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Screw Threads

Screw threads are used for the purpose of fastening (screws and bolts) and for the transmission of motion: e.g., a rotating screw spindle imparts a longitudinal motion to a nut mounted on it.

A screw thread conforms to a helix, a space curve that may be conceived as the hypotenuse of a right triangle wrapped round a cylinder (Fig.1) to form either a right-hand or a left-hand thread, according to the direction of wrapping. Depending on the shape of the groove, various types of screw threads are distinguished (see Fig.5).

A single-thread screw may be conceived as a cylinder with one continuous helical grove. The pitch of the thread is the distance, measured in the axial direction, between two corresponding points on adjacent turns of the thread; it is the distance a nut would travel in one complete revolution. For particular purposes a multiple thread may be employed-e.g., a double or a triple thread, which may be conceived as respectively two or three independent but parallel helical grooves around a cylinder.

In hardware stores, a machine screw or its larger cousin, a bolt, is described by length, the type of head and the thread. In the United States, the convention for describing threads is to give the number of threads per inch, preceded by a gauge number if the bolt is smaller than a quarter-inch in diameter, otherwise by the diameter in fractions of an inch. So, for example, in the United States one might ask for a 2-inch quarter-twenty bolt, which would be 2 inches long, have a nominal diameter of a quarter of an inch and have twenty threads to the inch.

Consider a body on an inclined plane (Fig.2). If the angle of inclination a is gradually increased, they body will begin to slide down the plane when this angle reaches a particular value a = r, called angle of friction. The magnitude of the angle of friction depends on the material and surface condition (roughness, smoothness) of the inclined plane and of the body, but is independent of the weight or loading of the latter. A screw thread may be regarded as a helical inclined plane, while the nut corresponds to the object placed on the plane.

For the nut to be self-locking, so that it will not slacken and move along the thread of its own accord, the thread could have a slope ?, which is smaller than the angle of friction r (screws and bolts for fastening). On the other hand, a screw thread for the transmission of motion should preferably have a larger value of a. In the absence of friction the amount of work done in moving a body up an inclined plane (Fig.3) would be G X h, where G is the load and h is the vertical distance traveled. If there is friction, the amount of work will be greater, namely, G X H (H greater than h), as though the plane were inclined at a steeper angle which exceeds a by an amount p.

Obviously for a thread with a small angle a the relative effect of friction (corresponding to the angle r) is greater than for a threaded with a large a (compare left-hand and right-hand diagram in Fig.3). To achieve good efficiency a screw thread for the transmission of motion should therefore have a large a and consequently a large pitch. Increased friction, to achieve self-locking can also be achieved by sloping the flanks of the threads.

From Fig.4 it is apparent that for a V-shaped screw thread the loading perpendicular to the plane of the thread (the normal force N) is greater than for the square thread show in the right-hand diagram. Since the friction force is proportional to the normal force, screw threads for the transmission of motion should preferably have the flattest possible flanks (square threads) to minimize friction, whereas fastening screws should have V-shaped threads for maximum friction.

Various forms of thread are shown in Fig.5. Most threads employed in engineering are of V form, some are square, and some are modifications of a V or square. In the Whitworth, or English Standard, thread, the angle made by the two flanks of the thread is 557°. In the metric thread and the sellers (American) thread it is 60°.

Knuckle threads (rounded threads) are used in cases where damage, clogging with dirt, or corrosion is liable to occur; the characteristic shape is a semicircle instead of a V. The acme thread with an angle of 29° between the flanks is a standardized thread of trapezoidal shape. The buttress thread is used in cases where large forces act in the longitudinal direction of the screw.