Basic calculation question

Multibody dynamics technique, real objects and its dynamical models

Basic calculation question

Postby Harry » Tue Feb 16, 2016 8:52 pm

Hello,
I have a basic question about how the motion calculations are carried out in UM7. A rigid body, e.g. a disk, consists of multiple mass points. Does the software consider a defined number of mass point to carry out motion calculatioons or does the software consider only the center of mass for a body to carry out the motion calculations?

I ask this because I have designed an overhanging spinning disk which rotates in a certain distance around a fixed pivot. If I run the simulation the spinning disk should precess around the pivot due to gyroscopic effects, however, the calculated motion results in a more ore less chaotic oscillationg motion (not nutation motion) but not in an unform precessing motion, which would be expected as a rule.
Is there a possibilty in UM7 to consider not only the center of mass of an object but consider a certain number of mass points of an object?
Thank you in advance for your feedback.

Best regards,
Harry
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Re: Basic calculation question

Postby Roman Kovalev » Fri Feb 19, 2016 5:40 am

Dear Harry,

As far as I understand, under a defined number of mass point you meant 'moment of inertia', see https://en.wikipedia.org/wiki/Moment_of_inertia. In terms of classical mechanics the distribution of mass within the volume of a body is described with the help 'moments of inertia'. For any rigid body in UM you can describe both mass and moments of inertia in see body properties in UM Input program, see attached snap shot.

Regards,
Roman
Attachments
mass and moments of inertia.PNG
Properties of rigid body
mass and moments of inertia.PNG (11.02 KiB) Viewed 80168 times
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Re: Basic calculation question

Postby Harry1 » Fri Feb 19, 2016 10:57 am

Dear Roman,

Thank you for your support. However, I don't mean rhe moment of inertia. The question is, whether UM 7 consider for the motion calculations only the center of the mass of a rigid body or a certain number of single mass points with different distances to the center of mass. The mass points at the outer diameter of a spinning flywheel behave different to applied forces respectively torques as mass points at the inner diameter of a spinning flywheel.
I hope that it is now more clear what I mean. If not I could send an example model for better understanding if you may provide an email address.

Regards, Harry
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Re: Basic calculation question

Postby Roman Kovalev » Fri Feb 19, 2016 5:06 pm

Dear Harry,

I am sorry. I did not understand what you mean. Please send me your model to support@universalmechanism.com.

Inertial properties of rigid bodies is fully described via mass, moments of inertia and position of center of mass. We do not artificially separate rigid body to several mass points.

Regards,
Roman
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