# Abaqus2 - Kinematic Coupling VS Rigids

1. Sep 29, 2010

### harsunra

What is the difference between Kinematic coupling in Abaqus and Rigids in Nastran?
Why it is named as Kinematic Coupling.... what is the physics behind the word kinematic and rigids?

2. Oct 2, 2010

### nvn

My vague impression is, kinematic coupling ties the displacement (motion) of node 2 to node 1, using a function such as Tx2 = function(Tx1), where T means translation, and x means global x axis. In its simplest form, the above kinematic coupling function could be Tx2 = Tx1, which means node 2 follows exactly the displacement of node 1. Or, you could make the kinematic coupling more advanced, such as Tx2 = 0.5*Tx1.

Another way to achieve Tx2 = Tx1 is to use a rigid bar finite element, such as RBAR or RBE2. A rigid element rigidly welds the parts together.

Aside: Keep in mind, RBE3 is not a rigid element. RBE3 creates force equilibrium, but not displacement compatibility. Therefore, generally always avoid RBE3, except in very simple situations where you are sure you know what you are doing. E.g., perhaps (?) use RBE3 only if you want to distribute a force or mass from a node that has no other element connected to it, but you do not want a valid displacement compatibility between the parts. You can get very strange, grossly incorrect results using RBE3. E.g., let's say you want to distribute a force from the center of a bolt hole to the hole edge. Sounds fairly benign, doesn't it? Well, the stresses in the plate can be completely wrong garbage. I am unable to clearly explain RBE3, except to say, generally always avoid RBE3, whenever possible.