DMU spherical joint

[marcofa]

I cannot do any simulation where spherical joints are involved.
Someone can help me learning how to solve enclosed mechanism?
Marco

 

[itsmyjob]

I just look at your video and I have the feeling your problem is that your spherical joints does not prevent the beige part to rotate along its own axis. you need to fix this.

I might look at your file next week if I find some time.

my solution to fix the rotation is not easy to define and may be more difficult to explain... so search and create some extra geometry and define extra joint...

Eric N.
indocti discant et ament meminisse periti ​

 

[marcofa]

I fixed Motor_axis rotation, but anyway mechanism cannot be simulated, (Exemple.jpg) because DOF=1 with one command.

If I add UNIVERSAL JOINT between Motor_axis and Part4(connecting rod)axis, DOF go to =0 and mechanism can be simulated. (Exemple1.jpg)

Seems that Spherical joint cannot work as is, but need extra joint.
In Exemple there is two Spherical joint, but only one Extra joint is needed. WHY?

I would like to have some clear rules to define Spherical joint (nothing found in documentation). To do something without knowing why, is not so good.
Marco

 

[jackk]

Marco,

You asked: "Seems that Spherical joint cannot work as is, but need extra joint.In Exemple there is two Spherical joint, but only one Extra joint is needed. WHY?

I would like to have some clear rules to define Spherical joint (nothing found in documentation). To do something without knowing why, is not so good."

I see in the attached files a very common 3D mechanism.

In your example with the two Spherical joints, the undefined Degree of Freedom is the rotation about the centerline of the Rod-Balls part. You must account for this rotation by using an extra command, or adding an extra joint that will prevent this rotation.

In your other example, the Universal Joint defines that centerline rotation, allowing the mechanism to be simulated.

Since DMU Kinematics is a motion study only (with no friction or gravity), you must be aware of all the degrees of freedoms of each part in order to mathmatically define the mechanism so it can be solved during simulation.