ASRB NET Machine Design: practice questions

1 · Knuckle and cotter joints · medium

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?

1. Only double shear matters, because pins never fail in bending
2. A single-shear calculation gives the correct diameter because the drawbar is one plate, and bending can be neglected in a short pin
3. The pin must be checked for bending and bearing as well as double shear, and it should be secured so it cannot work out
4. A loose fit is preferred because it reduces bending stress

2 · Bolted joints · medium

Why does a high preload improve the fatigue life of a bolt in a joint under fluctuating external load?

1. Preload makes the bolt stiffer than the clamped members, so the bolt safely carries a larger share of the load change
2. Preload removes the stress concentration at the thread root
3. The stiff clamped members absorb most of the load fluctuation, so the alternating stress in the bolt is small
4. Preload raises the endurance limit of the bolt material

3 · Springs · easy

Why is the spring index (D/d) of helical springs usually kept between about 4 and 12?

1. An index above 12 always makes the Wahl factor greater than 2
2. The index affects only the colour of the heat treatment
3. Below 4 the spring has no stiffness at all, and above 12 its stiffness becomes infinite, so it cannot deflect under load
4. Low indices are hard to coil and have high curvature stress; high indices tend to buckle and tangle

4 · Overload safety devices: shear bolts and slip clutches · easy

Why is a shear pin designed using the ultimate shear strength of its material rather than an allowable stress with a factor of safety?

1. Because it is meant to break at a chosen torque, so its breaking strength must be predicted, not kept below
2. Because a factor of safety would make the pin too small to fit the hub bore, so the drive could not be assembled
3. Because ultimate strength is always lower than yield strength
4. Because pins never carry fluctuating loads

5 · Overload safety devices: shear bolts and slip clutches · medium

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?

1. Weld the flange to the hub to remove the bolt
2. Use only the specified shear bolt grade and size; if bolts break often, reduce the load or fit an auto-reset slip clutch
3. Grease the shear bolt so it breaks less often
4. Use an even stronger bolt so it never breaks, since the gearbox failure shows the driveline needs a stiffer, stronger connection

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

Which transducer is widely used in tillage research to measure the horizontal (draft) and vertical forces on a soil-engaging tool?

1. The extended octagonal ring transducer with strain gauges
2. A tachogenerator
3. A thermocouple
4. A Bourdon tube pressure gauge

7 · Springs · hard

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?

1. 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
2. Design only for 900 N with the static shear yield strength
3. Ignore the curvature factor, because it applies only to static loading
4. Design for the average load of 600 N with the static shear yield strength

8 · Overload safety devices: shear bolts and slip clutches · hard

Shear pins on a forage harvester feed roll drive sometimes fail after long use at torques below their design breaking torque. What is the most likely mechanism, and what reduces it?

1. Fatigue of the pin under repeated torque peaks, aggravated by clearance; a tight-fitting pin in hardened bushes and a controlled neck reduce it
2. The ultimate shear strength rising with use; use a thicker pin
3. Creep of the steel at harvest temperatures; use a lower-strength pin
4. Corrosion alone; paint the pin

9 · Belt and chain drives · medium

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?

1. Belt dressing oil should be applied weekly
2. The belts are too tight (over-tensioned); loosen until they flap
3. The belts are slipping because of too little tension (or a worn groove); retension to the specified value and replace worn pulleys
4. A flat belt should replace the V-belts to increase grip