in contact with the barrel for three quarters
of its circumference, still further prevent any likelihood of slipping.
[Illustration: FIG. 119]
=Jibs.=—For the fixing of the gear a jib (fig. 119) is sufficient
to carry a fixed pulley. A jib is a single pole attached horizontally
to the standards or ledgers above the platform upon which it is
intended to deposit the material. It should project slightly more than
half the width of the load to be hoisted, usually from 18 inches to 2
feet.
[Illustration: FIG. 120]
It is useful at times to have a pulley block fixed over the work in
hand, especially for fixing purposes. In these cases, the loads being
usually heavy, the jib requires extra support, which is obtained by
fixing it as a continuous beam supported at both ends (fig. 120). This
is arranged by carrying up on the interior of the building a series of
standards and ledgers; these rise from each floor as the work proceeds.
The jib can be carried right across the work in hand and the gear fixed
as required.
[Illustration: FIG. 121]
Another form of jib known as the 'mason’s’ is shown in fig. 121. It
is of framed timber from 9 in. by 3 in. to 11 in. by 3 in., leaving
a 4-inch opening down the centre, and rests across the ledgers. It
allows the pulleys which are hung on to the iron movable axle, to be
shifted horizontally throughout its length. For heavy material this
is invaluable, as the load can be raised, moved to its position for
fixing, and lowered as required.
[Illustration: FIG. 122]
=Shears.=—The shears or shear legs is another contrivance for
supporting heavy weights. It consists of two legs forming two sides
of a triangle, and may carry a pulley at the apex as shown in fig.
122, or a jib as shown in fig. 123. In the first case the legs are
not rigidly fixed, but are kept in position by ropes, A and
B, which, on being slackened, allow the shears to move from
the perpendicular (fig. 124). In this manner loads can be lifted and
placed in a different position other than that immediately over the one
they first occupied. The range is, however, limited, as in practice
the shears should not move more than 18 inches to 2 feet from the
perpendicular.
[Illustration: FIG. 123]
[Illustration: FIG. 124]
Shears are useful for raising and lowering the machinery on Scotch
derricks, and are often fixed on gantries to carry jibs.
For heavy weights, the legs and also the jib can be of two or three
poles tied together.
=Gin.=—The gyn, or gin, consists of three legs usually from 12
to 13 feet long. They are set up and joined together at the top, thus
forming a triangulated frame. A pulley wheel or block is fastened at
the apex, and extra power can be gained if they are furnished with
a crab winch standing between two of the legs. They are useful in
lifting or in lowering material through a well or opening in the
working platform, as shown in fig. 125.
[Illustration: FIG. 125]
[Illustration: FIG. 126]
=Rails.=—Scaffolds of a particularly strong construction have,
when necessary, rails laid upon them, in order that light trucks may be
moved freely from place to place.
=Sack trucks= are also used on platforms to carry cement, &c.,
where required.
Other accessories for carrying purposes, the uses of which are obvious,
are described in the chapter on Scaffolding Accessories.
The attachment of material to the transporting power is within the
province of the scaffolder. To take each class of material separately:—
=Ironwork.=—Ironwork is principally used in the form of girders
and columns. These are sometimes slung by a chain round the middle, and
as evenly balanced as possible. There is considerable danger of the
chain slipping, however well balanced; more especially is this the
case if the load is tilted when swinging. This may happen by the load
receiving a jar through touching some part of the erection, and thus
allowing the material to fall. To prevent this 'softeners’—i.e. old
bags, sacks, or even pieces of wood—are placed between the chain and
load. Then, with the chain turned twice round the whole tightly, the
danger is minimised. An extra chain may also be run from each end of
the load to a point some distance up the supporting chain, as shown on
fig. 126.
=Timber.=—Timber in lengths can be carried in the same manner
as ironwork, but, owing to the greater friction set up, it is not so
likely to slip as the former. The same precautions should be taken.
To carry timber or ironwork vertically, the supporting chain is given
a timber hitch round one end of the pole, and a half hitch round the
end which is meant to rise first. It is sometimes advantageous to
substitute a cord lashing for the half hitch. Then, when the highest
end of the pole reaches the platform, the lashing can be removed and
the pole received horizontally. This method is useful where the load
has to be passed through a window.
=Bricks=, =slates=, &c., are slung in crates and baskets, and
on small jobs are carried in hods by labourers. These accessories are
described in Chapter VI.
Note should be taken that these fittings are in the first instance
strongly made, kept in proper repair, not overloaded, and that spring
hooks are used on the slings.
=Stone.=—Stone-work can be slung by means of the lewis, slings,
cramps, clips, or shears. Another method is to pass the chain several
times round the material, as for girder lifting. It is only suitable
for rough work, as any finished edges or chamfers may be flushed even
if 'softeners’ are used.