The propeller shaft, positioned adjacent to the gearbox in the transmission system, establishes a connection between the gearbox and the differential. At the propeller shaft's end is a universal joint that facilitates torque transmission from the gearbox to the differential. 

In this article, you will find all the details regarding this joint. 

A universal joint, also known as a U joint, universal coupling, Hardy Spicer joint, Cardan joint, or Hooke's joint, is a mechanism that connects two objects, typically shafts, allowing them to rotate relative to each other along two axes. 

It comprises two revolute joints with axes that are perpendicular and intersecting. The joint consists of a pair of closely positioned hinges, oriented at a 90° angle to each other and connected by a cross shaft. 

It's important to note that the universal joint does not maintain constant velocity throughout its rotation.

20MnVB and 20CrMnTi alloy steel are the predominant materials for producing universal joints. Here are the reasons why: 

• These materials exhibit exceptional physical characteristics, including wear resistance, high strength, flexibility, and corrosion resistance. 

• Additionally, they are cost-effective and readily accessible in the market. 

• Moreover, these materials can be machined easily and precisely, achieving the necessary close tolerances demanded by universal joints in diverse applications.

The universal joint comprises a cross-shaped spider and four roller bearings filled with grease. It is capable of transmitting substantial torque with minimal friction. 

The spider, equipped with journal pegs, and the outer rings of the bearing cases undergo either hot forging or cold forming, followed by machining. Subsequently, they are carburised in a furnace to enhance ruggedness and wear resistance.

The components are then meticulously ground to a specific size within strict tolerances. Manufacturers use oil seals and a dust cover to safeguard the internal journal area from foreign particles. 

Various grades and materials are selected to accommodate extreme conditions, including extremely high or low temperatures.

The following are the functions of a universal joint in automobiles: 

• It establishes a connection between two shafts that have intersecting axes. 

• It enables one shaft to rotate around its own axis while being driven by another shaft that rotates around its separate axis. 

• Universal joints facilitate the transmission of power at varying angles and during continuous changes in those angles.

There are multiple variations of universal joints. The variations are based on their applications and designs. Check out some of the most common universal joint types below:

1. Single Cardan Joint

The single cross-shaped universal joint is the prevailing type, connecting the two yokes. This design enables the joint to adjust to alterations in the relative positioning of the two shafts, facilitating the transmission of torque and enabling rotational movement.

2. Double Cardan Joint

In this type of universal joint, two cross-shaped connecting pieces are arranged in series to interconnect the two yokes. This configuration enables the joint to effectively accommodate greater changes in the relative orientation of the two shafts. It is commonly used in applications where the shafts experience more pronounced or extreme movements.

3. Cross and Bearing Joint

This particular type of universal joint utilises a cross-shaped connecting piece and a series of bearings to connect the two yokes. This design enables the joint to adapt to changes in orientation while effectively transmitting torque and facilitating the rotation of the shafts.

4. Constant Velocity Joint

The design of this specific universal joint aims to ensure a consistent rotational velocity between the two shafts, regardless of any variations in their relative orientation. This is accomplished by utilising a specialised arrangement of small balls or rollers. This arrangement enables the joint to accommodate changes in orientation while preserving a constant velocity.

5. Hooke’s Joint

This kind of universal joint employs a sequence of curved arms or levers to connect the two yokes. This design enables the joint to adapt to changes in orientation while effectively transmitting torque and facilitating the rotation of the shafts.

Universal joints function in diverse applications due to their capability to accommodate changes in the relative orientation of coupled shafts while transmitting torque and enabling rotational movement. Some common applications of universal joints include the following:

• Drive shafts in vehicles like cars, trucks, and tractors, where the shaft needs to adapt to changes in orientation while traversing uneven terrain.

• Power transmission systems such as those used in wind turbines, where the shafts experience significant and fluctuating forces, necessitate flexible and versatile coupling.

• Industrial machinery, such as conveyor belts, mixers, and other equipment, may require shafts that can move and rotate in different directions.

• Robotics, where universal joints are frequently utilised to connect different parts of a robot's body, granting it freedom of movement and rotation.

• Other mechanical systems demand flexible and adaptable coupling, such as cranes, lifting equipment, marine propulsion systems, and various other applications.

Here are the advantages of universal joints: 

• Flexibility: Universal joints can handle misalignments and angular variations, allowing for smoother operation and reduced stress on the system.

• Torque Transmission: Universal joints efficiently transmit torque between shafts, even when not in perfect alignment. This capability enables power transfer in systems where shafts may be positioned at different angles or have varying rotational speeds.

• Compact Design: Universal joints are relatively compact, making them suitable for applications with limited space. Their compact size allows for easy integration into mechanical systems without significant modifications.

• Reduced Wear and Tear: Universal joints can help dampen vibrations and shocks transmitted between shafts, thereby reducing wear and tear on the system and improving overall durability.

• Versatility: Universal joints are versatile and can be used in various applications across different industries. They can accommodate various shaft orientations and rotational movements, making them suitable for diverse mechanical systems.

• Cost-Effectiveness: Universal joints are often cost-effective, offering an efficient and affordable solution for transmitting torque and accommodating misalignments in mechanical systems.

Now that you know more about universal joints, you should check out their disadvantages for complete understanding: 

• Limited Angular Misalignment: Universal joints are designed to accommodate some angular misalignment between the input and output shafts. However, excessive misalignment can lead to accelerated wear and vibration. 

• Limited Torque Capacity: Universal joints have a limited torque capacity compared to other types of shaft couplings. U-joints can experience stress and wear when subjected to high torque loads, leading to premature failure.

• Maintenance Requirements: Universal joints must undergo maintenance periodically to deliver optimum performance and longevity. Lubrication is essential to reduce friction and wear, and regular inspection is necessary to identify any signs of damage or wear that could lead to failure.

• Noise and Vibration: U-joint design can contribute to noise and vibration transmission. As the joints rotate, they generate noise and introduce vibration into the system.

Overall, the advantages of universal joints make them a valuable component in many applications, contributing to smooth operation, durability, and efficient power transmission.