NCEES PE Agricultural and Biological Engineering · USA · NCEES (ASABE is the content partner)
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
NCEES PE Agricultural and Biological Engineering
Machinery systems: component design with supplied references, including shafts, bearings, belts, chains, gears, fasteners and welds, often in US customary units, and power-transmission selection with service factors.
How our questions for NCEES PE Agricultural and Biological Engineering spread across the units of Machine Design. Difficulty: 2 easy, 16 medium, 18 hard.
| Bolted joints | 5 | |
| Overload safety devices: shear bolts and slip clutches | 5 | |
| Shafts under combined bending and torsion | 4 | |
| Rolling and sliding bearings and bearing life | 4 | |
| Springs | 3 | |
| Belt and chain drives | 3 | |
| Spur gear design (Lewis and Buckingham) | 3 | |
| Measurement of force, torque and strain on machine elements | 3 | |
| Stress concentration and fatigue (Goodman and Soderberg) | 2 | |
| Welded joints | 2 | |
| The design process, materials and factor of safety | 1 | |
| Keys and couplings | 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
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 farmer keeps breaking shear bolts on a post-hole digger in stony ground and replaces them with high-tensile bolts. A week later the gearbox fails. What is the correct advice?
B. Use only the specified shear bolt grade and size; if bolts break often, reduce the load or fit an auto-reset slip clutch
The shear bolt is sized to fail below the gearbox's damaging torque. A stronger bolt moves the weak link to the gearbox, which is what happened. The fix is the correct bolt and, where overloads are frequent, a self-resetting device. Welding removes protection altogether.
Further reading: Srivastava et al., Engineering Principles of Agricultural Machines (ASABE)
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)
A spring-tine cultivator tine spring is subject to a load that fluctuates between 300 N and 900 N at every stone and clod. Which approach is correct for its design?
A. Find the mean and alternating shear stresses in the wire, apply the Wahl factor, and check against a fatigue criterion such as Goodman in torsion using the wire's torsional endurance strength
Fluctuating loading needs a fatigue check. The mean load is 600 N and the alternating load 300 N; each gives a stress through tau = K 8PD/(pi d^3), with the Wahl factor (curvature matters most in fatigue). The pair is checked against Goodman or Soderberg in shear. Static checks alone miss fatigue.
Further reading: Budynas & Nisbett, Shigley's Mechanical Engineering Design
A V-belt drive on a chaff cutter squeals and the belts glaze within weeks. The pulleys are aligned. Which cause is most likely and what is the fix?
C. The belts are slipping because of too little tension (or a worn groove); retension to the specified value and replace worn pulleys
Squeal and glazing are signs of slip, which heats and hardens the belt surface. Correct tension and sound grooves (a belt riding low in a worn groove loses the wedge action) stop it. Loose, flapping belts slip more, oil on belts is harmful, and a flat belt grips less than a V-belt.
Further reading: Bhandari, Design of Machine Elements
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