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# Free Body Diagram - Ball and Socket (both ends)

## Free Body Diagram - Ball and Socket (both ends)

(OP)
Good day fellow Engineers,

A co-worker and I are having a discussion about a ball and socket mechanical joint. He says the reason for a ball joint is so the ball can move freely, and the socket force load is ALWAYS normal to the center of the socket (cup). So, he says to set up a FBD. So, I was looking on the web to find one and I was not very successful. The crude pic attached shows my configuration.

-CP1 is the center point of both the ball and cup, as is CP2. Both center lines of the gray parts are assumed to be parallel. The far-right gray part (cup part) is fixed.

My thoughts:
The force is applied to face shown and translates to CP1. The reaction force on CP2, are vector components of the force from CP1 (the normal force on CP2 is at an angle from the original F1. Therefore, there is a moment about CP2.

There is buckling at both ends of the necked down diameter and the rest of the orange part is in compression.

Can I get some help on finding a good text or web page showing the FBD of this type of set up (or at least a close one)?

Thank you all in advance for any help.

### RE: Free Body Diagram - Ball and Socket (both ends)

The net force on each end has to pass through the axis of the link. There can be no net side forces.

### RE: Free Body Diagram - Ball and Socket (both ends)

your colleague is, what we say, wrong.

a 2 force member has loads that are co-linear, same line of action. You'd think this would align with the body of the link, but it doesn't Have to. The two points of contact (ie centers of their respective areas of contact and pressure) define one line.

Unless ... there is some moment being reacted that one or both ends. If the balls are sticky (from friction or clamp-up) then a moment can build up and it is no longer a two force member.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

### RE: Free Body Diagram - Ball and Socket (both ends)

pmbov,

A ball joint that completely encircles your ball, unlike your diagram, prevents motion in three linear axes, and allows motion in three rotational axes. If both ends of your beam are in ball joints, then the force entirely axial. If your beam somehow sees a side load, the ball joint will resist it.

--
JHG

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