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    2. Bearing elements - Rolling bearing and plain bearing basics
You will learn:
What are rolling and plain bearings?

How do rolling and plain bearings function and what are they use for?

What types of°rolling and plain bearings are there?

What advantages and disadvantages do rolling and plain bearings have?

    

Bearing elements - Rolling bearing and plain bearing basics

Bearing elements are central components of technical systems. They enable the guidance of moving components, absorb forces and reduce friction between components moving relative to one another. Different types of bearings are used depending on the requirements - in particular rolling or plain bearings.

Bearing elements

Lagerungselemente

    

Bearings fulfil three basic tasks:
Guidance for moving elements

Absorbing forces between components moving relative to each other

Reduction of frictional forces

Rolling bearings - functioning principle

Rolling bearings enable predominantly rolling motions with very little sliding. Rolling elements (balls or rollers) that transmit the relative movement, are located between an inner and outer ring. This principle significantly reduces friction compared to pure glide motions

Applications for rolling bearings

Walzläger bei einem Rad eines Fahrrads

Rolling bearings can perform a rolling motion with little gliding involved.°A simple example is their use on bicycle wheels

   

Wälzlager

Here you can see how the rolling bearing performs the rolling motion

What does a rolling bearing consist of?

  • Rolling elements: The rolling elements are the central elements of the bearing. They are positioned in a rolling element cage. In short, this is known as a roller cage.
  • Bearing outer race: The roller cage runs in a track in an outer ring raceway.
  • Bearing inner race: The bearing inner race supports the inner part of the roller cage. This also enables the guidance, as with the outer race.
  • Roller cage: The rolling elements are contained and separated within a cage to prevent friction caused by relative movement against one another.

   

Wälzlager Aufbau

Different types of rolling bearings

Kugellager   Nadellager   Kegelrollenlager   Tonnenlager   

There are different types of rolling bearings. The best known is probably the ball bearing. In addition to ball bearings, we differentiate between cylindrical roller bearings, needle roller bearings, tapered roller bearings and barrel bearings as standard designs.

    

Rolling bearings are categorised according to:
Type of rolling elements

Arrangement of inner and outer race

Application sector

Thrust angle (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.