Mechanical Fasteners and Joints: Design & Function

Posted on Jun 5, 2025 in Electromechanical Engineering

Mechanical Joints and Fasteners

Fixed Joints

Fixed joints are used for joining thin, flat pieces that cannot support welded parts, or where welds might cause internal tensions and deform the material. Riveting is a common method for creating fixed joints.

Types of Riveted Joints

  • Lap Joints: Where two pieces overlap and are riveted together.
  • Butt Joints with Single Cover Plate: Where two pieces are on the same plane and are linked together by a single riveted plate (cover plate) that joins both pieces.
  • Butt Joints with Double Cover Plates: Where two pieces are on the same plane and are linked together by two riveted plates, one on each side.

Threaded Fasteners

Screws perform essential clamping functions in mechanical assemblies.

Parts of Screw Fasteners

A screw fastener assembly typically consists of:

  • The Screw: Comprising a cylindrical rod or shank, a head, and a threaded section.
  • The Nut: The most important part for securing the assembly.
  • The Washer: Used to distribute load and prevent damage.

Understanding Threads

A thread is well-defined when its characteristics are known:

  • Thread Pitch: Formed by a grooved surface and a helical ridge (fillet) wound helically around a cylinder, called the male thread or screw. When the thread is in a hole, it is called a female thread or nut. The pitch refers to the distance between two adjacent turns.
  • Thread Direction:
    • If the thread coil starts from left to right, it is called a right-hand thread.
    • If it starts from right to left, it is called a left-hand thread.
  • Thread Geometry: Various geometric shapes of threads exist:
    • Triangular shapes: Used for holding parts.
    • Trapezoidal and square shapes: Used to transmit significant forces.
    • Rounded threads: Used for sealing, cutting, and tapping very thin plates.

Standard Threads

The ISO metric screw thread is a standardized form widely used for joining machine elements. Its triangular geometry metric thread belongs to the SI system and was adapted for the UNE standard.

Screws and Nuts

These elements serve to join two pieces together so that they can be assembled and disassembled.

Washers

Washers are generally ring-shaped pieces of mild steel. Their outer diameter typically protrudes about 2 or 3 mm beyond the nut or screw head. Washers are usually placed between the screw head and the workpiece, or between the nut and the workpiece, to secure the assembly.

When Washers Are Required:
  • When the material to be joined is very soft and needs protection from crushing.
  • When the surface where the nut or screw head rests is irregular.
  • To avoid scratching the surface by the nut.
  • To increase the clamping surface to prevent the screw from sinking in.

When purchasing screws and nuts, the thread metric, the shape of the screw head, and the screw length must be specified.

Common Screw Head Types:
  • Slotted
  • Phillips
  • Hex (Allen)
  • Square
  • Round
  • Wing (often a nut type, but listed here)

Keys and Pins

Keys are typically steel pieces inserted into notches or keyways on a shaft and a mating hole. The shapes of keys vary depending on the forces they need to transmit between connected elements.

Types of Keys and Pins

Depending on the type of force, we can distinguish between:

  • Transverse keys and pins
  • Longitudinal keys
  • Splined shafts

Transverse Keys and Pins

Transverse keys (or cotter pins) are used to join two cylindrical pieces that need to fit together when subjected to tension or compression, also allowing for interchangeability. Their geometric shape resembles a wedge. When forces are small, these wedges can be replaced by bolts or pins, which is one of the simplest and oldest known methods of joining parts.

A variety of pins are elastic pins, which can transmit torsional forces. Their shape is cylindrical, open down the middle, and they have a larger diameter than the hole, exhibiting a very high elastic limit.

Longitudinal Keys

The purpose of these keys is to transmit torque and also ensure the axial immobility of the joined pieces.

Feather Keys

These keys allow for longitudinal axial displacement of the pieces while still transmitting torque.

Another variety of keys are round or Woodruff keys, which have a semicircular bottom.

Splined Shafts

A fastening system used for assembling parts that must be rigidly attached to transmit significant forces involves a series of notches (splines) on the periphery of the shaft.