Machine Design
Two closely coiled helical springs with stiffness k₁ and k₂ respectively are connected in series. The stiffness of an equivalent spring is given by

(k₁ k₂)/ (k₁ + k₂)
(k₁ + k₂)/ (k₁ k₂)
(k₁ - k₂)/ (k₁ k₂)
(k₁ - k₂)/ (k₁ + k₂)

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Machine Design
Which of the following statement is correct regarding power screws?

The efficiency of Acme (trapezoidal) thread is less than that of a square thread
The efficiency of a self locking screw can not be more than 50%
If the friction angle is less than the helix angle of the screw, then the efficiency will be more than 50%
Both (A) and (B)

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