Lesson 4.2: Design of Joints (Cotter, Knuckle, Riveted, Welded, Bolted)

Machine joints are critical in transmitting loads between components. GATE and PSU exams often test strength, stress analysis, and design of various mechanical joints.

🔹 1. Introduction

• Definition: Joints connect two or more machine parts to transmit motion, torque, or load safely.

• Applications: Shafts, rods, structural members, frames

• Types of Joints:

  1. Cotter Joint

  2. Knuckle Joint

  3. Riveted Joint

  4. Welded Joint

  5. Bolted Joint

🔹 2. Cotter Joint

• Purpose: Connects two rods under axial load.

• Components: Socket, spigot, cotter (wedge)

• Design Considerations:

  • Axial stress in rods and cotter

  • Shear stress on cotter

  • Crushing stress on cotter and socket

• Example: Design cotter joint to transmit 50 kN load using mild steel. Compute dimensions considering allowable stress.

🔹 3. Knuckle Joint

• Purpose: Connect two rods under tensile load with angular motion possible.

• Components: Pin, fork, eye

• Design Considerations:

  • Axial stress in rods

  • Shear & bearing stress on pin

  • Factor of safety based on tensile and shear strength

• Example: Rod of 25 mm diameter, tensile load 30 kN → calculate pin diameter for steel with allowable shear stress 60 MPa.

🔹 4. Riveted Joints

• Purpose: Connect plates and structural members in tension/compression.

• Types: Lap joint, butt joint

• Design Considerations:

  • Shear on rivets: τ=P/nAr\tau = P / nA_rτ=P/nAr​

  • Bearing on plates: σb=P/Ab\sigma_b = P / A_bσb​=P/Ab​

  • Efficiency: % efficiency of joint = ratio of joint strength to plate strength

• Applications: Bridges, boilers, machine frames

🔹 5. Welded Joints

• Purpose: Permanent connection between two metal parts using heat/filler

• Types: Butt weld, fillet weld, groove weld

• Design Considerations:

  • Shear stress in weld: τ=P/Aw\tau = P / A_wτ=P/Aw​

  • Weld throat thickness → design parameter

  • Avoid stress concentration

• Applications: Frames, shafts, machines, pressure vessels

🔹 6. Bolted Joints

• Purpose: Removable joint connecting machine parts

• Design Considerations:

  • Axial and shear load on bolt

  • Bolt pre-tension to prevent slipping

  • Factor of safety in shear and tension

• Example: Bolt diameter for transmitting axial load 20 kN with allowable stress 100 MPa

🔹 7. Solved Examples (PYQ Style)

Example 1 (GATE ME 2017):
Design a cotter joint for mild steel rod under 40 kN load. Compute dimensions of socket, spigot, and cotter.

Example 2 (PSU Exam):
Design a single riveted lap joint to connect two 10 mm thick plates under 50 kN tensile load. Compute rivet diameter and pitch.

🔹 8. Practice Exercises

1. Calculate pin diameter for knuckle joint transmitting given tensile load.

2. Design welded joint with given load and allowable shear stress.

3. Determine rivet size and number for lap joint of given plate thickness.

4. Compute bolt pre-tension required for preventing slip in a bolted joint.

5. Compare efficiency of single, double, and triple riveted joints.

🔹 9. Summary

• Cotter & Knuckle Joints: Axial tensile load, rods connection, pin/cotter stresses

• Riveted Joints: Plates under tension/compression, shear & bearing stress

• Welded Joints: Permanent connection, shear stress & throat thickness critical

• Bolted Joints: Removable, tension & shear design, pre-tension important