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Participant

Join Date: Feb 2021
Posts: 2

Free Body Diagram - Vice Grip Plier

02/26/2021 4:29 PM

I don't understand why my thread was deleted so I' posting it again.
I ran into a few problems when solving the statics equations of the free body Diagram of these pliers:

A worker uses these pliers and applies 200 N on it to crush a hex nut. Very basic stuff, the worker does not apply torsion yet. I just want to make sure the diagram is correct.

So I broke it up into the following pieces:
It's modelled on solid works , so i measured everything there(i double checked the measurements).

I ran these equations on MAPLE and this is what I got:

P1 = 200 (N)

∑1Fx = bx - c*cos(α) = 0

∑1Fy = by + P1 - c*sin(α) = 0

∑1MB = by + P1*LBDx - c*sin(α) = 0 # LBDx= lenght from B to D on the x axis

bx= 403.247 by= 347.746 c= 680.173

∑2F = c - h = 0

h=680.173

∑3F = e - f = 0

∑4Fx = -bx + e*cos(γ) + ax + g*sin(20.4) = 0

∑4Fy = - by + ay + e*sin(γ) - g*cos(20.4) = 0

∑4MA = g*sin(20.4)*LAG1y +g*cos(20.4)*LAG1x +e*sin(γ)*LAEx +e*cos(γ)*LAEy-by*LABx -bx*LABy = 0

∑5Fx = -ax + f*cos(γ) + h*cos(α) - g*sin(20.4) = 0

∑5Fy = -ay - f*sin(γ) + h*sin(α) + g*cos(20.4)-P2 = 0

∑5MA = -g*sin(20.4)*LAG2y - g*cos(20.4)*LAG2x - f*cos(γ)*LAFy - f*sin(γ)*LAFx + h*sin(α)*LAFx - h*sin(α)*LAHy - P2*LP2 = 0

F = 1952.786 P2 = 200 , ax = 5338.525 ay= 4413.985

e = -6044.681 f = - 6044.681

The equations turned out fine and balanced, but the results seem a bit too high , and further more , the spring should be in compression and not in tension as demonstrated by the final result of e and f.

I don't understand what I'am doing wrong , but the error seems to be in the last 6 equations if anyone is willing to help me I would mighty appreciate it! Any suggestion is welcomed.
Thank you soo much for reading!

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#1

Re: FREE BODY DIAGRAM - VISE GRIP PLIER

02/26/2021 7:50 PM

I don't know if this helps, but I have a set of vice grips like that and the spring is definitely in extension when it is clamped down.

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#4
In reply to #1

Re: FREE BODY DIAGRAM - VISE GRIP PLIER

02/26/2021 9:51 PM

This is probably not how you're supposed to work it, but I would look at how much the handle moves versus how far the jaw moves. This mechanical advantage varies as you close the jaw, but should approach a limit, determined by the geometry. Input force x input motion equals output force x output motion.

It's just an idea, but it might give you a way to double-check your answer.

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#2

Re: FREE BODY DIAGRAM - VISE GRIP PLIER

02/26/2021 8:27 PM

It looks like you've got 200N from the top and the bottom which should be the total... ....no that's not right...nope no clue

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#3
In reply to #2

Re: FREE BODY DIAGRAM - VISE GRIP PLIER

02/26/2021 9:03 PM

Thanks for the video , I'll check it out maybe I'll find something I have overlooked!

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#5

Re: FREE BODY DIAGRAM - VISE GRIP PLIER

02/26/2021 11:15 PM

Further to Rixter’s suggestion, your force multiplier is just over 30, and your distance for the jaws is basically the strain on the handles and the stress on the object in the jaws, since the operator adjusts the geometry of the mechanism until the jaws touch the object.

From experience, the smaller the object in the jaws, it seems slightly more force is applied. But you always adjust the geometry with the adjustment screw so your closing stroke is fixed at about 15mm, maybe. So you really need to account for the object size in the jaws, and the location of the screw top/fulcrum, unless I missed that. Typically in use, I find a movement of the levers of about 15mm might result in jaw movement less than 0.5mm, which corresponds to a 30:1, as you calculated. I don’t know why it would be negative, though.

On some extreme cases, a very hard nut, the total movement could be limited to the elasticity of the handles and the pins, even higher than 30:1 movement.

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#6
In reply to #5

Re: FREE BODY DIAGRAM - VISE GRIP PLIER

02/27/2021 2:22 AM

A point is reached where the compressive force on the handles to operate the overcentre mechanism exceeds the torque that cam be applied by the hand.

There is also an even earlier point where the pinch factor of the fleshy hand overcomes any desire to continue clamping, this in theoretical physics is known as the ouch factor and can occur at any point in the compressive stroke.

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