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Deformation forms of mechanical parts

Date:Nov 8,2016

Mechanical parts under different external forces, will produce different forms of deformation. The main force and deformation are as follows:

1. Tension and compression

This type of deformation is equal to the size of the direction, the direction of the line and the pole piece of the axis of a pair of force caused by the performance of the length of the length of the rod to be extended or shortened [Figure 3-1 (a, b)]. Such as the heavy lifting of the cable, the truss rod, the piston rod of hydraulic cylinder and the deformation are tensile or compressive deformation. In the project, often to see the tensile or compression of the rod. For example fastening screws [figure 3-2 (a)], when the nut is screwed, the workpiece is pressed by the force of screw screw under tension; screw jack [figure 3-2 (b)] in lifting the top, are under compression. The former occurs elongation deformation, the latter occurs to reduce deformation, the straight rod along the axis by the size of the same, the direction of the external force, the occurrence of elongation or shortening of the deformation, known as the straight rod axial tension or compression. In this chapter, we only discuss the axial tension and compression of the straight rod.

If the axial tensile or compressive bar shape and stress condition were simplified, can be simplified into a force diagram shown in figure 3-1. In the figure are indicated by a solid line stress before the shape, the shape of the dotted line.

2. shear

In engineering, a member of the shear action is often seen. The common characteristic of the stress of the members is that the two sides of the component are equal in size and opposite in direction, and the action line is very close and the external force perpendicular to the pole axis. In this force, the main deformation bar is: with the cross section of the two force between M - M interface, two component along the surface relative dislocation. This type of deformation is called shear, the section m - M is called the shear plane, and the shear plane is parallel to the direction of the external force. When the external force is large enough, the component will be cut off along the shear plane. Only one shear surface, called the single shear, while the members of the compression, on both sides by other components of the extrusion, the local surface compression phenomenon known as extrusion. If the pressure is large, the local area of the contact surface will be a significant plastic deformation, the structure can not be used normally, this phenomenon is called extrusion damage.

In addition to shearing and extrusion, the coupling is often accompanied by other forms of deformation. For example, bending or stretching deformation. But because these deformation is secondary, it is generally not considered.

The form of this kind of deformation is caused by the force of equal size and opposite direction, the action line is parallel to each other, and it is shown that the two part which is affected by the shear member is relatively moved in the direction of the external force. Commonly used in mechanical coupling, such as keys, pins, bolts and other produce shear deformation.

3. Reverse

This kind of deformation form of the equal and opposite direction, face perpendicular to the two shaft caused by the couple. The relative rotation of an arbitrary two cross section of a member is shown to occur around the axis. Automobile transmission shaft, motor and the main shaft of the hydraulic turbine are all torsion bar. There are a couple in the plane perpendicular to the axis of the rod when the rod will produce torsion deformation of each cross section is the relative rotation around the shaft rod.

The torsion deformation of the rod has the following characteristics:

Force: the ends of the bar perpendicular to the rod axis in the plane of action of two couples, the moment is equal to the contrary.

Deformation: the axis of each cross section of the rod is rotated relative to the axis of the rod. The relative angle between the two cross sections is called the relative torsion angle.

Often encountered in the engineering of torsional deformation of the component. For example, an equal, the driver applied both hands on the steering wheel on the plane in the opposite direction, parallel to the line of action, they form a couple, role in the lever end, and in the end from the joystick steering reverse force couple, this lever is torsion.

4. Bend

This kind of deformation form of the perpendicular to the axis of the shaft lateral force, or by acting on the vertical plane containing the rod in a pair of equal and opposite to the direction of the couple by the performance for the bar axis changes from a straight line to a curve. One of the most frequently encountered cases in engineering is the curved bars. The bridge crane girder, deformation and various spindle cutter etc., all belong to bending deformation. The axis of the straight rod will be bent into a curve from the original straight line. The bending deformation of the rod is usually called the beam.

There are some basic deformation at the same time, such as the work of the lathe spindle bending, torsion and compression of the three basic deformation; drilling column at the same time stretching and bending two basic deformation. This kind of situation is called combined deformation.

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