What are lower pairs?

Short Answer:

Lower pairs are those kinematic pairs in which the two links or elements have surface or area contact with each other during relative motion. The contact between the two links is permanent and allows smooth transmission of motion.

Examples of lower pairs include sliding pair, turning pair, screw pair, cylindrical pair, and spherical pair. These pairs are widely used in mechanisms such as engines, pumps, and other machines where smooth and continuous relative motion between parts is required.

Detailed Explanation :

Lower Pairs

In mechanical engineering and kinematics of machines, a lower pair is a type of kinematic pair where the two elements are in surface contact with each other during their relative motion. This means that the motion takes place over a surface rather than a line or a point. The contact remains continuous throughout the movement, ensuring stability and minimizing wear between the moving parts.

Lower pairs are very common in mechanical systems because they provide a simple, strong, and durable way to transmit motion and force between two connected links. They are used in machines where one part slides, turns, or rolls over another part in a defined manner.

Features of Lower Pairs

  1. Surface Contact:
    The most important feature of lower pairs is that the contact between the two links occurs over a surface area. This reduces stress concentration and allows smoother operation.
  2. Permanent Contact:
    The two elements remain in contact at all times during motion. There is no separation between them during operation.
  3. Restricted Motion:
    The relative motion between the elements is restricted to certain types, such as sliding, turning, or helical.
  4. High Load Capacity:
    Because of surface contact, lower pairs can transmit large forces and withstand high loads without damage.
  5. Common in Machines:
    Most of the mechanical joints and machine parts such as pistons, shafts, bearings, and screws are examples of lower pairs.

Classification of Lower Pairs

Lower pairs can be classified into several types depending on the kind of relative motion between the two links:

  1. Sliding Pair:
    • In this type, one link slides over the other.
    • The contact between them is a surface, and the motion is purely sliding.
    • Example: The piston and cylinder of an internal combustion engine form a sliding pair. The piston moves back and forth inside the cylinder.
  2. Turning (or Revolute) Pair:
    • One link turns or rotates about a fixed axis of another link.
    • The motion is purely rotational.
    • Example: A crank and a connecting rod, or a door hinged to a frame, form a turning pair.
  3. Screw (or Helical) Pair:
    • One link moves relative to another through a combination of rotational and translational motion.
    • The motion occurs along a helical path, like that of a screw thread.
    • Example: A lead screw and nut in a lathe machine form a screw pair.
  4. Cylindrical Pair:
    • One link can both rotate and slide with respect to the other along the same axis.
    • The motion is a combination of translation and rotation.
    • Example: The piston rod inside a cylindrical guide.
  5. Spherical Pair:
    • One link can rotate in multiple directions relative to the other, forming a ball-and-socket type connection.
    • The center of the ball remains fixed while it can rotate in any direction.
    • Example: The ball and socket joint in the human body or automobile steering system.

Importance of Lower Pairs

Lower pairs are essential in mechanism design because they ensure controlled, smooth, and predictable relative motion between parts. Their applications and importance include:

  1. Smooth Operation:
    The surface contact ensures smooth and continuous movement with less vibration and noise.
  2. High Strength and Durability:
    The large contact area reduces wear and increases the life of machine parts.
  3. Efficient Power Transmission:
    They allow the easy transfer of motion and power from one component to another.
  4. Design Simplicity:
    Lower pairs are simple to design, manufacture, and maintain, making them suitable for most machines.
  5. Stability of Motion:
    Since contact is maintained throughout, the relative motion between links remains stable and predictable.

Examples in Real Machines

  1. Engine Mechanism:
    The piston and cylinder form a sliding pair, while the crank and connecting rod form a turning pair. Together, they convert reciprocating motion into rotary motion.
  2. Lathe Machine:
    The screw pair between the lead screw and nut helps in moving the carriage precisely for cutting operations.
  3. Robotic Arm:
    Revolute and cylindrical pairs help in achieving controlled movements in robotic joints.
  4. Automobile Components:
    The steering knuckle and ball joint form a spherical pair to allow flexible wheel movement.
  5. Door Hinges:
    The hinge and door frame create a turning pair, allowing rotation about a fixed axis.

Comparison with Higher Pairs

While lower pairs have surface contact, higher pairs (like gears and cam-follower mechanisms) have line or point contact. Lower pairs are more stable, durable, and preferred where smooth motion and higher load capacity are needed.

In contrast, higher pairs are used where precise motion is required over shorter durations, but they wear faster due to smaller contact areas.

Conclusion:

Lower pairs play a vital role in mechanical systems by ensuring smooth, stable, and controlled relative motion between machine components. Their surface contact makes them durable and efficient for transmitting motion and power. Most of the joints used in practical mechanisms, such as pistons, hinges, screws, and shafts, are examples of lower pairs, highlighting their importance in the design and functioning of mechanical systems.