Any complex mechanism in mechanical engineering consists of a set of simple elements. To understand how the system as a whole functions, it is necessary to analyze the operation of each node. And this, first of all, is an understanding of what a kinematic pair is.
Basic terms
If we take two objects (links) that are in contact with each other, and at the same time such a connection is movable, then we have a kinematic pair (KP). Its distinctive feature is a certain limitation in the movement of links.
A rigid body can have a limited ability to move, and then there is such a thing as a connection condition. Up to five communication conditions are counted, considering the interaction of links in a pair. Hence the division into classes. Five of them were brought out for kinematic pairs, each of them has its own degree of mobility. There are six degrees of mobility in the class. All modern mechanics is based on the use of the last three classes of kinematic pairs.
Each body (link) has its own geometry. Therefore, the elements are in contact with each other, the parts corresponding to this form. It turns out that the CP will be able to perform only such a movement,which will not contradict the geometry of the links. In addition, in order to make any movement relative to each other, one link must be permanently fixed, and the second in the pair is affected.
Each point in the link passes a path (trajectory) at the moment of movement. This trajectory may take the form of a curve located on a plane. When the planes of the curves of the path of the links in the pair are parallel to each other, this is a flat pair. If the curves of movement of the points of the links are located in three-dimensional space, the kinematic pair is spatial.
Views
The following types of mechanisms exist.
Pair of rotary type is a single-moving system. The links that make up such a pair are capable of performing only a characteristic rotation around the rod or axis. In this case, the contact of the elements is carried out on the surface of a cylindrical shape. Such a geometric system is closed and belongs to the lowest. The analogue mechanism in the area of higher pairs has a ball bearing.
The translational interaction pair is the same as the previous one in terms of single-motion. In such a system, the links can only perform translational movement in a straight line. The mechanism is the lowest pair, closed in geometric parameters.
Pair of type of interacting cylinders. This system is already two-movable, its geometry is closed. It is the lowest - the links can both rotate and move along in the forward direction.
Spherical type pairis a tripartite system. The freedom of such a pair has a degree that allows its links to rotate in three-dimensional space, describing the coordinate axes. It is also the lowest geometrically closed mechanism.
Pair of spherical type with a finger - two-movable. The movement (relatively independent rotation) of the links in this pair is limited by the pin and slot. A pair of lower order is geometrically closed.
The screw-type pair has a single-moving degree of freedom. The lower order mechanism is a geometrically closed system in which only helical movement is possible with a certain step. Movement in angular and linear direction is strictly unique.
Pair of flat type, plane-cylinder, plane-ball. In these mechanisms, force closure is used. According to the class, the first belongs to the lower, the rest - to the higher systems. In practice, such kinematic pairs of links have not been used.
Classification
KPs are classified as follows.
By the type of connection at the point of contact
Pairs of lower order are in contact with links along the surfaces. They have found wide application in mechanics, have a simpler design than higher pairs. Structurally, their links are in contact with planes and slide along them. Thus, there is a uniform distribution of the load inside the element, but the friction at the connection point of the links, respectively, increases. The positive aspect of low-order pairs is that it is possible to transfer large loads from link to link.
Higher kinematicpairs have links in contact along a curve or at points. Their main purpose is to reduce the degree of friction between the elements of the links during movement. A classic example of higher pairs is bearings or rollers. The internal design of these elements does not affect the movement of links connected in pairs. To simplify the mechanism, methods are used to replace higher kinematic pairs with lower analogues.
By the type of relative movement that the links of the pair make
- Rotational.
- Progressive.
- Cylindrical.
- Spherical.
- Screw.
- Flat.
If there are only pairs in the mechanism that use only the first four types of movement, then it is called lever.
According to the type of communication between links
- Due to force effects, such as spring pressure, body mass, compressed gas or liquid, inertial forces.
- Due to the geometric design of the elements of the pair.
- According to the degree of mobility of the links during movement.
- By the number of connection conditions.
Reversible and irreversible mechanisms
By the possibility of movement of links in the system relative to the choice of a conditionally fixed link, reversible and irreversible CPs are distinguished.
If in a mechanism any element in a free state repeats the relative movement of an element in a conditionally stationary state, then the kinematic pair is considered reversible (example - single-moving pairs).
If in the mechanism each element in a free state performsits relative motion is different from others, then such a pair is irreversible.
Types of gears in mechanics
Mechanical transmission is understood as a mechanical system that converts the kinematics and energy of the engine into a form acceptable for use by the working bodies of machines in order to function in a given mode.
Transmissions happen:
- Gear type. Such a connection is built on cylindrical and conical elements. The former transmit motion in one plane, the latter at an angle. Gears are characterized by compactness and the ability to transmit high power. They are highly efficient but generate noise and require lubrication.
- Screw type. In addition to the classic screw, this category includes hypoid and worm gears. The latter type of mechanical transmission is used when it is necessary to obtain a large gear ratio. They are also distinguished by their quietness and smoothness in operation and the ability to self-brake. The disadvantages include low efficiency and high wear.
- On flexible elements. Here, movement and energy are transmitted in the same plane through various belts and chains. Belt drives are simple and can cover long distances.
- Friction type. In bonds of this nature, the force of friction is applied. They are used in mechanisms, the operation of which takes place in difficult conditions.
Ball Type
The main purpose of the ball joint,so that the tie rod of the steering rack can be connected through the lever with the steering wheel strut. The design of the hinge includes a tip; crackers, a spring, a clamping cap, a ball pin, an oiler are built into it. The spring presses on the crackers of the ball joint, which hold the pin with spherical surfaces. This design ensures that the mechanism remains operational even when it is worn.
Hinge hinge
Hinges or canopies are mechanisms built on the basis of a cylindrical hinge. They serve to open and close doors, windows, furniture doors. The design of the hinge includes two rectangular sheets (cards) in which fastening holes and a rod are drilled. Hinges are made mainly from steel and various alloys.
Conclusion
It is interesting that human joints represent all the main types of kinematic pairs described above. Therefore, the need for understanding the processes occurring in mechanics is obvious.
