Short Answer:

A sliding pair is a type of lower pair in which one element slides over another, maintaining surface contact. Examples of sliding pairs include the piston and cylinder of an engine, and the crosshead and guide in a steam engine. These pairs allow only sliding motion between the two elements.

A turning pair is another type of lower pair where one part rotates about a fixed axis of another part. Examples of turning pairs include a crank and connecting rod, and a shaft rotating in a bearing. These pairs allow only rotational motion between the elements.

Detailed Explanation:

Sliding Pair and Turning Pair

In mechanical systems, different parts move relative to each other to transmit motion and power. The nature of this relative motion depends on how two elements are joined. Such connections between two elements that permit motion are called kinematic pairs. Among these, sliding pairs and turning pairs are the most commonly used types of lower pairs, as they allow surface contact and stable movement between the connected parts.

Understanding these pairs is essential for designing and analyzing machines like engines, pumps, and linkages, as they help in defining how motion and force are transmitted between the components.

Sliding Pair

A sliding pair is formed when two mechanical elements are joined in such a way that one element can only slide linearly over the surface of the other. The surfaces of both elements remain in contact throughout the motion. The relative motion between them is translational (i.e., movement in a straight line).

The sliding pair provides only one degree of freedom, meaning the motion can occur only in one direction — either forward or backward, depending on the machine’s design. The friction between the sliding surfaces plays a significant role in determining the efficiency and smoothness of motion.

Examples of Sliding Pair:

1. Piston and Cylinder in an Engine:
   The piston slides to and fro inside the cylinder. This relative sliding motion converts the reciprocating motion of the piston into rotary motion of the crankshaft through the connecting rod.
2. Crosshead and Guide in a Steam Engine:
   The crosshead moves along the guide, allowing only straight-line motion, which ensures that the piston rod moves smoothly and stays aligned.
3. Ram and Die in a Shaper Machine:
   The ram slides over the guideways of the shaper, producing the cutting motion needed to shape the workpiece.
4. Tailstock on a Lathe Bed:
   The tailstock slides along the bed of a lathe to adjust its position for different lengths of workpieces.
5. Slide of a Machine Tool:
   The slide in a milling or drilling machine moves linearly over the guideways to position the tool or workpiece precisely.

In all these examples, surface contact is maintained, and motion is purely sliding, with no rotation involved.

Turning Pair

A turning pair (also called a revolute pair) is formed when one element can rotate about a fixed axis relative to another element. In this type of pair, one component remains fixed while the other rotates or turns around it. The motion between the two elements is purely rotational, and they are connected in such a way that they share a common center of rotation.

The turning pair also possesses one degree of freedom, which means the motion is limited to rotation only. It is one of the most common types of kinematic pairs used in machines to produce continuous rotational movement.

Examples of Turning Pair:

1. Crank and Connecting Rod in an Engine:
   The crank rotates around its shaft while the connecting rod oscillates. Together they form a turning pair, converting reciprocating motion into rotary motion.
2. Shaft Rotating in a Bearing:
   The shaft turns freely within the bearing, forming a turning pair. This arrangement is common in motors, turbines, and gear systems.
3. Hinge Joint of a Door:
   The door rotates about the hinge axis, allowing angular motion only in one direction. This everyday example represents a simple turning pair.
4. Wheels Mounted on an Axle:
   The wheel rotates about the fixed axle, allowing continuous circular motion — an essential mechanism in vehicles.
5. Lever and Fulcrum:
   A lever rotates about a fixed fulcrum point, which is a basic example of a turning pair found in simple mechanical devices.

Comparison of Sliding and Turning Pairs

Although both sliding and turning pairs are lower pairs, they differ mainly in the type of relative motion and the nature of the surface contact.

• In a sliding pair, the motion is linear (sliding), while in a turning pair, the motion is rotational (turning or angular).
• The piston and cylinder form a sliding pair, while a shaft and bearing form a turning pair.
• Both have surface contact, but the direction of relative motion distinguishes one from the other.

The design of each pair depends on the purpose of the mechanism. For example, sliding pairs are suitable for reciprocating machines, while turning pairs are suitable for rotational or oscillatory mechanisms.

Applications of Sliding and Turning Pairs

• Engines: Sliding pairs (piston-cylinder) and turning pairs (crankshaft-bearing) work together to convert reciprocating motion into rotary motion.
• Machine Tools: Sliding pairs control the linear positioning of components, ensuring accuracy in shaping or drilling operations.
• Automobiles: Turning pairs are used in wheels, steering joints, and suspension systems for smooth rotational movement.
• Robotics: Both pairs are used in robotic arms, providing precise linear and angular motion.

Conclusion:

In conclusion, both sliding pairs and turning pairs are essential for the working of mechanical systems. A sliding pair allows relative motion in a straight line, while a turning pair enables rotational motion around a fixed axis. These pairs are the foundation of most machine mechanisms, from engines and tools to vehicles and robotic systems. Understanding their function helps engineers design machines that move smoothly, efficiently, and with precision.