## General Mechanical

Topics relate to Mechanical Enterprise, Motion, Additive Print and more

#### Needle tissue interaction

• Calum Bowmaker
Subscriber

Hi, does anyone know how to contstrain the needle so that it cant move in Z-direction during insertion. I would appreciate any help.

• Erik Kostson
Ansys Employee

Hi Calum

Please use symmetry on the face of the needle and block that are on the XY plane, and set sym. normal to Z on those faces (we basically need to restrain the Z movemenet on the nodes of that plane).

One can also use just a displacement boundary condition there and fix Z instead (that is what the symmetry will do anyway).

Thank you and all the best

Erik

• Calum Bowmaker
Subscriber

Thanks Erik!

• peteroznewman
Subscriber

It looks like the geometry is cut in half. Solid models that use symmetry in the X-Y plane need a Displacement boundary condition of Z = 0 on the faces that are in the X-Y plane. That is what prevents the needle from moving in the Z direction.

• Calum Bowmaker
Subscriber

Yes that is correct, I cut my geometry in half as the model was symmetrical. Thank you for the help, I obviosuly didn't apply the displacement boundary correctly but I will try again.

• Erik Kostson
Ansys Employee

Just to clarify a bit.

We do not need both displacement and symmetry condition (set sym. normal to Z) on those faces (XY-plane) in order to prevent them from moving in Z.

The symmetry in Z is applying a fixed DZ restrain on these nodes (it will be showing as a frictionless support) – this is clearly visible in the ds.dat file (this is the main file created by mechanical – you can find in the proj. dir.) that Ansys reads and solves with the APDL solver.

So this (symmetry condition) is enough on its own to prevent the movement in Z. You can like we said also skip this and use the other option a displacement condition with a fixed Z. So one or the other not both are needed.

Hope this makes it all clear.

Erik

ds.dat extract of symmetry condition with z-normal

/com,********* Frictionless Supports Z *********
CMBLOCK,_FRICSUZ,NODE,       21
(8i10)
3         4         5         6         7         8         9        10
11        38        39        40        41        42        43        45
46        49        50        53        55
cmsel,s,_FRICSUZ
d,all,uz,0
nsel,all

• peteroznewman
Subscriber

Good that you clarified that Erik, I should have been more precise in my words. I meant that when symmetric geometry has been sliced, a Z=0 Displacement condition can be used to support the faces in the X-Y plane.  I understand a Symmetry object in in the model will provide the same support so both would be redundant.  I didn't see your first reply until after I posted.