The use of block and tackle affords at least two advantages to the
user. One is the advantage of position. The user may stand on the
ground and pull downward--the most easy and natural way of exerting
force, while the resulting forces may be developed upward as in the
case of a hoist. The other advantage is mechanical. By the use of a
combination of lines and sheaves, force applied by the user can be
multiplied many times before it is transferred to act upon the body.
But where there is gain in pounds force applied, there is always a
counteracting loss due to an increase in the distance required to apply
the force compared with the distance the weight or load will travel; as
in Figure 96, a force of 100 lbs. on the free end of the rope will give
a resultant on the object of 200 lbs. (neglecting loss by friction in
rope and pulley) but distance travelled by the user will be two feet to
one foot travelled by the object.
The illustrations are shown in each case with an arbitrary force of
100 lbs. applied to the free end of the rope. The resulting force
(neglecting or disregarding friction) is then shown in all parts of the
set. In actual practice the friction of the sheave and the resistance
of the rope to bending gives rise to a loss of about 5% of the force
applied to the rope passing through each sheave. For example in Fig.
95 the force applied on the barrel would be 95% of that applied to the
free end of the rope or 95 lbs. In Fig. 96 the resultant force would
be 100 + (100 - 5) = 195 lbs. and in Fig. 97, the lift on the armature
would be 185½ lbs. instead of 200 as shown.
[Illustration: Fig. 95]
The ropes are also separated in the illustrations in order to show each
part clearly. The ropes are assumed to pull parallel to each other and
the figures represent the pounds resulting in different parts of the
set under those conditions. The illustrations show some of the most
typical applications of block and tackle for mechanical advantage or
advantage of position or both.
Fig. 95. The Single Whip affords only advantage of position
commonly used on a crane or derrick or perhaps for hauling an object
up to a wall or to the water’s edge. Theoretical advantage 1:1.
[Illustration: _Fig. 96_]
Fig. 96. The Running tackle is similar to the Single Whip except that
the object to be moved is attached at a different place. This gives a
theoretical advantage of 2:1.
Fig. 97. The Gun tackle A affords an advantage of position since the
user stands on the ground and pulls down and the resultant force is
applied to the object vertically upward. Theoretical advantage 2:1.
Fig. 98. The gun tackle B is the same as gun tackle A except that its
application is different, giving a theoretical advantage of 3:1.
Fig. 99. Whip-on-whip multiplies the mechanical advantage by two, where
applied as shown. If inverted and the top block applied to the load
with the loop snubbed the mechanical advantage would be 4:1. In both
cases two single blocks are used.
Fig. 100. The Luff tackle has many applications aside from the one
shown. Ordinarily consisting of one single and one double block and a
single rope, it gives a theoretical mechanical advantage of 4:1 in
the case shown.
[Illustration: _Fig. 97; Fig. 98; Fig. 99; Fig. 100_]
[Illustration: _Fig. 101; Fig. 102_]
Fig. 101. The Port tackle, consisting of Single Whip and a Luff tackle
may be applied when the level of operations changes from time to time
and it is undesirable to apply the amount of rope necessary to make the
Luff part of the set long enough to serve for all levels. A bale sling
is also shown in use.
Fig. 102. A Double Luff tackle has a four part line instead of a three
part line as in the Single Luff.
Fig. 103. A Single Spanish Burton (A) using two single blocks and one
rope gives a greater mechanical advantage than the same equipment used
as in Figure 97, the Gun Tackle. This is useful in shifting cargo, etc.,
where the distance hoisted in not great.
Fig. 104. A Single Spanish Burton (B) using three single blocks and
two ropes, gives the same hoisting range as the Type A Burton, but a
greater mechanical advantage.
Fig. 105. Three Fold Purchase using a six part line, gives a
theoretical mechanical advantage of 6:1 and an actual advantage of
5·03:1, assuming a loss of 5% of the force on the rope passing over
each sheave.
[Illustration: _Fig. 103; Fig. 104_]
[Illustration: _Fig. 105; Fig. 106_]
Fig. 106. Four Fold purchase using two four-sheave blocks, is commonly
used in derricks and hoists. The illustration shows the possibility of
using four two-sheave blocks, where the larger sizes are not available.
Fig. 107. The Double Burton (A), for one rope and two single blocks and
one double block, gives a limited hoisting range which is desirable in
shifting heavy weights when it is necessary to lift them but a small
distance.
Fig. 108. The Double Burton (B), while using exactly the same equipment
as is used in Fig. 91, shows the large differences in mechanical
advantage between different methods of threading up the set. The
illustration also shows a box sling in use.
Fig. 109. Double Burton (C), is a further application of the principle
of the Spanish Burton, using two ropes.
[Illustration: _Fig. 107; Fig. 108; Fig. 109_]
[Illustration: _Fig. 110_]
Fig. 110. Double Burton (D), using but one rope, illustrates the
possiblity of using it to greater mechanical advantage than would be
possible in a six fold purchase. However, in this case the hoisting
range is less than would be possible in a six fold purchase.
Fig. 111. Luff on Luff illustrates a common application of tackle to
secure mechanical advantage. It will readily be recognized that the
major tackle must be four times as strong as the other set if both are
to be used anywhere near to capacity.
Fig. 112. Another Double Burton which also illustrates the possibility
of combining two blocks in place of one, with the required number of
sheaves.
[Illustration: _Fig. 111_]
[Illustration: _Fig. 112_]