Screw Thread and Gear Metrology Questions and Answers

Answer: B

A left hand screw advances when turned anticlockwise.

A normal right-hand thread advances, or tightens, when turned clockwise. A left-hand thread is the mirror image, so it advances when turned anticlockwise and loosens when turned clockwise.

Left-hand threads are used where normal rotation would loosen a right-hand nut, for example on the left pedal of a bicycle, a grinder's left-hand wheel nut and gas cylinder fittings. Clockwise is the right-hand case. A thread cannot advance in any direction, because the helix fixes which way it travels.

Answer: B

Thread plug gauges check an internal thread.

A thread plug gauge is screwed into a tapped hole or nut. Its GO end must screw in fully and its NO GO end must not enter more than about two turns, which confirms the internal thread is within limits.

External threads are checked with thread ring gauges or thread caliper gauges. Plain plug gauges check plain holes, and plain ring or snap gauges check shafts; neither checks a thread.

Answer: C

The tap drill size is 10.25 mm.

For a metric thread the tap drill size is found by subtracting the pitch from the nominal diameter:

\[ \text{tap drill} = 12 - 1.75 = 10.25\ \text{mm} \]

(The standard coarse-thread drill for M12 is 10.2 mm, the nearest stock size.) 12 mm is the major diameter, which would leave no material for the thread. 13.75 mm adds the pitch instead of subtracting it. 10 mm is a rounded value that would leave too much material for the tap to cut.

Answer: D

The included angle between the flanks of an Acme thread is 29 degrees.

The Acme thread is a trapezoidal power-transmission thread, used for lead screws on lathes and for screw jacks. Its 29 degree angle gives a stronger thread, easier to cut and engage with a split nut than a square thread.

60 degrees is the angle of ISO metric and unified threads. 45 degrees appears on one flank of a buttress thread. A 90 degree included angle is not used for screw threads.

Answer: C

A nut is specified by the nominal diameter of the bolt.

A nut is named after the bolt it fits: an M12 nut fits an M12 bolt, whose nominal (major) diameter is 12 mm. The nut's own dimensions, such as width across flats and thickness, are then set by standard proportions of that nominal diameter.

The core (minor) and pitch diameters are thread dimensions used in design and gauging, not in naming the nut. The head of the bolt has nothing to do with the nut's size.

Answer: A

The radial distance between the pitch circle and the tip circle equals the module.

That distance is the addendum, the height of the tooth above the pitch circle. For standard full-depth involute gears in the module system, addendum = 1 × module (m). So the tip (outside) circle diameter is \(d + 2m\).

2 × module is the sum of the addenda of two meshing gears, the working depth. 1.157 × module is the dedendum, which lies below the pitch circle, and 0.157 × module is the clearance, the gap between the tip of one tooth and the root of the mating gear.

Answer: B

The standard dedendum is 1.157 X Addendum.

In the module system the addendum is 1 m and the dedendum is 1.157 m, so the dedendum equals 1.157 times the addendum. The extra 0.157 m is the clearance, which keeps the tip of each tooth from touching the root of the mating gear.

A dedendum equal to the addendum would leave no clearance. 2 × module is the working depth, and 0.157 × module is only the clearance itself. (Some standards use a dedendum of 1.25 m, but the 1.157 m figure is the one this paper uses throughout.)

Answer: A

An 8.8 marking means a tensile strength of 800 MPa.

In the ISO property class system for bolts, the first number times 100 gives the nominal ultimate tensile strength in MPa: 8 × 100 = 800 MPa. The second number is the ratio of yield strength to tensile strength, so 0.8 × 800 = 640 MPa is the nominal yield strength.

880 MPa and 88 MPa read the two digits as one number, and 300 MPa does not follow from the marking at all. A class 10.9 bolt, by the same rule, has a tensile strength of 1000 MPa.

Answer: D

All three are true, so the answer is All of the mentioned.

A buttress thread has one flank almost square to the axis and the other inclined, so it combines the low friction and high efficiency of a square thread with the strength at the root of a trapezoidal (V-type) thread.

Because only the square flank carries the load efficiently, it transmits power in one direction only. That is exactly what is needed in vices, screw jacks and breech mechanisms of guns, where heavy force acts one way.

Answer: B

The contact ratio is 1.50.

The contact ratio is the average number of tooth pairs in contact, and is found by dividing the arc of contact by the circular pitch:

\[ \begin{aligned} \text{Contact ratio} &= \frac{\text{arc of contact}}{\text{circular pitch}} \\ &= \frac{6}{4} = 1.5 \end{aligned} \]

A value of 1.5 means one pair of teeth is always in mesh and a second pair for half the time, which gives smooth, continuous drive; a contact ratio below 1 would mean moments with no teeth in contact. 3.20, 4.75 and 6.32 do not come from these two figures.