Why are mechanical engineers painting metal expansion joints with SolidWorks? — — The pain points of traditional CAD and the advantages of parametric modeling
To be honest, in the past few years, many factories still used AutoCAD to paint metal expansion joints, and the two-dimensional drawings were changed again and again. The corrugation depth and pitch were all calculated manually. Change the caliber and redraw the whole picture. When I arrived at the workshop, the workers took the two-dimensional diagram and the sheet metal blanking data, so they had to rework it. During that time, I met a customer who made high-temperature axial expansion joints. He complained, "It took two days to position the light guide tube, and the adjustment amount of the tie rod nut was not clearly marked in the assembly drawing."-This is the pain point of traditional CAD: no parameterization, no assembly logic, and changing one place to lead the whole body.
The strength of SolidWorks lies in parametric modeling. You define the key variables of wave wall thickness, wave height, wave pitch and layer number as global parameters, change a number, and the model will be updated automatically. With the equation and design table, a model of a universal corrugated expansion joint can be derived from the DN50 to DN2000 series. More importantly, once the matching relationship of the end tube, guide tube, tie rod and nut in the assembly is set up, it will not collapse when changing specifications. This is why now almost everyone in mechanical engineering has a set of SolidWorks to do SolidWorks modeling of metal expansion joints.
Taking the general corrugated expansion joint as an example, the core steps of SolidWorks parametric modeling are disassembled: the matching logic of corrugation, end tube, guide tube and tie rod
Take the most sophisticated general-purpose corrugated expansion joint in our station. The first step in modeling is not to draw a picture, but to determine parameters. You need to list: design pressure, compensation amount, number of corrugated layers, single layer wall thickness, wave height, wave pitch, end tube length, guide tube inner diameter, number and length of tie rods. Write these into SolidWorks equations, such as "wave height = corrugation outer diameter-corrugation inner diameter/2", so that they are quoted directly when scanned or rotated later.
I recommend "scan" plus "curve array" for bellows body. Draw a sketch of the ripple section, then sweep out individual waves along the axis path, and then array them by wave pitch. Note: The number of instances of the array must be tied to the wave number in the equation. The end pipe is easy to handle, and the rotating boss is done. The conductor should be positioned with special care-it should have a gap with the inner wall of the bellows, but it should never interfere with the corrugations. The specific function of the guide tube is talked about in a special article in our station. Simply put, it is to guide the flow direction of the medium and avoid the direct scouring of ripples. When modeling, make the outer diameter of the guide tube equal to the inner diameter of the bellows minus 0.5~1mm, and use the constraint of "concentricity" + "distance" in the matching.
The most test kung fu is the tie rod assembly. Tie rod is the key to withstand internal pressure thrust, and its length directly determines the compensation amount of expansion joint. Many novices draw the tie rod to a fixed length, and as a result, they find that the nut can't be screwed to the head when assembling. Correct approach: Set the length of the tie rod to "spacing between two end plates-free length of bellows-length of end pipe", and then distribute the tie rod evenly along the circumference with "linear array". The adjustment amount of the nut should be set aside in the modeling stage, and the parameters can be directly changed when simulating the cold tightening condition.
Don't wait for physical testing: Use SolidWorks Simulation to analyze the fatigue life of bellows to avoid stress concentration in advance
Make the prototype and test it again? Not to mention the high cost, the cycle is long. Now it's smarter to run the simulation at the model stage. SolidWorks Simulation can directly reuse your 3D model without resetting the geometry. For bellows, the fatigue life analysis focuses on two places: the bending stress at the crest and trough of the wave, and the stress concentration at the weld between the end pipe and the corrugation.
I am used to doing static analysis first-applying design pressure and axial displacement (such as 30mm compensation amount) to see if the maximum stress exceeds the yield strength of the material. If it exceeds the standard, adjust the wave height or increase the number of layers. Then using the fatigue analysis plug-in, input the cycle times (usually 1000~10000 times for pipeline system), calculate the fatigue life safety factor. When the power station industry in the station used corrugated expansion joints for simulation, we found that the stress on the inside of the trough was always about 30% higher than that on the outside. Later, the fillet of the trough was changed from R3 to R5, and the safety factor was directly doubled. In the physical test, the mold has to be changed twice, which costs thousands of dollars. In the simulation, it takes ten minutes to change the parameter.
From model to workshop: Practical experience of automatic labeling of engineering drawings, BOM export and docking of unfolding blanking data
The model is done, and the next step is to draw out. Many engineers let go after drawing models in SolidWorks, and let the craftsmen mark them themselves. In fact, you can use the "Model Project" function to import dimensions with one click-provided that the sketch is fully defined when modeling. In the engineering drawing of the general corrugated expansion joint, I focus on the inner diameter, outer diameter, wave number, total length, diameter of the center circle of the tie rod, and cold tightness. These parameters directly reference the equation, the model changes, and the drawing is automatically updated.
The BOM table export is simpler. Right-click Configure, select "Generate BOM Table", and list the materials, weight, surface treatment and remarks of the parts. Note that bellows are typically multi-layer structures, with each layer listed separately. The weight table of metal expansion joints in the station can be referred to, but your model may deviate from the standard weight, so the actual model will prevail. When unfolding the blanking data docking workshop, I suggest using the "sheet metal" module to unfold the bellows into a flat plate and export the DXF file to the laser cutting machine. Expansion length = corrugation diameter × π × wave number. This formula is directly generated by "matrix flange" in SolidWorks, and the accuracy is much higher than manual calculation. Two days ago, a customer asked me, "Does the screw of the expansion joint need to be disassembled?" I said, if you set the correct assembly relationship in the model, and the engineering drawing is marked with "remove the tie rod after cold tightening", then the worker will naturally know how to do it.
Record of Pit Stepping in Design: Compensation Setting and Cold Tightening Treatment Easily Overlooked in Modeling of High Temperature Axial Expansion Joints and Large Diameter Thick Wall Expansion Joints
High temperature axial type expansion joint. The material creep is not negligible when the design temperature exceeds 600℃. Many engineers only calculate the compensation amount according to the elastic modulus at normal temperature when modeling. As a result, as soon as the temperature rises on site, the expansion joint is crushed or cracked. Correct approach: Set a "thermal expansion coefficient" parameter in SolidWorks, compensation amount = pipe thermal elongation × coefficient (usually 0.5~0.7). The cold tightening amount should also be marked at the adjustment position of the tie rod nut. The product information of the high-temperature axial expansion joint in the station clearly states that "the pre-stretching amount is 50% of the design compensation amount", which must be switched by configuration in the model.
Large diameter thick wall expansion joint. When the diameter exceeds DN1000 and the wall thickness is ≥6mm, the rebound after corrugation forming is serious. I have seen a project in which the wave height in the three-dimensional model is 50mm. As soon as the sample is pressed out, the wave height changes to 48mm, and the compensation amount is directly reduced by 4%. Later, we added the "rebound compensation coefficient" to the equation, and the wave height target value = the design value × (1+0.03). In addition, if the guide tube of thick-walled parts is directly made of rotating boss, the wall thickness direction will be inconsistent with the entity. It is best to use "rotating curved surface" and then "thicken" to ensure uniform wall thickness. There is also the problem of how to adjust the tie rod nut-many tie rods of large-diameter expansion joints are hinged at both ends, so "ball head + ball socket" should be used for modeling, otherwise the constraints will be reported in the simulation. This kind of detail can only be remembered after stepping on a pit.
solidworks modeling of metal expansion joints is not a one-time sale. The higher your parameterization level, the faster the subsequent modification, simulation and drawing will be. Don't be afraid to spend time building a skeleton at the beginning, and it will be worry-free to run later.