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How to analyze toggle clamp friction

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cjccmc

Mechanical
Oct 11, 2012
111
I have a design in work that uses a custom designed toggle clamp to close a spring loaded door using only hand force applied to the clamp handle. The clamp end travels freely until final .20 inch of travel where the load in closing the door increases linearly from 0 to 50 lbs. In this range I found that the overall mech advantage is approx 5:1, getting near 20:1 as the links approach over-center.

requirements are:
1. clamp must fully close the door with 10 lb max hand force applied to the handle

2. clamp handle will be held closed by a locking tab and when this tab is removed, the handle (clamp) must spring open to the fully released position. (handle rotates about 30 deg)

I'm desiging the nominal closed position to be 2 deg before overcenter so the clamp can spring open as required. Also think I will need to incorporate an internal spring that will help the clamp pop open when the locking tab is released.

I can figure out the free body diagram if the latch were at overcenter, but elsewhere I am not really sure how to analyze this beast. At overcenter I find that the 3 pins in line have a radial load of approx 65 lb, the 4th pin about 100 lb. Using .25 dia pins with dry lube I'm assuming a friction factor of .20 and I can find the breakaway torque at closed. What I find most confusing is how to calculate the frition losses as the clamp is being closed by the hand force.

I can't be the first to be lucky enough to land this puzzle. Can someone point me to a fairly simple method to calculate this? Attach pic is similar to my design.
 
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