Ball valve load analysis

[salmon2]

Hello,

I struggled to figure this out and has been a while... Need serious help. This is not college homework, this is a problem in my work.

Please see attached pic top section, which is a ball valve. This is a top view, if I change it to bottom, you will see another pin at the symmetric position. Basically two pins are welded to a piston, not shown, and are siting on two flat slots on the ball. When the piston/(two pins) move from left to right, the pins will drive the ball rotate and close the ball. Similarly, open the ball in the opposite direction. The two slots are two flat slots cut on the ball with 45 deg relative the axis.

what shown is the initial position where the pin is 1.0 inch vertically away from the ball center. The full stroke for the piston or pins will be from left 45 deg to right 45, or two inches in distance; and the ball will rotate 90 deg during one stroke.

Assume the resistance torque on the ball is known as T, a constant, how much axial force F needed on the piston or two pins to drive the ball rotation?

As you can see I have done two calculations for the initial position in the middle section of attachment, I think the right one, method 2, is the correct one. My question is how to dissolve the for F? Method 1 or 2 or #3? In the pic, arm is the vertical distance for the torque.

But in the lower section of attachment, you can see I also run a solidworks motion study and found the curve for piston force which seems to indicate the method 1 is correct. If the method 2 is correct, then I should see a horizontal line with value at T.

Can anyone help? Thanks a lot.

 

[tr1ntx]

The force required to turn the ball will vary as the piston moves through its range. The highest force required will be at the beginning of the stroke, when the pin force is at the greatest angle to the slot surface. Your Solidworks motion study is leading you astray. It is showing inverted results.

F(max) = T / (1 in. * cos45deg) [neglecting friction]

Find a tutorial for basic engineering statics.

The pin doesn't move radially in the slot, does it? I'm assuming the piston acts in line with the arrow shown for F, i.e. a constant distance from the ball centerline.

 

[chicopee]

You would be better off to draw your vector diagram showing radial and tangential components. The torque value will be then directly proportional to the radial component. Both the radial and tangential components will change in magnitude and direction as the ball rotates.
The torque equation neglecting friction will be proportional to the radial force and the distance between the pin and the ball centers. That distance will also change during the ball rotation explaining the slots to allow for the movement of the pins.

 

[chicopee]

To add to my first reply, while the tangential force within the slot is to overcome the resisting torque from the ball, the radial force within the slot will be to overcome friction which will be a product of the coefficient of friction and the radial force.

 

[gerhardl]

.. but the really big question comes when the theoretical calculation is solved: what are you going to use the calculation for, and what natural variation (safety factor) do you need to use when selecting the actuator force (if that is the case)? 20, 30, 100, 500 or 1000% - all can be the factual value according to application.