2 Replies Latest reply on Apr 8, 2010 8:15 AM by Roland Schwarz

    Can I use Simulation to solve a statically indeterminate problem?

    Tom Nuessen

      I currently have a 4 bar linkage with the hinges connected via pin joints:

      a fixed base

      two unequal length arms

      a fixture that rotates between level and ~65* ccw

       

      The two arms each have one end attached to the base, and the other end attached to the stand.  They are unequal in length, like a standard 4-bar linkage.  There is also a hydraulic cylinder that moves one of the arms and causes the mechanism to rotate, but that is not being considered in this instance.  I have the center of gravity of the object that is held in the stand, and I need to perform a static analysis on this mechanism to find/verify the forces that are transmitted to the pin joints so I can then load those forces into Simulation and perform an FEA study on each respective member of the mechanism.  I've searched all over, even using different programs such as Pro-Mechanica and Algor, but I haven't found anything that will show me how the forces are distributed to the pin joints (think elementary beam loading problem with a free body diagram--- i.e. a beam is fixed at point A with a pin on one end, and a roller support at point B on the other end and loaded with a force of x-newtons at a location y-inches from point A.  What are the reaction forces at points A and B?)  Currently, I have more variables than equations and even with assumptions to simplify the model, my values still indicate the model is statically indeterminate.  Is there a feature in SolidWorks that will give reaction forces to a simple 2D or 3D model? 

       

      Thank you for your time and help,

       

      Tom

        • Re: Can I use Simulation to solve a statically indeterminate problem?
          Ian Hogg

          Hi Tom,

          Statically indeterminate system are only an issue if you can't account for part stiffness.

           

          If all you are after are the static pin loads at different positions, then SW Simulation can handle that. The level of fidelity is up to you. You can represent the physical pins as simulation connector pins and then not only get pin loading but also based on the strength of the pin check if the loads exceed their capacity (this uses a rigid pin so it depends on if that assumption makes sense in your case). Otherwise you can use no penetration contact for parts in the joints to get more accurate pin bearing stresses in the system (but it takes longer to solve).

           

          If you're interested in dynamic forces (forces due to inertia, acceleration during movement etc) then SolidWorks Motion may be suitable. The key question is if you a having tight fit pins on all connections or if you can assume some of those pins are only carrying forces, but not moments (like a self aligning bearing). Motion doesn't like statically indeterminate systems (called redundancies in that discipline) and while it can remove them automatically, it is best to minimize them so it gives the most relevant joint representation. This is because it assumes parts are rigid bodies (like Pro/Mechanica motion would). There are other ways to tackle this in motion as well, and if you want to pursue that, please make a post under the motion thread.

           

          Hope this helps,

           

          Ian

          • Re: Can I use Simulation to solve a statically indeterminate problem?
            Roland Schwarz

            A 4-bar linkage as you describe is not "indeterminate", it is underconstrained.  Indeterminate condition arises from an excess of constraints.  Without some force or additional member (like your actuator) to completely constrain, there is no solution.

             

            I solve problems nearly identical to this, 4-bar lift mechanisms with electric actuators in SW Motion.  You need to do one of two things:

            1. Add the actuator and drive it
            2. Add a rotational motor to one of the joints to move the 4-bar

             

            I usuall install the actuator as two components, a "base" and a "rod", then add a linear motor to move them relative to one another at a set speed for a specified time.  Go slow to minimize inertial effects.

             

            Once the mechanism has gone through the motion, the forces at each joint, as well as the force required to move the actuator, are available as results.  You will be able to get graphs of the forces at each joint through the full range of motion.

             

            If your system is truly indeterminate (overconstrained), add bushing parameters to each of the joint.  You will find this in the mate dialog when Motion addin is active.  Without the bushings, force results at overconstrained joints are likely to be wrong.