Shear Movement: Shearing Force

The shear is a tool that has many applications in the metalworking industry. The shear and its movement are used to cut plastics, sheet metal, thin wood and even paper. When the materials are thicker, a shear driven by an electric motor is used.

What is a shear?

Shears are similar to scissors, but considerably larger and suited to industry. The shear is made up of two sharp blades that face each other as they press on the surface to be cut; thanks to the pressure applied, the resistance of the surface can be overcome. This is thanks to tension, and that is why the surface can be pierced, broken and a single piece separated in two.

It is worth noting that the edges of the cut piece are quite irregular, since, in precision work, the piece may require a grinding process that can be carried out with grinding machines or by manual methods. But what is the principle of shears? What are the shear and its movement like?

To make the cut it is necessary to exert a certain pressure. This pressure is achieved by applying leverage between one of the arms, which is located at the bottom, and another that moves up and down applying pressure. The movement of the shear, in manual tools, is made by the operator. However, there are also automatic shears, which are large in size.

Types of shear

There are different types of shear to be found, classified according to the use they are given:

• Sheep shears: for textiles.
• Pruning shears: for gardening.
• Metal shears: used to cut thin metals and, in turn, divided according to the type of cut they make: straight and curved, to the right and to the left
• Jaws: for rescue work
• Industrial: specialised machine tool for industrial work. This is the type of shear used in metalworking, since it is the one that makes vertical cuts when pressure is applied to sheets of different materials. There are versions with an electric motor, to exert greater pressure.

The hydraulic or industrial shear

The hydraulic shear works when the mechanism of the machine puts pressure on a liquid or gas. This machine tool is divided, in turn, into two types: guillotine and swing-beam. The first is used to cut sheets of metal and can be mechanical or hydraulic; the hydraulic or swing-beam shear, for its part, uses spacers and screens that make it possible to see the speed, the gauge and the cuts being made.

The hydraulic shear, as its name indicates, has a hydraulic mechanism. It has two blades: one located at the top, which makes the movement; and another at the bottom, which is fixed. The hydraulic shear and its movement are characterised by a gap between the blades being used to apply the cutting force to the workpiece.

Shear: movement and force

When analysing the shear and its movement, the so-called shear force comes into play. It is a compound force; the shear force is always parallel to the surface on which it is exerted and splits the piece. This is how the workpiece is separated in two. The shear force is characterised by the application of two forces applied at the same time to the same object and at two specific points on its surface.

These forces also have particular characteristics. As well as being applied at specific points on the surface of the piece, they must have the same magnitude; furthermore, they must be applied in opposite directions but always along parallel lines of action.

To understand the shear and its movement, the example of a pencil that is to be snapped is used. Two forces are applied and the pressure is concentrated on specific points along the length of the pencil, which deforms or breaks it.

Shear stress

The shear force and the effects it has on the piece depend on the magnitude of the force applied, as well as on the area to which that force is applied. This is how the concept of “shear stress” arises, since two factors are taken into account in order to cut a piece in two. In some sectors, such as construction, shear stress is also known as shearing stress or cutting stress.

Shear stress, in the International System, is measured in newtons per square metre, a unit that is called the pascal. To calculate the mean stress, a mathematical formula must be followed in which the force is divided by the area over which the force is applied. However, sometimes the force applied is not enough, so only deformation of the piece occurs which, when the application of that stress ends, returns to its original shape. Here it is said that shear stress and strain are proportional.

The so-called shear modulus, or modulus of rigidity, also comes into play; it is also measured in pascals and its value varies according to the type of material to be cut.