lifting tong is it statically indeterminate 3

[nighthawk123]

I am trying to figure out the grip force of a tong; however I think it is overconstrained and therefore is statically indeterminate. I've attached a bitmap of the design. Any opinions? Castigliano's theorem? Deflection method?

 

[rb1957]

not so sure that it isn't statically determinate ... the two jaws (that pick up the work piece look like simply supported beams, and the rest of the mechanism solves accordingly (or maybe accordian-like !)

 

[hydtools]

Do free body diagrams of each link. Line of action of each link force at each joint aligns with each respective link.

Ted

 

[chicopee]

Nighthawk123-shame on you for presenting this Rube Goldberg contraption. This lifting clamp will not work as the vertical elements have locked up the clamp in one position and will not lift anything as it can not clamp on the object to be lifted. If you want it to work properly, remove the center vertical element and slot the two other vertical elements at their lower pin connections. The system can then be analyzed and will work properly.

 

[tmacm]

chicopee, the only thing that would do is to not allow the fingers to remain parallel. The bottom pair of elements connected to the above two by the vertical member will just follow the above two and keep the fingers parallel (assuming that the lenghts of the two bottom pairs of links are the same and that they are parallel with each other).

 

[chicopee]

tmacm- right now the thing is useless as it can not move. If you carry out my suggestion, the two slotted vertical elements will remain parallel as they clamp on a load to be lifted and now you would have something to analyze.

 

[SaskGear]

If the center vertical link is removed the device will not clamp when lifted. As a vertical force is applied to the top vertical member, the device starts to close and a clamping force is applied to the load being lifted. To analyze this device you would need to know the lengths of the links not just the over all width and height. You need more dimensions

 

[NomLaser]

If the bottom joint in the center isn't a slot then the contraption won't move. Something needs to be slotted or self-adjusting in order for the parts to move. See the attached animation. Notice how the top holes at the end are never the same distance from the next set below it?

 

[btrueblood]

Deddie,

The tongs work just fine if you keep the pin-to-pin lengths of the two members attached to the jaws equal. They aren't equal in your model.

 

[NomLaser]

Thanks, I missed that point in my quickness to recreate the model

 

[spider007]

i'll work.

 

[nighthawk123]

I appreciate the response to this "chinese finger trap". spider007 has nailed the motion. I attempted to recreate the mechanism in Analytix but cannot yield the grip force. I keep getting a redundant dimension.

If in spider007's recreation the tong was lifting 100 pounds, what would the grip force be at, say, 1[in]?

 

[btrueblood]

Yup. Just to point out, though, the length of the upper set of members can be varied, the length of the upper members relative to the jaw members determine how much "squeeze" the jaws will produce for a given lifted load, which in the final anlysis will affect how low the friction coefficient between the jaws and the lifted object can be before the object will slip out.

my quick & dirty model:

 

[MintJulep]

Anyway, back to the original question.

The three-pin links cause it to be indeterminate.

I haven't looked closely, but it's possible that HALF of the device could be analyzed using statics.

 

[nighthawk123]

ok, so can a powerful analysis program be used to model such a beast?

my problem is that, for a standard tong in my statics books, the tong point only contains an x-force component. easily solvable. sum the moments about the main pin, using half the product weight as a known vertical force at the lever tip and solving for the grip force directly.

this model has a hinge point instead, containing x and y components. a free body of the main link has x and y components of the main pin and x and y components at the tip. summing the moments and forces allows for 3 equations, but there are 4 unknowns.

not to mention the center link component or the fact that forces through the side vertical links have y components along the same force line.

i am in the process of physically modeling the system and taking physical measurements. i will advise...thanks everyone.

 

[btrueblood]

Oh, and one more thing, back to the OP - the jaws are indeterminate unless something rigid is placed between them, or they bump together. One also must assume (for a static analysis) that the load is lifted vertically, and the c.g. of the load is below the upper pivot. In terms of CAD restraints, the upper member must be restrained to move parallel to the jaw faces.

 

[rb1957]

i thought the lifting fingers were three force bodies, so that the three forces' lines of action intercept at a point. this implies that the links are subject to bending (for non-axial forces).

 

[btrueblood]

Mmmm...the members are all pinned at each end, not sure how you'd get bending forces into them unless you assume non-ideal joints with friction.

I'll take back what I said about the upper member needing to move parallel to the jaw faces, it's not really necessary unless you are trying to simulate a vertical lift (and keep the analysis simple). The jaws should work for a horizontal drag just as well; a drag or lift/drag combination will just impose a different set of motion constraints to the upper link.

 

[rb1957]

no, the pins can transfer load that is not axial to the links putting the links in bending. i'm not suggesting that there is moment transfer ... if there were (but there isn't) then the fingers wouldn't be three force members.

the links up by the handle are two force members, so the forces are co-linear (and axial along the link). these links join up to the scissor links which are also 3 force members, with one of the forces directed along the short link (the other 2 force member). this actually solves the mechanism, defining all the forces' lines of action. you don't need FE (which no-one has suggested), some graph paper and/or some H.S. trig will do it.

 

[nighthawk123]

i did a mock up dimensioned drawing and attempted to solve the linkage forces. the part size is 4 inch and weighs 100 lb. The tong weighs 10 lb. Any corrections or suggestions would be greatly appreciated. Would also like to eventually solve if picking up a 2 inch part.