A mechanic uses a screw driver to install a ¼-20 UNC bolt into a mechanical brace. What is the mechanical advantage of the system? What is the resistance force if the effort force is 5 lb.

15.7 ; 78.5

Explanation:

Mechanical advantage of a screw = Circumference / pitch

Circumference = pi × d

Where :

pi = 3.142, D = diameter

Therefore ;

Circumference = 3.142 × (1/4) = 0.785 in

Pitch = 1/TPI

TPI (thread per inch) = 20

Pitch = 1/ 20 = 0.05

Mechanical advantage = 0.785 / 0.05 = 15.7

Resistance force if effort force is 5lb

Mechanical advantage = Fr / Fe

Fe = effort force, Fr = resistance force

15.7 = Fr / 5

Fr = 15.7 × 5 = 78.5 lbs

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