How do ball and roller bearings work? Types and durability calculation. DIN ISO 281

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Bearings are used to reduce the friction between two objects that are in linear or circular motion against each other.

The first intuition came from Leonardo Da Vinci in his studies regarding friction.
But the first patent was filed only in 1794 by Philip Vaughan of Carmarthen.
Then, a Parisian bicycle mechanic named Jules Suriray, was able to patent the first ball bearing on August 3rd 1869. This invention allows him to win the first cycling race in history: the Paris-Rouen race.

Now let’s take a closer look at a classical ball bearing. By decomposing and sectioning it, its components can be analysed.
The first elements that immediately get our attention are the steel balls.
They represent the rolling elements, and they’re held together by a cage which can come in different shapes and it’s made of low friction materials.
Then, we have the inner ring (usually the one that moves) and the outer ring, both with a groove, which can be more or less deep, in order to make the rotary elements glide.
It is very common for the bearing to also be shielded. The shield can be made of metal for dust protection, or it can be made of plastic, preventing liquid infiltrations. They are placed in specific locations between the two rings, so that they can protect the balls and the cage from external agents. At the same time they can retain oil or grease inside the bearing, keeping it lubricated for its entire life cycle.
In order to work correctly, bearings need a certain radial and axial clearance, so that when they reach the working temperature, their thermal expansion won’t lead to overtightening (which would cause the bearing’s wear), or in the worst case, even a seizure!

Ball bearings are among the most used bearings. The radial ones can be classified as single-row bearings (the most common type) or double-row bearings (which can withstand high radial loads).
Among the radial ball bearings we also find the self-aligning bearings, that have two rings of balls. This type has an inner ring with two grooves placed side by side, while the outer ring has only one hemispherical groove. This system allows the inclination of the two rings’ axis during the functioning.
There are also angular contact ball bearings, that have either one or two rings of spheres. They typically have an oblique position (respective to the axis of rotation) of the load line passing through the contact points between the balls and the rings. They can withstand higher radial loads and axial forces.
Finally, it should be noted that all these varieties can also have an axial configuration, in order to withstand a mostly axial load. They are also called thrust bearings.

The other major type of rotary bearings are the so called roller bearings, used when very high accuracy is required. They have the same categories: radial, angular contact and axial bearings. But roller bearings have different shapes depending on the type: this is another difference between these and ball bearings.

Radial bearings with cylindrical rollers are the most common type. If they have small rollers, they’re called needle roller bearings, and thanks to their compactness they are used when space is limited. They can only withstand radial loads.
There is another variety: spherical roller bearings, which have cask-shaped rollers, in order to allow inclining the rotation axis of the two rings.
Tapered roller bearings are an oblique version and are used in case of combined load. This type of bearings are generally used in pairs and they usually have inverse taper.
All these variations can also have an axial configuration, in order to mainly withstand an axial load. They are also called thrust bearings.

Bearings are one of the most used mechanical parts and there are many varieties of them. So it is important to consider the area of use when choosing the right bearing.
The reference standard for bearings is DIN ISO 281, which provides a formula for the calculation of bearings’ durability: given the dynamic load coefficient of the bearing “C”, the equivalent dynamic load “P” and a specific coefficient, which is 3 for ball bearings, and ten thirds for roller bearings. The “n” allows us to obtain the durability of the bearing expressed as millions of turns.
The equivalent load “P” is calculated on the basis of the radial and the axial load which act on the bearing. The coefficient of dynamic load is instead determined by the fatigue behaviour of a significant lot of equal bearings.

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