Crane’s Balance

4- Realization of prototypes
Authors : Hélène Merle(plus d'infos)
Muriel Guedj(plus d'infos)
Summary :
The children prepare and assemble the various elements of a crane, according to the plans they have established.
Publication : 1 February 2004
Objectives :
Session 1: Procedure goals: learning mapping, anticipating construction actions, taking into account technical constraints. Know-how goal: choose a method of assembly “base/mast/jib” and use the tools: - allowing the rotation of the jib relative to the mast (mobile link); - providing a rigid connection between the mast and the base; - use the tools for sawing with the miter box, nailing. Session 2 Know-how goal: a system to guide the cable along the mast and boom, a wire winding system, a system for fixing the cable.
Material :

Session 1:
For each group of pupils

- materials required when drafting their diagram

For the classroom:

- Reserve of strips from different sections, squares, bases already cut by the teacher according to the children’s patterns or salvaged planks from which they choose.

- Long nails, wood screws, wood glue.

- Hammers, screwdrivers, hand drills or tendrils.

- Saws and miter boxes.


 Session 2:

For each group of pupils

- prototype during manufacture


For the classroom:

- various strings, fishing line

- screw hooks, identifying tacks, fences

- various bobbins or cylinders...

Session 1

Pedagogical approach



1. Collective introduction
Each group has its construction plan. The materials are available on a central table.
The teacher recalls the specifications. He presents the materials and tools, specifies the vocabulary (names of tools) and provides instructions on usage and safety
Individual: read the specifications
The bases designed by the children during sequence 3 have been cut  by the teacher with a jigsaw. The children will have to cut the pieces of wood.

2. Group work

The pupils in each group divide tasks of measuring and cutting the pieces of wood.
Need for a classroom with open space.

Possible solutions:

- Fixing the mast relative to the base: wood glue and many nails (only one nail would lead to the rotation of the mast/base)

- Securing the jib to the mast: children generally use a nail or a wood screw.

The nail: it ensures the rotation but the teacher will ask the pupils to observe the fixed and moving parts.
With a long enough nail, the friction at the mast is more important in the jib, the nail is attached to the mast. The jib therefore revolves around the nail, but with friction. Make them discover the need to drill a hole in the jib, with a diameter slightly larger than the nail, before the final fixing (Warning: if the diameter of the hole is too big, the movement will be too wide).


Wood screw: it allows the rotation, but it must be ensured that it doesn’t unscrew when the jib turns. Therefore, preference is for wood screws with a non-threaded portion of at least 20mm in the screw head.


Counterweight: the children reflect on their models and materials needed for the next session.
 The most common idea is to use various boxes that they can fill add more or less weight. Pupils imagine various non-definitive fastening systems (elastic plates, string). This will change the mass of the counterweight and its position.

Foreseeable difficulties
- improper positioning for sawing, nailing, etc.
- miter box that slides: the need for clamps
- misuse of the hammer: learn to hold the handle at the end
- difficulty to screw: need for pre-drilled holes with an auger or a drill
- use of wood glue: reading the instructions, the importance of setting time

In all the models, the jib is attached to the mast so that the counterweight is close to the fixed point, the load in  contrast is far from this point; the pupils reproduce the cranes observed during the first sessions.
3. Collectively
The teacher organizes an session for each group to present the progress of their work. Pupils should particularly report the difficulties encountered and the solutions adopted.


The teacher will highlight the issues raised and suggested solutions.

Word from La main à la pâte

If it appears that there are problems with the crane’s balance during the assembly of the different parts of the prototype, it is possible to work the role of the base and its ballast. (Séquence 6)

Session 2

Pedagogical approach



Collective introduction :
The teacher recalls that the objective of the session; after having fixed the counterweight, is to guide the cable that lifts the load and find a winding system for the cable.
It presents additional equipment, including equipment possibly provided by the children.
Group work:
The pupils imagine a solution for each problem
Possible solutions:
Pupils can distribute hooks, staples or tacks along the mast and the jib to act as the guide.
For the winding of the cable, they nail a bobbin or a cylinder at the bottom of the mast and optionally add a crank.
The pupils then set the counterweight.