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Impossible simulation with spherical joints

marcofa

New member
A machanism composed by two 90° oriented cams and connected by link rod with two spherical joints cannot be simulated because it's impossible to resolve one DOF.
If somebody would like to help me I will send files by mail.
Thanks Marco
 

MrCATIA

Super Moderator
My guess is the link rod has a rotational DOF on the axis between the spherical joints. Try changing one of the spherical joints to a revolution joint, or maybe a U-joint.
 
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marcofa

New member
Exacly! That is the point! I tried to fix rotation but in this case , after creating automatic joints, there is couple not resolved.
With rotational joint the mechanism dont work because there is relative rotation between crank and rod ( two crank lever are 90° oriented).
What you means U joint?

BTW its impossible that spherical joint cannot be simulated. Where is a mistake?
Marco
 

MrCATIA

Super Moderator
Marco,

Try replacing one of the spherical joints with a Universal Joint (a "U-joint"). That will usually take out the extra rotational DOF.

You say "automatic joints"; I never had alot of success converting assembly constraints into kinematic joints, so I always defined all the joints manually.

You might have to create some dummy parts in your mechanism to resolve some of the extra DOF.

Another trick to try is to go ahead and run the simulation with 2 DOF (as if you have 2 input commands), and just use "0" as the rotation of the link rod.
 
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marcofa

New member
I dont understand U joint, I dont find constant-velocity joint.

Its not possible to go ahead, is not possible to create the second joint and simulation cannot start with free dof.

If you supply me Email adress I will send you the file, (catia r17)
Marco
 

MrCATIA

Super Moderator
Hello Marco. Good News - I was able to get your mechanism to work! :)

I downloaded your parts and assemblies and converted your assembly constraints into Kinematic Joints. As you said, it created a mechanism with 2 DOF.

(some of the parts were missing in the link rod product, so I replaced it with a new link rod part).

I replaced one of the Spherical Joints with a Universal Joint. This eliminated the rod from rotating on it's axis and left only 1 DOF. (see attachment)

I made one of the Revolute Joints "Angle Driven" and then I was able to simulate the mechanism.
 

Attachments

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marcofa

New member
Sorry but one part was forgotten.
Beautifull solution, like a "Columbus egg", I tried to fix rotation in many different way but unsuccesfull.
My big machinery (horse simulator) work fine now.
Thank you very much
Marco
 

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