Brazing. Image credit

Brazing. Image credit

ZIMSEC O Level Combined Science Notes:Types of joints

  • The main function of a joint is to transmit load from one part of a structure to another
  • This usually results in the joint being subjected to a lot of stress and weakening the structure
  • Certain types of joints allow stress to pass uniformly accords the joint
  • Examples are the butt-weld and scarf joints
  • The former is used when joining metal and the later is used when joining wood/timber
  • There are various other types of joints including:
  • Lapped joints
  • Mortised joints
  • welded joints
  • bolted, screwed or nailed joints
  • brazed and soldered joints and
  • riveted joints
  • Joints are named in accordance with the material they are holding together e.g rivets,
  • These are also known as pinning
  • the material/method used to hold the materials together e.g. nailed joints
  • Or the way in which the members of the joint are brought together e.g mortised joints

Lapped joints

Joint types and pinnings

Joint types and pinnings

  • In a lapped joint which is also known as a lap joint
  • In a lapped/lap joint the two pieces being joined overlap without over one another
  • They overlap over each other hence the name lapped joint
  • The two adjacent pieces may be glued, nailed, screwed, welded, bolted or riveted together
  • the load is transferred at the two ends of the joint and
  • stress will be at greatest at these points
  • The strength of these joints depend on their width rather than upon the length of overlap
  • Joints can be secured in different ways
  • This is known as pinning
  • There are various types of pinning:

a) bolted and nailed joints

  • lapped joints in wood are often bolted or nailed together
  • bolts, nails and screws resist sheer

b) Mortised joints

Dowels and mortised joints

Dowels and mortised joints

  • In mortised joints
  • A hole(or more) is made in  one of the timber beams
  • the adjoining beam is carved out to fit into this hole
  • the end of this beam is glued into the hole of the first beam

b) Dowels

  • Are similar to mortised joints
  • Dowels are often used in making furniture
  • Holes are made into both beams and
  • Small wooden cylinders are inserted and glued to hold the beams together

c) Welded joints

  • Are commonly used in steel-work
  • It has several advantages:
  • This is because they are cheaper than rivets
  • They are also lighter
  • They also offer less streamlining resistance to air or water
  • Welding also creates joints that spread stress evenly
  • Welding is achieved in various ways
  • In electric arc-welding electricity is used to heat an electric rod
  • The rod melts and the molten metal is spread along the joint and solidifies joining the metal in the process
  • Welding has several disadvantages:
  • It is difficult to obtain a perfect weld thus there is always the possibility of human error during the welding process creating inherent weaknesses in the joint
  • Welded joints also do not stop cracks from developing and spreading through steel structures

d) Brazed and soldered joints

Soldering is often used in electronics. Image credit

Soldering joints are often used in electronics. Image credit

  • involve the joining of metal plates with molten metal that solidifies to make a continuous joint
  • Solder is an alloy of tin and lead
  • It has a lower melting point
  • A small hot iron element is used to melt it
  • Brazing uses an alloy of zinc and brass and a very hot flame
  • Brazing creates a more flexible joint
  • Brazed joints pass stress more evenly

e) Gluing

  • Is often used in making furniture
  • Often in scarf joints the two pieces of timber are glued together
  • A glued joint passes stress evenly

f) Riveted joints

  • involves the use of rivets
  • White hot rivets are placed in a hole drilled through a lapped joint
  • Their ends are then hammered flat
  • Upon cooling the rivet contracts and pulls the joint together
  • The more rivets used in making a joint the stronger it is
  • Because they share the stress
  • Rivets have several advantages:
  • Riveted joints are reliable
  • the joint stops cracks because of its discontinuity
  • The joints can slip a little thus redistributing the load and avoiding stress concentrations
  • The rivet holes can fill up with rust thus locking the joint and allowing it to transmit shearing forces equally between the joined plates
  • It acts like glue and as a result rivet joints improve with age

To access more topics go to the Combined Science Notes page.