What is Bearing | Bearing Types | Classification Of Bearing
A bearing is a mechanical element that limits relative motion to only the desired motion and at the same time it reduces the frictional resistance to the desired motion. Depending on the design of the bearing, it may allow free rotation around a fixed axis (such as the case of shafts) or free linear movement, or both in some cases.
Classification Of Bearing / Types Of Bearing :
Bearings may be classified according to the type of operation, the motions allowed, or to the directions of the loads applied to the parts. The most broad classification of bearings is according to the type of operation where they are classified in two groups;
- Rolling-contact bearings
- Sliding-contact bearings
Rolling-contact bearings, also called “rolling bearings” or “antifriction bearings”, have rolling elements (balls or rollers) that supports the loads and allow connected parts to move freely in the desired direction. Unlike sliding-contact bearings, rolling bearings have very small frictional resistance at both starting and running conditions (the starting friction is about twice the running friction).
In general, rolling-contact bearings consist of four main parts (as seen in the figure);
- Inner ring
- Outer ring
- Balls or Rollers
- Separator (also called retainer or cage)
Bearings are used to support shafts and they are designed such that they can support radial load or thrust load or combination of both. However, they are not supposed to be subjected to bending moment where that will shorten their life significantly. For this reason, always, a minimum of two bearings spaced apart from each other are needed to support any shaft (if the shaft is short, a single double row bearing can be used).
When rolling bearings are to be used in a mechanical system, the mechanical designer does not design the bearing but rather he selects a bearing according to the design requirements at hand (loads it will support, desired life, speed, available space, etc.).
Rolling-contact Bearing Types
Rolling-contact are divided in two main groups according to the type of rolling elements used in the bearing:
- Ball bearings
- Roller bearings
Ball bearings: are the most commonly used type because they are more versatile and, in general, they can support both radial and thrust loads.
The most common types of ball bearings are shown in the figure:
(a) Deep groove bearing: it takes radial and some thrust load.
(b) Filling notch bearing: it has more balls. It takes more radial load, but very little thrust.
(c) Angular contact bearing: it takes more thrust than deep groove bearings because of the added support in the axial direction, but thrust should be applied in one direction.
(d) Shielded bearing: it has protection shields that prevent dirt and foreign particles from entering inside the bearing.
(e) Sealed bearing: it has seals that prevent lubricant from going out of the bearing and dirt or any liquid from entering inside the bearing.
(f) Self-aligning bearing: it can withstand high shaft misalignments.
(g) Double row bearing: it takes twice the load of single row, but with less parts and space than two bearings.
(i) Thrust bearing: it takes thrust load only (no radial load).
(j) Self-aligning thrust bearing: it takes thrust load only and can withstand high shaft misalignments.
Roller bearings: are generally capable of supporting higher loads than ball bearings of similar size because of the larger contact area of the rollers. Roller bearings need more perfect geometry and heavier cages to retain the rollers in position.
The most common types of roller bearings are shown in the figure:
(a) Straight roller bearing: it takes high radial load, but does not take thrust load.
(b) Spherical-roller thrust bearing: it takes both radial and thrust loads and it is useful when heavy loads & misalignments occur.
(c) Thrust bearing: it takes thrust load only.
(d) Needle bearing: It is somehow similar to straight roller bearing but it is more useful when radial space is limited.
(e) Tapered-roller bearing: it takes both radial & thrust loads (higher loads than angular contact ball bearings).
In addition to the previous types, there are some other types of bearings such as:
- Instrument bearings: high precision, made of stainless steel.
- Non precision bearings: no separator, made of sheet metal.
- Ball bushings: permit rotation & sliding (shown in the figure).
The sliding-contact bearings may be considered the simplest kind of bearings in terms of construction where it could simply consist of a shaft rotating inside a hole (with appropriate clearance between them).
Unlike rolling-contact bearings which generally have a limited life, sliding-contact bearings could have a very long life if they are well designed, lubricated and working under reasonable operating conditions.
Sliding-contact bearings are generally more applicable for extreme operational conditions (high loads and rotational speeds).
Also, they are used for low demand applications (without external lubrication) because they are more cost effective than antifriction bearings.
SLIDING CONTACT BEARINGS – CLASSIFICATION
Sliding contact bearings are classified in three ways.
1. Based on type of load carried
2. Based on type of lubrication
3. Based on lubrication mechanism
Bearing classification based on type of load carried
a. Radial bearings
b. Thrust bearings or axial bearings
c. Radial – thrust bearings
Bearing classification based on type of lubrication
The type of lubrication means the extent to which the contacting surfaces are separated
in a shaft bearing combination. This classification includes
(a) Thick film lubrication
(b) Thin film lubrication
(c) Boundary lubrication
Bearing classification based on lubrication mechanism
a. Hydrodynamic lubricated bearings
b. Hydrostatic lubricated bearings
c. Elastohydrodynamic lubricated bearings
d. Boundary lubricated bearings
e. Solid film lubricated bearings
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