Car Maintenance Basics

Chapter 2 Project 2 Basics of Vehicle Maintenance Technology ([-])
Turn the worn parts at an angle or turn them over, and replace the worn parts with unworn parts. This method is called the turn and flip repair method.For example, after the keyway on the shaft is worn out, the shaft can be turned at an angle to reprocess the keyway, as shown in Figure I-12(a).After the uniform pin hole on the part is worn, it can be changed to a certain angle and re-drilled, as shown in Figure 1-12(b).After the tooth end face of the ring gear of the flywheel is worn out, just remove the ring gear, turn it over and press it on the flywheel again for use.

(a) Repair of worn key tip Repair of worn bolt holes

The advantages of the machining repair method are: economical materials, simple process, good quality, prolonging the service life of parts, and suitable for repairing expensive parts.Its disadvantages are: the precision of machining is high, and the hardness and strength of parts with high hardness and alternating load must be guaranteed.

[-]. Pressure processing repair method
The pressure processing repair method uses external force to cause plastic deformation of the metal of the part under heating or normal temperature, and restores the geometric shape and size of the part with metal displacement.It is suitable for restoring the shape and size of the surface of worn parts and for bending and twisting correction of parts.

When using the pressure processing repair method, attention should be paid to the nature of the part material.Such as low-carbon steel, aluminum, copper and other materials with good plasticity can be carried out at room temperature; for materials with poor plasticity such as medium-carbon steel and high-carbon steel, it needs to be heated to a certain temperature before processing.

Pressure processing repair methods include upsetting method, punching method, reduction method, elongation method, embossing and correction.

①Upsetting method.The upsetting method is a method of increasing the outer diameter of a solid part by reducing the height of the part.Such as increasing the working surface of the valve, etc.

② Chong Dafa.The punching method is a method of restoring the worn outer diameter by expanding the inner diameter of the hollow part and increasing the outer diameter.It is suitable for piston pins, bronze sleeves, etc.

③ reduction method.The reduction method is a method of reducing the inner diameter of hollow parts by extruding the outer diameter.It is suitable for repairing bushes, outer rings and other hollow parts.

④ elongation method.The elongation method is a method of restoring its length by elongating rod-like parts.It is suitable for tie rods, valve stems, etc.

⑤ Embossing method.The embossing method is to use a toothed knurling knife to squeeze the worn surface of the part to produce grooves or peaks to increase the outer dimension of the wear.It is suitable for restoring interference surfaces of static fit parts.

⑥ correction method.The correction method is used to repair twisted or bent rods or shaft parts. There are two types of cold press correction and hot press correction.Generally, cold pressure correction is used, and hot pressure correction is used when the deformation is large.It works for bent crankshafts, twisted connecting rods, etc.

The advantages of pressure processing repair method are: simple process and material saving.The disadvantage is: for parts with complex shapes, more complicated tooling needs to be processed, which is only suitable for parts with certain plasticity.

Three, electro-O processing repair method

Electroplating is to immerse the metal workpiece in the electrolyte, use the part as the cathode, and pass it through direct current. Under the action of the current, the electrolyte will undergo electrolysis, so that the metal in the solution will precipitate and accumulate on the surface of the plated part to form electroplating. layer.

The electroplating repair method can not only restore the size of the parts and improve their surface properties; at the same time, because the temperature in the electroplating process is not high, it will not cause deformation of the parts, nor will it affect the original heat treatment performance.Electroplating is one of the important methods of auto parts repair.At present, the most widely used are chrome plating and copper plating.

1. The basic principle of electroplating

In the electrolyte, electrolyte molecules are electrolytically separated into negatively charged negative ions and positively charged positive ions.For example, in the copper sulfate electrolyte, the copper sulfate molecules are decomposed into positively charged copper ions and negatively charged sulfate ions. The equation is expressed as inserting two electrodes into the sulfuric acid solution, with the parts as the cathode and the copper plate as the anode (Fig. 1-13), turn on the DC power supply, at this time the positive ions (copper ions) in the electrolyte move towards the cathode, and after reaching the cathode, they obtain the electrons they lack on the cathode, and transform into neutral copper atoms and attach to the cathode ; At the same time, negative ions (acid ions) move to the anode and react chemically with the copper plate of the anode to generate copper sulfate.Copper sulfate is dissolved in water again to replenish the lost ions in the electrolyte.Through a continuous chemical reaction, the surface of the part is coated with a layer of copper.This is the basic principle of electroplating.

2. Chrome plating

Chromium plating is used most in auto repair.When chrome plating, the parts are used as the cathode, the lead plate is used as the anode, and the aqueous solution of chromic anhydride is used as the electrolyte by adding a certain amount of sulfuric acid.After the direct current is turned on, metal chromium precipitates on the cathode and adheres to the surface of the plated part to form a chromium plating layer.The chrome-plated layer has the advantages of high hardness, heat resistance, corrosion resistance and wear resistance.The disadvantage is that the mechanical properties of the chrome-plated layer deteriorate as the thickness increases, so the thickness of the chrome-plated layer should generally not exceed 0.5mm.In automobile maintenance, electroplating is often used to repair piston pins (Figure 1-14), valve lifters (Figure 1-15), camshaft journals, steering knuckle shafts, kingpins and other parts.

3. Copper plating

Copper plating is a process that is relatively easy to obtain a good coating in electroplating. The coating can be thicker, but the coating is soft and wear-resistant.It is mostly used to repair static fit parts, such as the outer surface of bronze bushing and bearing shell surface, to increase the outer diameter.It can also be used as the bottom layer of chrome plating, iron plating and nickel plating, and the copper plating on the nut can also play the role of anti-loosening.

Four, metal spraying processing repair method
Metal spraying is also called metal spraying.It uses the high-speed airflow of compressed air to blow the metal powder or the metal melted by the heat source into a mist, and then spray it onto the prepared rough and clean workpiece surface to form a metal coating.

Metal spraying is widely used in auto repair and has a history of many years.Metal spraying mainly includes electrospraying and gas flame spraying, and there are also high-frequency electrospraying, plasma spraying and explosive spraying.Electrospraying uses an electric arc to melt the wire, and gas flame spraying uses an oxygen-acetylene flame to melt the wire.Both use high-pressure air to evenly impact and adhere molten metal particles to the surface of the part to form a spray coating.In automobile repair, metal spraying is mainly used to fill the cracks of cast iron parts, such as cylinder blocks and the shells of various parts; to restore the size of worn parts, such as crankshafts, camshaft journals, valve lifters, etc.; to prevent metal rust and Decoration, such as spraying zinc and lead on the bumper, door handle and the inner wall of the gasoline tank, etc.

Advantages of the metal spraying repair method: various metals can be sprayed onto the surface of the parts as needed to obtain a spray coating of 10-15mm, so it can repair the parts with serious wear.The sprayed coating of the repaired parts has high hardness and porosity, so it has good wear resistance.

Its disadvantages are: the bonding strength between the sprayed layer and the surface of the part is not high, and the sprayed layer is prone to fall off. When spraying small parts, there is a lot of metal loss; the sprayed layer is hard and brittle, and it is not easy to machine.

Five, welding repair method
The welding repair method is to use high temperature to melt the welding repair material and the local metal of the parts to connect the metal parts.Welding is divided into fusion welding and brazing.

Fusion welding is the process of locally heating the parts to the melting point and using the cohesive force of molecules to connect the metal parts.Fusion welding is divided into arc welding and gas welding.

(1) arc welding (electric welding)

① Working principle of arc welding.The working principle of arc welding is shown in Figure 1-16.One pole of the electric welding machine is connected to the welding rod through the welding tongs, and the other pole is connected to the weldment.When welding, the welding rod and the weldment are short-circuited instantaneously, because the short-circuit current passes through the contact,

A large resistance heat is generated, and the temperature of the metal at the contact point rises rapidly and melts.When the electrode is lifted up, a "thin neck" is formed at the contact point, and a large current passes through the "thin neck", which further increases the temperature, and makes the surrounding metal vapor and hot air violently heated and ionized.When the electrode is lifted 2-4mm away from the weldment, under the action of voltage, the electrons escaping from the negative electrode also impact the gas molecules on the way to the positive electrode, so that the gas is further separated, so the charged ions continue to increase, and the gas between the electrodes is absorbed Breakdown produces strong light and heat.

The heat released by the arc discharge heats the metal in the joint part of the weldment to a molten state to form a weld, and a layer of slag crust is formed on the surface of the weld.

②Main equipment and tools for arc welding:

a. Electric welding machine (Figure 1-17).There are two types of welding machines, AC and DC.The arc of the DC welding machine is relatively stable and the combustion is uniform. By connecting the parts to the positive or negative pole, the heating degree of the parts can be properly controlled, so the welding quality is good; but the equipment of the DC welding machine is more complicated, the efficiency is low, and the cost is high. Weld repairs.AC welding machine has simple equipment, high efficiency, and is widely used in automobile repair; but the arc is unstable and the temperature is not easy to control.

b. Welding pliers (Figure 1-18).Welding pliers are used to hold the welding rod and conduct current.The jaws are made of metal with good electrical conductivity, and the shell is made of insulating material.

c. Cable.Welding cables are used to conduct welding current.Two cables are used, one is connected to the welding tongs, and the other is connected to the weldment, and its specification is determined according to the capacity of the electric welding machine.

③ Electrode (Figure 1-19).The welding rod consists of a welding core and a coating covering the outside.

a. Solder core.The core is used to conduct current and act as a filler metal.The diameter of the welding core is the diameter of the welding rod, and the diameters of commonly used welding cores are 3.2mm, 4mm, and 5mm.

b. Drug skin.The role of the coating is to stabilize the arc, form a protective layer, prevent air from entering the weld, remove harmful elements such as oxygen, sulfur, and phosphorus, and allow gold elements such as manganese, titanium, and chromium to penetrate into the weld to increase its strength.

④Basic operation of arc welding:

a. Joint form.Commonly used joint forms include butt joint, lap joint, corner joint and T-shaped joint, as shown in Figure 1-20.The joint form should be selected according to the weldment thickness, structural form and strength requirements.Since the depth of arc melting metal is only 3-4mm, thicker weldments must be grooved to penetrate.

(a) on the pole (b> lap joint (c> angle pole (d> T-shaped joint)

b. Welding specification.The selection of the welding rod should be the same as the material of the parts to be welded. The diameter of the welding rod depends on the thickness of the weldment. The thicker the weldment, the larger the diameter of the welding rod.The selection of welding current should be determined according to the diameter of the electrode. The larger the diameter of the electrode, the larger the current, and the smaller the diameter of the electrode, the smaller the current.

⑤ Matters needing attention during arc welding:

a. The shell of the electric welding machine should be connected to the ground wire, and the insulation between the welding tongs and the cable should be reliable.When operating, wear protective masks, gloves and rubber-soled shoes.

b. The joints of the electric welding machine circuit must be in close contact to avoid heating due to poor contact.The welding tongs should not be placed on the workbench to avoid short circuit of the welding machine.After welding, the power should be cut off.

c. The workplace should be ventilated and have exhaust equipment.

(2) Gas welding
① Working principle of gas welding.Gas welding is to use the flame when acetylene and oxygen are mixed through the welding torch as a heat source, heat the welding metal to a molten state, and form a welding pool, and then continuously feed the welding wire into the welding pool, and fuse them into one, and form a weld after cooling. seam.As shown in Figure 1-21.

② Gas welding equipment.The connection mode of equipment and pipeline system used for gas welding is shown in Figure 1-22.

a. Acetylene bottle.The acetylene bottle is a device for storing dissolved acetylene.When in use, the acetylene dissolved in acetone will continuously escape, the pressure in the bottle will decrease, and the remaining acetone can be used for refilling.The surface of the acetylene bottle is painted white and has the words "Acetylene" written in red paint.

b.Oxygen bottle.The oxygen cylinder is a container for storing and transporting high-pressure oxygen, with a volume of 40L and a maximum pressure of 14.7MPa (150kg/cm2).The exterior of the oxygen cylinder is painted sky blue, and the word "oxygen" is written in black paint.

(End of this chapter)