# Machine Design Questions and Answers – Couplings

This set of Machine Design Multiple Choice Questions & Answers (MCQs) focuses on “Couplings”.

1. A coupling is a mechanical device that temporarily joins two rotating shafts to each other.
a) True
b) False

Explanation: A coupling permanently joins two rotating shafts.

2. In distortion energy theorem, if a unit cube is subjected to biaxial stress, then S(yt) is given by which of the following?
a) √ (σ₁²- σ₁σ₂ +σ₂²)
b) σ₁²- σ₁σ₂ +σ₂²
c) √ (σ₁²+ σ₁σ₂ +σ₂²)
d) σ₁²+ σ₁σ₂ +σ₂²

Explanation: S(yt)=√[(σ₁ -σ₂)²+(σ₂ -σ₃)²+(σ₃ -σ₁)²]/2, for σ₃=0; expression a is obtained.

3. Oldham coupling i used to connect two shafts having intersecting axes.
a) True
b) False

Explanation: Oldham coupling is used to connect two parallel shafts separated by a small distance.

4. A machine started malfunctioning due to some issues with the coupling. The coupling emplaced in the machine was Oldham. The only coupling available in the workshop is Hooke’s coupling. So Oldham coupling can be replaced by Hooke’s coupling.
a) True
b) False

Explanation: It is not possible to replace Oldham by Hooke’s coupling as Hooke’s is used to connect intersecting axes and not parallel axes shafts.

5. A muff coupling is connecting two shafts. The torque involved is 650N-m. The shaft diameter is 45mm with length and breadth of the key being 14mm and 80mm respectively. Find the shear stress induced in the key.
a) 30.2N/mm²
b) 25.8N/mm²
c) 34.4N/mm²
d) None of the listed

Explanation: τ =2M/dbl.
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6. The region of safety for biaxial stresses is of which shape in the case of maximum distortion energy theorem.
a) Ellipse
b) Circle
c) Rectangle
d) Square

Explanation: S(yt)= √ (σ₁²- σ₁σ₂ +σ₂²) or S²(yt)= (σ₁²- σ₁σ₂ +σ₂²). This is an equation of ellipse.

7. Distortion energy theorem is not recommended for ductile materials.
a) True
b) False

Explanation: Experimental investigations have shown that distortion energy theorem gives good results for failure of ductile component.

8. Among maximum shear stress theory and distortion energy theory, which gives the higher value shear yield strength?
a) Maximum shear stress theory
b) Distortion energy theory
c) Both give equal values
d) Vary from material to material

Explanation: Maximum shear stress theory gives S(sy)=0.5S(yt) while Distortion energy theorem gives S(sy)=0.577(Syt).

9. A muff coupling is connecting two shafts. The torque involved is 650N-m. The shaft diameter is 45mm with length and height of the key being 14mm and 80mm respectively. Find the compressive stress induced in the key.
a) 70.1 N/mm²
b) 51.6N/mm²
c) 45.5N/mm²
d) None of the listed

Explanation: σ=4M/dhl.

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