Line of action question for a mechanism 2

[mod231]

Hello all, nice forum you have here.
My question is this: I have to lift a component with a type of fork attached to a cylinder. The fork will contact the lifted object at 2 distinct points, not symmetrical to the centerline of the cylinder. How do I determine where the imaginary centerpoint of these contact points is so that I can place it on the cylinder centerline (line of action)? Is there some way to calculate a centroid for the contact points, or is there a simple geometric way to do this? Is what I am looking for the same thing as instant center or no?

regards,
Mod231

 

[desertfox]

Hi mod231

Post a sketch it speaks a thousand words.

desertfox

 

[mod231]

Ok here is a pdf from cad... Basically the arrangement is a mobile cylinder attached to hook component with a pin. The hook is used to lift the edge of a large welded basin. (this is a section view). The hook has multiple holes so that it can be extended as needed. It works fine at the attachment hole shown in the sketch, but when moved to the higher holes it wants to kick out. This is a redesign originally done by others... anyway the hook contacts the basin under it's edge and at the side of the piece of angle. I want to line up the "centroid" of these contact points with the line of action of the cylinder.

Regards.

 

[mod231]

Forgot the attachment...

 

[mod231]

And here is the same arrangement extended:

 

[hydtools]

Neglecting friction, point contact forces are perpendicular to the hook contact surfaces. Resolve a force diagram based on a free body diagram of the hook to determine that the pin force goes through the cylinder line of action.

Ted

 

[desertfox]

Hi mod231

If I understand the the diagrams correctly the hook has its own pivot independent of the cylinder, unfortunatly the lower jaw of the hook will always take most of the weight of the lifted component, the lower jaw being below the screw head.
The upper hook jaw seems to have only a tangential contact with the component but the force will act in a different direction to that of the lower jaw.
You're problem stems from the line of action of the cylinder creating an anticlockwise moment around the jaw pivot when its extended, if you look when the cylinders collapsed the line of force passes through the jaw pivot point and therefore causes no problem.
Sorry I can't be of further help.

desertfox

 

[zekeman]

It matters little where you place the contact points.

The only solution is that if you draw the line of action, the CM must be (absent friction)

1) below the cylinder

2) the cylinder must be vertical

3) the line of action must pass thru the CM

The reason is that the virtual motion of the cylinder ram must be colinear with the vertical virtual motion of the CM; otherwise you have a residual moment in the cylinder.

 

[zekeman]

In looking over the design again, it does matter where you place the contact points but you must meet the 3 conditions set forth above.

The problem with the existing design is that if the CM is to the right of the cylinder at the start, it can never achieve the required design criteria, since the contact point is to the right .
You must move that point to the left of the cylinder center line and have another flat that can take the additional vertical force needed after the load rotates CW to get in line with the cylinder.

If, however the CM is to the left of the cylinder, the intial contact point is OK and the mass will then swing CCW to get under the cylinder, and another point will make contact at the bottom of the hook to satisfy the 3 conditions.

 

[mod231]

thanks everyone for the insight