ASRB NET — Farm Machinery & Power / Land & Water Management Engineering / Agricultural Structures & Process Engineering · India · Agricultural Scientists Recruitment Board (ICAR)
Original practice questions written to the published syllabus · not past papersHow mechanisms move and transmit power: linkages, cams, gears, belts, flywheels, governors, balancing and vibration.
In the exams: GATE AG Sections 2 and 3 (power transmission, belts and gears recur); ICAR AIEEA-PG Unit III
ASRB NET — Farm Machinery & Power
Applied kinematics and dynamics of farm machines: tractor power train (clutch, gearbox, differential, final drive), balancing and vibration of rotating implements, dynamometers for tractor testing.
How our questions for ASRB NET spread across the units of Theory of Machines. Difficulty: 8 easy, 12 medium, 18 hard.
| Differential gear | 5 | |
| Governors | 5 | |
| Balancing of rotating and reciprocating masses | 5 | |
| Clutches | 5 | |
| Free and forced vibrations and critical speed | 4 | |
| Brakes and dynamometers | 4 | |
| Cams and followers | 3 | |
| Gears and gear trains: simple, compound and epicyclic | 3 | |
| Flywheels and turning-moment diagrams | 2 | |
| Mechanisms, links, pairs and Grashof's law | 1 | |
| Belt, rope and chain drives: tension ratio and power | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
A cam turning at 300 rpm lifts a follower 40 mm with cycloidal motion during 120° of cam rotation. What is the maximum acceleration of the follower?
C. 56.5 m/s²
For cycloidal motion a max = 2πhω²/θo² = 2π x 0.04 x 987.0/4.3865 = 56.5 m/s², reached at one quarter and three quarters of the lift, with zero acceleration at the ends. Dropping the 2π gives 9.0; 44.4 and 36.0 m/s² are the SHM and uniform-acceleration peaks.
Further reading: Rattan, Theory of Machines
Why are double-helical (herringbone) gears used instead of single helical gears in some heavy drives?
D. The opposite helices cancel the axial thrust that a single helical gear produces
A single helical gear produces an axial force proportional to the tangential load and tan of the helix angle, which the bearings must carry. Two opposite helices cancel it. Right-angle intersecting shafts use bevel gears; large single-stage ratios use worms.
Further reading: Rattan, Theory of Machines
When should a tractor operator disengage the differential lock?
A. Before turning, because a locked axle resists turning and can cause loss of steering control
The lock makes both rear wheels turn at the same speed, which is useful for traction in straight-line work on slippery ground but fights the different wheel paths on a turn. Operators' manuals therefore require it to be released before turning and when travelling at speed.
Further reading: Liljedahl et al., Tractors and Their Power Units
A thresher drum has a residual unbalance equivalent to a 50 g mass at 250 mm radius. What rotating force does this unbalance exert on the bearings at 900 rpm?
C. 111 N
F = mω²r = 0.05 x 94.25² x 0.25 = 111 N. Using rpm for ω gives 10125 N; using rev/s gives 2.81 N. The force rises with the square of speed, which is why a drum that is acceptable at low speed shakes badly at working speed. 11.3 N divides the correct force by g.
Further reading: Rattan, Theory of Machines
A rigid fan rotor running at 3000 rpm is to be balanced to grade G6.3 under the ISO rotor-balancing standard, where the grade number is the permissible eccentricity times angular speed in mm/s. What is the permissible eccentricity of its centre of mass?
A. 20.1 μm
e x ω = 6.3 mm/s with ω = 314.2 rad/s, so e = 6.3/314.2 mm = 0.0201 mm = 20.1 μm. Using rev/s instead of rad/s gives 126 μm; using rpm gives 2.1 μm. Multiplying e by the rotor mass gives the permissible residual unbalance.
Further reading: ISO 21940-11 (formerly ISO 1940-1)
A machine is mounted on springs to reduce the force transmitted to its foundation. For the transmitted force to be less than the exciting force, the ratio of forcing frequency to natural frequency must be:
C. greater than √2
Transmissibility falls below 1 only when ω/ωn > √2, whatever the damping. Below √2, and especially near r = 1 (resonance), the mounts amplify the force. Mounts therefore need to be soft enough to put the natural frequency well below the running frequency.
Further reading: Rao, Mechanical Vibrations
A grain cleaner running at 1500 rpm is to be mounted on undamped springs so that only 10% of its unbalanced force reaches the floor. What static deflection of the springs is needed?
B. 4.37 mm
Undamped TR = 1/(r² - 1) = 0.1 gives r² = 11, r = 3.317. Running frequency = 25 Hz, so fn = 25/3.317 = 7.54 Hz. δ = g/(2πfn)² = 9.81/(47.36)² = 4.37 mm. Using r² = 10 (forgetting the minus one) gives 3.98 mm; setting the natural frequency equal to the running frequency gives 0.40 mm, which would put the machine at resonance; forgetting 2π gives a deflection far too large.
Further reading: Rao, Mechanical Vibrations
Two rotors with moments of inertia 3 kg m² (rotor A) and 1 kg m² (rotor B) are at the ends of a uniform shaft 1.2 m long. Where is the node for torsional vibration?
A. 0.3 m from rotor A
Both parts of the shaft must give the same frequency: l_A I_A = l_B I_B with l_A + l_B = 1.2 m. So 3 l_A = 1 x (1.2 - l_A), l_A = 1.2/4 = 0.3 m. The node lies nearer the heavier rotor. 0.9 m is the distance from rotor B, the classic swap; 0.6 m assumes the node is always at mid-length.
Further reading: Rao, Mechanical Vibrations
A single-plate clutch with both sides effective has friction surfaces of 300 mm outer and 200 mm inner diameter, an axial force of 4 kN and μ = 0.3. What torque does it transmit if uniform pressure is assumed, as for a new clutch?
D. 304 N m
Uniform pressure: T = n(2/3)μW(R1³ - R2³)/(R1² - R2²) = 2 x (2/3) x 0.3 x 4000 x (0.003375 - 0.001)/(0.0225 - 0.01) = 304 N m. This is slightly more than the uniform-wear value (300 N m), which is why uniform wear is the safe design assumption. Using the outer radius only gives 360 N m. 152 N m is the one-surface value.
Further reading: Rattan, Theory of Machines
A clutch plate has an outer radius of 150 mm and an inner radius of 100 mm and carries an axial force of 4 kN. Assuming uniform wear, what is the maximum pressure on the lining?
B. 127.3 kPa
Uniform wear: p r = C and W = 2πC(R1 - R2), so C = 4000/(2π x 0.05) = 12732 N/m. The pressure is greatest at the inner radius: p max = C/R2 = 12732/0.10 = 127.3 kPa. 84.9 kPa is the minimum (at the outer radius); 101.9 kPa is the uniform-pressure value. 12.7 kPa is a factor-of-ten slip.
Further reading: Rattan, Theory of Machines
Ten here, 38 in the app. The Laisha app has every practice question we have written to ASRB NET in this subject, marked as you go, with timed papers, a jump-to-unanswered list and a certificate when you pass at 70 per cent.
Free for ten days, then one subscription of US$10 a month. We would rather say so here than have you find it later.
An AI operating system for agriculture: free guides, manuals and tools written for every country.