ICAR AICE-JRF/SRF (PhD) — Agricultural Engineering subjects 38–43 · India · National Testing Agency for ICAR
Original practice questions written to the published syllabus · not past papersSizing the parts of farm machines so they do not fail: shafts, keys, joints, springs, bearings, drives and overload devices.
In the exams: GATE AG Section 2 Machine Design (belts, spur gears, shear pins recur); ICAR AICE subject 39 Units 1–2; NCEES PE machinery component design
ICAR AICE-JRF/SRF (PhD) — Farm Machinery and Power Engineering (subject 39)
Design of farm machine components at postgraduate depth: fatigue under variable soil loads, Goodman and Soderberg design, bearing selection, overload protection, and measurement of force, torque and strain for design data.
How our questions for ICAR AICE-JRF/SRF spread across the units of Machine Design. Difficulty: 1 easy, 20 medium, 22 hard.
| Measurement of force, torque and strain on machine elements | 7 | |
| Stress concentration and fatigue (Goodman and Soderberg) | 5 | |
| Overload safety devices: shear bolts and slip clutches | 5 | |
| Bolted joints | 4 | |
| Rolling and sliding bearings and bearing life | 4 | |
| Spur gear design (Lewis and Buckingham) | 4 | |
| Welded joints | 3 | |
| The design process, materials and factor of safety | 2 | |
| Shafts under combined bending and torsion | 2 | |
| Keys and couplings | 2 | |
| Knuckle and cotter joints | 2 | |
| Belt and chain drives | 2 | |
| Springs | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
A steel link has a stress amplitude of 100 MPa and a mean stress of 150 MPa. Its corrected endurance limit is 250 MPa, yield strength 400 MPa and ultimate strength 600 MPa. What is the factor of safety by the Goodman criterion?
C. 1.54
Goodman: 1/N = sigma a/Se + sigma m/Sut = 100/250 + 150/600 = 0.400 + 0.250 = 0.650, so N = 1.54. Soderberg uses Syt: 1/N = 0.400 + 0.375, N = 1.29. Gerber (parabola) gives 1.92. Se/sigma a = 2.50 ignores the mean stress.
Further reading: Budynas & Nisbett, Shigley's Mechanical Engineering Design
A steel link has a stress amplitude of 100 MPa and a mean stress of 150 MPa. Its corrected endurance limit is 250 MPa, yield strength 400 MPa and ultimate strength 600 MPa. What is the factor of safety by the Soderberg criterion?
D. 1.29
Soderberg: 1/N = sigma a/Se + sigma m/Syt = 100/250 + 150/400 = 0.400 + 0.375 = 0.775, so N = 1.29. Using Sut (Goodman) gives 1.54. Syt/(sigma a + sigma m) = 1.60 is only the first-cycle yield check. Multiplying the ratios instead of adding them is an algebra slip: 1/(0.400 x 0.375) = 6.67.
Further reading: Budynas & Nisbett, Shigley's Mechanical Engineering Design
The hitch pin of a trailed implement is a clevis-and-pin (knuckle-type) joint. In service the pin bends and wears oval. Which statement about its design is correct?
C. The pin must be checked for bending and bearing as well as double shear, and it should be secured so it cannot work out
In a clevis the pin acts as a short beam between the fork prongs, so bending often governs; bearing pressure causes the oval wear seen. Double shear is also checked. Clearance increases bending and impact, and the securing clip matters because a lost pin causes run-away implements.
Further reading: Bhandari, Design of Machine Elements
Why does a high preload improve the fatigue life of a bolt in a joint under fluctuating external load?
C. The stiff clamped members absorb most of the load fluctuation, so the alternating stress in the bolt is small
With preload, only the fraction C = kb/(kb + km) of the external load change reaches the bolt while the joint stays closed, so the bolt's stress amplitude is small even though its mean stress is high. Preload does not change material properties or thread geometry, and it does not change stiffnesses.
Further reading: Budynas & Nisbett, Shigley's Mechanical Engineering Design
A shaft of 60 mm diameter is welded to a plate by a circumferential fillet weld of 6 mm leg and carries a torque of 1 kN m. What is the shear stress in the weld throat?
A. 41.7 MPa
Throat t = 0.707 x 6 = 4.24 mm. For a thin ring of throat t at radius d/2, J = pi t d^3/4 and tau = T (d/2)/J = 2T/(pi t d^2) = 2 x 10^6/(pi x 4.24 x 60^2) = 41.7 MPa. Using the leg size gives 29.5 MPa; dropping the 2 in 2T/(pi t d^2) gives 20.8 MPa; 16T/(pi d^3) = 23.6 MPa is the stress in the solid shaft, not the weld.
Further reading: Bhandari, Design of Machine Elements
A roller chain of 15.875 mm pitch connects sprockets of 17 and 51 teeth at a centre distance of 635 mm. The centre distance can be adjusted slightly, and the chain length is to be rounded up as recommended for roller chains. What chain length, in links, should be used?
A. 116 links
Ln = 2C/p + (z1 + z2)/2 + ((z2 - z1)/(2 pi))^2 p/C = 80.0 + 34 + 0.73 = 114.73, rounded up to the next even number, 116, to avoid an offset link. Rounding up to 115 links, an odd number, would need an offset (cranked) link, which is weaker; forgetting to halve (z1 + z2) gives 148.73, rounded up to 150 links, and using C/p instead of 2C/p gives 74.73, rounded up to 76.
Further reading: Bhandari, Design of Machine Elements
A 20-tooth pinion meshes externally with a 40-tooth gear; module 4 mm, face width 40 mm, and load-stress factor K = 1.2 N/mm^2. What is Buckingham's wear strength Sw = b Q dp K?
A. 5120 N
Q = 2 zg/(zg + zp) = 2 x 40/60 = 1.333; dp = m zp = 80 mm. Sw = 40 x 1.333 x 80 x 1.2 = 5120 N. Using zp in the numerator of Q gives 2560 N; using the gear diameter gives 10240 N. 3840 N takes Q = 1, ignoring the ratio factor.
Further reading: Bhandari, Design of Machine Elements
A single active strain gauge (quarter bridge) with gauge factor 2.0 is excited at 5 V. The part strains by 1000 microstrain. What is the approximate bridge output?
C. 2.5 mV
Quarter bridge: Vo = V GF epsilon/4 = 5 x 2.0 x 0.001/4 = 0.0025 V = 2.5 mV. A full bridge of four active gauges gives about 10 mV, a half bridge 5 mV. 1.25 mV omits the gauge factor.
Further reading: Doebelin, Measurement Systems
Four active strain gauges in a full Wheatstone bridge, each with gauge factor 2.0, measure a strain of 1000 microstrain (two gauges in tension, two in compression, arranged so their outputs add). The excitation is 5 V. What is the approximate output?
C. 10 mV
For a full bridge with four equal active arms of alternating sign, Vo = V GF epsilon = 5 x 2.0 x 0.001 = 10 mV, four times a quarter bridge (2.5 mV). 40 mV multiplies the full-bridge value by 4 again.
Further reading: Doebelin, Measurement Systems
Which transducer is widely used in tillage research to measure the horizontal (draft) and vertical forces on a soil-engaging tool?
A. The extended octagonal ring transducer with strain gauges
The extended octagonal ring, fitted with strain gauges at nodes that respond selectively to horizontal and vertical force, separates draft and vertical load and can also give moment. A Bourdon tube measures fluid pressure, a thermocouple temperature and a tachogenerator speed.
Further reading: Srivastava et al., Engineering Principles of Agricultural Machines (ASABE)
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