First to answer a basic question: Why do expansion joints need to be netted?
The bellows absorbs the displacement by the deformation of the wall surface, and when the internal pressure is combined, the stresses on the trough and the peak are completely different. If it is just a bare pipe, the pressure exceeds a certain value, and the bellows will be bulged and even unstable. At this point, the outer braided mesh came into play-it was equivalent to adding a circle of constraint to the bellows. When you look at the structure of the metal hose and the general-purpose corrugated expansion joint, the braided mesh layer is not decoration, it is directly involved in the force. Without this layer of net, the withstand voltage level will be directly reduced by one gear.
To be honest, in the eyes of many users, the process of netting is a simple action of setting a layer of stainless steel wire mesh. Isn't it just putting the bellows on the net? What technical content can it have? Let's take it apart and see. The fatigue life and stability of bellows have much more to do with this layer of net than you think.
Core parameters of weaving machine: number of strands, pitch, tension
Number of strands, pitch, tension. The number of strands determines the coverage density of the net. If there is less, it can't be wrapped, and if there is more waste of steel wire; If the pitch is too loose, the steel wire will run as soon as it is pushed on the bellows. If the pitch is too tight, it will affect the lateral displacement of the bellows.
What's really hard is tension control. The tension was too great, and the wire slashed into the bellows wall like a knife; The tension is too small, and the woven mesh surface is loose and baggy, so you can press it down with your fingers. How exactly does the web weaving machine control the tension? It relies on the synchronous cooperation of the damping of the payoff frame and the braiding head. If the damping is slightly biased, the tension will run away. Corrugated pipes with different diameters have different requirements for weaving speed. Thin pipes are easy to say, while thick pipes fluctuate obviously when the speed is high. In the final analysis, this work is still experienced.
Different expansion joint products have completely different requirements for weaving
If you take the process of weaving ordinary axial expansion joints to set large-diameter thick-walled expansion joints, it will definitely not work. Large diameter and thick-walled bellows have high rigidity and need greater braiding tension. The requirements for spindle rigidity and traction of mesh weaving machine are completely two levels.
Another example is the high-temperature axial expansion joint. The temperature-resistant stainless steel wire used is harder than that at normal temperature, and the rebound is greater. Although the number of strands remains unchanged during weaving, the pitch and tension have to be readjusted, otherwise the steel wire won't be able to comply at all. There is also a double hinge transverse expansion joint. This product has a large transverse displacement, and the bellows is bent in operation. If the net is weaved according to the state of a straight pipe, the net surface will be torn apart as soon as the machine is installed. The weaving structure must follow the displacement mode, and the weaving density on which side is stressed with greater force should be adjusted. There is no standard answer here, it all depends on the deep understanding of the product.
The few details that are easiest to miss when selecting a model
Many customers ask about metal expansion joint mesh weaving machine. The first sentence is: How many meters can your machine weave in an hour? The question itself goes off the rails. No matter how fast the equipment runs, what's the use of a messy mess of networks?
Guess what, two days ago, I met a customer who made expansion joints, saying that their machine shook badly when running at high speed. I asked him to check the spindle bearing clearance, and it was really out of tolerance. The rigidity of the spindle is insufficient, the whole braiding head shakes at high speed, and the wear of the steel wire guide wheel also intensifies, and the tension suddenly increases and decreases, let alone the product consistency. Also, the matter of modulus matching is easy to overlook. If the gear modulus of the braiding head does not match the diameter of the steel wire, the wire feeding will not be smooth, and the wire will be broken sooner or later.
Therefore, when selecting the model, don't just look at the speed parameters, spindle rigidity, modulus matching, the material and wear degree of the steel wire guide wheel. These are the things that really affect the long-term stability.
Poor weaving quality, the consequences are worse than you think
What can go wrong with poor quality braiding? Wire fracture, mesh slack, local bulging, these three are the most common. The problem is that these defects are not necessarily exposed during the factory stress test.
The bellows itself did not leak, and the hydraulic test could pass. However, at the scene, as soon as the pipeline heated up and the internal pressure rose, the force on the mesh layer increased sharply, and the weak point collapsed. The failure of the expansion joint is often not the bellows leaking first, but the weaving mesh collapsing first. Therefore, the tension sensor and broken wire shutdown function on the equipment are not decorations. Regular inspection and calibration are more important than anything else. If you ask me what I should value most when choosing a metal expansion joint mesh weaving machine, in a word: stability. No matter how fast the speed is, the stability is not good, it is just an embroidered pillow.