Bending Forming of Pipes in Pressure Vessels
There are various methods of bending pipes, generally divided into manual bending and mechanical bending. There are various mechanical bending methods, such as pressing bending, rolling bending, back bending, and squeezing bending. Regardless of the bending method, the main contradiction in the entire bending process is how to overcome the problem of local deformation. The most widely used bending processes in engineering are manual bending and backbending. The back bending method is used to bend pipes on a rotary pipe bending machine, which can be divided into two types: mold bending and non mold bending.
Manual bending does not require specialized equipment or complex process equipment. It can bend bends of various radii, angles, and spatial orientations. But this bending method has high labor intensity, low productivity, and unstable quality.
Manual bending of steel pipes often uses hot bending, while cold bending is recommended for stainless steel and non-ferrous metals. Before bending, fill the pipe with filling material. The filling material for steel pipes is usually pure and dry fine sand. Low melting point substances such as rosin and lead should be used for stainless steel and non-ferrous metal filling to prevent wrinkles and reduce ovality.
This type of pipe bending machine uses mold bending. There are two main types of molds: circular groove wheels and sheep horn core heads. When using a disc shaped groove wheel to bend the pipe, the bending die is located on the outer side of the pipe, with half of the bend lying in the groove, and the other half of the pipe in the bending area is pressed by a small roller with a groove (also known as a pressing roller). The end of the pipe is fixed on a disc shaped bending die by a clamp. If the clamping roller does not move, the disc shaped bending die actively rotates to complete the bending, which is called a pull bending type; If the pipe is pushed and the circular bending die is driven to rotate to complete the bending, it is called push bending; If the disc shaped bending die does not move, the pressing roller presses the pipe and rotates around the disc shaped bending die to complete the bending, which is called the bending type.
When using a sheep horn core head for bending, the bending mold is located on the inner side of the pipe. The sheep horn core head is shaped like a sheep horn, with an axis length of 1/4 circumference and a bending radius of the same as the bent pipe. The maximum bending angle is 180 °. The fine end of the sheep horn core is slightly thinner than the inner diameter of the tube blank. The tube blank is inserted from the fine end, and the coarse end is the outlet, which is slightly thicker than the inner diameter of the tube blank. When bending, the tube blank is heated before being inserted into the mold. Under the action of thrust, the tube undergoes two processes: bending and expansion. When it slides out of the end of the core head, it is bent and formed.
The bending machine used in this bending method does not require a specific bending die to determine a certain bending radius. It can be divided into two types: pull bending and push bending. The pipe bending machine has a rotating arm that can be extended and retracted. When bending, the pipe is fixed on the rotating arm, and the distance from the center axis of the pipe to the center of rotation of the rotating arm is the bending radius. During operation, the pipe is covered with a copper induction coil, which heats the pipe locally to 900-950 ℃ using medium frequency (for thicker pipes) or high frequency (for thin pipes) electricity, and then bends the pipe. If the rotating arm actively rotates to bend the pipe back into shape, it is called a tension bending type; If the end of the pipe is pushed, the pipe pushes the rotating arm to rotate, thus achieving the bending back of the pipe, which is called push bending. Next, spray water from a small hole on the inner side of the sensing ring to the bent pipe section, allowing it to cool quickly. The purpose of water spraying is to limit the heating section and deformation zone to a very small range, thereby preventing wrinkles and flattening of the pipes.