science

Constant-velocity joint

Constant-velocity joint is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Constant-velocity joint rather than just read about it. In short: A constant-velocity joint (also called a CV joint and homokinetic joint) is a mechanical connection between two rotating shafts, that keeps them rotating at the same speed, while allowing the shafts to be at an angle to each other as they rotate. This joint operates without an appreciable increase in friction or backlash and compensates for the angle between the two shafts, within a certain range of angles.

Key takeaways

Constant-velocity joint belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Constant-velocity joint to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Constant-velocity joint from memory before moving on to harder problems.

Reference excerpt

A constant-velocity joint (also called a CV joint and homokinetic joint) is a mechanical connection between two rotating shafts, that keeps them rotating at the same speed, while allowing the shafts to be at an angle to each other as they rotate. This joint operates without an appreciable increase in friction or backlash and compensates for the angle between the two shafts, within a certain range of angles. A common use of CV joints is in front-wheel drive vehicles, where they are used to transfer the engine's power to the front wheels while still allowing the wheels to steer.

History

The predecessor to the constant-velocity joint was the universal joint (also called a Cardan joint) which was invented by Gerolamo Cardano in the 16th century. A short-coming of the universal joint is that the rotational speed of the output shaft fluctuates despite the rotational speed of the input shaft being constant. This fluctuation causes unwanted vibration in the system and increases as the angle between the two shafts increases. A constant-velocity joint does not have this fluctuation in output speed and therefore does not possess this unwanted vibration. Also, although universal joints are simple to produce and can withstand large forces, universal joints often become "notchy" and difficult to rotate as the angle of operation increases. The first type of constant-velocity joint was the Double Hooke's (Double Cardan) Joint which was invented by Robert Hooke in the 17th century. This design uses two universal joints connected by a shaft and offset by 90 degrees thereby cancelling out the speed variations inherent in each individual joint. Many other types of constant-velocity joints have been invented since then.

Types

Double Cardan Joint

Double Cardan Joints are similar to Hooke's use of two universal joints except that the length of the intermediate shaft is shortened leaving only the yokes; this effectively allows the two Hooke's joints to be mounted back to back. Double Cardan Joints are typically used in steering columns, as they eliminate the need to correctly phase the universal joints at the ends of the intermediate shaft, which eases packaging of the intermediate shaft around the other components in the engine bay of the car. They are also used to replace Rzeppa style constant-velocity joints in applications where high articulation angles, or impulsive torque loads are common, such as the driveshafts and halfshafts of rugged four-wheel drive vehicles. To be truly constant-velocity, Double Cardan joints require a centering element that will maintain equal angles between the driven and driving shafts. This centering device requires additional torque to accelerate the internals of the joint and does generate some additional vibration at higher speeds.

Tracta joints

The Tracta joint works on the principle of the double tongue and groove joint. It comprises only four individual parts: the two forks (a.k.a. yokes, one driving and one driven) and the two semi-spherical sliding pieces (one called male or spigot swivel and another called female or slotted swivel) which interlock in a floating (movable) connection. Each yoke jaw engages a circular groove formed on the intermediate members. Both intermediate members are coupled together in turn by a swivel tongue and grooved joint. When the input and output shafts are inclined at some working angle to each other, the driving intermediate member accelerates and decelerates during each revolution. Since the central tongue and groove joint are a quarter of a revolution out of phase with the yoke jaws, the corresponding speed fluctuation of the driven intermediate and output jaw members exactly counteracts and neutralizes the speed variation of the input half member. Thus the output speed change is identical to that of the input drive, providing constant velocity rotation.

Rzeppa joints

A Rzeppa joint (invented by Alfred H. Rzeppa in 1926) consists of a spherical inner shell with 6 grooves in it and a similar enveloping outer shell. Each groove guides one ball. The input shaft fits in the centre of a large, steel, star-shaped "gear" that nests inside a circular cage. The cage is spherical but with ends open, and it typically has six openings around the perimeter. This cage and gear fit into a grooved cup that has a splined and threaded shaft attached to it. Six large steel balls sit inside the cup grooves and fit into the cage openings, nestled in the grooves of the star gear. The output shaft on the cup then runs through the wheel bearing and is secured by the axle nut. This joint can accommodate the large changes of angle when the front wheels are turned by the steering system; typical Rzeppa joints allow 45°–48° of articulation, while some can give 54°. At the "outboard" end of the driveshaft a slightly different unit is used. The end of the driveshaft is splined and fits into the outer "joint". It is typically held in place by a circlip.

Birfield joints The Birfield joint is a type of constant-velocity joint based on the Rzeppa joint but confines the travel of the six balls using elliptical tracks. They have improved efficiency and are widely used in modern cars for the outboard driveshaft joints. The Birfield joint was developed by Birfield Industries and came into widespread use with the development of front-wheel drive cars such as the Mini.

Tripod joints

Tripod joints are used at the inboard end of car driveshafts. The joints were developed by Michel Orain, of Glaenzer Spicer of Poissy, France. This joint has a three-pointed yoke attached to the shaft, which has barrel-shaped roller bearings on the ends. These fit into a cup with three matching grooves, attached to the differential. Since there is only significant movement in one axis, this simple arrangement works well. These also allow an axial 'plunge' movement of the shaft, so that engine rocking and other effects do not preload the bearings. A typical Tripod joint has up to 50 mm of plunge travel, and 26 degrees of angular articulation. The tripod joint does not have as much angular range as many of the other joint types, but tends to be lower in cost and more efficient. Due to this it is typically used in rear-wheel drive vehicle configurations or on the inboard side of front-wheel drive vehicles where the required range of motion is lower.

… excerpt ends here. Continue reading the full article.

Illustrations

Constant-velocity joint: A Rzeppa-type CV joint — A Rzeppa-type CV joint

Constant-velocity joint: Animation of a universal joint — Animation of a universal joint

Worked examples

Example 1 — a first encounter with Constant-velocity joint

Start with the simplest possible case. Write down what Constant-velocity joint claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Constant-velocity joint before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Constant-velocity joint ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Constant-velocity joint

In research: Constant-velocity joint appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Constant-velocity joint in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Constant-velocity joint is common in secondary-school and first-year university syllabi. It links to neighbouring topics Automotive transmission technologies, Rotating shaft couplings, so understanding it makes those chapters shorter.
In everyday life: Look for Constant-velocity joint outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Constant-velocity joint in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Constant-velocity joint means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Constant-velocity joint out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Constant-velocity joint in simple terms?

Why does Constant-velocity joint matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Constant-velocity joint?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Constant-velocity joint.