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
Machine screws are

Used to prevent relative motion between two parts
Similar to small size tap bolts except that a greater variety of shapes of heads are available
Slotted for a screw driver and generally used with a nut
Similar to stud

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