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    5. Éléments de roulement – Principes de base des roulements et des paliers lisses

Écrit par Martin Ahner

    

Vous apprendrez :
Que sont les roulements et les paliers lisses ?

Comment fonctionnent les roulements et les paliers lisses, et pour quoi sont-ils utilisés ?

Quels sont les différents types de roulements et de paliers lisses ?

Quels sont les avantages et les inconvénients des roulements et des paliers lisses ?

    

Éléments de roulement – Principes de base des roulements et des paliers lisses

Les éléments de roulement sont des éléments essentiels des systèmes techniques. Ils permettent le guidage de composants mobiles, absorbent les forces et réduisent le frottement entre les composants en mouvement relatif. Selon les besoins, différents types d’éléments de roulement sont utilisés, notamment des roulements et des paliers lisses.

Les éléments de roulement

Lagerungselemente

    

Les roulements remplissent trois fonctions essentielles :
Guidage d’éléments mobiles

Absorption des forces entre des éléments en mouvement relatif

Réduction des forces de frottement

Roulements – Principe de fonctionnement

Les roulements permettent principalement un mouvement de roulement avec une faible composante de glissement. Entre la bague intérieure et la bague extérieure se trouvent des éléments roulants (par ex. des billes ou des rouleaux) qui transmettent le mouvement relatif. Ce principe réduit considérablement les frottements par rapport aux mouvements de glissement purs

Exemple d’application des roulements

Walzläger bei einem Rad eines Fahrrads

Les roulements peuvent effectuer un mouvement de roulement avec une faible composante de glissement. Un exemple simple est leur utilisation sur une roue de vélo

   

Wälzlager

Vous pouvez voir ici comment le roulement effectue le mouvement de roulement

De quoi se compose un roulement ?

  • Éléments roulants : les éléments roulants constituent l’élément central du roulement. Ils sont logés dans une cage. On appelle l’ensemble une couronne d’éléments roulants.
  • Bague extérieure de roulement : la couronne d’éléments roulants est guidée dans sa trajectoire par une bague extérieure de roulement.
  • Bague intérieure de roulement : la bague intérieure de roulement délimite la couronne d’éléments roulants depuis l’intérieur. Tout comme la bague extérieure de roulement, elle permet le guidage.
  • Cage d’éléments roulants : les éléments roulants sont regroupés dans une cage afin d’éviter les frottements dus aux mouvements relatifs.

   

Wälzlager Aufbau

Les différents types de roulements

Kugellager   Nadellager   Kegelrollenlager   Tonnenlager   

Il existe différents types de roulements. Le plus connu est sans doute le roulement à billes. Outre les roulements à billes, on peut également citer les roulements à rouleaux cylindriques, les roulements à aiguilles, les roulements à rouleaux coniques et les roulements à rouleaux sphériques parmi les modèles standard.

    

La classification des roulements s’effectue selon :
le type d’éléments roulants

la disposition des bagues intérieure et extérieure

le domaine d’application

l’angle de contact (α)

° ° ° ° ° ° °°

Another influencing factor for differentiating between various rolling bearings is the thrust angle. We can also distinguish between axial bearings and radial bearings based on the thrust angle (alpha).

                  Radiallage                                         Axiallager                                            

°(Radial bearing)thrust angle°α: 0 - 45°(Axial bearing)             thrust angle°α: 45 - 90°

Druckwinkel 

    

Bearing-specific code consisting of letters and numbers

A standard helps to identify the correct rolling bearing - DIN 623. Codes for naming the bearings are standardised here. The codes are composed of the prefix, the basic character and the suffix.

Bearing-specific code°for tapered roller bearings:

°Prefix: Which individual parts of complete bearings (rings, cages) were used?

°Basic symbol: Which bearing series and bore code are involved?

°Suffix: What other information is known (seals, cage design, tolerances, etc.)?

  

The prefix provides information about the single parts and materials used. By tapered roller bearings the HR stands for a reinforced version (increased permissible load).

The basic symbol 313 08 indicates that it is a tapered roller bearing in width series 1 and diameter series 3 and has a bore code 08.

In this example the suffix describes that the tapered roller bearing has a thrust angle and raceway diameter according to DIN ISO 355.

Advantages and disadvantages of rolling bearings

Rolling bearings are frequently used standard components. This is why they're also cost-effective, widely available and can be easily installed and replaced. Despite requiring little lubrication and are easy to lubricate, they have a long service life. Additionally, they have no minimum speed and combined loading is possible.°

Although rolling bearings have no minimum speed, the maximum speed is limited to prevent overheating. Furthermore, it should be taken into account that rolling bearings may cause vibrations in the system. They are also highly susceptible to shocks and very large or very small sizing are only achieved with difficulty.

Plain bearings - functioning principle and applications

Plain bearings also absorb forces and guide the motion of moving bodies. This motion is a sliding motion where the supported element rotates in the plain bearing.

Gleitlager bei einem 3D-Drucker

As an example, here you can see plain bearings being used in a 3D printer. Plain bearings are also used in other guides and joints and are often used on crankshafts or camshafts in combustion engines.

  

Further applications can be found by:
Guides and joints

Internal combustion engines (crankshafts and camshafts)

Pumps and generators

Rolling mills and presses

Gas turbines

° °

An intermediate medium is used between the bearing and the shaft to enable the body in motion to slide smoothly. Do you know what can be used here? 

  • Gas is used as an intermediate medium to separate sliding surfaces where the application is subjected to extremely low temperatures.
  • When hydrodynamic plain bearings are required to release oil during rotation to enable lubrication, they're saturated with oil.
  • Thick (greases) and solid lubricants are used when e.g. a vacuum, high temperatures or centrifugal forces make the use of oil lubrications impossible. Graphite, molybdenum disulphide or a PFTE (Teflon) is often used.
  • Magnetic fields are used where wear and dirt are not permitted (e.g. clean rooms).

Designs and materials for plain bearings

The design of plain bearings are distinguish by split and one-piece bearings. They can also be thick-walled or thin-walled.

One-piece bearings: Only one bearing bush is used

Split bearings:°Two or more bearing shells are used

Thick-walled:°Thick-walled plain bearings retain their shape and enable functional fulfillment even with no or a light interference fit

Thin-walled:°Thin-walled plain bearings rely on sufficient pressure between the housing and bearing

Note: As with rolling bearings, we differentiate between radial and axial bearings

Dry bearings

Dry bearings can be made from various materials. They are called a solid bearing if just one material type is used, if two or more bonded materials are used it is called a composite bearing. In addition to plastic, aluminium alloys or bronze, they can also be made from e.g. white metal alloys, ceramic or synthetic carbon. When selecting the material, certain requirements should be taken into account.

   

Plain bearing materials
Plastic

Aluminium alloys

Bronze

White metals

Ceramic

Synthetic carbon

Naturally, the material should posses good glide qualities and favourable cold-start characteristics. It should also have high strength and not wear quickly.

   

Trockenlager

Hydrodynamic and hydrostatic plain bearings

There are also hydrodynamic and hydrostatic plain bearings. By these, a viscous lubricant film is added, thereby completely separating the shaft from the plain bearing surface. This type is recommended by frequent startup processes, as the lubrication film prevents rubbing.

hydrostatische Gleitlager

Note: In hydrodynamic plain bearings, the lubricating film is not generated by the rotation of the shaft, but is supplied separately by an activated pump.

Advantages and disadvantages of plain bearings

As with rolling bearings, plain bearings are very simple in design and installation. They require only a little space and are very unsusceptible to knocks.

Advantages:

  • Simple design
  • Space-saving
  • Impact resistant
  • Low noise
  • Suitable for very large constructions

Disadvantages:

  • Higher friction coefficient by insufficient lubrication
  • Possible start-up wear
  • Somewhat higher lubrication and maintenance requirements

Summary

Rolling and plain bearings are used to guide moving parts. The type of motion depends on their composition. Rolling bearings guide rotating movements. In plain bearings, the shaft rotates freely around its axis.

To design bearings correctly, one should be aware of the load during operation and see to sufficient lubrication as well as cleanliness. Rolling and plain bearings differ in advantages and disadvantages. Their common features are that they are inexpensive and have a simple design, making them easy to install.

   

You have learnt:
Rolling and plain bearings are used to guide moving parts.

Rolling bearings guide rotating motions. In plain bearings, the shaft rotates around its axis.

To design the bearings correctly, one should be aware of the load during operation and see to sufficient lubrication as well as cleanliness.

Rolling and plain bearings are cost-effective and have simple designs. As a result, they're easy to install.