Any complex mechanism in mechanical engineering consists of a combination of simple elements. To understand how the system as a whole functions, it is necessary to disassemble the operation of each node. And this is, first of all, an understanding of what a kinematic pair is.
Key Terms
If we take two objects (links) that are in contact with each other, and at the same time such a connection is mobile, then we have a kinematic pair (CP). Its distinctive feature is a certain limitation in the movement of links.
A solid body can have a limited ability to move, and then a concept such as a condition of communication arises. Up to five communication conditions are counted, considering the interaction of the 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. The class has six degrees of mobility. All modern mechanics is based on the application of the last three classes of kinematic pairs.
Each body (link) has its own geometry. Therefore, the elements that correspond to this form are in contact with each other. It turns out that the CP can only perform such a movement that will not contradict the geometry of the links. In addition, in order to make any movement relative to each other, one link must be stationary fixed, and the second in the pair is affected.
Each point in the link at the time of movement passes a path (trajectory). This trajectory may take the form of a curve located on a plane. When the planes of the location of the paths of the links in a pair are parallel to each other - this is a flat pair. If the motion curves of the points of the links are located in three-dimensional space, the kinematic pair is spatial.
Kinds
The following types of mechanisms.
The rotary pair is a single-motion system. The links making 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 lower ones. The analogue mechanism in the field of higher pairs is a ball bearing.
A pair of translational interaction is the same as the previous one, in terms of one-mobility. In such a system, links can only perform translational motion in a rectilinear direction. The mechanism is the lowest pair, closed in geometric parameters.
A pair of type interacting cylinders . This system is already bi-mobile, its geometry is closed. It is inferior - the links can both rotate and move along in the forward direction.
A pair of spherical type is a three-moving 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 a lower geometrically closed mechanism.
A pair of spherical type with a finger is two-movable. The movement (relatively independent rotation) of the links in this pair is limited by a finger and a slot. The lower order pair is geometrically closed.
A screw-type pair has a single-moving degree of freedom. The lower order mechanism is a geometrically closed system in which only a helical movement is possible with a certain step. The movement in the angular and linear direction is strictly unambiguous.
A pair of flat type, plane-cylinder, plane-ball. These mechanisms use a power short circuit. According to the class, the first refers to the lower ones, the rest to the higher systems. In practice, such kinematic pairs of links have not found application.
Classification
KP have the following classification.
- By type of communication at the point of contact.
Vapors of a lower order contact the links on the surfaces. They are widely used in mechanics, have a simpler design than higher pairs. Structurally, their links are in contact with the planes and glide over them. Thus, a uniform distribution of the load inside the element occurs, but the friction at the connection point of the links, respectively, increases. The positive thing about lower order pairs is that large loads can be transferred from link to link.
Higher kinematic pairs have contact links on 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 coupled. To simplify the mechanism, apply methods of replacing higher kinematic pairs with lower analogues.
- By the kind of relative movement that the links of the pair make.
- Rotational.
- Progressive.
- Cylindrical.
- Spherical.
- Screw.
- Flat.
If the mechanism contains only pairs that use only the first four types of movement, then it is called lever.
- By type of communication between the links.
- Due to force action, for example, spring pressure, body mass, compressed gas or liquid, inertial forces.
- Due to the geometric design of the elements of the pair.
- By the degree of mobility of the links during movement.
- By the number of communication conditions.
Reversible and irreversible mechanisms
If possible, the movement of the links in the system regarding the choice of a conditionally fixed link distinguishes between reversible and irreversible gearboxes.
If in the mechanism any element in a free state repeats the relative motion of the element in a conditionally stationary state, then the kinematic pair is considered to be reversible (for example, one-moving pairs).
If in the mechanism each element in a free state makes its relative motion different from the others, then such a pair is irreversible.
Types of gears in mechanics
By mechanical transmission is meant a mechanical system that converts the kinematics and energy of the engine into a form suitable for use by the working bodies of machines to function in a given mode.
Transmissions are:
- Toothed 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 transfer large powers. They are highly efficient, but produce noise during operation and require lubrication.
- Screw type. In addition to the classic helical gear, this category also includes hypoid and worm gears. The latter type of mechanical transmission is used when it is necessary to obtain a large gear ratio. They also differ in noiselessness and smoothness in work and ability to self-braking. The disadvantages include low efficiency and high wear.
- On flexible elements. Here, movement and energy are transmitted in the same plane due to various belts and chains. Belt drives are simple and can cover long distances.
- Friction type. In bonds of this nature, friction force is applied. They are used in mechanisms whose operation takes place under severe conditions.
Ball joint type
The main purpose of the ball joint is so that the steering rod tie rod can be connected via a lever with a swivel wheel strut. The hinge includes a tip; rusks, a spring, a clamping cap, a ball finger, an oil can are built in it. The spring presses on the crackers of the ball joint, which hold the finger with spherical surfaces. This design ensures the operability of the mechanism even when it is worn.
Hinge loop
Hinges or awnings - mechanisms built on the basis of a cylindrical hinge. They serve to open and close doors, windows, furniture doors. The loop design includes two rectangular sheets (cards) in which the mounting holes and the core are drilled. They make hinges mainly from steel and various alloys.
Conclusion
Interestingly, human joints are all the basic types of kinematic pairs described above. Therefore, the need for understanding the processes occurring in mechanics is obvious.