From the perspective of production practice, sometimes the straightening head can not be straightened no matter what, sometimes it can be straightened with a little adjustment, indicating that there is a prerequisite for smooth straightening.
Prerequisites for straightening:
1. Need to use a sizing machine. The sizing machine actually provides two (for two- and four-roll straightening heads) fixed fulcrums for straightening, which are matched with the movable fulcrums of the straightening head to form two same directions The force application point and a force application point in the opposite direction act together on the tube body to complete the straightening. Therefore, whether it is a two-roller, a four-roller or an eight-roller, it needs to be completed by a sizing machine. Another significance of adjusting with a sizing machine is The magnitude and direction of the straightening force (formation rolling force) provided by the sizing machine need to be stable, which is why the straightness of the welded pipe will bend and wobble once the sizing machine is unstable.
2. The stress size and direction distribution on the welded pipe in the sizing machine needs to be stable. The reason why the welded pipe can be straightened is that the magnitude and direction of the straightening force applied by the straightening head and the internal stress of the welded pipe in the sizing machine reach a short-term dynamic balance. As a result, once this balance is disturbed, such as the thickness change of the tube blank, the change of the coolant flow rate, the change of the welding heat, the damage of the roll bearing, etc., the originally straight welded pipe will immediately bend, and the straightening head needs to change the force applied to the welded pipe. Regain the balance of the size and direction to restore the straightness of the welded pipe.
3. The position of the weld seam should be kept stable in the sizing machine. According to the welding principle and welding process, there is a welding heat-affected zone at both ends of the weld seam. Since this area absorbs a large amount of heat energy during the pipe welding process, it is During the cooling process of the welded pipe (after the extrusion point), a large amount of shrinkage stress (compressive stress) will be accumulated. This stress of the same magnitude and direction forces the welded pipe to bend to the side of the weld. The straightening force in the right direction and direction is applied to the point where the force is not needed (may not need to be so large, or it may not be in this direction of force), thereby disrupting the previous stress balance, turning the welded pipe from straight to straightening Before the head finds a new balance point, the welded pipe has been in a bent state. If the position of the weld is always unstable, the straightening head needs to constantly find a new balance point. The welded pipe will constantly change between bending and straightening. If the straightness of the welded pipe is good, the position of the weld must be fixed first.