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BCS Agriculture cadre Machine Design: practice questions

BCS Agriculture cadre — Agricultural Engineering (subject code 821) · Bangladesh · Bangladesh Public Service Commission

Original practice questions written to the published syllabus · not past papers
10worked here, free
27written to this exam in this subject
12syllabus units

About BCS Agriculture cadre

BCS Agriculture cadre — Agricultural Engineering (subject code 821) is a government recruitment exam run by Bangladesh Public Service Commission, for candidates in Bangladesh. Preliminary 200-mark MCQ (2 h); written 900 marks; viva 200 marks. Code 821 is a 200-mark post-related paper.

Source: bpsc.gov.bd.

Checked against the exam body's published information. Formats and fees change; confirm the current edition there before you book.

Machine Design in BCS Agriculture cadre

Sizing the parts of farm machines so they do not fail: shafts, keys, joints, springs, bearings, drives and overload devices.

Our 27 BCS Agriculture cadre questions in this subject: 15 easy, 12 medium, 0 hard, across 12 units.

10 practice questions, worked

Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.

These 10 were written to BCS Agriculture cadre and are not shown on any other exam's page.

Question 1 · easy

Why is grey cast iron a common material for gearbox housings and machine beds but not for highly stressed tension members?

  1. It cannot be machined to close tolerances, so it is only used where no dimensional accuracy is needed
  2. It casts easily, damps vibration and is strong in compression, but it is brittle and weak in tension
  3. It is ductile and strong in tension but too expensive to machine
  4. It has a higher modulus of elasticity than steel, so it deflects less, but it rusts too quickly to be used in tension
Answer and working

B. It casts easily, damps vibration and is strong in compression, but it is brittle and weak in tension

Grey cast iron flows well into complex moulds, its graphite flakes damp vibration and give good machinability, and its compressive strength is several times its tensile strength. The same flakes make it brittle and weak in tension. Its modulus is lower than that of steel, not higher.

Question 2 · medium

The rods of a cotter joint carry an axial tensile load of 50 kN. The allowable tensile stress is 80 MPa. What is the minimum rod diameter?

  1. 14.1 mm
  2. 19.9 mm
  3. 25.0 mm
  4. 28.2 mm
Answer and working

D. 28.2 mm

P = (pi/4) d^2 sigma t, so d = sqrt(4P/(pi sigma)) = sqrt(4 x 50000/(pi x 80)) = 28.2 mm. Omitting the 4 gives 14.1 mm; using 2 gives 19.9 mm, treating it like a double-shear pin; sqrt(P/sigma) = 25.0 mm treats the rod as a square bar.

Question 3 · medium

A bracket plate is welded with two parallel fillet welds of 8 mm leg size and carries 50 kN along the welds. The allowable shear stress on the throat is 60 MPa. What length is needed for each weld?

  1. 52.1 mm
  2. 147.3 mm
  3. 36.8 mm
  4. 73.7 mm
Answer and working

D. 73.7 mm

Parallel fillets are in shear on the throat: P = 2 x 0.707 h l tau, so l = 50000/(1.414 x 8 x 60) = 73.7 mm each. Using leg size instead of throat gives 52.1 mm; ignoring the second weld gives 147.3 mm. 36.8 mm halves the answer again, counting the two welds twice.

Question 4 · medium

Two 12 mm plates are joined by a full-penetration butt weld 150 mm long. The allowable tensile stress in the weld is 100 MPa. What tensile load can the joint carry?

  1. 360 kN
  2. 180 kN
  3. 127 kN
  4. 180000 kN
Answer and working

B. 180 kN

For a butt weld the throat equals the plate thickness: P = sigma t l = 100 x 12 x 150 = 180000 N = 180 kN. Applying the fillet-weld factor 0.707 gives 127 kN; adding both plate thicknesses (24 mm) gives 360 kN; 180000 kN forgets to convert N to kN.

Question 5 · easy

At the moment a heavily loaded journal bearing starts from rest, which lubrication regime exists?

  1. Full hydrodynamic lubrication
  2. Elastohydrodynamic lubrication of rolling contact
  3. Boundary lubrication
  4. Hydrostatic lubrication without an external pump
Answer and working

C. Boundary lubrication

A hydrodynamic film needs relative sliding speed to pump oil into the wedge. At start-up the speed is near zero, so the surfaces touch through thin adsorbed films: boundary lubrication, with higher friction and wear. Hydrostatic lubrication needs an external pressure supply.

Question 6 · easy

What is the minimum number of teeth on a 20-degree full-depth involute pinion to avoid interference when it is generated by a rack cutter?

  1. 18
  2. 13
  3. 32
  4. 14
Answer and working

A. 18

The theoretical limit is 2/sin^2(20 degrees) = 17.1, so at least 18 teeth. For 14.5-degree teeth it is 32, which is why the 20-degree system allows smaller pinions. 13 is the minimum when the pinion meshes with an equal gear rather than a rack; 14 is the figure for 20-degree stub teeth.

Question 7 · medium

What does the property class 8.8 marked on the head of a metric bolt indicate?

  1. A bolt of 8.8 mm shank diameter
  2. Yield strength about 88 MPa (8.8 × 10)
  3. Nominal diameter 8 mm and pitch 0.8 mm
  4. Ultimate tensile strength about 800 MPa and yield strength about 80 per cent of that (about 640 MPa)
Answer and working

D. Ultimate tensile strength about 800 MPa and yield strength about 80 per cent of that (about 640 MPa)

In ISO 898-1 the first number times 100 gives the nominal ultimate strength in MPa, and the second number times 10 gives the yield-to-ultimate ratio in per cent, so 8.8 means about 800 MPa ultimate and 640 MPa yield. Size is given separately, for example M16.

Question 8 · medium

Which bearing is best suited to a long implement shaft supported on a light frame where some misalignment between bearings is unavoidable?

  1. A pair of angular-contact ball bearings preloaded against each other, so the preload holds the shaft true despite frame flexing
  2. A self-aligning bearing, such as an insert bearing with a spherical outer ring, or a spherical roller bearing
  3. A cylindrical roller bearing rigidly mounted
  4. A needle roller bearing
Answer and working

B. A self-aligning bearing, such as an insert bearing with a spherical outer ring, or a spherical roller bearing

Self-aligning designs let the inner ring tilt with the shaft, so frame deflection and mounting errors do not overload the rolling elements. Cylindrical, needle and preloaded angular-contact bearings are intolerant of misalignment.

Question 9 · easy

What is the difference between a shaft and an axle?

  1. A shaft is always hollow; an axle is always solid
  2. An axle transmits torque to the wheels; a shaft only supports rotating parts and carries bending but no significant torque
  3. They are the same thing
  4. A shaft rotates and transmits torque; an axle supports rotating parts and carries bending but no significant torque
Answer and working

D. A shaft rotates and transmits torque; an axle supports rotating parts and carries bending but no significant torque

A shaft is a rotating member that transmits power, so it carries torque and usually bending. An axle, which may be fixed or rotating, carries bending (for example a trailer wheel axle) but does not transmit torque. Either may be solid or hollow.

Question 10 · medium

A pinion with 20 teeth drives a gear with 60 teeth. The pinion runs at 900 rpm. What is the gear speed, and what happens to the torque (ignoring losses)?

  1. 2700 rpm; torque is divided by 3
  2. 300 rpm; torque is divided by 3
  3. 300 rpm; torque is multiplied by 3
  4. 900 rpm; torque is unchanged
Answer and working

C. 300 rpm; torque is multiplied by 3

Speed ratio = zg/zp = 60/20 = 3, so gear speed = 900/3 = 300 rpm. With power constant, torque rises in the same ratio, three times the pinion torque.

A study plan for Machine Design in BCS Agriculture cadre

This plan follows where our questions actually fall, so the time goes where the marks are likely to be. It is our reading of the syllabus, not the exam body's weighting.

  1. First, the heavy units (52% of the 27 questions): Knuckle and cotter joints (4, 15%); Welded joints (4, 15%); Bolted joints (3, 11%); Keys and couplings (3, 11%).

    Read the manual chapters for these first and do every worked question you can find on them.

  2. Then the middle band: Rolling and sliding bearings and bearing life (3, 11%); Shafts under combined bending and torsion (2, 7%); Spur gear design (Lewis and Buckingham) (2, 7%); The design process, materials and factor of safety (2, 7%).

    Aim for one sitting per unit, then mixed questions across them.

  3. Then the lighter units: Belt and chain drives (1, 4%); Overload safety devices: shear bolts and slip clutches (1, 4%); Springs (1, 4%); Stress concentration and fatigue (Goodman and Soderberg) (1, 4%).

    Learn the definitions, classifications and standard numbers; these units are more often recalled than worked.

  4. Bank the easy marks: Belt and chain drives (1 easy of 1); Overload safety devices: shear bolts and slip clutches (1 easy of 1).

    Do these early in revision so they are fast on the day.

  5. Finish timed. Sit the subject as a timed set without notes, mark it, and send each wrong answer back to its unit above.

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