SPRING WITH INITIAL FORCE 1

[Maraach]

Is supported any spring element that...

F = K*X+CONSTANT ???

with an initial force with a null displacement between DOFs???.

 

[mendinho]

At least in Cosmos/M it is possible to define gap elements with an initial force. The beam element offers the possibility to define a pre-deformation wich is equivalent to prestress the element.

 

[GBor]

You can use a truss or beam in virtually any software that allows a thermal difference between the reference temperature and a preset temperature. A quick calculation using a thermal expansion number will give you the pre-stress.

 

[Maraach]

Hi;

Thanks for the answers..I'm performing an analisys about a connecting rod, so i have to simulate the fix between the two parts by a screw. I'm using the implicit non-linear SOL600 solver (In MSC.NASTRAN). Can I use, like you are saying, an inital temperature in the screw to simulate the pretension?.

What about the GAP element in Nastran/Patran???. I think the pretension in a screw (or in a part of the structure) is a very common situation ( at least in mechanical engineering) so it must be supported by any element/load/element property.

It could be modeled by "cutting" the screw in two separated part (the length of the separation corresponding to the elongation of the pretension) and before, in the analisys, fix the nodes in the opposited faces and constrain the dofs. It could be performed by fixing the relative-tangencial movements and using a spring between the face's nodes with a very high constant K and with a very high force too corresponding to null displacement...

F=K*X+C

with K>> and C>>

THNKS.

 

[GBor]

Think the answers are:

1) Yes you can use the thermal difference to simulate the preload.

2) You should be able to use the Gap element and apply a preload, but I don't use Patran.

3) Not completely sure I'm following your "split element" precess. Are you suggesting that the spring go in the split of the element, or between the connecting points of the ends of the screw? I also don't really understand the "tangential" movements. Do you mean axial? As in the expansion along the length of the screw from the head to the tip?

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group

 

[Maraach]

Well..

i've found that GAP element can be used only in SOL106 and others... but it can't be used in SOL600 (it can be used only like linear stiffness, without an initial force).

The best way to simulate the pretension is using thermal difference.

But, in static analisys (SOL 600 and SOL 101, for example), You can't only apply temperature in the screw (using TEMP Bulk data entry), so the unique way to simulate temperature only in the screw is using two materials in your model: One meterial with non-zero thermal expansion coefficient and one with zero th. ex. coef. and you must apply an unique temperature in your model.

Is it?

 

[GBor]

Afraid I'll have to defer to someone with Patran experience. In most software packages that I use, you can apply a unique temperature to individual parts. The set-up can be non-sensical. For instance, if I want to create a beam with a stiffness of 1000, I give it a cross section resulting in an easily calculated I, a length dependent upon how long it is in the model, and then set the E value to whatever it needs to be so that EI/L = 1000.

The same is true if I want a certain preload. F=kx, so I make the stiffness "k" calculable. I know what "F" needs to be since it is the preload I'm looking for. Then, all I have to do is determine how to get the "x" that I calculate.

Well, since "x" is the strain that I need in my element, I just have to insert a coefficient of expansion "a" in inches per degree C (or whatever temperature units) and apply the delta temperature that, when multiplied by the thermal expansion, results in the "x" that I need.

Sounds much more complicated than it really is.

Hope this helps more than it confuses...

Garland

 

[rb1957]

if the initial force is constant, why not superimpose it on the varying spring load ?

and, btw, how does this happen ? it's not a simple spring if it can have a load with a displacement ??