Joints and Fasteners

Welded Joints

 A permanent joint which is obtained by the friction of edges of two parts to be joined, with or without the application of pressure and a filler material.

• Used in fabrication as an alternative method for casting and forging.
• Used as a repair medium.
• If filler material is used  ↠  Strength of Joint > Base metal strength.
• If filler not used  ↠  Joint strength < Base metal strength.
• Grinding Finish  ↠  Welding at top and bottom off the plate.
• Machining Finish  ↠  Welding at bottom surface after welding machining operation.
• Want to learn more about welded joint, click here.

Designing of Butt Joint

Design of Lap Joint or Fillet Joint

Lap or Fillet joint is "formed by the overlapping of plates and then welding the edges of plates". There are two types of fillet joints:

1. Single and Double Transverse Fillet Joints  ↣  are designed for tensile loading.
2. Parallel Fillet Joints  ↣  are designed for shear loading.

Screw Joints

Screw Threads are formed by "Cutting the continuous helical groove on a cylindrical surface or rod".

• Pitch is the reciprocal of the number of threads forms per inch 'N'.
• Lead 'l' is the distance the nut moves parallel to the screw axis when the nut is given one turn.
• For a single thread  ↠  Lead = Pitch.

Thread Standards

1. American National Thread Standard

• Thread size is specified by the number of threads per inch 'N'.
• Thread angle is 60-degrees.
• Crests of thread may be either Flat or Rounded.

There are two series of American National Thread Standard:

1. UN Series  ↠  we have sharp edges at root (no radius).
2. UNR Series  ↠  we add root radius which increases its thread and fatigue strength.

2. Metric Thread Standard

• Specified by writing the nominal major diameter and pitch in millimetres.
• Thread angle is 60-degrees.
• If we add pitch with M12  ↠  it is Fine-pitch thread otherwise it is Coarse-pitch thread. 

There are two series of Metric Thread Standard

1. Without Root Radius  ↠  denoted by 'M'.
2. With Root Radius  ↠  denoted by 'MJ'.

Square and Acme Threads

Square and Acme threads are used on screw when power is to be transmitted.

• Square Threads  ↠  thread angle is 0-degrees, interlocking of threads is good but have wear.
• Acme Threads ↠  thread angle is 29-degrees, high load carrying capacity.

Power Screw

It is defined as, "a device used in machinery to change angular motion into linear motion to transmit power".

• Bolt Preload or Pretension  ↠  Clamping force which stretches the bolt when nut gets tighten (or assembly force which nut apply to bolt).
• Spring Rate  ↠  Ratio between the force applied to the member and the deflection produced by that force.
• Grip  ↠  Total thickness of clamped material (including both member and both washers).
• Self-locking Screw  ↠  which requires a positive torque to lower the load.
• Over-hauling Screw  ↠  in which friction is too low so that load lower itself by causing the screw to spin without external effort.

Riveted Joints

Riveted joints are used as a permanent fasteners in buildings, bridges, boilers, tanks, ship.

• Strength of rivets < threaded fasteners of same diameters.
• No disassembly of joint.
• The minimum rivet spacing for structural steel work is 3 times rivet diameter and maximum rivet spacing is 16 times the thickness of outside plate.

Reasons for choosing rivets over threaded fasteners are:

1. Rivets will not loose due to shaking.
2. Cheap, especially in assembly.
3. Light weight.

Failure of Riveted Joints

Modes of failure of a bolted or riveted connection are:

1. Shear Loading 
2. Bending of rivets
3. Shearing of rivets
4. Tensile failure of members
5. Bearing of rivets on members OR Bearing of members on rivets.

Failure of Riveted Joints

References:

• Material from Class Lectures + Book named Mechanical Engineering Design by Shigley (8th Edition) + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube.