lifting tong is it statically indeterminate
lifting tong is it statically indeterminate
(OP)
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?





RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
Ted
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
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.
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
If in spider007's recreation the tong was lifting 100 pounds, what would the grip force be at, say, 1[in]?
RE: lifting tong is it statically indeterminate
my quick & dirty model:
RE: lifting tong is it statically indeterminate
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.
RE: lifting tong is it statically indeterminate
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.
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
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.
RE: lifting tong is it statically indeterminate
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.
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
if you're lifting 100 lbs, then you know the load in links AB and AC = (100/2)/sin 27deg. links BDF (and CDE) are three force members; you know the direction of the force at B, and at D (GD is a 2 force member, so the force has to be axial), this means you know the direction of the force at F (thru the intersection of the line of action of the forces at B and D, which would be A). GD and GH are two force members, so the force in these links is axial. then (finally) the lifting finger, EHJ, is a 3 force member, the force at E has a line of action thru A, the force at J is horizontal, and the force at H is thru G; and when you extend these 3 lines of action they should all intersect at the same point.
i would move link GD so that it's between GH and GI (links GI and GH are separated by the thickness of the lifting finger. this may have been what the other posters were noticing, that the mechanism is alittle "twisted".
RE: lifting tong is it statically indeterminate
Also, don't forget there is also a vertical force acting at J, being the friction force, or mu*Fhorizontal. The friction generated (and/or deformation of the lifted object, but this is statics, not strength of materials and tribology) is what lifts the block, there will be a minimum mu below which you can't lift something.
RE: lifting tong is it statically indeterminate
thanks for putting the thought into this... where i disagree with the geometric approach from rb1957 is that GD contributes to link BDF as a 2 force member in the y direction, but the contribution from link CDE in the x direction is not included. this moves the intersection away from point A.
On link FIK (or EHJ) the product weight at J is not included. Doing so sends the force vector at an unknown angle (not horizontal), and still leaves the force directions at E&F unknown.
RE: lifting tong is it statically indeterminate
i accept btrue's comment about friction, so that the force vector at J isn't horizontal, but i think it should be close to it. the point of the tongs is to squeeze the thing being lifted. if the thing is too heavy (or if the friction is too low) for the amount of squeeze, the thing will slip out of the tongs. the amount of squeeze force is determined by the force applied at the top of the tong, the geometry of the links is determined by the size of the thing being lifted.
the geometrical approach (for looking at force vectors) is valid, if somewhat old fashioned.
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
therefore you have only one unknown on FIK.
i think you're doing a "dead lift" at ptA, so that the squeeze force at ptK is determined by the geometry of the links ...
RE: lifting tong is it statically indeterminate
I can't recall the full details but during a an in-house course I took about under the hook lifting and the devices thereof it was mentioned that there is a limit on the amount of multiples that can be used in operation of a gripper. There was also a discussion about having a problem with over center locking when trying to increase the grip by increasing the mechanical advantage.
http://www.pnl.gov/contracts/hoist_rigging/bth_l
ifting.asp
RE: lifting tong is it statically indeterminate
I think your tongs are statically indeterminate and further more I beleive it won't work, your analysis assumes you have picked the 100lb weight up.
Imagine that the weight is resting on the floor and you place the tongs around it, how do you apply an external force to the tongs,your anaylsis shows a crane hook operating the tongs;therefore the only applied force to the 100lb weight is due to the mass of the tongs as the crane hook moves upward;therefore unless you apply a force greater than that required for sliding friction that block is going nowhere. Assuming friction coefficient to be 0.25 those jaws would require to apply 25lb minimum to prevent the tongs slipping off during lift.
regards
desertfox
RE: lifting tong is it statically indeterminate
Friction force = 100/2 = 50
Normal force = 50 * 4 = 200
Ted
RE: lifting tong is it statically indeterminate
RE: lifting tong is it statically indeterminate
Yes your right I was thinking of the block sliding horizontally along the floor thanks for the correction.
desertfox
RE: lifting tong is it statically indeterminate
Thanks to everyone's help I feel that a close graphical solution has been attained and the grip force will be quite high, in the neighborhood of 4.5 : 1 (ie 450 lb on a 100lb lift)
I built a scale model of the tong and it has a very high grip force, but maybe too high. It would have been nice to get a working force diagram that I could put into Excel with trig., thereby allowing me to adjust the strap lengths to exactly what they needed to be as well as the proportions of the lower parallel links.
The reason for using this "parallel linkage" design is that I would like to maintain parallel contact with the part throughout a gripping range.