What is a Universal Joint?
A universal joint is a connection between two objects, typically shafts, that allows relative rotation in two axes. It is made up of two revolute joints with perpendicular and intersecting axes.
A universal joint (universal coupling, U-joint, Cardan joint, Spicer or Hardy Spicer joint, or Hooke’s joint) is a joint or coupling connecting rigid rods whose axes are inclined to each other and is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges located close together, oriented at 90° to each other, and connected by a cross shaft. The universal joint is not a constant-velocity joint.
When shafts are connected using a universal joint, each shaft terminates in a revolute joint with its axis perpendicular to the shaft’s rotational axis. This allows rotary motion to be transferred between the shafts while allowing misalignment in both remaining rotational degrees of freedom.
A single rotational degree of freedom is constrained (the shaft rotation) as well as all relative translations, giving a universal joint two degrees of freedom (2-DOF).
The universal joint is not a constant-velocity joint. If the input shaft is rotating at a constant velocity, the output shaft’s velocity will oscillate. They will have the same average velocity but the output shaft’s velocity will be somewhat higher or lower than this average at any given time.
The amount of oscillation in the output shaft depends on the amount of misalignment between the shafts, if the shafts are coaxial then the output shaft will in fact have a constant velocity.
It is possible to create a constant velocity joint by combining a number of universal joints. A double Cardan joint is an arrangement of two universal joints, with a short connecting shaft between them, 90° out of phase with each other.
If any bend angle is shared equally between the two universal joints, then the two joints will cancel out velocity oscillations so that the final output shaft has a constant velocity. However, the oscillation of the intermediate shaft will cause vibrations and supports are required to maintain equal angles.
Purpose of Universal Joint
Universal joints allow drive shafts to move up and down with the suspension while the shaft is moving so power can be transmitted when the drive shaft isn’t in a straight line between the transmission and drive wheels.
MORE: What is Transmission in a Car?
Rear-wheel-drive vehicles have universal joints (or U-joints) at both ends of the drive shaft. U-joints, connect to yokes that also allow drive shafts to move fore and aft as vehicles go over bumps or dips in the road, which effectively shortens or lengthens the shaft.
Front-drive vehicles also use two joints, called constant velocity (or CV) joints, but they are a different kind that also compensates for steering changes. On rear-drive vehicles, one sign of a worn U-join is a “clank” sound when a drive gear is engaged.
On front-drive vehicles, CV joints often make a clicking noise when they’re worn. CV joints are covered by protective rubber boots, and if the boots crack or are otherwise damaged, the CV joints will lose their lubrication and be damaged by dirt and moisture.
Structure of Universal joint
The universal joint consists of a spider(cross) and 4 grease-filled roller bearings. It can transmit large torque at low friction. Spider with journal pegs and bearing cases as outer rings were hot forged or cold formed and machined. They are later carburized in the furnace so that increased ruggedness and wear resistance is obtained.
Parts are finally ground to designed size under severe tolerance. Oil seals, as well as a dust cover, are also in use to protect the internal journal area from the intrusion of foreign material. A variety of grades and materials are selected to respond to extreme conditions such as very hot / very cold temperatures.
Types of Universal Joint
There are two types of universal joints, defined by their number of bending joints:
- Single joint: has only one bending aspect and is capable of operating at up to a 45-degree angle.
- Double joint: utilizes two bending joints, the double u-joint can operate at angles up to 90 degrees. Additionally, it also accommodate parallel offset between 2 shafts with an operating angle of the central section from 0 to 45 degrees.
Universal joints vary based on their material composition, hub type, and the applications for which they are designed.
Steel is the most common material used, either in stainless form; or alloyed with other metals to handle greater torque and temperature.
Plastics and thermoplastics are often used in constructing universal joints, as this lends greater rust and corrosion resistance, as well as electrical and magnetic insulation in applications where this is required.
Hub Styles
U-joints are available with two hub styles:
- Solid: solid hub universal joints are solid and have not been machined, and as a result, do not have a hole.
- Bored: bored styles of u-joints generally derive their name from the shape of the hole in their hub, as with round, hex or square styles.
The two bored styles that deviate from the convention of round, hex, or square styles are:
- Splined: has longitudinal grooves inside of the bore.
- Keyed: has keyways to prevent rotation of the u-joint on the corresponding shaft.
Application of Universal join
Today, universal joints are used universally in many machines in which there are long rotating shafts. Most notably, such joints are found between the transmission and rear driveshafts of rear-wheel-drive cars and in old-fashioned tower clocks.
Typical applications of universal joints include aircraft, appliances, control mechanisms, electronics, Instrumentation, medical and optical devices, ordnance, radio, sewing machines, textile machinery, and tool drives.
Universal joints are available in steel or in thermoplastic body members. Universal joints made of steel have the maximum load-carrying capacity for a given size. Universal joints with thermoplastic body members are used in light industrial applications in which their self-lubricating feature, lightweight, negligible backlash, corrosion resistance, and capability for high-speed operation are significant advantages.
Universal joints of special construction, such as ball-jointed universals are also available. These are used for high-speed operation and for carrying large torques. They are available both in miniature and standard sizes.
Advantages of Universal joint
- Universal coupling is more flexible than knuckle joint.
- It facilitates torque transmission between shafts which have angular misalignment.
- It is cheap and cost-effective.
- It is simple to be assembled and dismantled.
- Torque transmission efficiency is high.
- The joint permits angular displacements.
Disadvantages of Universal Joint
- Wear may occur if the joint is not properly lubricated.
- Maintenance is often necessary to avoid wear.
- Universal joint produces fluctuating motion
- It does not support axial misalignment.
