The bolted joints, though removable in nature, are required to stay firm without becoming loose, of their own accord. However, the joints used in the moving parts of a machinery, may be subjected to vibrations. This may slacken the joint, leading to serious breakdown. To eliminate the slackening tendency, different arrangements, as discussed further, are used to lock the nuts :

Lock Nut

This is the most commonly used locking device. In this arrangement, a second nut, known as lock nut is used in combination with a standard nut (fig a). The thickness of a lock nut is usually two-thirds D, where D is the major diameter of the bolt. The lock nut is usually placed below the standard nut. To make the joint, the lock nut is first screwed tightly and then the standard nut is tightened till it touches the lock nut. Afterwards, the locknut is then screwed back on the standard nut, which is held by a spanner. The threads of the two nuts become wedged between the threads of the bolt.

When the lock nut is first screwed into its position, the top flanks of it press against the bottom flanks of the bolt (fig b). Figure c shows the condition between the flanks of the nuts and the bolt, when the second nut is locked in position. It may be observed that in this position, the top flanks of the top nut, press against the bottom flanks of the bolt, whereas, the bottom flanks of the lock nut press against the top flanks of the bolt.

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Lock Nut

Locking by Split Pin

A split pin, made of steel wire of semi-circular cross-section is used for locking the nut. In this arrangement, the split pin is inserted through a hole in the bolt body and touching just the top surface of the nut. Then, the ends of the pin are split open to prevent it from coming out while in use.

Locking by split pin

Locking by Castle Nut

A castle nut is a hexagonal nut with a cylindrical collar turned on one end. Threads are cut in the nut portion only and six rectangular slots are cut through the collar. A split pin is inserted through a hole in the bolt body after adjusting the nut such that the hole in the bolt body comes in-line with slots. This arrangement is used in automobile works.

Castle nut

Wile’s Lock nut

It is a hexagonal nut with a slot, cut half-way across it. After tightening the nut in the usual manner, a set screw is used from the top of the nut, compressing the two parts. For this purpose, the upper portion of the nut should have a clearance hole and the lower portion tapped.

Wile’s lock nut

Locking by Set Screw

In this arrangement, after the nut is tightened, a set screw in fitted in the part, adjoining the nut, so that it touches one of the flat faces of the nut. The arrangement prevents the loosening tendency of the nut.

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Locking by set screw

Grooved Nut

It has a cylindrical grooved collar, integrally provided at the lower end of the nut. This collar fits into a corresponding recess in the adjoining part. In this arrangement, after tightening the nut, a set screw is inserted from one end of the upper part, so that the end of the set screw enters the groove, preventing the loosening tendency of the nut.

Grooved nut

Locking by Screw

In this, a cap nut with an integral washer and with a threaded hole in the cylindrical cap, is used. A corresponding tapped hole at the top end of the bolt is also required for the purpose.

In this arrangement, a set screw fitted through the cap and through the bolt end, prevents the loosening tendency of the nut, when the pitches of the main nut and the set screw are different. This type of arrangement is used for fitting the propeller blades on turbine shafts.

Locking by screw

Locking by Plate

A locking plate is grooved such that it fits a hexagonal nut in any position, at intervals of 30° of rotation. It is fixed around the nut, by means of a machine screw, as shown below.

Locking by plate

Locking by Spring Washer

In this arrangement, a spring washer of either single or double coil is placed under the nut and tightened. The spring force of the washer will be acting upwards on the nut. This force makes the threads in the nut jammed on the bolt threads; thus preventing the nut from loosening.

Locking by spring washer