NCEES FE Other Disciplines Machine Design: practice questions

NCEES FE Other Disciplines · USA · NCEES

Original practice questions written to the published syllabus · not past papers

6worked here, free

22written to this exam in this subject

6syllabus units

What this subject covers

Sizing 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

The design process, materials and factor of safety
Stress concentration and fatigue (Goodman and Soderberg)
Shafts under combined bending and torsion
Keys and couplings
Knuckle and cotter joints
Bolted joints
Welded joints
Springs
Rolling and sliding bearings and bearing life
Belt and chain drive design
Spur gear design (Lewis and Buckingham)
Overload safety devices: shear bolts and slip clutches
Measurement of force, torque and strain on machine elements

Where the 22 questions fall

How our questions for NCEES FE Other Disciplines spread across the units of Machine Design. Difficulty: 12 easy, 8 medium, 2 hard.

The design process, materials and factor of safety	6
Stress concentration and fatigue (Goodman and Soderberg)	6
Measurement of force, torque and strain on machine elements	6
Springs	2
Shafts under combined bending and torsion	1
Welded joints	1

6 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.

1 · The design process, materials and factor of safety · easy

Steel with a tensile yield strength of 400 MPa is used for a part in pure shear. According to the distortion energy theory, what is its yield strength in shear?

400 MPa
200 MPa
283 MPa
231 MPa

Answer and working

D. 231 MPa

Distortion energy theory gives Ssy = Syt / sqrt(3) = 0.577 x 400 = 231 MPa. 200 MPa is the maximum shear stress theory value (0.5 Syt). 283 MPa is 400 / sqrt(2), which is not a failure theory result.

Further reading: Budynas & Nisbett, Shigley's Mechanical Engineering Design

2 · The design process, materials and factor of safety · easy

A shaft is specified in AISI/SAE 1045 steel. What does the designation tell the designer?

It is a free-cutting steel with 4.5 per cent sulphur
It is a plain carbon steel with about 0.45 per cent carbon
It is an alloy steel with 10 per cent nickel and 0.45 per cent chromium
It has a tensile strength of about 1045 MPa

Answer and working

B. It is a plain carbon steel with about 0.45 per cent carbon

In the AISI/SAE four-digit system, the first two digits (10) mean plain carbon steel and the last two give carbon content in hundredths of a per cent (45, so about 0.45 per cent). The number is not a strength, and sulphurised free-cutting grades are in the 11xx series.

Further reading: Budynas & Nisbett, Shigley's Mechanical Engineering Design

3 · Stress concentration and fatigue (Goodman and Soderberg) · easy

A ductile steel part with a sharp groove carries only a steady load. How is stress concentration normally treated in its design?

It is applied as Kf, because Kf is usually larger than Kt
It is always applied in full, as Kt times the nominal stress, because a ductile part fails at the notch peak
It is usually neglected
It is applied to the modulus of elasticity rather than the stress

Answer and working

C. It is usually neglected

Under static load a ductile material yields locally at the notch and the load spreads to the surrounding material, so the peak stress does not cause failure; stress concentration is normally ignored. It must be applied for brittle materials and for all materials under fatigue loading. Kf is never larger than Kt. The reason is that local yielding at the notch redistributes the stress.

Further reading: Budynas & Nisbett, Shigley's Mechanical Engineering Design

4 · Stress concentration and fatigue (Goodman and Soderberg) · medium

Which material behaviour is correct for fatigue design?

Steels usually show an endurance limit, while aluminium alloys have no true endurance limit
No metal has an endurance limit; all fail eventually at any stress
An endurance limit exists only under compressive stress
Aluminium alloys have an endurance limit of about half their ultimate strength, just as wrought steels do

Answer and working

A. Steels usually show an endurance limit, while aluminium alloys have no true endurance limit

The S-N curve of steel flattens beyond roughly 10^6 to 10^7 cycles, giving an endurance limit. Aluminium and most non-ferrous alloys keep falling, so a fatigue strength at a stated life (for example 5 x 10^8 cycles) is used. The 0.5 Sut estimate applies to wrought steel.

Further reading: Budynas & Nisbett, Shigley's Mechanical Engineering Design

5 · Measurement of force, torque and strain on machine elements · medium

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?

10 mV
5 mV
2.5 mV
1.25 mV

Answer and working

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

6 · Measurement of force, torque and strain on machine elements · hard

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?

40 mV
5 mV
10 mV
2.5 mV

Answer and working

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

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