Basic knowledge of automobile machinery 1

The effect of carbon mass fraction on the properties of iron-carbon alloys:
The structure of iron-carbon alloy of any composition at room temperature is composed of two phases of ferrite and cementite. It is just that with the increase of the carbon mass fraction, the amount of ferrite is relatively reduced, while the amount of cementite is relatively increased, and the shape and distribution of cementite also change, thus forming different organizations, but the organization is different, and the performance is different.
When the mass fraction of carbon is less than 0.9%. As the mass fraction of carbon increases, the strength and hardness of steel increase linearly, while the plasticity and toughness continue to decrease. This is because as the mass fraction of carbon increases, the amount of cementite in the steel increases and the amount of ferrite decreases; when the mass fraction of carbon is greater than 0.9%, the secondary cementite has formed relatively complete along the grain boundary As a result, the strength of the steel began to decrease significantly, but the hardness was still increasing, and the plasticity and toughness continued to decrease. In order to ensure sufficient strength, certain plasticity and toughness of industrial steel tools, the mass fraction of carbon in steel generally does not exceed 1.3%. White cast iron with a carbon mass fraction greater than 2.11%, due to the large amount of cementite in the structure, high hardness and poor plasticity and toughness, is difficult to cut and cannot be processed by forging methods, so it is rarely used in mechanical engineering .
Force couple refers to the simplest force system composed of two forces of equal size and opposite direction but not collinear on a rigid body. (M=Fd) The three elements to measure the rotating effect of the couple are: the size of the couple's moment, the turning of the couple and the acting surface of the couple. Nature: The couple cannot be combined into a force; the moment of the couple at any point in the plane is equal to a constant, which is equal to the size of the couple moment itself, and has nothing to do with the position of the centroid.
The reasonable structure design of the shaft must meet the following basic conditions: 1. Accurate positioning and fixing of the shaft and the parts on the shaft. 2. The shaft structure must have good manufacturability. 3. Minimize stress concentration as much as possible. 4. The size of each part of the shaft should be reasonable.
The key connection is mainly used for the circumferential fixation between the shaft and the parts on the shaft to transmit torque, and some can also achieve axial fixation or axial movement at the same time. This kind of connection has a simple structure, reliable work, and convenient assembly and disassembly.
Spline connection: When the torque to be transmitted is very large and ordinary flat keys cannot meet the requirements, spline connection should be used.
Pin connection is usually used to determine the relative position between parts and is an important auxiliary part for combined processing and assembly. Can be divided into cylindrical pins and tapered pins. The former relies on the interference fit with the pin hole and is suitable for occasions that are not often disassembled. The latter is suitable for occasions that are frequently disassembled.
Shaft-hub connection is a common method for making circumferential static or dynamic connection between the parts on the shaft and the shaft. The flat key and spline connection are the most widely used.
Sliding bearings are suitable for the following situations: 1.Rotating speedExtremely high, heavy load, especially high rotation accuracy requirements. 2. Withstand huge shocks and vibrations. 3. Bearings with split structure must be used. 4. The warp dimension is required to be extremely small.
Analysis of the motion diagram of internal combustion engine mechanism:pistonIt forms a low pair with the frame, the piston is connected with the connecting rod, and the piston transmits force to theCrankshaftMakeCrankshaftRotate the crankshaft to move the gear along with the gear, and the gear drives another gear to rotate, and the other gear drives the cam to rotate the cam to make the top rod move up and down.switchValveExhaust the valve. The whole process reciprocates like this in an instant.
Identification of the type of hinged four-bar mechanism: In the hinged four-bar mechanism, if the sum of the length of the shortest rod and the longest rod is less than or equal to the sum of the lengths of the other two rods, the rod adjacent to the shortest rod is used as the frame. It is a crank and rocker mechanism. When the shortest rod is taken as the frame, the mechanism is a double crank mechanism. When the rod opposite to the shortest rod is taken as the frame, the mechanism is a double rocker mechanism.
The advantages of belt transmission are: simple structure, convenient maintenance, and low manufacturing and installation accuracy requirements. The belt is flexible, can absorb vibration, run smoothly, and has low noise. It is suitable for transmission between two shafts with larger center distance. It can slip on the pulley to avoid hard damage to other parts. The disadvantages are: the transmission ratio cannot be ensured, and the anti-expansion strength is small and can transmit large power.
The characteristics of V-belt transmission: The friction force generated during transmission is relatively large, so the transmission power is also relatively large. The wheel groove adopts wedge-shaped pressurization technology. It can slip and play the role of a protective belt.
The characteristics of the synchronous belt drive: its production is thin and light, with high strength, and the belt speed can reach 30M/S. The transmission efficiency is high, which can be close to 98%.
Belt drive maintenance: The drive belt should be prevented from contacting with acids, alkalis, oils and other media that are corrosive to rubber to extend its service life. For V-belt drives, all V-belts should be replaced when replacing them, so as to avoid uneven load distribution caused by the mixed use of new and old belts, resulting in rapid loss of new belts. When installing the transmission belt, the two shafts must be parallel, and the grooves of the two pulleys must be aligned. Otherwise it will accelerate the wear of the belt. When installing, you should first reduce the center distance, loosen the tension wheel, and adjust the belt to a proper tension after the belt is installed. The belt cannot be forced in.
The advantage of chain drive: Because it is a meshing drive, the average transmission ratio is kept constant. No initial tension is required during installation, so it is beneficial to prolong the life of the bearing during work. Can work in harsh environments. It has high strength and can transmit large circular force.
Disadvantages: poor running stability, and a constant instantaneous transmission ratio cannot be guaranteed during work. The noise and vibration are large. The requirements for manufacturing and installation are relatively high. Overload cannot provide protection.