I tried to model rotation of crane parts using temperature loads. For this calculation I use advanced nonlinear static calculation (see below picture with model).
Temperature load were applied to one node of cylinder. Cylinder modeled using one rod element. Results for this model look fine. See below graphs with axial stress in jib cylinder with loads and without loads.
As expected when we calculate model without any loads only temperature we have rotation of jib but stresses in all parts are close to 0.
Next calculation was done using same model but with bottom parts (see below)
To this model temperature load were applied to both cylinders (on node). For calculation used same analysis type. But now results look different (for jib cylinder). See below graphs with axial stresses in cylinders (calculation without loads only temperature).
So from graph above we can see big stresses in jib cylinder. And stress close to 0 for boom cylinder. I expected receive all stress 0 when I calculate model without loads (as in previous model).
Question is why I have in same models stress in jib cylinder? How I can avoid this stress in parts when I calculate model without loads (only temperature)?
Solved! Go to Solution.
Nice trick, I have used before as well to move joints in assemblies.
For me -without the model in hand- the problem is related to TEMP(INIT) case control, if not specified initial temperature is assumed to be zero.
Thanks for answer. I specify initial condition and recalculate model with taken into account initial temperature condition (all zero). But still I have big stresses in jib cylinder.
Boom cylinder all stresses close to 0. Jib cylinder in cases where we have elongation gives big stresses. In cases with boom rotation (jib cylinder static) jib cylinder stresses is 0 (almost zero). But in previous model (not full model) we have only jib rotation and stresses was zero.
Define two time curves: assign one for applying thermal load in the jib and another time curve in the boom (for thermal load select the two nodes of the CROD element). Both times curves will have the same starting & ending time, but you can play with the loading, this way you can isolate the problem, not need to use only ONE time curve for both bodies.
For instance, you can let the function value 0 for the thermal loading on the boom and study what happens with the JIB cylinder using a ramp function, then decrease the JIB load and start the BOOM load, this way you can study what happens with the BOOM, activating & deactivating loading effects.
Thanks for all recommendations. I use different function for temperature loads (separate for JIB load and for BOOM load), but still received big stresses in JIB cylinder. Also I removed temperature loads from JIB cylinder and calculate only with temperature on BOOM cylinder. And even without any load on JIB cylinder I still receive big stresses.
For me it looks strange because now to model applied only temperature load to BOOM cylinder. From JIB cylinder I removed all temperature influence but still we have big stresses.
See below picture with applied loads and results.
There may be some convergence difficulties related to a "no load" analysis. The convergence criteria requires (for example) a small error between the sum of all internal energies and the external work. In an analysis when both of those are zero, the energy criterion may well have some difficulties with "tiny numbers" compared to "other tiny numbers".
It might not be the cause of your current issue, but I recommend that you apply tighter convergence tolerances AND you should have a "small" load applied somewhere so that the convegrence criteria have something to work with. The load should cause enough strain to be mathematically meaningful, but otherwise be of no consequence to the answers you are looking for. You may need to apply some other convergence criterion other than just energy.
Thanks all for help. Problem solved. Solution is to increase number of steps.
See below graph with results (axial stresses in cylinder).