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What is the transmission ratio

In a mechanical transmission system, the ratio of the angular velocity or rotational speed of the driving wheel at the beginning and the driven wheel at the end.
Transmission ratio (i) = ratio of driving wheel speed (n1) to driven wheel speed (n2) = inverse ratio of gear index circle diameter = ratio of driven gear teeth (Z2) to driving gear teeth (Z1).
That is: i=n1/n2=D2/D1 i=n1/n2=z2/z1
For multi-stage gear transmission
1: The transmission ratio between every two shafts is calculated according to the above formula
2: The total transmission ratio from the first axis to the nth axis is calculated according to the following formula: Total transmission ratio ι=(Z2/Z1)×(Z4/Z3)×(Z6/Z5)……=(n1/n2)× (N3/n4)×(n5/n6)……

For multi-stage gear transmission

For multi-stage gear transmission

Extended information

The ratio of the angular velocities of the two rotating components in the mechanism is also called the speed ratio. The transmission ratio of component a and component b is Ⅰ=ωa/ωb=na/nb, where ωa and ωb are the angular velocities (radians/sec) of components a and b, respectively; na and nb are the rotational speeds of components a and b respectively ( Rpm) (Note: a and b after ω and n are subscripts).
When the angular velocity in the formula is an instantaneous value, the obtained transmission ratio is the instantaneous transmission ratio. When the angular velocity in the formula is an average value, the obtained transmission ratio is the average transmission ratio. For most gear transmissions and friction wheel transmissions with the correct tooth profile, the instantaneous transmission ratio is unchanged; for chain transmission and non-circular gear transmission, the instantaneous transmission ratio is variable.
For meshing transmission, the transmission ratio can be expressed by the number of teeth Za and Zb of wheel a and wheel b, i=Zb/Za; for friction transmission, the transmission ratio can be expressed by the radius Ra and Rb of wheel a and wheel b, i=Rb/Ra, At this time, the transmission ratio generally means the average transmission ratio.
In hydraulic transmission, the transmission ratio of the hydraulic transmission element generally refers to the ratio of the turbine speed S and the pump wheel speed B, that is, =S/B. Hydraulic transmission elements can also be combined with mechanical transmission elements (generally with various gear trains) to obtain various transmission ratios of different values ​​(see gear trains for gear train transmission ratios).

Description of lining material for rubber lining valve

Overview of rubber-lined valves:
Rubber-lined valve (lined valve) is a valve suitable for pipelines containing corrosive media and high sealing requirements. Generally, the applicable temperature should not be too high, and it is not suitable for use in media

double-wing-check-valve-3

double-wing-check-valve-3

containing particles to avoid scratches. Scratches or scratches the sealing surface, causing the valve to leak. Rubber-lined valve is a kind of plastic product produced by the chemical reaction of many chemical substances. The main materials are: soft rubber, hard rubber, butyl rubber, natural rubber, EPDM rubber, fluorine rubber, Lining silicone rubber, etc., its main working purposes are: diversion, adjustment, throttling, cut-off, check, diversion, overflow, etc., which means that as long as it is a general fluid valve, it can be controlled.

The main lining materials of rubber-lined valves:
【Neoprene, Code CR】
Applicable temperature: -10℃~105℃ Animal oil, vegetable oil, inorganic lubricating oil and corrosive slurries with a wide range of pH value, good wear resistance.
【Soft Rubber Code BR】
Applicable temperature: -10℃~85℃
Applicable medium: good wear resistance. Mainly used for sulfuric acid below 50%, sodium hydroxide, potassium hydroxide, neutral salt bath solution and ammonia solution, cement, clay, cinder ash, granular fertilizer and solid fluids with strong abrasiveness, and thick viscous liquids of various concentrations Wait. _
【Butyl Rubber Code IIR】 Applicable temperature: -10℃~120℃
Applicable medium: corrosion resistance and wear resistance. Can withstand most organic acids, alkalis and hydroxide compounds, inorganic salts and inorganic acid element gases, alcohols, aldehydes, ethers, ketones, esters, etc., ≤30% sulfuric acid, phosphoric acid, hydrofluoric acid, animal oil , Vegetable oil, caustic alkali and a variety of lipids. _
【Hard Rubber Code NR】 Applicable temperature: -10℃~85℃
Applicable medium: hydrochloric acid, fluorosilicic acid, formic acid and phenolic acid, hydrochloric acid, 30% sulfuric acid, 50% hydrofluoric acid, except strong oxidants (such as organic solvents such as acid, chromic acid, concentrated sulfuric acid and hydrogen peroxide) Acid, 80% phosphoric acid, alkali, salt, metal plating solution, sodium hydroxide, potassium hydroxide, neutral salt solution, 10% sodium hypochlorite, wet chlorine, ammonia, most alcohols, organic acids and aldehydes, etc.

Working principle diagram of swing check valve

Working principle diagram of swing check valve
Swing check valve, also known as check (flow) valve, is an automatic valve used on one-way flow pipelines to prevent the medium from flowing back. It depends on the pressure of the pipeline medium to open or close the valve. It can be divided into single valve type , Double valve type and multi valve type, are one of the more commonly used valves. Swing check valves are generally suitable for pipelines with relatively clean fluid media. They are not suitable for

swing-check-valve-working-principle-diagram

swing-check-valve-working-principle-diagram

working conditions with high viscosity or solid particles. Otherwise, the check valve will be insensitive to opening and cannot achieve a complete seal. The answer is not reliable enough. The swing check valve only allows the medium to flow in one direction, which can effectively prevent the medium from flowing back to prevent accidents.

The working principle of the swing check valve:
The disc of the swing check valve and the rocker are connected together, and can rotate a certain angle around the pin shaft. When the pipeline fluid flows in the specified direction (from left to right), the inlet pressure of the disc is higher than the outlet pressure. At this time, the disc is pushed away from the valve seat and rotates around the pin to a certain position, and the valve is in the open state. When the pressure difference between the two sides of the valve flap decreases to a certain level, the valve flap falls and returns to the closed state. When the fluid flows from right to left, that is, reverse flow, the pressure on the right side of the valve disc is higher than the pressure on the left side, and the force generated by the pressure difference on both sides presses the valve disc on the valve seat, and the fluid cannot pass through, the medium Can not flow back; and the greater the fluid pressure, the tighter the sealing surface is, and the better the sealing effect. To

The installation position of the swing check valve is not restricted. It can be installed horizontally in the pipeline, or on a vertical or inclined pipeline, but if it is installed on a vertical pipeline, the flow direction of the medium should be from bottom to top.

TH Valve is a professional manufacturer of butterfly valvegate valvecheck valveglobe valveknife gate valve, ball valve with API, JIS, DIN standard, used in Oil, Gas, Marine industry, Water supply and drainage, fire fighting, shipbuilding, water treatment and other systems, with Nominal Diameter of DN50 to DN1200, NBR/EPDM/VITON, Certificates & Approvals: DNV-GL, Lloyds, DNV, BV, API, ABS, CCS. Standards: EN 593, API609, API6D

Related news/knowledge:
Basic knowledge of valve;
What is double plate swing check valve;
Butterfly valve use principle and installation instructions;
Wafer type double disc swing check valve introduction

Working principle diagram of lift check valve

Lifting check valve structure:

Lifting check valve is a kind of valve that prevents pipeline media from flowing back, mainly composed of valve body, valve seat, valve flap, valve cover and other related parts. The lift check valve is equipped with a spring to ensure that the disc is always in a dynamic equilibrium state under the action of the spring. Regardless of the medium pressure at the inlet of the valve, the valve can maintain a balanced operation.

lift check valve Working principle diagram

lift check valve Working principle diagram

The working principle of the lift check valve:

The lift check valve has a guide rod on the disc, which can move up and down freely in the guide hole of the valve cover. When the pipeline medium flows in in the specified direction (from left to right), and the pressure below the valve flap exceeds the pressure above it, push the valve flap to rise along the center line of the guide hole of the valve cover, and the valve will automatically open to allow the medium to flow; if the fluid flows from the right to the right Left flow, that is, when it flows backward, the pressure above the valve flap is greater than the pressure below it. The pressure difference between the upper and lower pressures and the weight of the valve flap press the valve flap on the valve seat, so that the medium cannot pass through, that is, the fluid cannot flow back; and pipeline fluid The greater the pressure, the tighter the sealing surface is, and the better the sealing effect.

The seat sealing surface of the lift check valve can be welded, or it can be made into a seat sealing ring and then expanded or threaded on the valve body; when the valve body is made of stainless steel, the sealing surface can also be on the valve body It is directly processed from above. The sealing surface of the disc can be directly processed on the disc, or can be processed after surfacing or welded on the disc with the inner and outer periphery of the sealing ring. The straight-through lift check valve can only be installed on the pipeline in a balanced way, and the center line of the valve flap is vertical to the horizontal plane, while the vertical lift check valve is not subject to this restriction.

What is butt welding? (8)- typical workpieces

Butt welding of typical workpieces
1, butt welding of small section workpieces

wire with diameter d≤5mm is mostly used for resistance butt welding,
Small diameter wires, wires of different materials, and wires and stamping parts (such as resistors and diode end caps) can be welded by capacitive energy storage type, which is characterized by very hard welding conditions and

resistance butt welding-Schematic diagram

resistance butt welding-Schematic diagram

extremely narrow heating range, which greatly reduces The influence of the thermal and physical properties of the welded metal on the formation of the joint.
2. Butt welding of rods
is mostly used in the butt welding of steel bars in the construction industry, usually resistance butt welding is used for diameter d<10mm; continuous flash butt welding for d>10mm; preheated flash butt welding for d>30mm. When using a manual butt welding machine, because the power of the welding machine is small (usually not more than 50KVA), when d=15-20mm, it is generally necessary to use preheated flash butt welding.

Semi-circular or V-shaped clamp electrodes can be used for butt welding of rods. The latter can be used in various diameters, so it is widely used. The rods are of solid cross-section, with greater rigidity, and longer extension lengths can be used.
3, pipe butt welding
Pipe butt welding is widely used in boiler manufacturing, pipeline engineering and petroleum equipment manufacturing. Choose continuous or preheated flash butt welding according to the section and material of the pipe. The clamp electrode can be semicircular or V-shaped. Usually when the ratio of pipe diameter to wall thickness is greater than 10, a semicircle can be selected to prevent the pipe from being crushed. V-shape can be used when the ratio is less than 10. To prevent the tube from slipping in the clamp electrode, the clamp electrode should have an appropriate working length. When the pipe diameter is 20-50mm, the length of the workpiece is 2-2.5 times the pipe diameter; when the pipe diameter is 200-300mm, it is 1-1.5 times.

Because the tube has an expanded cross-section, the heat dissipation is faster, the liquid metal on the end surface is easy to cool, and it is difficult to extrude during upsetting. The area is scattered, and the self-protection effect is weakened during the flashing process. Therefore, when the process parameters are not selected properly, non-metallic inclusions will remain in the interface to form gray spot defects. Maintain stable flash, increase flash and upsetting speed, and adopt gas protection to reduce or eliminate gray spots.

After the pipe is welded, the internal and external burrs need to be removed to ensure that the external surface of the pipe is smooth and there is a certain channel aperture inside. Deburring requires special tools.

4, thin plate butt welding

flash butt welding-schematic diagram

flash butt welding-schematic diagram

Thin plate butt welding is widely used in the continuous production line of rolled steel plate in the metallurgical industry. The width of the plate is from 300 to 1500mm or more, and the thickness is from less than 1mm to more than ten mm. The materials include carbon steel, alloy steel and non-ferrous metals and their alloys. After the plate is butt welded, the joint will undergo rolling and produce great plastic deformation, so it must not only have a certain strength, but also a high plasticity. For steel plates with a thickness of less than 5mm, continuous flash butt welding is generally used, and a plane electrode is used for single-sided conduction. When the plate is thick, preheated flash butt welding is used for double-sided conduction to ensure uniform heating along the entire end surface.

When welding thin plates, due to the relatively large cross-section length and width, the area is scattered, the joint cools quickly, and the self-protection effect is weak during the flashing process. At the same time, the liquid lintel is small and the liquid metal layer on the end surface is thin. Prone to oxidation and solidification. Therefore, the flashing and upsetting speed must be increased. After welding, the burr must be removed with a burr cutting device while it is hot.
5. Butt welding of ring parts
When welding ring parts (such as wheel rims, chain rings, bearing rings, jet engine mounting edges, etc.), in addition to considering the general rules of butt welding process, attention should be paid to the influence of shunting and ring deformation elasticity. Due to the shunt, the required power should be increased by 15-50%. Although the diameter of the ring decreases, the cross section increases, and the material resistivity decreases.

When the ring parts are butt welded, the upsetting pressure must consider the influence of the deformation rebound force, but because the shunt has the effect of heating the ring back, the increase in the upsetting pressure is not large.

Bicycles, motorcycle rims, and automobile rims all use continuous flash butt welding, and the front mouth of the clamp electrode must match the cross-section of the workpiece. During upsetting, in order to prevent the rebound force from affecting the quality of the joint, or even pull the joint apart, it is necessary to extend the time of no current upsetting.

Chain links such as anchor chains and drive chains are mostly used in the manufacture of low-carbon steel and low-alloy steel. Resistance butt welding can be used when diameter d<20mm, and preheating flash butt welding can be used when d>20mm. The purpose of preheating is to heat the interface. Uniform, easy to produce certain plastic deformation during upsetting.
6, tool butt welding
When cutting tool butt welding, one of the current process methods used to manufacture blanks in tool manufacturing is mainly the butt welding of high-speed steel (W8Cr4V, W-9Cr4V2) and medium carbon steel. Tool butt welding has the following characteristics:

1) The thermal conductivity and resistivity of high-speed steel and medium-carbon steel are quite different. At room temperature, medium carbon steel λ=0.42W/(cm℃), ρ0=18-22uΩcm; high-speed steel λ=0.23W/(cm℃), ρ0=48Ωcm. In order to make the temperature distribution on both sides of the joint surface basically the same , The extension length of high speed steel should be 30-50% smaller than that of medium carbon steel. Under normal circumstances, the extension length of high-speed steel is (0.5-1.0)d. In order to prevent excessive heat dissipation, the extension length is not less than 10mm.

2) High-speed steel has a high tendency to quench, the hardness after welding will be greatly increased, and quenching cracks may occur. In order to prevent cracks, preheating flash butt welding can be used. During preheating, heat the metal in the range of 5-10mm near the interface to 1100-1200℃. After welding, it is annealed in an electric furnace at 600-700℃ for 30 minutes.

3) When high-speed steel is heated to a high temperature, it will produce grain growth or the formation of ledeburite eutectic on the semi-melted grain boundary, making the joint brittle. The ledeburite eutectic cannot be eliminated by heat treatment. Therefore, it is necessary to use sufficient upsetting to eliminate this structure.

TH Valve is a professional manufacturer of butterfly valvegate valvecheck valveglobe valveknife gate valve, ball valve with API, JIS, DIN standard, used in Oil, Gas, Marine industry, Water supply and drainage, fire fighting, shipbuilding, water treatment and other systems, with Nominal Diameter of DN50 to DN1200, NBR/EPDM/VITON, Certificates & Approvals: DNV-GL, Lloyds, DNV, BV, API, ABS, CCS. Standards: EN 593, API609, API6D

Related news/knowledge:
What is butt welding? (1);
What is butt welding? (7)- Flash butt welding of common metals;
What is butt welding? (9)- New technology of flash butt welding;
What is butt welding? (5)- flash butt welding

What is butt welding? (6)- Flash butt welding of common metals

Flash butt welding of common metals
Almost all steel and non-ferrous metals can be flash butt welded, but to obtain high-quality joints, it is necessary to take necessary process measures according to the relevant characteristics of the metal. The analysis is as follows:

flash butt welding-schematic diagram

flash butt welding-schematic diagram

(1) Electrical and thermal conductivity For metals with good electrical and thermal conductivity, higher specific power and flashing speed, and shorter welding time should be used.

(2) High temperature strength For metals with high high temperature strength, the width of the temperature plastic zone should be increased and a larger upsetting force should be used.

(3) Crystallization temperature range The larger the crystallization temperature range, the wider the semi-melting zone. A larger upsetting pressure and upsetting allowance should be used to squeeze all the molten metal in the semi-melting zone so as not to stay in the joint Causes defects such as shrinkage, porosity and cracks.

(4) Thermal sensitivity There are two common situations. The first is quenched steel. After welding, the joint is prone to quenching structure, which increases the hardness and reduces the plasticity. In severe cases, quenching cracks will occur. Quenched steel usually adopts preheating flash butt welding with a wide heating zone, and adopts measures such as slow cooling and tempering after welding. The second type is metals strengthened by cold work (such as austenitic stainless steel). The joint and heat-affected zone soften during welding, which reduces the strength of the joint. Welding such metals usually uses a larger flash speed and upsetting pressure to minimize the softening zone and reduce the degree of softening.

(5) Oxidation The oxide inclusions in the joint have a serious hazard to the quality of the joint. Therefore, preventing and eliminating oxidation is the key to improving the quality of the joint. The composition of the metal is different, and its oxidative generation is also different. If the melting point of the formed oxide is lower than that of the metal to be welded, the oxide will have better fluidity and will be easily squeezed out during upsetting. If the melting point of the formed oxides is higher than that of the welded metal, such as SiO2, Al2O3, Cr2O3, etc., they must be discharged when the welded metal is still in a molten state. Therefore, when welding alloy steels containing more silicon, aluminum, chromium, and a class of elements, strict process measures should be taken to completely eliminate oxides.

What is butt welding? (5)- flash butt welding

(4) Flash speed vf A sufficiently large flash speed can ensure the strong and stable flash. However, if vf is too large, the heating zone will be too narrow, which will increase the difficulty of plastic deformation. At the same time, due to the increase in welding current required, it will increase the depth of the fire hole after the lintel blasting, which will reduce the joint quality. The following factors should also be considered when choosing vf:

flash butt welding-schematic diagram

flash butt welding-schematic diagram

1) The composition and performance of the material being welded. For materials with a lot of easily oxidizable elements or good electrical and thermal conductivity, vf should be larger. For example, it is larger when welding austenitic stainless steel and aluminum alloy than when welding low carbon steel;

2) Whether there is preheating. When there is preheating, it is easy to excite the flash, so vf can be improved.

3) There should be a strong flash before upsetting. vf should be large to ensure a uniform metal layer on the end face.

(5) Upsetting flow rate δu δu affects the removal of liquid metal and the magnitude of plastic deformation. If δu is too small, liquid metal will remain in the interface, which will easily form defects such as looseness, shrinkage, cracks, etc.; when δu is too large, the crystal lines will bend severely and reduce the impact toughness of the joint. δu is selected according to the cross-sectional area of ​​the workpiece and increases with the increase of the cross-sectional area.

During upsetting, in order to prevent the interface from oxidizing, the current should not be cut off immediately before the end face interface is closed. Therefore, the upsetting flow should include two parts-current upsetting allowance and non-current upsetting allowance. The former is the latter. 0.5-1 times.

(6) Upsetting speed vu In order to avoid the difficulty of liquid metal removal and plastic metal deformation due to metal cooling in the interface area, and to prevent the end surface metal from oxidizing, the faster the upsetting speed, the better. The minimum upsetting speed depends on the properties of the metal. The minimum upsetting speed for welding austenitic steel is twice that of welding pearlitic steel. The welding of metals with good thermal conductivity (such as aluminum alloy) requires a high upsetting speed (150-200mm/s). For the same metal, if the temperature gradient in the interface area is large, the upsetting speed needs to be increased due to the fast cooling rate of the joint.

(7) The upsetting pressure Fu Fu is usually expressed by the pressure per unit area, that is, the upsetting pressure. The size of the upsetting pressure should ensure that the liquid metal in the joint can be extruded and a certain degree of plastic deformation will be produced at the joint. If the upsetting pressure is too small, the deformation will be insufficient and the strength of the joint will decrease; if the upsetting pressure is too high, the deformation will be too large, the crystal lines will bend seriously, and the impact toughness of the joint will be reduced.

The size of the upsetting pressure depends on the metal properties, temperature distribution characteristics, upsetting allowance and speed, and the shape of the workpiece section. High-temperature and strong metals require large upsetting pressure. Increasing the temperature gradient will increase the upsetting pressure. Because the high flash speed will increase the temperature gradient, when welding metals with good thermal conductivity (copper, aluminum alloy), a large upsetting pressure (150-400Mpa) is required.

(8) Preheating flash butt welding parameters In addition to the above process parameters, preheating temperature and preheating time should also be considered.

The preheating temperature is selected according to the cross-section of the workpiece and the material properties. When welding low carbon steel, it generally does not exceed 700-900 degrees. As the cross-sectional area of ​​the workpiece increases, the preheating temperature should be increased accordingly.

The preheating time is related to the power of the welding machine, the size of the workpiece section and the performance of the metal, and it can be changed in a relatively large range. The preheating time depends on the required preheating temperature.

In the preheating process, the amount of shortening caused by preheating is very small, and it is not specified as a process parameter.

(9) The clamping force Fc of the clamp must ensure that the workpiece does not slip during the upsetting. Fc is related to the upsetting pressure Fu and the friction coefficient f between the workpiece and the clamp. Their relationship is: Fc≥Fu/2f. Usually F0=(1.5-4.0) Fu, the lower limit is taken for low carbon steel with compact section, and the upper limit is taken for cold-rolled stainless steel plate. When the clamp is equipped with a top support device, the tightening force can be greatly reduced, and Fc=0.5Fu is sufficient at this time.

3. Workpiece preparation

The preparation of the workpiece for flash butt welding includes: the geometry of the end face, the processing of the blank end and the surface cleaning.

When flash butt welding, the geometry and size of the butt surface of the two workpieces should be basically the same. Otherwise, the heating and plastic deformation of the two workpieces will not be guaranteed to be consistent, which will affect the quality of the joint. In production, the difference in diameter of round workpieces should not exceed 15%, and the difference between square workpieces and tubular workpieces should not exceed 10%.

When flashing butt welding large-section workpieces, it is best to chamfer the end of a workpiece to increase the current density to facilitate the laser flash. In this way, the secondary voltage can be increased without preheating or initial flashing.

The butt welding blank end can be processed on a shear, punch, lathe, or plasma or gas flame cutting, and then the end face can be removed.

During flash butt welding, the end metal is burned out during flash, so the end face cleaning is not very strict. However, the cleaning requirements for the contact surface between the clamp and the workpiece should be the same as for resistance butt welding.

What is butt welding? (4)- flash butt welding

2. Resistance and heating of flash butt welding

The contact resistance Rc during flash butt welding is the total resistance of the liquid metal lintel between the end faces of the two workpieces, and its size depends on the number of lintels and their cross-sectional area at the same time. The latter two are related to the cross-sectional area of ​​the workpiece, the current density and the

flash butt welding-schematic diagram

flash butt welding-schematic diagram

approach speed of the two workpieces. With the increase of these three, the number of lintels and their cross-sectional area simultaneously existing increases, and Rc will decrease.

The Rc of flash butt welding is much larger than that of resistance butt welding, and it exists throughout the flashing stage. Although its resistance value gradually decreases, it is always greater than the internal resistance of the workpiece. Rc does not disappear completely until the upsetting starts. Figure 14-5 shows the general law of Rc, 2Rω and R changes during flash butt welding. The gradual decrease in Rc is due to the increase in the approach speed of the workpiece as the end surface temperature increases during the flashing process, and the number and size of the lintels increase accordingly.

Because Rc is large and there is the entire flashing stage, the heating of the joint during flash butt welding mainly depends on Rc.

Three, flash butt welding welding cycle, process parameters and workpiece preparation

1, welding cycle

The welding cycle of flash butt welding is shown in 14-7, and the reset time in the figure refers to the time from loosening the workpiece to returning to the original position. There are two preheating methods: resistance preheating and flashing preheating. The figure (b) uses resistance preheating.

2, process parameters

The main parameters of flash butt welding are: extension length, flash current, flash flow, flash speed, upsetting flow, upsetting speed, upsetting pressure, upsetting current, clamping force, etc. Figure 14-8 is a schematic diagram of each flow rate and extension length of continuous flash butt welding. The following describes the influence of various process parameters on welding quality and the principles of selection:

(1) Elongation length l0 is the same as resistance butt welding, l0 affects the temperature distribution along the axial direction of the workpiece and the plastic deformation of the joint. In addition, with the increase of l0, the impedance of the welding circuit increases, and the required power also increases. Under normal circumstances, bar and thick arm tube l0=(0.7-1.0)d, d is the diameter of the round bar or the side length of the square bar.

For thin plates (δ=1-4mm), in order not to lose stability during upsetting, generally l0=(4-5)δ.

When different metals are butt welded, in order to make the temperature distribution on the two workpieces consistent, usually the metal with poor electrical and thermal conductivity should be smaller. Table 1 is the l0 reference value for flash butt welding of different metals.

(2) The flash current If and the upsetting current Iu If depend on the cross-sectional area of ​​the workpiece and the current density jf required for the flash. The size of jf is related to the physical properties of the welded metal, flashing speed, the area and shape of the workpiece section, and the heating state of the end surface. In the flashing process, as vf gradually increases and contact resistance Rc gradually decreases, jf will increase. During the upsetting, Rc disappears quickly, and the current will increase sharply to the upsetting current Iu. When welding large-section steel parts, in order to increase the heating depth of the workpiece, a smaller flash speed should be used, and the average jf used generally does not exceed 5A/mm2. Table 2 shows the reference values ​​of jf and ju for flash butt welding of workpieces with a cross-sectional area of ​​200-1000mm2.

The magnitude of the current depends on the no-load voltage U20 of the welding transformer. Therefore, in actual production, the secondary no-load voltage is generally given. When selecting U20, in addition to considering the impedance of the welder circuit, when the impedance is large, U20 should be increased accordingly. When welding large-section workpieces, the method of adjusting the secondary voltage in stages is sometimes used. At the beginning, a higher U20 is used to excite the flash, and then it is reduced to an adaptive value.

(3) Flash flow rate δf The flash flow rate should be selected so that there is a molten metal layer on the end of the workpiece at the end of the flash, and the plastic deformation temperature is reached at a certain depth. If δf is too small, the above requirements cannot be met, which will affect the welding quality. If δf is too large, it will waste metal materials and reduce productivity. When choosing δf, you should also consider whether there is preheating, because the δf of preheating flash butt welding can be 30-50% smaller than continuous flash butt welding.

What is butt welding? (2)

2. Welding cycle, process parameters and workpiece preparation for resistance butt welding

1, welding cycle

During resistance butt welding, the two workpieces are always pressed tightly. When the end face temperature rises to the welding temperature Tω, the distance between the end faces of the two workpieces is as small as a few

resistance butt welding-Schematic diagram

resistance butt welding-Schematic diagram

angstroms. The atoms between the end faces interact to produce common crystal grains on the joint. Form joints. There are two welding cycles in resistance butt welding: equal pressure and increased forging pressure. The former is simple and easy to implement. The latter is conducive to improving the quality of welding, and is mainly used for

resistance butt welding of alloy steel, non-ferrous metals and their alloys. In order to obtain sufficient plastic deformation and further improve the quality of the joint, a current upsetting procedure should be set.

2, process parameters

The main process parameters of resistance butt welding are: extension length, welding current (or welding current density), welding energization time, welding pressure and upsetting pressure.

(1) Overhang length l0 is the length of the workpiece over the end face of the clamp electrode. When selecting the extension length, two factors should be considered: the stability of the workpiece during upsetting and the heat dissipation to the clamp. If l0 is too long, the workpiece will lose stability during upsetting. If l0 is too short, due to the enhanced heat dissipation to the jaws, the workpiece will be cooled too strongly, which will increase the difficulty of plastic deformation. For the workpiece with diameter d, generally low carbon steel: l0=(0.5-1)d, aluminum and brass: l0=(1-2)d, copper: l0=(1.5-2.5)d.

(2) Welding current Iω and welding time tω In resistance butt welding, the welding current is often expressed by the current density jω. jω and tω are the two main parameters that determine the heating of the workpiece. The two can be adjusted accordingly within a certain range. High current density and short time (strong condition) can be used, or low current density and long time (weak condition) can be used. But when the condition is too strong, it is easy to produce incomplete penetration defects; when it is too soft, it will cause serious oxidation of the interface end surface, coarse grains in the joint area, and affect the strength of the joint.

(3) The welding pressure Fω and the upsetting pressure Fu, Fω have an effect on the heat generation and plastic deformation of the joint. Reducing Fω is good for heat generation, but not good for plastic deformation. Therefore, it is easy to use a smaller Fω for heating and a much larger Fu for upsetting. However, Fω should not be too low, otherwise it will cause splashing, increase end surface oxidation, and cause looseness near the interface.

3. Workpiece preparation

In resistance butt welding, the shape and size of the end faces of the two workpieces should be the same to ensure that the heating and plastic deformation of the workpieces are consistent. The end surface of the workpiece and the surface in contact with the clamp must be strictly cleaned. Oxide and dirt on the end face will directly affect the quality of the joint. The oxides and dirt on the surface of the workpiece in contact with the clamp will increase the resistance of the contact, which will cause the surface of the workpiece to burn, increase the wear of the jaws, and increase the power loss.

The workpiece can be cleaned by mechanical means such as grinding wheels, wire brushes, etc., or pickling.

Oxide inclusions easily occur in resistance welding joints. For rare metals, certain alloy steels and non-ferrous metals with high welding quality requirements, protective atmospheres such as argon and helium are often used to solve the problem.

Although resistance butt welding has the advantages of smooth joints, small burrs, and simple welding process, the mechanical properties of the joints are low, and the preparation of the end face of the workpiece is high, so it is only used for butt joints of small section (less than 250mm2) metal profiles.

What is butt welding? (1)

Butt resistance welding (hereinafter referred to as butt welding) is a type of resistance welding method that uses resistance heat to weld two workpieces along the entire end surface at the same time. Butt welding has high

welding

welding

productivity and easy automation, so it is widely used.
Application of butt welding:
Butt welding has high productivity and easy automation, so it is widely used. Its application range can be summarized as follows:
(1) The length of the work piece, such as the butt welding of strip steel, profile, wire, steel bar, rail, boiler steel pipe, oil and natural gas transportation pipeline.
(2) Butt welding of annular workpieces, such as car rims and bicycles, motorcycle rims, butt welding of various chain links, etc.
(3) Assembling welding of components The simple rolling, forging, stamping or machining parts are butt-welded into complex parts to reduce costs. For example, butt welding of automobile steering shaft shell and rear axle housing, butt welding of various connecting rods and tie rods, and butt welding of special parts.
(4) Butt welding of dissimilar metals can save precious metals and improve product performance. For example, the butt welding of the working part of the tool (high-speed steel) and the tail (medium carbon steel), the butt welding of the head (heat-resistant steel) and the tail (structural steel) of the exhaust valve of an internal combustion engine, the butt welding of aluminum and copper conductive joints, etc. .
Classification of butt welding:
Butt welding is divided into resistance butt welding and flash butt welding.
Resistance butt welding
Resistance butt welding is a method in which the end faces of the two workpieces are always pressed tightly, heated to a plastic state by resistance heat, and then upset pressure is quickly applied (or no upset pressure is applied and only the pressure during welding is maintained) to complete the welding.
One, resistance and heating of resistance butt welding
The resistance distribution during butt welding is shown in the figure. The total resistance can be expressed by the following formula:
R=2Rω+RC+2Reω
where Rω-the internal resistance of the conductive part of a workpiece (Ω);
Rc–the contact resistance between two workpieces (Ω);
Rω–The contact resistance between the workpiece and the electrode (Ω);
The contact resistance between the workpiece and the electrode is usually ignored due to its small resistance and far from the joint surface.
The internal resistance of the workpiece is proportional to the resistivity ρ of the welded metal and the length l0 of the workpiece extending from the electrode, and inversely proportional to the cross-sectional area s of the workpiece.
Same as spot welding, the contact resistance of resistance butt welding depends on the surface condition, temperature and pressure of the contact surface. When the contact resistance has obvious oxides or other stolen goods, the contact resistance is large. The increase in temperature or pressure will reduce the contact resistance due to the increase in the actual contact area. At the beginning of welding, the current density at the contact point is very large; after the end surface temperature rises rapidly, the contact resistance decreases sharply. When heated to a certain temperature (steel 600 degrees, aluminum alloy 350 degrees), the contact resistance disappears completely.
Like spot welding, the heat source during butt welding is also the resistance heat generated by the resistance of the welding zone. In resistance butt welding, the contact resistance exists for a very short time, and the heat generated is less than 10-15% of the total heat. But because this part of the heat is generated in a very narrow area near the contact surface. Therefore, the temperature in this area will increase rapidly, and the internal resistance will increase rapidly. Even if the contact resistance disappears completely, the heat generation intensity in this area is still higher than other places.
The harder the welding rod used (that is, the larger the current and the shorter the energization time), the smaller the pressing force of the workpiece, and the more obvious the influence of contact resistance on heating.