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PeterBrown1-Visitor
1-Visitor
November 21, 2011
Question

MDO & MDX

  • November 21, 2011
  • 4 replies
  • 2623 views

Hola -


First, I know just enough about MDO and MDX to get into some real trouble. Anyway, I have a mechanism that I will best describe as a single-cylinder engine. It's not, but it has a crank with a common throw, 2 connecting rods, and a load at the end of the rods. It really screams along, 6,000+ rpm. I have the mechanism all modeled and running in Mechanism well enough, I have a servomotor applied, etc. I want to know what the max load is on the rods from accelerating and decelerating. Will Mechanism do this? Or am I in Mechanica Structure territory now?



TIA



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4 replies

C
ChrisKaswer16-Pearl
16-Pearl
November 21, 2011
Peter,

Yes - MDO will do this for you. If you have no other external loads applied to the rods or crank, the forces you can measure at the rod journals (or wrist pins if you have them) would be your accel/deccel loads. Since you have additional loads at the rods, you'll need to back these out. The easiest way to do it is to run one analysis without the external loads to get the forces due to translational and rotational accelerations. I f you wish to keep the external loads added, you should be able to create a custom measure that will multiply the accel at the points of interest by the effective mass (can't forget v^2 for rotating parts). The second approach is more complicated since you'll need to deal with translational and rotational accels and their effective masses, so I'd go with option #1.

Hope this helps,

Chris

Christopher Kaswer
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PeterBrown1-VisitorAuthor
1-Visitor
November 21, 2011

When I attempt to run an Analysys that is Kinematic, I get a failure that the mechanism constraints could not be satisfied. This does not happen with a Position analysys... what's going on here?

R
randyspeed1-Visitor
1-Visitor
November 22, 2011
Peter,

You have more degrees of freedom in your model than the solver can tolerate. Be tedious in understanding the number and directions of the DOFs in each joint. "Loose" DOFs (those that you do not want to be moving) must be controlled either by a load (force, spring, etc.), or position driver. Best way to understand DOFs is to count up how many you desire, and add those to the number of constrains (constrained DOFs). That should equal to the total number in your model. Most likely, the number of DOFs in your model is higher than this number.

Randy Speed
President & CEO
Speed Consulting, LLC
4715 Harvest Hill Road
Dallas, Texas 75244
(214) 213-4440
(214) 853-9221 fax
www.speedconsulting.com
R
randyspeed1-Visitor
1-Visitor
November 22, 2011
Peter,

MDO will tell you the load magnitude at the either end of the rods. Plot "joint reactions". You can plot the load at each time step versus time, or versus any other measure, like crank rotation angle for example. Structure would be used to tell you what the maximum Allowable load is for the rod, etc. due to internal stresses reaching the material limits. Joint reactions are the loads going through the joints. If your applied load at the end of the rods represents combustion or some other real force like fluid pressure then that load will be accounted for in the joint reaction. If it is a real load and contributes to the maximum joint load then it should be kept. I am not sure from your post what those loads represent, but they must not apparently be to drive the "motor" because you have a servo driving the motor.

Randy Speed
President & CEO
Speed Consulting, LLC
4715 Harvest Hill Road
Dallas, Texas 75244
(214) 213-4440
(214) 853-9221 fax
www.speedconsulting.com

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