Car Maintenance Basics

Chapter 3 Project 3 Basics of Vehicle Maintenance Technology ([-])
c. Pressure reducer.The pressure reducer is an instrument used to reduce the high-pressure oxygen (or acetylene) in the oxygen cylinder (or acetylene cylinder) to the working pressure required for welding, and to keep the pressure basically stable during the welding process, as shown in Figure 1-23.When using a pressure reducer, first slowly open the valve of the oxygen cylinder (or acetylene cylinder), and then turn the pressure regulator handle until the pressure reaches the required value.When stopping work, first loosen the adjusting screw, and then close the valve of the oxygen cylinder (or acetylene cylinder).

d. Welding torch.The welding torch is a tool for mixing acetylene and oxygen in a certain proportion to obtain a gas welding flame. The appearance of the welding torch is shown in Figure 1-24.When working, first open the oxygen and then open the acetylene valve, the two gases will be evenly mixed in the mixing tube, and then sprayed out from the welding nozzle, and then ignited to burn.By controlling the size of each valve, different mixing ratios of oxygen and acetylene can be adjusted.The general welding torch has 5 kinds of welding nozzles with different diameters, so as to be used for welding workpieces with different thicknesses.The most widely used welding torch in my country is the H01 type. Table 1-3 lists the basic parameters of two types for reference. The meanings of each part in the H01-2 (or 6) model are as follows: H" represents the welding torch, "1" represents the injection suction type. "2" (or "6") represents the maximum thickness of the weldable low carbon steel plate 2 (or 6mm ).

③ welding wire and flux.

a. Welding wire.During gas welding, the welding wire is melted and filled into the weld, so the quality of the welding wire has a great influence on the performance of the welding.When welding various metals, corresponding welding wires should be used.

The diameter of the welding wire is mainly determined by the thickness of the workpiece. For the diameter of the carbon steel gas welding wire, refer to Table 1-4.

b. Flux.The role of the flux is to remove oxides from the surface of the weld and protect the weld pool metal.When gas welding low carbon steel, because the flame itself has a considerable protective effect, no flux is required.When gas welding cast iron, non-ferrous metals and alloy steel, corresponding flux is required.

Commonly used fluxes are: CJ101 (used for welding stainless steel, heat-resistant steel, commonly known as stainless steel welding powder), CJ201 (used for cast iron), CJ301 (used for copper alloy), CJ401 (used for aluminum alloy).

④ Gas welding flame.During gas welding operation, adjusting the oxygen valve and acetylene valve of the torch can change the mixing ratio of oxygen and acetylene to obtain three different gas welding flames: neutral flame, carbonization flame and oxidation flame.

a. Neutral flame.A neutral flame is obtained when the ratio of oxygen to acetylene is 1.1-1.2.The flame center is bright white, the inner flame is orange red, the outer flame is light blue, and the inner flame has the highest temperature, about 3150°C.Neutral flame is used for welding low carbon steel, medium carbon steel, alloy steel, red copper and aluminum alloy, etc. It is the most widely used gas welding flame, as shown in Figure 1-25(a).

b. Carbonization flame.The carbonizing flame is obtained when the ratio of oxygen to acetylene is 0.85-0.95.Due to the small amount of oxygen and incomplete combustion, the entire flame is longer than the neutral flame, and the higher the proportion of acetylene in the flame, the longer the flame.When there is too much acetylene, there will be black smoke.Carbonization flame is used for welding cast iron, high carbon steel, hard alloy and magnesium alloy, etc., as shown in Figure 1-25(b).

c. Oxidation flame.The oxidizing flame is obtained when the ratio of oxygen to acetylene is 1.3-1.7, the flame becomes shorter and only consists of the flame core and the outer flame.Because there is more oxygen, the combustion is intense, the flame is obviously shortened, the flame center is conical, and there is a strong "hissing" sound.Oxidizing flames are easy to oxidize metals, and are generally not used except for welding brass, as shown in Figure 1-25(c).

⑤Basic operation method.The basic operation of gas welding has several steps such as ignition, flame adjustment, welding and flame extinguishing.

a. Ignition.When igniting, first open the oxygen valve slightly to blow off the residual debris in the gas path, then open the acetylene valve and ignite the flame, which is a carbonization flame at this time.

b. Adjust the flame.After the flame is ignited, gradually open the oxygen valve to adjust the carbonization flame to a neutral flame.

c. Welding.When welding, hold the welding torch in your right hand and the welding wire in your left hand.At the beginning of welding, in order to heat and melt the workpiece as soon as possible to form a weld pool, the inclination angle of the welding torch is close to the vertical workpiece, as shown in Figure 1-26.During normal welding, the inclination angle of the welding torch is generally kept between 40°-50°.At the end of welding, the inclination angle should be reduced to better fill the arc crater.

d. Turn off the flame.When stopping welding, the acetylene valve should be closed first, and then the oxygen valve to avoid tempering.

⑥Safety Precautions:
a. Oxygen cylinders must not be impacted or dried at high temperatures. They should be stored in the basement. Oil is prohibited. The valve of the cylinder can only be lubricated with talcum powder or glycerin.

b. Fireworks are strictly prohibited near acetylene cylinders and oxygen cylinders are not allowed.

c. When tempering, close the acetylene valve immediately.

2. Brazing
Brazing is a method of melting and welding parts using low-melting tin, lead, copper, silver and other metals.

The difference between welding and brazing is that the weldment does not have to be melted during brazing, and the melting point of the solder is always lower than the melting point of the weldment.Soldering parts with eutectic solder tin or lead with a melting point lower than 40C is called soft soldering; soldering parts with refractory solder copper or silver with a melting point higher than 550C is called hard soldering.

During brazing, since the workpiece does not melt, the mechanical properties of the workpiece components are not affected, and the solder is soft and easy to process after welding.The brazing process is simple and the cost is low, but the brazing connection strength is low.

Soldering is used to repair parts that do not require high strength, such as floats, gasoline pipes, etc.; brazing is used to repair parts that do not meet the required soldering strength, such as brake oil pipes, compressed air pipes, etc.The following takes soldering as an example to briefly introduce brazing.

Reliable fire-fighting measures and good ventilation equipment should be taken in the workplace.

⑴ Welding tools

① Soldering iron.Soldering work is mainly carried out with a soldering iron.Soldering irons are usually made of copper.Copper absorbs more heat, transfers heat faster, and can quickly transfer more heat to the workpiece to be welded; at the same time, copper oxidizes slowly, which can prolong the use time of the tip of the soldering iron.

②Auxiliary tools.During the brazing process, auxiliary tools used to repair the soldering iron and clean the welding part and weld bead, such as files, scrapers, wire brushes and pliers, etc.

(2) Solder and flux

①Solder.The solder used for soldering is an alloy of tin and lead. Pure tin has poor fluidity, high price, and is rarely used. Generally used solder contains 50% tin and [-]% lead.

② Welding flux.The role of flux is to remove the dirt at the weld during welding, protect the metal from oxidation, help the solder flow, and increase the welding strength.

(3) Basic method of brazing

① Use a file, scraper or wire brush to remove oil from the weld.

②Clean the soldering iron and remove copper oxide with a wire brush.

③ Apply flux to the welding part.

④ Apply solder with a heated soldering iron, stop for a while at the welding part to heat up the weldment, and then move slowly to make the solder evenly flow into the weld to form a clean and smooth weld bead.

⑤ When the welding seam is long, the welding parts can be fixed, pressed firmly and coated with flux, spot welding first, and then welding all the welding seams.

(4) Safety precautions for brazing
①The soldering iron should be placed steadily to prevent it from falling, so as not to cause fire or burns.

② When using a soldering iron, you should first pay attention to whether the power supply voltage is consistent with the soldering iron voltage.In case of inconsistency, it is not allowed to use.After the power is turned on, you can't leave it casually.After use, the power should be disconnected.

③ When testing the temperature of the soldering iron, use solder instead of touching it with your hands to prevent burns.

[-]. Adhesive repair method
The bonding repair method is a method of bonding and repairing parts of the same or different materials with little stress and low working temperature using adhesives.

Parts are repaired by bonding, which has simple process, simple equipment, low cost, and will not cause deformation of parts and changes in metal structure. Therefore, it is widely used to repair cracks and fill holes left in parts manufacturing.

1. Epoxy resin bonding

Epoxy resin is a synthetic polymer resinous compound.It can form chemical bonds with the surface of many kinds of materials, resulting in greater adhesion.Therefore, the glue made from it has a wide range of uses and can bond various metal or non-metal materials, such as steel, wood, rubber, ceramics, glass, plastics, etc.It also has the advantages of acid, alkali and salt corrosion resistance, is not afraid of water and oil, and has high electrical insulation.Its disadvantage is that it is fragile in nature, not resistant to impact, low in tensile strength, and the bonding strength will decrease when the temperature exceeds 100-C.

Epoxy resin is used in repairing cracks in automobile repairs, such as cracks in distributor covers, cylinder blocks and cylinder heads (non-stressed parts), carburetors, etc., can be repaired; in repairing wear, such as shaft Such parts can be restored to their original size with glass cloth dipped in epoxy resin and pasted on the outside of the shaft; in addition, it can also be leak-proof and sealed to solve oil, water, and air leaks.

2. Inorganic adhesive bonding

Inorganic adhesives are formulated from inorganic substances such as copper chloride and phosphoric acid, so they are called inorganic adhesives.Its advantages are high temperature resistance and high strength. Generally, the temperature resistance can reach a high temperature of 700C for a short time, and it can be used at 200C for a long time.Its disadvantages are high brittleness, low plane bonding strength, and resistance to oil, water, acid, and alkali erosion and corrosion.

Inorganic adhesives are widely used in the bonding or inserting of cylinders, brake master cylinders, various oil seals, journals, etc. in auto repair.For example, when repairing the screw hole of the cylinder head, the damaged screw hole can be increased to 15.5mm, and then the corresponding screw can be selected, coated with inorganic adhesive and screwed into the screw hole. After heating and curing, it can be processed into The original standard screw holes.

task implementation
In the process of task implementation, they will learn the repair methods of auto mechanical parts and the operating procedures and maintenance methods of equipment, and implement them in groups to complete the work within the specified time.

[-]. Technical standards and requirements
① All operations comply with safety technical standards.

② All operations meet the technical requirements for maintenance of mechanical parts.

③ Safety accidents are not allowed during the operation.

[-]. The work to be done

①Practical training on the use of arc welding.

②Practical training on the use of gas welding.

③Practical training on the use of electric soldering iron.

④ Complete the training report.

learning assessment
[-]. Self-evaluation

1. Summarize the repair methods of auto mechanical parts.

2. Please write down your experience of this learning task.

3. Self-evaluation of the learning environment (including dress, learning attitude, mastery of knowledge and skills, completion of work pages, etc.).

thinking and practice
1. What are the requirements for car disassembly?
2. What are the methods for removing carbon deposits?

3. What are the cleaning methods for metal parts?

4. How to clean non-metallic parts?
5. What are the inspection methods for parts?

6. How are the parts sorted?

7. What are the requirements for car assembly?
8. What are the safety rules for using gasoline?
9. What are the safety rules when starting the engine?
10. What are the rules for working safely under the vehicle?
11. What are the safety rules for the use of batteries?
12. What is the machining repair method?
13. What does the machining repair method include?
14. What does the pressure processing repair method include?
15. What is electroplating repair method?
16. Briefly describe the basic principles of electroplating.

17. What are the characteristics of chrome plating?
18. What is metal spraying processing repair method?
19. What is the role of metal spraying in auto repair?
20. What is the welding repair method?
21. Briefly describe the working principle of arc welding.

22. How to choose welding rod?
23. Briefly describe the principle of gas welding.

24 Briefly describe the function and operation method of the pressure reducer.

25 What are the flames of gas welding?
26 Briefly describe the operation method of gas welding.

27 What are the precautions for gas welding?

28 What is brazing?

29 What is the difference between welding and brazing?
30 Briefly describe the basic method of brazing.

31 What are the safety precautions for brazing?

32 What is the adhesive repair method?
33 What are the characteristics of the bonding repair method?
34 What are the characteristics and uses of epoxy resin bonding?

35 What are the characteristics and uses of inorganic adhesives?

(End of this chapter)