A screw jack has a pitch of 12 mm with a mean radius of thread eq
A. 6.2° , 4.5°
B. 4.85°, 5.7°
C. 6.37°, 4.42°
D. 4.2°, 5.5°
Please scroll down to see the correct answer and solution guide.
Right Answer is: B
SOLUTION
Concept:
Screw threads are frequently used in jacks, presses, and other machines.
Screw thread acts as an inclined plane of the angle of inclination equal to the helix angle α as shown in the figure.
The helix angle of the thread is α
\( \tanα =\frac{p}{2\pi r}\)
where, p = thread pitch of screw, r = mean radius of a screw.
The friction angle ϕ = tan-1 μ
where μ = coefficient of static friction.
Calculation:
Given:
p = 12 mm, r = 22.5 mm. μ = 0.10, length of lever = 600 mm, load = 1800 kg.
The helix angle of the thread is α, where,
\( \tanα =\frac{p}{2\pi r}\)
\(α=\tan^{-1}({12\over2\times\pi\times 22.5})\)
α = 4.85°
The friction angle
ϕ = tan-1 μ
ϕ = tan-1 (0.10)
ϕ = 5.71°
