Design of Threaded Fasteners Questions and Answers

Answer: A

Efficiency is the ratio of work done by W per revolution to work done by F per revolution.

Efficiency is always useful work out over work in. In one turn of the screw the effort F moves through the circumference at its point of application and does the input work; the load W is raised by one lead, and the work done on it is the output. So \(\eta = W \times \text{lead} / (F \times 2\pi R)\), which is less than 1 because of thread friction.

W/F alone is the mechanical advantage, not the efficiency, and ignores the distances moved. F/W is its inverse. The last option inverts the efficiency and would always exceed 1.

Answer: C

Pre-tensioning is used to increase stiffness of the bolted joint.

Tightening the bolt clamps the members together under compression. Under external load, the clamped members unload while the bolt carries only a fraction of the load, so the joint deflects much less than a loose one, stays closed and resists separation and leakage. The bolt also sees a smaller alternating load, which improves its fatigue life.

Pre-tension does not aim to strain-harden the bolt head, and it does not prevent yielding at the thread root; excessive pre-tension can in fact cause yielding there. It increases, not decreases, the joint's effective stiffness.

Answer: D

Machine tool tables use the T-headed bolt.

Machine tool tables have T-shaped slots cut along them. The head of a T-bolt slides into the slot and is held under its lips, so the bolt can be positioned anywhere along the slot to clamp a workpiece or fixture, and the head cannot turn.

A countersunk-headed bolt sits flush in a conical recess, a hook bolt is used where a hole cannot be drilled through one of the parts, and an eye-bolt is used for lifting heavy machines. None of them is designed to locate in a T-slot.

Answer: B

A screw jack is self-locking if its efficiency is less than 50%.

A self-locking screw holds the load without an applied effort; the load cannot drive the screw back down. This happens when the friction angle \(\phi\) is at least the helix angle \(\alpha\). Working through the efficiency of the screw, \(\eta = \frac{\tan\alpha}{\tan(\alpha + \phi)}\), the condition \(\phi \ge \alpha\) keeps the efficiency below one half.

So efficiency of 100% is impossible with friction, more than 50% means the screw would overhaul under the load, and 'equal to 40%' is only one value inside the self-locking range, not the condition itself.

Answer: D

The shank is made equal to the minor diameter of the threads.

In an ordinary bolt, the threaded portion has the smallest area, at the root (minor or core) diameter, so under a shock load the stress and the strain energy concentrate there. A bolt of uniform strength has its shank turned down, or bored hollow, to the same area as the thread core. The stress is then the same along the whole length, and the long shank can absorb more strain energy before failure.

Making the shank equal to the nominal or major diameter is the ordinary bolt, which is what the design avoids. The pitch diameter is still larger than the core, so the shank would remain stiffer and lower-stressed than the threads.