Mechanical Joining is a process of coupling metal parts together using screws or nuts and bolts etc.
The threaded holes employed for mechanical joining are vulnerable to fractures. In ductile materials, the facture can come from fatigue, while in brittle materials, the fractures can simply result from mechanical overloading. This mechanical joints must be designed with fatigue failure and brittle fracture in mind. Another issue to be considered when designing mechanical joints is galvanic corrosion that affects different types of metals that are in contact with each other.
However, these may be temporary joints which are easily separable without causing any deformation to the parts. The processes may be regarded as mechanical fastenings which includes bolts and nuts, set screws, studs and other locking devices.
The reason for temporary joining processes or mechanical fastening as it is sometimes called is to enable parts to be removed for replacement, overhaul or to allow access to other parts of the component.
Bolts and Nuts
Bolts and nuts may be regarded as the most widely used of all the mechanical fastening devices and are used in conjunction with washer for a more reliable tightening. The use of bolt and but tends to make the joining process simpler as the only requirement for this exercise is to ensure that the parts to be joined are provided with clearance holes to enable the bolt pass through there by providing the bolt head and the nut accessible for tightening with spanner.
The length of bolts are taken from under the head of the bolt to the end of the screwed part and not from the top of the head to the end of the screwed part or beyond. The hexagonal headed bolt is widely used especially where heavy tightening forces are required.
Bolts are of types like: Cheese, round head, countersunk, hexagonal or socket popularity called Allen Key, while the ones with slot are made to require the use of screwdrivers.
Set Screw
The set Screw are designed for places where the use of bolt and nuts are not possible. In this type of joint, only one part of the components is provided with ordinary hole while the other is provided with threaded hole to receive the set Screw. However, set screws are meant for holding small parts together in assemblies, example of such fittings is assemblies of electrical junction box.
Set screws are of various types similar to bolts. The use of set Screws eliminate the need for long bolts when parts of thicker sections are required to be tightened. There are two outstanding disadvantages of set screws:
i. Breaking the threads in the threaded holes as a result of frequent removal and replacement of the set Screw.
ii. Breaking into the threaded component thereby blinding the hole.
Studs
Studs are threaded on both ends. It is a special mechanical fastener which is required when heavy tightening forces are needed especially where compression is to be maintained. A typical example is when joining the cylinder head of an engine, in this case, the stud is rigidly fastened into the tapped hole and may remain in this position when the nut is removed.
The outstanding Disadvantage of Stud is that if it breaks as a result of over tightening, it becomes more problematic in removing the broken part that remains in the threaded hole which may lead to damaging the component of sufficient technicalities are not employed in it's removal process.
Locking Devices
It is noticeable that when bolts and nuts fastening are used on components and are subjected to severe vibrations, there is the tendency for them to lose and this could lead to serious damage to the engine or machine. To prevent this, locking devices are made available for use.
These devices include: lock nut, spring washer, splint pin, castle nut, tab under the nut, etc.