Fig.2Head of Jib, showing movable and fixed pulley blocks.
crane, it would perhaps be well to give some account of their functions, and the reasons for the particular method of construction that has been adopted. Apart from the base,
the derrick consists essentially of the following component parts :the jib,
the operating cables 19, the vertical member 4, and the two side members or ties 3.
These units combine to absorb all the strains and stresses generated in the crane while it is at work.
Most of the strain on the jib head
is converted into compressive force applied to the jib itself,
the remainder being taken by the cables 19.
bolted in the first and fourth holes of the Girders 28.
A pair of 2" Strips bolted to the outer ends of the 5 1/2" Flat Girders provide bearings for two 2" Rods, to which are pivoted the pulley blocks of the jib head. The outer block, which forms part of the hoisting mechanism, consists of two 2 ½” Strips and one 3" Strip, in which are journalled 1" Rods.
Two 1" loose Pulley Wheels are mounted on one of these Rods, and the Strips are spaced as required by Washers and Collars. The inner or rear pulley block of the jib is built up of two 2 1/2" Strips and two 31/2".
and are separated by 2 1/2" Strips.
In each case the Pulley Wheels are spaced by Washers so that they may rotate quite freely.
Fig.3Top of vertical member, showing method of attaching the tie members.
in directions corresponding to their respective positions, and as the two forces are not in direct opposition they combine in producing a force that tends to depress the upper ends of the tie members 3. This force takes the form of a downward thrust on the vertical member 4.
Therefore the latter,like the jib, must be designed
to withstand compression. Accordingly it is constructed from a pair of 18 1/2" Angle Girders
bolted together at each extremity, as well as at a point near the lower end, by 11" Angle Girders 5 (see Figs. 3 and 4).
a bearing 8 built up fromtwo 2 1/2" Strips bolted across the base Girders 1. Two Collars should be placed on this Rod, one above the Sprocket Wheel 7 and one below the 2 1/2" Strips 8 (Fig. 4).
which also act as guide Pulleys, are mounted
on horizontal Axle Rods journalled in the Girders of the vertical member 4.
The latter is secured to the Strips by two nuts (see Standard Mechanism No. 262) and its shank forms a pivot about which turns the upper end of the vertical member 4.
Fig.4Swivelling base of jib.
The Gear Box
Electric Motor, which The Meccano supplies the power for operating the Derrick, may be of either the 4-volt or the high-voltage type. It is bolted to a 5 1/2" Flat Plate supported on two 12 1/2" Angle Girders 15, and owing to its position
outside the triangular base it helps to counterbalance the weight of the jib and its load.
Motor is led through two 3 : 1 reduction gears,each consisting of a y Pinion Wheel and a 57-teeth Gear Wheel, and then is transmitted via a f *
Sprocket Wheel and a length of Sprocket Chain 17 to a 2" Sprocket Wheel on the Rod 16. A second 2" Sprocket Wheel on this Rod is connected with a 1* Sprocket Wheel on the Rod 25.
is communicated to the Rod 33.AWorm on this Rod operates a further 57-teeth Gear Wheel on the vertical Rod 27, which, in its turn, carries a 1" Sprocket Wheel that swivels the jib by means of a Sprocket Chain passing round the 3" Sprocket Wheel 7.
hole engaging two Collars on the Rod 16, causes the latter to slide in its bearings.
member 4, round one of the Pulley Wheels 32, and over a similar Pulley Wheel near the top of the member 4. It is then led in turn round each of the sheaves in the pulley blocks on the jib and the vertical member,
and is tied finally to the tail of the block on the jib.
Fig.5 Plan view of Gear Box,showing High-voltage Motor in position for driving the model.
By the use of this arrangement of pulleys a considerable mechanical advantage is obtained, which enables the jib to be luffed by the application of a very small force, even when the load Hook is carrying a considerable weight.
the swivelling movement of the crane while the handle 24
the raising and lowering of
the load on the Hook and
the luffing of the jib.
Reversing is effected, of
course, by means of the
starting handle of the Electric Motor.
position the greater will be the strains upon it in proportion to the load. This statement may be verified quite easily by applying the well-known principle of the Triangle of Forces,
which may be summarised as follows :
If three forces meet at a point and are in equilibrium, and we know one of the forces, we may determine the other two by drawing a triangle, making each side parallel to the direction of one of the forces,
and comparing the dimensions of the three sides. It will be found that these dimensions are in the same proportion as the three forces.
ten tons respectively.
In the case of a crane the three forces are
(c) the jib, which acts as a strut to withstand the compressive force, exerted by the combination of (a) and (b). All three forces meet in the head of the jib and counterbalance each other : that is to say, they are in equilibrium.
the disposition and proportion of the strains or forces vary according to the particular type of crane.
described in the specification as capable of raising with safety a load of 20 tons at a radius of 20 feet may be able to lift only 10 tons or thereabouts at a radius of 35 feet.
set up in the structure and machinery.
It is to minimise the possibility of such a mistake
being made that a radius indicator is fitted to the majority of luffing cranes.
A glance at this indicator tells the operator the position of the jib and the maximum load that he can handle safely without increasing the angle of the jib. Hence the device not only saves time but also obviates a considerable risk of accidents.
Fig. 6 shows the indicator attached to the side of the jib in the Derrick, and the actual position of the device in relation to the remainder of the model is indicated more clearly by the arrow in Fig. 7.
Fig.6 The meccano Radius Indicators attached to de jib of Derrick
The Coupling 1 is free to turn about the li" Axle Rod 2, which is gripped in the boss of a Crank 3 bolted to the upturned flanges of the jib girders. It carries in its upper end a further 1 1/2 " Rod 4 and in its lower end a 1" Rod on which is secured the Worm 5. The weight of the latter serves to keep the Rod 4 always vertical, no matter in what position the jib is placed. A dial 6, shaped from a piece of stout cardboard, is bolted at 7 to an Angle Bracket attached to the jib. The Rod 2 passes through a hole in the dial and carries two or three Washers to space the Coupling 1 away from the card so that the Worm 4 will clear the edges of the girders forming the jib. The jib should now be placed in different positions and the radius covered by the load hook for each position should be marked on the card.
Parts required for building the Stiff-Leg Derrick