Influence of friction in pivot point(s)

[jrmichler]

Let's work on the static FBD by itself. Your solution has a horizontal force at C that is not counteracted by a moment at D, so the sum of moments about D is not zero. It needs to be zero.

A good way to double check your work is by taking the sum of moments about A, B, C, D, and G. All of them must sum to zero. You only need the resultant force to calculate bearing loads.

You can embed images in the post by converting to JPG format, then using Attach files > Insert > Full image. That's the best way to do it.

 

[Mech_LS24]

I re-calculated the forces and added a moment around D, without the moment indeed, the force Cx didn't sum to zero.

Afterwards I checked the forces with the FBD and moment around A, G and B. I found a silly mistake here, I missed the force in point B and C. I thought these were internal force and therefore didn't take them into account. Could you explain when those are internal forces and how to recognize them?

The moment in D doesn't play a role in the friction as I understand? This feels strange haha.

See attached the files.

 

[jrmichler]

In the case of this linkage, an internal force would be the force inside a link. Link AB, for example, has an internal compressive force (and stress). In a linkage, the forces at each pinned joint are external forces, so each link needs its own FBD.

You are correct, the moment in D does not play a role in calculating friction. It may feel strange at this time, but with more practice at analyzing linkages and trusses, the strangeness will go away.

The next step in calculating friction is a diagram similar to the one in Post #22. It is good practice to have all angular velocity arrows on a single diagram in the same direction - clockwise or counterclockwise. If a particular link is rotating opposite to the arrow, that angular velocity will be negative.

 

[Mech_LS24]

Link AB, for example, has an internal compressive force (and stress).

What do you mean with internal compressive force?

Is this meant with the angular velocity arrows?

Besides, I have read the SKF documentation for the friction calculation. They refer to a torque calculator on their site named, SKF bearing tool. I am not quite familiar with that yet. Is there an other more general way to calculate the friction?

 

[jrmichler]

The diagram in Post #25 is well done, but has wrong angular velocities. The diagram in Post #22 is well done, especially the dashed line diagram showing the linkage position after small displacement of the input link. Now compare the rotations of each link to your calculated angular velocity for each link. Focus on the direction of the angular velocity.

The SKF documentation is the easy way to calculate rolling element bearing friction. If you want to learn more, try search terms antifriction bearings calculations. The first hit was this: https://web.iitd.ac.in/~hirani/lec32.pdf. It's a good summary of friction in rolling element bearings. Note how many of the inputs are found experimentally.

 

[Mech_LS24]

Now compare the rotations of each link to your calculated angular velocity for each link. Focus on the direction of the angular velocity.

Do you mean I should focus on the 'links' rather than the rods? The rotation of each link should be...?:
C = CW
B = CW
D = CCW
A = CCW

The SKF documentation is the easy way to calculate rolling element bearing friction. If you want to learn more, try search terms antifriction bearings calculations. The first hit was this: https://web.iitd.ac.in/~hirani/lec32.pdf. It's a good summary of friction in rolling element bearings. Note how many of the inputs are found experimentally.

Going to read/study this, thanks.

 

[Mech_LS24]

I tried to calculate the friction with the support of the documentation you have sent in post #41 @jrmichler , this are my results. The documentation link is here.

Total lost of momentum = 23.192 Nmm (see calculation)

I compared it with the SKF calculator and the outcome for the SKF was 19.1 Nmm, but this is for each bearing I think. In my calculation I multiplied the Mp (Moment due to load) by 4 because I have 4 pivot points. When I multiply the SKF data by 4 I will have an outcome of 76.4 Nmm.

I guess I miss something in my calculation but can't figure out what.

Bearing: 6001-2RSH

Calculations:

 

[jrmichler]

Get two bearings - one a deep groove ball bearing such as a 6001-2RS, the other a pillow block bearing about the same size. Hold them in your hands and rotate the inner race. You will notice that one of them has far more friction torque than the other.

There are many different designs and types of oil seals, and each type will have different friction. The SKF calculator is for the specific seals used in their bearings. The PDF - who knows? The CR Seal catalog, with its 225 pages of seal goodness, will give you an idea of the different seals available: https://www.skf.com/binaries/pub12/...1_CRSeals_Handbook_Jan_2019_tcm_12-318140.pdf.

Sometimes, and this is one of those times, you have to get the best information you can find, make your best estimate, add a safety factor, and hope for the best.

 

[Mech_LS24]

Thanks @jrmichler . This has been a great learning moment for me. Bit by bit I getting better in approaching mechanical challenges (I guess ). I really appreciate your patience and offered support.