×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

lifting tong is it statically indeterminate
3

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

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 !)

RE: lifting tong is it statically indeterminate

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

Ted

RE: lifting tong is it statically indeterminate

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.

RE: lifting tong is it statically indeterminate

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).

RE: lifting tong is it statically indeterminate

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.

 

RE: lifting tong is it statically indeterminate

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

RE: lifting tong is it statically indeterminate

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?

RE: lifting tong is it statically indeterminate

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.

RE: lifting tong is it statically indeterminate

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

RE: lifting tong is it statically indeterminate

(OP)
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]?
 

RE: lifting tong is it statically indeterminate

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:  

RE: lifting tong is it statically indeterminate

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.  

RE: lifting tong is it statically indeterminate

(OP)
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.  

RE: lifting tong is it statically indeterminate

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.

RE: lifting tong is it statically indeterminate

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).

RE: lifting tong is it statically indeterminate

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.

 

RE: lifting tong is it statically indeterminate

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.

RE: lifting tong is it statically indeterminate

(OP)
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.

RE: lifting tong is it statically indeterminate

nice sketch ...
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

Oh, sorry, yes I see what you are saying rb.  

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

(OP)
rb / btrue,
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

GD is on the plane of symmetry of the part ... the load from CDE and the opposite link offset one another.  also GD has only two pins so the load must (assuming no friction moments at the pins) be aligned along GD.

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

you know the force in GI (consider ptG).

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

Once you get this thing going here is short synopsis of what's required to get this gripper in use.

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

Hi nighthawk123

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

desertfox, I think the normal force in your example would be 200 lbs. not 25.  Friction force = normal force * friction coefficient.
Friction force = 100/2 = 50
Normal force = 50 * 4 = 200

Ted

RE: lifting tong is it statically indeterminate

Just for the heck of it. Can you use 2 smaller grabs on a spreader bar. These grabs would the simpler 2 link models.  

RE: lifting tong is it statically indeterminate

hi hydtools

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

(OP)
The tong has points that imbed into the product.  Therefore a gripping force of 100lb for a 100lb lift would be ok.  If I had a product on a larger tong, say 20,000lb, a grip force of 20,000lb on 2 small points would easily imbed into the product, unless picking up something harder, like titanium.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources