CREO 3.0 Mechanism Dynamic Option

aburdi-2 · ‎Aug 02, 2016

Hi all,

I am trying to use CREO MDO to get the torque needed to put an assembly in rotation considering acceleration, masses, frictions and so on.

I think that the model is well implemented but the results given by CREO are not consistent with my hand calculation so after some tests on the main assembly I decided to make a simple test using a small assembly in which I have a simple beam with a pin joint on one of the edge (please refer to attached picture MDO_TEST_01).

To get a result I've applied a force on a point laying on the beam (120000 mN) (refer to picture MDO_TEST_02) and i asked the software to give me back the torque calculated (radial moment) on the pin joint (refer to picture MDO_TEST_03).

I was expecting to get a result similiar to the one below:

F=120000 mN

Distance from the center= 80 mm

Torque= 120000*80= 9600000 mNmm=9.6 Nm

Instead I am getting as result something like 33665 Nmm=33.665 Nm (please refer to picture MDO_TEST_04).

What's wrong in the model and/or in my calculations?

Thank you all.

Andrea.

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346gnu · ‎Aug 03, 2016

Andrea,

Some observations:

The units of your model are wrong for your hand calculation. Your model units are kg-mm-S. You should change them all to N-mm-S
The pin joint offers no resistance; it is frictionless. Your force will accelerate the lever's rotation about the pin joint.
Your measure is measuring 'radial moment' and not the moment about the joint axis. The measurement you have measures the moment trying to tilt the pin joint (axis normal to the cylindrical axis). The moment you calculated will depend on the vertical position of your load application point and as this vertical position doesn't change and therefore neither does the moment. This agrees with your result. I would expect that if the datum point is exactly halfway through the thickness then the moment would be zero and not the number you got (regardless of units). So can't understand that. Check the position of your point
Your force is directional. This direction does not vary. You have it pointing in Z (there is a direction tab in the Force or Torque Definition dialogue). You might want the force to follow the lever i.e. always be tangential to the circle drawn by the application point.
The force is initially at right angles and therefore the largest moment. The lever will be accelerated by the force and by its own inertia might be carried past 90 degrees and then be acting against your constant force. Eventually it will act against the force. So I would have expected a sinusoidal response if you measure joint position or radial force. Without damping your model will oscillate for ever ... or until the analysis time runs out.

I think you need to add a spring or something that resists the turning moment due to the force then measure what is happening at the spring or the angle through which the arm is deflected (angular) and multiply by torsional stiffness.

Add some damping as well else it will oscillate.

Hope this helps

aburdi-2 · ‎Aug 04, 2016

Hi Charles,

thanks for your reply.

As for point 1:

You're right I am using Kg/mm/sec but when the system asks for the force units displayed (refer to picture 2) are mm Kg/sec^2 which are mN so I was supposing that the system was converting the units while making the calculations.

Point 2: Yes the pin joint is actually frictionless, I am not interested in applying friction in this simple case;

Point 3: As far as I understood from PTC learning center Radial moment should the moment about the joint axis and not the til moment. The point is laying on the external surface of the beam and not on it's center;

Point 4: My fault. I was thinking that the force was following the point

Point 5:

My fault again I was supposing CREO inserting springs and dampers automatically as another software I am more familiar with does.

However I'll change units and force direction. I'll add a big friction to the foint and I'll measure the moment again.

I'll be back later.

Regards.

Andrea.