: Calculation of Linear Bearing (Carriage) Load

[berdan]

Hi Guys, please please assist me!
I need to pick for a project at my work linear carriages which should work at this configuration :
As you can see, there is a piston which moves the plate on 4 bearings from the side (yes, I know, not the best configuration, but that's what we have given the spatial constraints).

From looking at manufacturer site for linear bearings, I can see that the parameter I need to calculate is Static Load C0, given in Newtons.
(BTW we built that system already, and we have a "caging", in a lack of better word. When system starts moving, one side with bearing 3 & 4 goes up before 1 & 2, and then that's it. No more movement because of that tilt).

So,Static Load C0, that's what I should calculate? Then why there are three moment parameters at manufacturer site (allowable moments) ?

Should I calculate those moments as well ? How the hell do I solve that issue ? Please assist, I'm getting nuts here.

 

[Lnewqban]

While help arrives, could you provide more specific information?
As the system is built, rather than m, L1 and L2, show us those actual values.
The manufacturer from which you have copied that table would also help.

The way the cylinder and the guides are anchored, if adjustable or fixed.
What type of bearings and guides are currently installed?
Why do you think are the visible reasons for the current layout to deform and get stuck?

 

[jrmichler]

We need to see your free body diagram (FBD) of this assembly. And, as @Lnewqban posted above, with actual values. There are other factors to consider to fully solve this problem, but the FBD comes first.

The allowable moments on a linear bearing normally apply only when one bearing has moment loads. In your case, the moment load from the actuator shows up as side loads on the linear bearings. You only need to use the C and Co values.

When system starts moving, one side with bearing 3 & 4 goes up before 1 & 2, and then that's it.

First the FBD, then we tackle this. It's a straightforward problem to solve, but first things first.

 

[berdan]

We need to see your free body diagram (FBD) of this assembly. And, as @Lnewqban posted above, with actual values. There are other factors to consider to fully solve this problem, but the FBD comes first.

The allowable moments on a linear bearing normally apply only when one bearing has moment loads. In your case, the moment load from the actuator shows up as side loads on the linear bearings. You only need to use the C and Co values. First the FBD, then we tackle this. It's a straightforward problem to solve, but first things first.

Hi, well that is really my question .
How the hell I solve this FBD??

BTW, you guys asked for numbers, but can we solve it parametrically ? I'm not sure how the numbers help, and all I need to understand actually is indeed how to make this FBD. That is my question.

If you say it is important I put the numbers here, but really all I need is just helping me understand how to solve this FBD.

From that, I understood that I need to calculate C and C0. So I understand I calculate them from the forces on the bearing.
So how on earth do I find the reactions on bearings?

From what I remember, this frame is statically indeterminable?
There is a solution from the THK site, but I don't understand how to do it :( .

 

[Tom.G]

Let's try an empirical approach.

Are you sure the piston travel is absolutely parallel to the rails without any horizontal force applied by misalignment? You can check this by disconnecting the table-cylinder connection and see if it works better when the table is moved manually.

1) This part applies if the table is being moved horizontally, it won't help (much) if it travels vertically.

As a simple 'first attempt', can you move the attachment point of the cylinder to a point along the edge of the table to a point half way between the bearings? That is 'half way down the edge it is currently attached to' in your sketch.

2) If the table is moving vertically, how about instead of attaching the cylinder to the table edge, you attach a bar to the cylinder that runs under the table and attaches to the under side of the table at the center of mass of the table + contents. Make the bar-table attachment with some minor amount of freedom with a slot in the bar (horizontally in your sketch) so the table can self-align on the rails.

Cheers,
Tom

 

[Lnewqban]

Hi, well that is really my question .
How the hell I solve this FBD??

BTW, you guys asked for numbers, but can we solve it parametrically ? I'm not sure how the numbers help, and all I need to understand actually is indeed how to make this FBD. That is my question.

If you say it is important I put the numbers here, but really all I need is just helping me understand how to solve this FBD.

If the piston is perfectly anchored and parallel and coplanar with the guides, there will be no force on any of your bearings, except minimum needed to prevent horizontal rotation about the cylinder itself.
Evidently, that is not the case.

That is why dimensions, or at least proportions, could be important in this case.
Even better, rather than calculation of bearings, the suggested solutions for improving the system are recommended.

It seems to me that the guides are too flimsy to correct any problems introduced by the piston force and trajectory, and possible missalignments.

 

[jrmichler]

Hi, well that is really my question .
How the hell I solve this FBD??

Your Post #4 shows a diagram. Now decompose that into two free body diagrams - one looking at it from the side, the other looking at it from the front. The side view FBD will be used to calculate the force components in the "front" direction, the front view FBD will be used to calculate the force components in the "sideways" direction. At this point, it is helpful to define X, Y, and Z directions on the figure in Post #4 and add them to the figure. And be sure to define which direction is vertical.

When system starts moving, one side with bearing 3 & 4 goes up before 1 & 2, and then that's it.

You can show this in one or both of the FBD's by sketching the carriage (which should be shown as a simple rectangle) at the appropriate tilt. Be sure to correctly indicate if it tilts in one or both directions.

A FBD does not need to be a professional quality CAD drawing. A hand sketch is sufficient.