سوال ۱۶ کنکور ارشد مهندسی مکانیک ۱۴۰۱

PART C: Reading Comprehension

PASSAGE 1:

A stress concentration is present in the screw where the load is transferred through the nut to the adjoining member. This arises because the force in the screw must shift outwardly to the region near the boundary as it is transferred from the screw to the nut. Under ideal conditions, the tension in the screw and the compression in the nut should be reduced uniformly, starting from full load at the first contact between screw and nut. However, the tension increases the pitch in the screw, and the compression decreases the pitch in the nut so that correct compliance between the loaded parts is not maintained. The major portion of the load is transferred at the first pair of contacting threads and a large stress concentration is present here. Although the stress concentration is somewhat relieved by bending of the threads and expansion of the nut, most bolt failures occur at this point.

Stress concentration factors for threads with static loads are usually determined by photoelastic analysis. Three-dimensional tests have indicated a stress concentration factor of 3.85 at the root of the first engaged thread. Other investigators, however, have found higher as well as lower values. Various methods are used for increasing the flexibility of the nut and thus increasing the area over which this transfer of force takes place. A tension nut, or a nut with a tapered lip, has been successfully used in fatigue service. The reduced cross-sectional area of the lip deforms in tension with the bolt, and the load is accordingly spread over a greater number of threads. Another method is to cut the thread of the nut on a very small taper, thus reducing the contact area for the first few threads. Since these threads will bend and carry less load, additional threads must come into service. This method is expensive because of the close dimensional tolerance required. Nuts made of a material with a modulus smaller than the bolt have also been successful in spreading the load over a larger area. The material in the nut, however, must have a sufficient reserve of ductility to deform without breaking.

In nut-bolt joints, nuts are ----------.