What Is Universal Joint?- Function, Types, And Uses

What is a Universal Joint?

A universal joint, often referred to as a U-joint, Cardan joint, or Hooke’s joint, is a mechanical connection that links two objects most commonly shafts so they can rotate relative to each other along two different axes.

What’s interesting about this design is that it brings together two hinges (or revolute joints), each set at right angles to the other, and links them with a central cross-shaped shaft.

When engineers need to transfer rotary motion between shafts that aren’t perfectly aligned, the universal joint is a classic solution. You’ll see it in action in everything from car drive shafts to various industrial machinery.

Each shaft connects to one of the revolute joints, and the axes of these joints are perpendicular to the direction of shaft rotation. This clever arrangement lets motion move smoothly from one shaft to another even when there’s some misalignment while still restricting motion to just two rotational degrees of freedom. In other words, the joint allows the shafts to “wiggle” in two directions but keeps their ends fixed together.

One important thing to remember about universal joints is that they’re not constant-velocity joints. Even if you spin the input shaft at a steady speed, the output shaft’s speed will fluctuate slightly.

The average speed stays the same, but moment-to-moment, the output might run a bit faster or slower than the input. How much it fluctuates depends on the angle between the shafts the more misaligned they are, the more pronounced the speed changes. If the shafts are perfectly lined up, there’s no fluctuation and the joint acts as if it’s transmitting constant velocity.

To address this limitation, engineers sometimes use a double Cardan joint. This setup uses two universal joints connected by a short shaft, and the joints are set 90 degrees out of phase with each other.

When the angle is split evenly between the two joints, the fluctuations in velocity from one joint cancel out those from the other, and the result is a much steadier output. However, the shaft in the middle can experience some vibration due to these oscillations, so additional supports are usually needed to keep everything running smoothly and at equal angles.

Related Post:

• Cotter Joint: Types, Assembly & Application
• Knuckle Joint: Definition, Assembly, and Application

Purpose of Universal Joint

Universal joints, often referred to as U-joints, play a crucial role in how drive shafts function, especially as vehicles encounter bumps or dips in the road. Essentially, these joints let the drive shaft flex up and down with the suspension, ensuring that power keeps flowing even when the drive shaft isn’t perfectly straight between the transmission and the drive wheels.

If you’re looking at rear-wheel-drive cars, you’ll typically find U-joints at both ends of the drive shaft. These joints attach to yokes, which is a fancy way of saying they can slide a bit to accommodate changes in length as the vehicle moves. So, as the car goes over uneven ground, the drive shaft can get a little longer or shorter without causing any problems.

Front-wheel-drive vehicles use something similar, but with a twist. Instead of traditional U-joints, they rely on constant velocity (CV) joints. CV joints do everything U-joints do, but they’re also designed to handle steering, since the front wheels turn left and right. It’s pretty clever engineering.

Now, when these joints start to wear out, they usually let you know. In rear-wheel-drive setups, a bad U-joint often announces itself with a distinct “clank” noise whenever you put the car in gear. For front-wheel-drive cars, worn CV joints tend to make a clicking sound, especially when you’re turning.

CV joints also have rubber boots around them to keep grease in and dirt out. If these boots crack or get damaged, it’s only a matter of time before dirt and moisture work their way in, robbing the joint of lubrication and causing damage.

Structure of Universal joint

A universal joint is made up of a spider, sometimes called a cross, along with four roller bearings packed with grease. This setup allows it to handle high levels of torque while keeping friction to a minimum.

The spider itself features journal pegs, and its bearing cases act as the outer rings; both are typically produced through hot forging or cold forming before being machined to precise dimensions.

After shaping, these components go through carburizing in a furnace a process that toughens the surface and boosts wear resistance, making the joint more durable.

Once the main parts are formed and treated, they’re ground down to their exact final size, with very tight tolerances to ensure a perfect fit. To keep the inner journal area clean and running smoothly, oil seals and dust covers are put in place to block out dirt and other contaminants.

It’s also worth noting that manufacturers choose from a range of material grades, tailoring their choices to make sure the joint can perform reliably even in harsh environments whether it’s exposed to extreme heat or freezing cold.

Types of Universal Joint

Universal joints generally come in two main types, distinguished by how many pivoting points they have. First, you have the single joint—this design has just one bending point, which allows it to operate effectively at angles up to 45 degrees.

Then there’s the double joint (sometimes called a double u-joint), which, thanks to its two bending sections, can manage much steeper angles—up to 90 degrees.

One extra advantage of the double joint is its ability to deal with parallel shaft misalignment; the central part can flex between 0 and 45 degrees, making it a versatile option for more complex setups.

Another thing to consider with universal joints is the variation in materials, hub styles, and their intended applications. Steel is the go-to material for most universal joints, available both as stainless steel and in alloyed forms. The reason for this popularity? Steel can handle high torque and elevated temperatures, which is crucial in demanding environments.

On the other hand, plastic and thermoplastic universal joints are also quite common, especially in situations where you need resistance to rust and corrosion, or when electrical and magnetic insulation is necessary. These properties make them a practical choice in industries where those particular conditions are a concern.

Hub Styles

A solid hub universal joint is exactly what it sounds like it’s a single, unmachined piece without any hole through the hub. There’s no internal opening; the entire hub is left solid during manufacturing.

On the other hand, bored U-joints are defined by the shape of the opening that’s been machined into the hub. You’ll often see these referred to by the type of hole they have, such as round, hexagonal, or square. The terminology is pretty straightforward if the bore is round, it’s a round-bored U-joint; if it’s hex, it’s a hex-bored one, and so on.

However, there are two styles of bored U-joints that don’t follow the usual round, hex, or square convention:

• Splined: In this style, the bore isn’t smooth but instead has a series of grooves running lengthwise along the inside. These grooves called splines help the joint grip onto a matching splined shaft.
• Keyed: This type features keyways, which are slots designed to fit a key that keeps the U-joint from spinning on the shaft. Basically, the keyway ensures the joint stays locked in place when it’s mounted.

So, whether you need a completely solid hub or a bored one (and which bored style), it really depends on your specific application and the type of shaft connection you’re dealing with.

Application of Universal joint

These days, universal joints have become a staple component in all sorts of machines basically, anywhere you’ve got long rotating shafts, you’re likely to find them. A classic example is in rear-wheel-drive cars, where the universal joint sits between the transmission and the rear driveshaft, quietly doing its job. You’ll even find them in old tower clocks, helping keep those giant hands turning.

Universal joints aren’t limited to cars and clocks, though. They’re used across a surprising range of fields: from aircraft and home appliances to control systems, electronics, instrumentation, medical devices, and even optical equipment.

You’ll spot them in ordnance, radios, sewing machines, textile machinery, and tool drives essentially, anywhere there’s a need to transfer rotational motion between misaligned shafts.

When it comes to construction, universal joints typically come in two main materials: steel and thermoplastic. Steel joints are your go-to for heavy-duty applications since they handle the most load for their size.

On the other hand, thermoplastic versions are perfect for lighter industrial uses. What makes these stand out is their self-lubricating property, light weight, minimal backlash, resistance to corrosion, and their ability to operate smoothly at high speeds.

There are also universal joints designed for special purposes. Take ball-jointed universals, for example they’re built for high-speed operation and can handle hefty torque loads. Whether you need something tiny for a precision device or a standard size for a larger machine, there’s usually a universal joint out there to fit the bill.

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.

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