Storage concepts
The bearing of shafts is one of the standard design tasks in mechanical engineering. Bearings, generally consisting of at least two bearings or guides, limit the degrees of freedom of two elements that are moved relative to one another. When shafts are supported, they can rotate around their own longitudinal axis while maintaining a defined position in space. In order to meet this requirement, there are a number of common storage arrangements or storage concepts. The concepts presented below are mainly implemented in practice with rolling bearings .
conditions
Basically, the following requirements are placed on all shaft bearing concepts:
- Guiding the wave or restricting it to desired degrees of freedom
- Introducing the forces generated by the operating load into the surrounding structure
- Enables length compensation of the shaft due to thermal / mechanical expansion
- Minimizing friction
Bearing arrangement
The bearing arrangement must meet the principle of uniqueness. This means that cause and effect must be clearly related. Furthermore, the bearing must ensure the defined management of the power flow. If a bearing fulfills these basic principles, it is considered statically determined.
Fixed-lot storage (FLL)
In the case of fixed-loose bearings , the axial forces are absorbed in both directions by a bearing, the so-called fixed bearing.
The function of the fixed bearing is to clearly position the shaft in the axial direction. The fixed bearing must be able to absorb both radial forces and axial forces and direct them into the surrounding structure. In principle, this function can be fulfilled by a single warehouse. In the case of very high axial or radial loads, two bearings are often used to perform the locating function. The function of absorbing axial and radial forces is thus divided between the axial bearing and the radial bearing, i.e. two bearings.
If you want to implement the principle of separation of functions , it must be ensured in the design that the respective bearing can and may only absorb axial or radial loads.
If the fixed bearing function is to be implemented by a single bearing, it must be able to absorb axial and radial forces. This is achieved by non-divisible bearings. Neither the outer ring in the housing nor the inner ring on the shaft can be displaced, which ensures that the shaft is fixed in the longitudinal direction.
There are several options for realizing the floating bearing function. A non-divisible bearing is often chosen. The inner ring of the bearing is usually fixed on the shaft and the floating bearing function is ensured by the sliding outer ring inside the housing. Another option is to use a divisible bearing, e.g. B. a cylindrical roller bearing, the inner ring and outer ring of the bearing must be fixed here. The necessary displaceability to fulfill the floating bearing function now takes place within the bearing.
- Advantages of fixed-lot storage
- no axial play
- Expansion of the shaft (e.g. due to temperature) is not critical
- suitable for changing axial loads
- Uniqueness of the forces acting
- disadvantage
- increased constructive effort
- relative expensive
Support bearing (SL)
In support bearings (also known as support-support bearings ), the axial alignment of the shaft is taken over by one or the other bearing, depending on the direction of the force introduced.
Floating support bearing (SLS)
The floating support bearing (SLS) has a few tenths of a millimeter clearance in the axial direction, i.e. the shaft is not clearly fixed in the axial direction. This allows mechanical or thermal changes in length to be absorbed without the bearing becoming warped. Depending on the axial load direction, one of the two bearings takes on the fixed or floating bearing function. The outer and inner ring are only axially fixed on one side (opposite), which is why no divisible bearings (non-self-retaining bearings, floating bearings, e.g. needle bearings, tapered roller bearings) can be used for this type of bearing . The floating support bearing can only be used if the axial play - which this bearing concept entails and which is also absolutely necessary to fulfill the function, so that no tension occurs - has no influence on the function to be fulfilled.
- Advantages of floating support bearings
- inexpensive
- low construction effort
- Expansion of the shaft (e.g. due to temperature) in the area of play is not critical
- disadvantage
- not clearly fixed in the axial direction
- not suitable for changing axial loads
- only if axial play does not affect the function
Support bearing employed (SLA), mutual guidance
The setting up of a bearing means a defined tensioning of the two bearings - usually two mirror-inverted angular contact ball bearings or tapered roller bearings - against each other. It is named after the position of the pressure lines (lines of force flow) through the roller bearings - when the center of pressure is between the bearing points, one speaks of an X-arrangement, outside the bearing points of an O-arrangement. In an O-arrangement, the bearing can absorb a greater tilting moment than in an X-arrangement, since in an O-arrangement the distance between the pressure centers is greater. If the shaft heats up more than the housing during operation, the bearing preload increases with an X arrangement and decreases with an O arrangement. In general, it is useful to use the X-arrangement when there is a predominant force application between the bearing points, and the O-arrangement when force is applied outside the bearing points.
Individual evidence
- ↑ a b Roloff, Matek: machine elements. Standardization, calculation, design. 24th edition. Springer, 2019, ISBN 978-3-658-26280-8 , Chapter 14.2.1, Figure 14.21, p. 549.
2. Europe teaching material: Fachkunde Metall, 58th, revised edition 2017. ISBN 978-3-8085-1290-6 , Chapter 6 Machine Technology, Figure 3, p. 453