Theory of Machines and Machine Design MCQ With Answers for JKSSB Junior Engineer (Mechanical)

Technical · 2026-10-21 · 6 min read

Theory of machines and machine design test mechanisms, gears, governors, belts, fasteners, joints and failure theories. Most questions are definition or formula based. This page gives a revision list and then solved MCQs with explanations.

Quick revision list

  • Mechanisms: lower and higher pairs, the four-bar chain with one degree of freedom, and the Whitworth quick-return mechanism in shapers
  • Gears: module equals pitch circle diameter divided by the number of teeth, and the standard pressure angle is 20°
  • Governors and flywheels: a governor controls the mean speed and a flywheel smooths the fluctuation within a cycle
  • Design: keys transmit torque, square threads suit power transmission, and stress concentration is reduced by fillets
  • Failure theories: maximum shear stress (Guest or Tresca) for ductile materials and maximum principal stress for brittle ones

Worked numerical problems

  • Gears: a driver gear of 20 teeth runs at 600 rpm and drives a gear of 40 teeth. The driven gear runs at 600 × 20 ÷ 40 = 300 rpm
  • Module: a gear with a pitch circle diameter of 120 mm and 40 teeth has a module of 120 ÷ 40 = 3 mm
  • Belt drive: with μ = 0.3 and an angle of contact of π radians, the tension ratio is e^(0.3π) = 2.57
  • Degrees of freedom: a four-bar chain has 4 links and 4 joints, so F = 3(4 − 1) − 2(4) = 1
  • Shaft torque: a shaft transmitting 10 kW at 600 rpm carries a torque of 10,000 × 60 ÷ (2π × 600) = 159 N·m
  • Rivet: one rivet of 20 mm diameter with a shear strength of 60 MPa can carry π/4 × 20² × 60 = 18.85 kN in shear
  • Spring: with G = 80 GPa, wire diameter 5 mm, coil diameter 40 mm and 10 turns, the stiffness is Gd⁴ ÷ (8D³n) = 9.77 N/mm

Mechanisms in brief

A kinematic link is a body that transmits motion. A kinematic chain is a set of links joined so that the motion is constrained. A mechanism is a chain with one link fixed. The Grashof condition says that in a four-bar chain, if the sum of the shortest and longest links is not more than the sum of the other two, at least one link can rotate fully. A cam and follower changes rotary motion into reciprocating motion in a set pattern, as in valve operation.

Gears and gear trains

Spur gears have teeth parallel to the axis. Helical gears have teeth at an angle and run more quietly. Bevel gears connect shafts that meet at an angle, and worm gears give a large speed reduction. In a simple gear train, each gear has its own axle, and an idler gear changes the direction without changing the ratio. In a compound train, two gears share an axle, so the ratio is the product of the individual ratios.

Balancing, vibration and governors

Static balancing makes the resultant centrifugal force zero. Dynamic balancing also makes the resultant couple zero. A governor controls the mean speed under changing load, whereas a flywheel evens out speed variations within a cycle. In a Watt governor, the height is inversely proportional to the square of the speed.

Design basics

  • A key transmits torque between a shaft and a hub
  • A square thread is used for power transmission, and a metric thread for fastening
  • The factor of safety is the ultimate stress divided by the working stress
  • A fillet at a change of section reduces stress concentration
  • A ball bearing has lower starting friction than a plain bearing

Questions candidates ask

What is a higher pair? A joint with point or line contact, such as a cam and follower or gear teeth, as opposed to a lower pair with surface contact.

Why is a flywheel heavy at the rim? It stores energy in proportion to its moment of inertia, which rises when mass is placed far from the axis.

What is the factor of safety for a ductile material based on? The yield strength is divided by the working stress for ductile materials.

Practise

Solve the MCQs below, then read the exam guide. Next: manufacturing and machining.

By Shivam · Written with AI assistance and fact-checked · Questions on this page are original · Editorial policy · Report an error

Practice questions

Q1. A kinematic pair in which the relative motion is completely constrained is called:

  1. An open pair
  2. A higher pair
  3. A free pair
  4. A closed pair
Show answer

A closed pair — Closed pairs are held together by their own form or by external forces.

Q2. Grubler's equation for the degrees of freedom of a plane mechanism with only lower pairs is:

  1. F = 3(n − 1) − 2j
  2. F = (n − 1) − j
  3. F = 2(n − 1) − 3j
  4. F = 3n − 2j
Show answer

F = 3(n − 1) − 2j — n is the number of links and j the number of joints.

Q3. A four-bar chain with all lower pairs has how many degrees of freedom?

  1. One
  2. Zero
  3. Two
  4. Three
Show answer

One — 3(4 − 1) − 2(4) = 1.

Q4. A cam and follower is an example of a:

  1. Higher pair
  2. Lower pair
  3. Turning pair
  4. Sliding pair
Show answer

Higher pair — The contact is along a line or point rather than a surface.

Q5. A whitworth quick-return mechanism is used in:

  1. Milling machines
  2. Drilling machines
  3. Lathes
  4. Shaping machines
Show answer

Shaping machines — The return stroke is faster than the cutting stroke.

Q6. The velocity ratio of a gear pair is the ratio of:

  1. Diameters of driven to driver gear
  2. Speed of driver to speed of driven gear
  3. Torque of driver to driven gear
  4. Number of teeth of driver to driven gear
Show answer

Speed of driver to speed of driven gear — It equals the inverse of the ratio of teeth.

Q7. The module of a gear is the ratio of:

  1. Circular pitch to the pressure angle
  2. Pitch circle diameter to the number of teeth
  3. Number of teeth to the pitch circle diameter
  4. Addendum to the dedendum
Show answer

Pitch circle diameter to the number of teeth — m = D / T.

Q8. The standard pressure angle for involute gear teeth is commonly:

  1. 60°
  2. 45°
  3. 5°
  4. 20°
Show answer

20° — 14.5° was used in older practice.

Q9. In a gear train, an idler gear changes the:

  1. Number of teeth
  2. Direction of rotation of the driven gear
  3. Torque ratio
  4. Speed ratio
Show answer

Direction of rotation of the driven gear — It has no effect on the overall gear ratio.

Q10. A flywheel's function is to:

  1. Convert linear to rotary motion
  2. Maintain a constant mean speed
  3. Smooth out speed fluctuations by storing kinetic energy
  4. Reduce friction
Show answer

Smooth out speed fluctuations by storing kinetic energy — A governor, not a flywheel, controls the mean speed.

Q11. In a Watt governor, the height of the governor is inversely proportional to:

  1. The cube of the speed
  2. The square of the speed
  3. The speed
  4. The mass of the ball
Show answer

The square of the speed — h = g / ω².

Q12. In a belt drive, the ratio of tensions on the tight and slack sides, T1/T2, is:

  1. μθ
  2. e^(μ/θ)
  3. sin(μθ)
  4. e^(μθ)
Show answer

e^(μθ) — It applies for a flat belt on the verge of slipping.

Q13. Static balancing of a rotating mass system requires that the:

  1. Resultant couple is zero
  2. Resultant centrifugal force is zero
  3. Torque is zero
  4. Speed is constant
Show answer

Resultant centrifugal force is zero — Dynamic balancing also requires the resultant couple to be zero.

Q14. In a clutch, the torque transmitted for a uniform wear assumption is:

  1. Zero
  2. Larger than that for uniform pressure
  3. Smaller than that for uniform pressure
  4. Independent of the radii
Show answer

Smaller than that for uniform pressure — Uniform wear is the usual design assumption for a worn clutch.

Q15. The device used to join two shafts that can be disengaged while running is a:

  1. Coupling
  2. Bearing
  3. Key
  4. Clutch
Show answer

Clutch — A coupling joins shafts permanently, while a clutch can be engaged and disengaged.

Q16. A key is used to:

  1. Lock a nut
  2. Reduce vibration
  3. Transmit torque between a shaft and a hub
  4. Support the shaft
Show answer

Transmit torque between a shaft and a hub — It sits in a keyway cut in the shaft and hub.

Q17. The standard metric thread profile has an included angle of:

  1. 55°
  2. 60°
  3. 90°
  4. 29°
Show answer

60° — Whitworth threads have a 55° angle.

Q18. A square thread is mainly used for:

  1. Pipe joints
  2. Fastening
  3. Power transmission, as in a lead screw or jack
  4. Locking
Show answer

Power transmission, as in a lead screw or jack — It has low friction and no radial thrust.

Q19. Which of these is a locking device for nuts?

  1. Split pin
  2. Gib
  3. Cotter
  4. Washer plate
Show answer

Split pin — A split pin passes through a hole in the bolt and the castle nut.

Q20. A rivet joint fails by:

  1. Shearing of the rivet, crushing of the rivet or plate, or tearing of the plate
  2. Buckling of the rivet
  3. Only shearing of the rivet
  4. Only tearing of the plate
Show answer

Shearing of the rivet, crushing of the rivet or plate, or tearing of the plate — The designer compares the strengths for each mode.

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