Boundary conditions - PROBLEM

[matos11]

Hello.

Could you help me, please, with setting of boundary conditions in Ansys Workbench.
I have model of pliers (something like this Link)

I need to do static analysis, Handles are subjected to loading with certain force. But I dont know which other boundary conditions I have to set (supports,...). I tried to do something, but the solving fails.
Can you help me? Thanks.

 

[kranmeste]

Hi Matos11,

if you look at the pliers you can see it probably consists of 3 parts, Left Handle, Right Handle and a Lock Pin (2 parts if the lock pin in integrated into one of the handles). I have disregarded the insulation on the handles. If you know your beam theory, one could chose to look at a single handle as a simple supported beam with overhang at one end. The displacement conditions should be were the object is being firmly griped and at the lock pin. The load condition should be, as you point out yourself, at the handle. You should only model one handle to begin with. The above is a simple way of looking at the problem, but it will get you started.

Best Regards

 

[matos11]

Hi Kranmeste.

Thank you for your reply.
My pliers consist of 2 handles with cutting edges at the end and one pin. Loading is on the handles and I have to investigate how they bend when I have certain force whether they touch together or not.
Solver in Ansys Workbench solves something, but it always announce that there is som mistake and it stops (when the handles touch each other).

I define boundary condition:
Fixed support on pin, contact between two areas on handles (when they touch each other - they create a contact) and forces on handles.
My task is to define boundary condition in cutting edges. It has to be define gap between two cutting edges 5 mm. And I dont know how can I define this boundary condition.

Thanks for help.

Martin

 

[kranmeste]

Hi Matos11,

first of all my main area is not FEA so it might be that my information to you is not 100% correct .

Comments to your boundary condition:
When looking at the pliers, using the link you referred to in your first comment, it can be seen that the pin connection can not be modelled as a fixed support. In real life that would mean the pin connection can obtain a moment and therefore the pliers would not be able to open or close for that matter. I would use a cylindrical support which allows for a rotational DOF. If needed you can lock the pin from radial and axial movement.
Regarding using contact areas i believe the FEA will be treated as non-linear, due to the friction. This typically requires a lot more specialised settings than i can advise you about.

Not sure i understand your goal for this task, but if it is to check if the handles touch under the gripping force, could you not just measure the shortest distance between handles unloaded. Then add some measure points on each handle and see how much they displace at a specific force? Deflection of the handles should be proportional to the force, at least as long as you are within the yield stress of the material grade used.

Best Regards

 

[matos11]

Hi.

Sorry, that I reply a little bit later, but I was busy with another things, that were need to be done.

Ok. So.
Pin which connects two handles I set as revolute joint (ground-body) and has 1 degree of freedom - only rotation.
Then I set for handles revolute joint (body-body) where reference body is a pin and the handle is mobile and can rotate only.
Because of the fact that I am acting with certain force on the handle - that is acting of force on an "arm". I have to set some force on the cutting edge (resp. a point of cutting edge). This force is higher because, this force acts on shorter arm - I need to have a moment balance.

Am I right?
In addtion, I set bonded contacts between bodies with MPC formulation...
But, the solution is not still convergent. I dont know why. I turn on large deflection too because of nonlinearities..

Best regards.

 

[kranmeste]

Hi,

In my opinion you are partly right regards your moment balance. You are correct that there needs to be a moment balance, but if you intend to use a manually calculated reaction force at the cutting edge then i am sure that the model will never converge. The reason being that you will never be able to calculate the exact force at the cutting edge to make sure the model is in equilibrium.

I would apply the following conditions:

1: Force created from the hand gripping strength.
2: 1 DOF (Rotational) constraint @ the pin connecting upper and lower plier.
3: 0 DOF (Fixed) @ the cutting edge (Lower)
4: 0 DOF (Fixed) @ the cutting edge (Upper)

Furthermore i would not use Large Deflection, since i assume that the material used is a steel grade and you most likely intend to stay within the yield stress of the material. If i remember correct large deflection is typically used when plastics or elastomer's are analysed. Or in the case that you know you will have a large deflection of your steel structure, for example a long slender crane boom made of extra high strength steel.

I would also leave out the contact areas. Maybe you need to have a contact area at the pin because you are analysing an assembly.

If you could upload a picture of you analysis showing the boundary conditions this would help as well.

Best Regards

 

[matos11]

Hello.

Ok, so.

I need to investigate the stress- and deformation analysis of pliers.

I have pliers (see pic.). I set this boundary conditions:
1. forces on handles
2. forces on cutting edges
3. joints (reference-mobile): revolute - DOF only around z-axis
3.1. revolute: right handle-left handle with centre of rotation in the centre of the
pin
3.2. revolute: pin-upper cutting edge with centre of rotation in the centre of the
pin
3.3. revolute: left handle-pin with centre of rotation in the centre of the
pin
3.4. revolute: right handle-pin with centre of rotation in the centre of the
pin
3.5. revolute: pin-lower cutting edge with centre of rotation in the centre of the
pin
4. contacts (all bonded):
4.1. between handles
4.2. between handles and cutting edges
4.3. between handles and pin

forces applied to the handles are 400N, forces acting on the cutting edges 1200N...principle of lever.
I set iterative solver and large deflection.
I dont know what I did bad.

Thanks

Martin

https://drive.google.com/file/d/0B0OilJPwBKJ8TnJSR0dEbmMyNDA/edit?usp=shari

 

[kranmeste]

Hi

If the stress and deformation on the pliers are the main focus on the analysis, you could also just analyse a single handle.
But again i always try to keep problems as simple as possible, because if it is one thing i have learned over the years it is that engineers tend to make a simple problem very complex, including me .

I have found a youtube clip which i believe sums up exactly what you need for your plier analysis. It is probably good to watch the whole video, but the fun part begins at approx. 15min.

https://www.youtube.com/watch?v=W5gHfi8qsTs

Below i have linked to another techniche called sub-modelling. It might be that you will need this to. It depends on how much in detail you will need to go with your stress analysis.

https://caeai.com/sites/default/files/Submodeling145_CAEA.pdf

I hope the above will give some good hints to your problem.

Best Regards