Only a few components are as important in industrial machine manufacturing as the rolling bearing. This article gives an overview of how these bearings are designed and what you should know when selecting them.
Rolling bearings - also referred to as ball bearings - are bearings that facilitate the movement of components due to a rolling motion and reduced friction. Typical rolling elements are balls and cylinders (small types are also called needles). They are guided in cages.
Design of the rolling bearing
The design of a rolling bearing is generally relatively simple. First there are the rolling elements, such as balls, ball rollers, needle rollers or tapered rollers. In addition, a rolling bearing has an inner and an outer ring with the rolling element raceways, the cage (a holder with pockets for the rolling elements), seals (they protect against contamination and lubricant leakage) and the lubricant. It is important to know that the shape of the rolling elements essentially determines the properties as well as the name of the bearing (e.g. "ball bearing" or "needle bearing").
In modern rolling bearings, the rolling elements are usually guided in a cage. This cage keeps them at an even distance from each other and prevents them from touching each other. It also prevents the rolling elements from falling out of separable bearings.
The materials used for rolling bearing
Bearing rings and rolling elements are mostly made of completely hardened chrome steel, but case-hardened steel is also used. Materials such as corrosion-resistant special steels, ceramics or plastics, on the other hand, are frequently used in special bearings and extreme operating conditions.
For special operating conditions, rolling bearings are available in the following designs: Made of stainless steel; hybrid bearings (two materials), where the bearing rings are made of steel, the rolling elements are made of ceramic; ceramic bearings, where both the bearing rings and the rolling elements are made of silicon nitride, zirconium oxide or silicon carbide; plastic bearings with rolling elements made of glass or ceramic against aggressive acids or alkalis in the chemical and food industries, and bearings with plastic cages for low-noise running.
Today, steel is used as the standard material for sheet metal cages. Solid cages are available in brass, steel, hard fabric and other materials. Cages made of glass fiber reinforced polyamide offer an extremely low weight as well as very good sliding and fail-safe properties. They are offered, for example, as so-called window cages in single row angular contact ball bearings or in needle roller bearings.
Different designs of rolling bearings
Each type of rolling bearing has characteristic properties that make it particularly suitable for certain applications. Depending on the type of rolling element, a distinction is also made between different types of rolling bearings. The shapes of the rolling elements range from balls, ball rollers to barrel rollers, needles, tapered rollers and cylindrical rollers. A brief summary:
- The deep-groove ball bearing (DIN 625) absorbs radial forces.
- The angular contact ball bearing is suitable for radial and axial forces in one direction. Bearings of this type are used in bicycle construction, but also in automotive engineering.
- The self-aligning ball bearing (DIN 630) can support both axial and radial loads.
- The cylindrical roller bearing (DIN 5412) has a large radial load carrying capacity, however it cannot be loaded in the axial direction or can only handle very low load.
- The tapered roller bearing (DIN 720, ISO 355) can withstand very high radial and axial loads.
- The needle bearing (DIN 617) has needles as rolling elements and can only be loaded radially and is suitable for small installation spaces.
- The self-aligning ball bearing (DIN 630) can be loaded both radially and axially.
For special applications, special bearings are tailored precisely to the customer's requirements. An example is a bearing combination of roller and ball bearings for particularly high loads.
Selection of rolling bearings
Rolling bearings are generally standardized components - and yet they are functionally critical machine elements when installed. However, generally applicable rules for selecting the right rolling bearing can only be established to a limited extent. The designer should consider several factors. The work of the developer can be facilitated by the fact that very often at least one of the main dimensions of the rolling bearing - usually the bore diameter - is determined by the surrounding construction. Nowadays, computer-aided programs that guide the designer safely on his way to the optimal bearing selection also make it easier for the designer to make the right choice.
When searching for the right rolling bearing for their application, designers should consider the following factors:
- Volume and type of load
- Working life/service life
- Installation situation (available installation space) and type of lubrication of the bearing
- Operating parameters of the bearing (speed, heat balance/operating temperature)
- Ambient conditions (dirt, vibrations, ...)
- Accuracy requirements
- Requirements for assembly and disassembly
Frequently, it is also the case that the rolling bearing does not support a one-sided load but a combined load. This is why axial angular contact ball bearings, spherical roller bearings, deep groove ball bearings, tapered roller bearings and cylindrical roller bearings are used.
However, installation space and load direction alone are not sufficient to select a rolling bearing for a specific application. The required service life and operating conditions raise many questions which either confirm or negate the bearing selection.
In addition the following questions arise, which need to be answered: How long does the bearing work? Can it handle the required speeds? What lubrication does the rolling bearing require?
The lubrication of the rolling bearing
Optimum lubrication and maintenance are essential for rolling bearings to work reliably. It is intended to prevent or reduce machine damage caused by bearing failures and to increase the service life of rolling bearings. The selection of a suitable lubricant and its lubrication method is therefore essential for every bearing application. Knowing the speed of the rolling bearing and/or the operating temperature is of great importance.
This article was first published by MM MaschinenMarkt.