Forms of Nuts
In order to mack bolt and studs into effective fastenings, the nut is required. Nuts are usually in the form of hexagonal or square prisms. Besides these, the cylindrical and other form is used in a particular requirement.
Hexagonal Nut
This is the most common form of nut used for the general fastening purpose in conjunction with a hexagonal-headed bolt. A hexagonal nut is shown in Fig.
Square Nut
A square nut, shown in fig. is used in conjunction with a square-headed bolt. The spanner used for the nut can have a better hold on a square nut than on hexagonal nut.
Ring Nut
It is the form of a ring provided with slots in the curved surface for a special c-spanner. A ring nut is shown in the figure. These nuts are generally used in parts, one nut acting as a lock nut the other.
Cap Nut
It is a hexagonal nut provided with a cylindrical cap at the top to protect the end of the from corrosion. It also prevents leakage through the threads. Cap nuts commonly used in smoke-boxes locomotives.
Cylindrical or Capstan Nut
This nut is cylindrical in shape and has holes drilled in the curved surface for turning it with a tommy bar.
Dome Nut
It forms the cap nut having a special dome at the top as shown in the figure.
Wingnut or Thumb Nut
A wing nut is shown in the figure. This nut can be easily operated by the thumb and a finger and is used where it is required to be adjusted frequently.
Locking of Nuts
When a nut is tightened over the bolt, a small working clearance exists between the threads of a nut and bolt even in the highest class of fits.
The types of nuts and bolts used in the moving parts of machinery will be subjected to continuous vibration and in addition, sometimes, required to carry varying axial load also, for example, as the crossheads and connecting rods of an engine.
Under such conditions because of the working clearance existing between the mating threads, the nut develops a tendency to work loose or unscrew by itself.
This consequently may result in a serious breakdown. To prevent this, some means of locking the nut from unscrewing is necessary. The various locking devices for nuts are classified into positive and friction types. However, the tendency of unscrewing is reduced if the threads of the nut and bolt are of the good fit.
The positive methods of locking make use of a split pin, or a screw, or a lock plate, or a tab washer to lock the nut. In the friction method either an additional nut called lock nut, or a spring washer is used to lock the nut.
Split Pin Locking
The simplest method of locking a nut using a split pin is shown in the figure. After the nut has been finally tightened, a small hole is drilled through the blot close to the top face of the nut. The split pin is then inserted in the hole and the split ends of the pin are opened so as to prevent it from coming out while in use.
The main objection to using this type of locking is that the hole drilled in the blot reduces its strength considerably. The other equally important objection is that after continuous use swing to the stretch of the bolt the split pin may not rest on the top face of the nut which may reduce the locking effect.
Set Screw Locking Using Grooved Nut
A hexagonal nut provided with a cylindrical grooved collar at its the lower end is called Penn to ring or grooved nut. The end of the bolt hole is counterbored to receive the cylindrical lower grooved portion of the nut as shown in the figure. Locking of the nut is done by a set screw screwed through the nearest face of the workpiece.
The projecting dog-end of the set-screw enters the groove in the cylindrical portion of the nut and prevents the slackening of the nut. This method of locking is possible if the bolt hole is close to the nearest vertical edge as in the case of the marine engine connecting rods.
When the bolt hole is not close to the vertical edge of the workpiece, this nut is used in conjunction with a separate collar as shown in the figure. The dowel pin screwed in bearing surface prevents the rotation of the collar.
Locking By a Lock Plate
This type of locking is employed in heavy engineering work, as in the case of connecting rods, wheel shaft etc. The empirical proportions of the lock plate are shown in the figure. Initially, the nut is tightened and then the lock plate is inserted through the nut.
The plate is grooved in such a way that the grooves in the plate receive the hexagonal corners of the nut at every 30° rotation. The plate is fixed to the bearing surface by a tap bolt screwed into it. The figure showed the assembly of the locking of a nut by the lock plate. Since the nut is held tight within the grooves of the lock plate, the nut is prevented from slackening.
Locking By a Tab Washer
A washer provided with a rectangular projection is called the tab washer. This method of locking of nut or bolt head is suitable when the nut or bolt head is placed near the vertical edge of the workpiece.
After the nut is tightened the tab and the projecting portion of the washer itself are bent to bear against the vertical surface of the workpiece and one of the lateral faces of the nut or, the bolt head.
Locking By a Lock Nut
In the type of locking, the friction between the mating threads of bolt and nut locks the nut. When the nut is tightened over a bolt, the lower flanks of the threads of the bolt will be in contact with the upper flanks of threads of the nut.
When another nut is screwed down hard over the lower nut, it tends to pull the bolt through the lower nut. This virtually makes the upper nut to take up all to load, While the lower nut to function simply as a washer since there will not be any contact between the threads of the bolt and the lower nut.
Now the upper nut is held by a spanner While the lower nut is pull in the opposite direction. This cause the lower flanks of threads of the lower nut to come in contact with the upper flanks of the threads of the bolt causing a wedging action between them.
This makes the lower nut to act as a lock nut. Theoretically, the lower nut which is a lock nut may be the thin nut as shown fig. But as it required a special thin spanner to turn it, in practice it is more usual to place taking nut below the lock nut as shown in Fig. such cases sometimes two nuts of equal thickness are used as shown in Fig.
Locking By Spring Washer
In this method of locking a coiled spring, the washer is placed underneath the nut as shown in Fig. when the nut is tightened, the spring force of the spring exerts an axial force on the underside of the nut and holds it by the friction grip. This prevents the slackening of the nut.
A single coiled spring washer will be sufficient for light classes of work. When the vibrations are high either a double coiled or triple coiled spring washer is used.
Screw Pin Locking
A nut may be locked by a screw pin, screwed in the bearing surface adjoining the nut touching one of the lateral faces of the nuts as shown Fig. This type of locking is employed when the nut is expected to remain without any adjustment for a long time.