Numerical - Number of screw threads - Direct shear stress

Q.  A single start square threaded power screw supports  load of 30 kN which has outer diameter of 30 mm and a pitch of 8 mm. Maximum shear stress of 30 N/mm2 is induced in a screw body. Determine number of screw threads considering direct shear stress.
- Published on 14 Sep 15

a. 3 threads
b. 2 threads
c. 4 threads
d. None of the above

ANSWER: 3 threads

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    Discussion

  • Sravanthi   -Posted on 04 Nov 15
    Direct shear stress acts on screw threads. The formula used to calculate direct shear stress is:

    τ = W / (π d t Z)

    Given: W = 30 kN, pitch = 8 mm, diameter = 30 mm, maximum shear stress = 30 N/mm2

    Formula: 1) direct shear stress (τ) = W / (π d t Z)

    here, d = diameter, W =load, t = p/2 i.e pitch/2

    Solution:

    Substituting the given values we get,

    1) t = 8/2 = 4 mm

    1) τ = W / (π d t Z)

    30 = 30 x 103 / (π x 30 x 4 x Z)

    Z = 2.65

    Therefore, Z = 3 threads

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