Learning Forum

[ENV159]

Hello,

I am trying to create a shaft with bearings and a force applied on the end. This is a rotating shaft for a spindle. For the bearing locations I want to place a spring in place of the bearings because there is a preload force with a corresponding stiffness value. I am reading online that COMBIN14 is used for what I am trying to accomplish. How do I apply this method correctly in workbench? I am not too experienced in the command window but I am willing to learn. Here is an example shaft I am trying to experiment on and a spring layout from a paper. Another question, if I do not use a spring connection and only fixed supports, will there be a significant change in deflection?

Thanks

 

 

[Sandeep Medikonda]

Hello,

  You might find this section from the help very useful in getting started.

Regards,
Sandeep

[peteroznewman]

Hello,

Don't use fixed supports, it will have a significant effect on deflection. Create two Remote Points, each one scoped to one of the faces you had fixed. Apply the springs to those remote points. On one of the remote points, you also have to add a Fixed Rotation along the shaft axis, leaving the other two rotations free to avoid a zero pivot error on the solver.

If you didn't need a preload, you would just use a Bearing connection.

Also, instead of four springs at 90 degree spacing, consider using eight springs at 45 degrees. That is what ANSYS does for the Bearing connection which, without a preload, only requires four springs. The reason to use 45 degree spacing is that with 90 degree spacing, the stiffness in the 45 degree directions is only 70.7% of the stiffness in the 90 degree directions.

 

Regards,
Peter

[ENV159]

Thank you.

So I would add 8 springs from the “connections” section in workbench to each bearing location and I apply those boundary conditions you stated? For the stiffness of each spring, will they each be equal to each other? For the stiffness value will this be equal to the stiffness on the bearing sheet?

[ENV159]

Thank you I will check it out.

Sadly, the link for the forum requires a nonstudent account.

 

[peteroznewman]

This is the workaround for opening the ANSYS Help when someone provides a link on this forum and you are using the Student 19 license.

[peteroznewman]

COMBIN14 doesn't appear to offer a preload.  You are going to have to work hard to get that preload. Where physically is the preload coming from?

COMBIN40 offers a GAP capability which is useful if you want to implement preload. If you use two COMBIN40 springs that have a GAP capability where one spring is opposite the other and the nodes are moved toward each other to provide compression preload, you will get the centering force profile shown below.  Is this what you want?

If you just use a single spring, you get this force-displacement curve

I recommend you build your model without the preload first, learn what you can from that model before you go to the effort of implementing preload.

Regards,
Peter

[ENV159]

Thank you so much Peter, I am just starting ANSYS out and I will take your advice without the preload. Most importantly, I am not sure my model is correct and I when I try to modify the fixed rotation settings for the remote point, the box is grayed out.

Adding the springs to the remote points, I am not sure if this is correct but the spring originates in the center of the shaft when I click the face. Since I want to have springs modeling a bearing, the springs should be on the outer surface where the remote point lies correct? if so, how would I fix this? Adding 8 springs to each remote point is what you have recommended, so what is the correct method to do this? if its possible could you upload a simple shaft with this example? I apologize if this seems rudimentary. 

Thank You 

[peteroznewman]

Now that you don't need opposing springs to generate a preload, you can just have springs on one side.
When you Insert a Bearing you automatically get 4 springs at 45 degrees apart. You don't see them, but they are there.

You scope the bearing to the surface on the axle that the bearing seats on. A remote point is created at the center of the shaft and has a spider of Constraint Elements from that point out to the bearing surface, so the springs are effectively spread out around the entire surface.

The figure of the four springs in my post above shows the direction of the springs for which you provide the stiffness values in the Bearing Details: K11, K12, K21 and K22.

Read this post for more information.

Regards,
Peter

[ENV159]

Ok thanks so when I apply the bearings to different diameters, the bearings does not fit around the shaft properly. It seems to fit inside the shaft even though I click the face on the outside What is the reason for this? I downloaded the example in the link you provided and the bearings fit around the face

Selection

Thanks again

[peteroznewman]

The Bearing graphic is just an icon illustrating the plane in which the four springs are located. There is a point at the center and CEs out to the surface.

In this example, the icons are much larger than the bearing seat.  It might even be the case that the icon changes size depending on the zoom of the view. There is a resize icons button for when you do a large change in the view zoom.

Regards,
Peter