What kind of joints do we need to apply and why?

In mechanical design, the choice of joint depends on the required motion, load, and assembly constraints. Common joint types include:

1. Rigid (fixed) joints – no relative motion between parts. Use when parts must stay in a fixed relationship, such as a welded frame or a bolted flange. They provide high stiffness and are ideal for load‑bearing structures.

1. Pivot (revolute) joints – allow rotation about a single axis. Examples are hinges, cam‑shaft bearings, and robotic arm joints. They are chosen when a single rotational degree of freedom is needed and the load is primarily axial or radial.

1. Sliding (prismatic) joints – allow linear motion along one axis. Common in telescoping mechanisms, linear actuators, and piston assemblies. They are used when precise linear displacement is required.

1. Spherical (ball‑and‑socket) joints – provide three‑axis rotational freedom. Used in universal joints, shoulder joints, and some robotic wrists.

1. Universal (cross‑shaft) joints – allow rotation between shafts at an angle, transmitting torque while accommodating misalignment.

The selection criteria include the required degrees of freedom, load magnitude and direction, manufacturing feasibility, maintenance, and cost. For example, a robotic arm that needs to reach around obstacles would use a combination of revolute and spherical joints to maximize workspace while keeping the structure lightweight. In contrast, a structural beam that must resist bending would use rigid joints to maintain integrity. Understanding these trade‑offs is essential for efficient, reliable mechanical design.