Short Answer:

Oldham’s coupling is a type of mechanical coupling used to connect two parallel shafts that are slightly misaligned. It transmits torque from one shaft to another while allowing small lateral displacement between them. The coupling consists of three main parts – two flanges with slots and a middle disc with a tongue on each side, which fits into the slots of both flanges.

This coupling is especially useful when shafts are not perfectly aligned but need to rotate at the same speed. It ensures smooth power transmission and reduces vibrations caused by misalignment in machines.

Detailed Explanation:

Oldham’s Coupling

The Oldham’s coupling is a special form of flexible coupling designed to transmit torque between two shafts whose axes are parallel but have a small lateral (offset) misalignment. It is named after John Oldham, who invented it to overcome alignment issues in machinery. This coupling is widely used in mechanical systems where perfect alignment between driving and driven shafts is difficult to maintain.

The Oldham’s coupling ensures that power transmission between the shafts is smooth and uniform, even when there is slight displacement of one shaft relative to the other. It helps prevent excessive stresses, vibration, and wear that could result from misaligned shafts.

Construction of Oldham’s Coupling

The Oldham’s coupling consists of three main parts:

1. Two Flanges (or Hubs):
   Each flange is mounted on the end of the shafts that are to be connected. Both flanges have a slot cut across their face. The slots are parallel to each other on each flange but are at right angles (90°) to the slot on the other flange.
2. Central Disc (or Intermediate Plate):
   The central disc is placed between the two flanges. It has two tongues or keys on opposite sides. These tongues fit into the slots of the flanges. The disc is free to slide within the slots during rotation.
3. Fasteners or Keys:
   Set screws or keys are used to secure the flanges to their respective shafts. This ensures that each flange rotates with its shaft without slipping.

Working of Oldham’s Coupling

The working principle of Oldham’s coupling is based on the sliding motion between the central disc and the slots in the two flanges.

• When the driving shaft rotates, its flange also rotates.
• The tongue on one side of the central disc engages in the slot of the driving flange, causing the disc to rotate.
• As the disc rotates, the tongue on the other side of the disc drives the slot of the driven flange, thus transmitting torque to the driven shaft.
• During this rotation, the central disc slides alternately in both slots, accommodating the lateral misalignment between the two shafts.

This sliding action allows the shafts to remain parallel but displaced by a small amount, without interrupting smooth power transmission. Hence, the coupling acts as a compensating element between two misaligned shafts.

Kinematic Analysis

Let the offset between the two shafts be ‘e’. During one complete rotation of the coupling, the central disc moves back and forth twice in a direction perpendicular to the shaft axis.
This relative motion causes some sliding friction between the tongues and slots, but it ensures continuous and uniform angular velocity of the driven shaft.

The speed of the driven shaft remains equal to that of the driving shaft, making it ideal for applications where synchronized rotation is required even with slight misalignment.

Advantages of Oldham’s Coupling

1. Compensation for Misalignment:
   It allows small lateral misalignments between two parallel shafts without affecting power transmission.
2. Smooth Power Transmission:
   Provides continuous and uniform speed of rotation between the shafts.
3. Simple Construction:
   It is easy to manufacture, assemble, and maintain.
4. No Backlash:
   The coupling operates with minimal backlash, ensuring precise motion transfer.
5. Protection Against Stress:
   Reduces bending stresses and vibrations caused by misalignment.

Limitations of Oldham’s Coupling

1. Wear and Friction:
   Continuous sliding of the disc causes friction and wear, requiring regular lubrication.
2. Limited Misalignment Compensation:
   Can only handle small lateral displacements, not angular or axial misalignments.
3. Material Fatigue:
   The central disc may weaken or crack under excessive torque or high-speed conditions.
4. Regular Maintenance Needed:
   Needs periodic inspection and lubrication to ensure smooth functioning.

Applications of Oldham’s Coupling

Oldham’s coupling is used in various mechanical systems where slight shaft misalignment is unavoidable. Common applications include:

• Machine tools: Such as milling machines, lathes, and drilling machines.
• Pumps and compressors: For connecting motor and pump shafts.
• Robotics and automation: For precise torque transmission in compact setups.
• Printing and textile machinery: Where accurate and smooth rotation is essential.
• Engines and generators: To connect shafts of small power units with alignment flexibility.

Comparison with Other Couplings

Oldham’s coupling differs from rigid and flexible couplings:

• Rigid couplings require perfect alignment and cannot tolerate misalignment.
• Flexible couplings use elastic elements to absorb misalignment but may introduce vibration.
• Oldham’s coupling, on the other hand, provides an ideal middle solution — it allows small misalignment while maintaining rigid torque transmission and minimal vibration.

Conclusion

The Oldham’s coupling is a highly effective and simple device for connecting two parallel but misaligned shafts. It works on the principle of sliding motion of an intermediate disc between two flanges, ensuring smooth and uniform torque transmission. Despite minor frictional losses, its advantages — such as simplicity, alignment compensation, and reliability — make it a preferred choice in many mechanical systems. Proper lubrication and maintenance can enhance its life and performance, making it a vital coupling in modern engineering machinery.