Short Answer:

An Oldham’s coupling is a mechanical device used to connect two parallel shafts that are slightly misaligned. It transmits rotary motion and power from one shaft to another smoothly, even if their axes are not perfectly in line. The coupling works with the help of an intermediate disk that slides between the two shafts, allowing for small angular or parallel misalignments.

It is commonly used in applications where perfect shaft alignment is difficult to achieve, such as in pumps, compressors, and small machinery. The design of Oldham’s coupling ensures smooth torque transmission without vibration or excessive wear.

Detailed Explanation :

Oldham’s Coupling

An Oldham’s coupling is a type of flexible mechanical coupling used to connect two parallel but slightly misaligned shafts for the purpose of transmitting torque. It is an important inversion of the double slider-crank mechanism, where one of the sliding links is fixed. This coupling is widely used in machinery where the shafts cannot be aligned perfectly due to manufacturing tolerances, thermal expansion, or operational vibrations.

The design of Oldham’s coupling helps maintain constant speed transmission and smooth motion, even when slight parallel displacement exists between the connected shafts.

Construction of Oldham’s Coupling

Oldham’s coupling consists of three main parts (links):

Driving Shaft (Link 1): This is the input shaft that provides rotary motion. It has a circular flange with a central slot or key that fits into the intermediate disk.
Driven Shaft (Link 2): This is the output shaft that receives motion and torque from the driving shaft. It also has a similar flange with a slot, but its slot is perpendicular to the slot on the driving shaft.
Intermediate Disk (Link 3): This is the central element that connects both shafts. It has two tongues or projections on opposite faces, arranged at right angles. These projections fit into the slots of the two shaft flanges.

When the driving shaft rotates, its slot causes the intermediate disk to slide and rotate simultaneously. The intermediate disk then drives the second shaft through its perpendicular slot. This arrangement allows torque transmission even when the shafts are slightly misaligned.

Working of Oldham’s Coupling

When the driving shaft rotates, it transmits motion to the intermediate disk through its slot. As the disk rotates, it slides alternately in both directions within the slots of the driving and driven shafts. This relative sliding action compensates for the parallel misalignment between the shafts.

The intermediate disk moves in a circular path while simultaneously rotating about its own center. The motion of the disk ensures that both shafts rotate at the same angular speed, maintaining uniform torque transmission.

The unique feature of the Oldham’s coupling is that it allows parallel misalignment between shafts while keeping their rotational motion synchronized. However, it does not compensate for large angular misalignments or axial displacements.

Kinematic Relation to Double Slider-Crank Mechanism

Oldham’s coupling is derived as one of the inversions of the double slider-crank mechanism. In this inversion:

One of the sliding links (the frame) is fixed.
The intermediate disk acts as the coupler connecting the two shafts.
The slots and projections represent the sliding pairs that allow relative motion.

This relationship with the double slider-crank mechanism helps understand how linear sliding motion in perpendicular directions is converted into rotary motion for torque transmission between misaligned shafts.

Advantages of Oldham’s Coupling

Compensates for Misalignment: It can handle small parallel misalignments between shafts efficiently.
Smooth Torque Transmission: The intermediate disk ensures uniform speed and smooth power transfer without vibration.
Simple Design: The construction is compact and easy to assemble or disassemble.
Low Maintenance: Requires little lubrication and maintenance compared to other flexible couplings.
Cost-Effective: The coupling is relatively inexpensive and easy to manufacture.

Limitations of Oldham’s Coupling

Limited Misalignment Range: It can only compensate for small parallel misalignments, not angular or axial ones.
Wear on Sliding Surfaces: Continuous sliding between the disk and slots causes wear and may require periodic replacement.
Torque Limitation: It is suitable for light and medium torque applications only.
Lubrication Required: Sliding surfaces need occasional lubrication to reduce friction and wear.
Not Suitable for High-Speed Applications: The presence of sliding motion makes it unsuitable for very high rotational speeds.

Applications of Oldham’s Coupling

Oldham’s coupling is used in many mechanical systems where parallel misalignment between shafts is unavoidable but torque transmission must remain smooth and reliable. Some common applications include:

Rotary pumps and compressors – for connecting the motor and pump shafts.
Printing machines – where precise speed synchronization is required between components.
Machine tools – such as milling machines and lathes, to link misaligned drive shafts.
Robotic systems – for transmitting power between drive components.
Automotive systems – in certain cases where compact motion transmission is needed.

In all these applications, the Oldham’s coupling ensures proper motion transfer without vibration or damage caused by shaft misalignment.

Working Example

Imagine two shafts positioned side by side but slightly displaced parallel to each other. If they were connected directly, the misalignment would cause vibration, noise, and bearing wear. By introducing an Oldham’s coupling between them, the intermediate disk compensates for the displacement and ensures both shafts rotate smoothly and at the same speed. This makes the coupling an ideal choice for precision machines.

Conclusion

In conclusion, Oldham’s coupling is a special type of flexible coupling that effectively connects two parallel but slightly misaligned shafts. Its working is based on the double slider-crank mechanism, where the intermediate disk slides and rotates to transmit motion between the shafts. It is simple, compact, and reliable, making it suitable for many mechanical and industrial applications that require smooth torque transmission and alignment flexibility. Although it cannot handle large misalignments or high speeds, it remains one of the most efficient solutions for small to moderate shaft misalignments.