Bearings are one of the simplest yet crucial machine elements used to get desired motion. It assists with rotation, support shafts or load and reduces friction between two moving parts.
For example to reduce unwanted vibration or deviation of a shaft; a bearing is used in between. It also acts as a protective device that wears down; sacrificing itself to the expensive shafts.
Not just they are remarkably good at carrying loads but help facilitate the desired motion. At first glance, these bearings may look quite simple and basic. But it is so fundamental to our industries; they won’t exist without them.
You can find them everywhere from airplanes to automobiles and from generators to air-conditioners. Not just that they are found in the refrigerator, pumps, fan, vacuum cleaner, etc.
In short, where there is a shaft there must be a bearing!
Developed towards the end of the 19th century, it consists mainly of smooth rollers or metal balls and their races.
A race is that smooth surface in or outside the bearing body; upon which these metal balls roll against. They are made based on the simple concept that; things roll well than they slide.
These bearings are classified based on its application; load, motions allowed and operation type. The simplest of them named ball and roller bearing; based on the type of rolling body.
Depending on the application a bearing typically encounters one of these two forces; axial and radial. The axial forces act parallel to the shaft; while the radial forces act in perpendicular.
Different Types Of Bearings
A mechanical bearing can be classified into eight major subtypes; based on its construction, operation, load, working and more.
These eight distinct categories are; friction bearings, non-friction or anti-friction bearing, journal bearings, pivot, thrust, Slipper, plain, cylindrical or round bearings.
1 ) Friction Bearings
A friction or sliding contact bearings is the one where; the bearing surface is in contact with the moving object or the shaft.
Here the shaft surface slides over the bush of the bearing thus generating friction and wear.
Now to minimalize wear and friction oil film is used as a lubricating medium.
Since the bearing is subjected to sliding friction it’s often called sliding contact bearing or just friction bearings.
Made of material such as brass, white metal, cast iron, and bronze; these are much quieter and takes less radial space.
These are relatively simple in design, easy to manufacture, low cost and are less sensitive to contaminants or foreign particles. In some way, it remains free from rust or corrosion for their lifetime.
Because of their simpler construction, they are often broadly termed as plain bearings.
Friction bearings are used mostly where there are high load requirements; such as crack shaft, intermediate shaft and rocker arm of I.C Engine. The simplest example of such a bearing type is journal bearing.
The frictional bearing can itself be classified into different types:
A ) Solid Bearing
Solid bearings or sometimes referred to as the plain bearing is the simplest bearing type possible; because of its one-piece construction.
Made of cast iron it includes just a hollow passage in the center with bearing surface; as to allow shaft tp pass with running fit.
With simple construction, they are quite easy to install. The example of such a bearing is journal bearing.
Here, a journal is the part of the shaft supported by the bearing. While the part matching the journal is called a bearing.
Therefore, these bearing types are best utilized for application where; shaft speed is low and load is low to moderate.
For example, the main bearing of a marine diesel engine.
B ) Self-aligning Bush Bearing
A self aligns bush bearing is used when it is hard to obtain alignment of the shaft and the housing.
These bearing types are adjustable up to 5 degrees of misalignment in either direction. Suitable for high loads its spheric is secured against distortion.
These bushing type adjustable bearings are used; as a cost-effective alternative to the adjustable ball and roller bearings.
The main drawback of using this bearing is increased wear rate, noise, and occasional lubrication breaks down.
Above all, It consists of a movable bush made of brass and a cast iron body. To keep them from moving a screw is fixed at ends.
C ) Split Bearings
A split bearing is much like a solid/plain bearing with the addition of split arrangement.
In other words, you no longer need to open the entire bearing assembly for maintenance or repair.
For instance, now repair work can be done when removing pulleys and couplings from the shaft.
Similarly, they are built not in one piece but in halves and then assembled later. In addition, to their wide-reaching benefits; these are quite versatile and require much less maintenance than others.
These are used for bearing positions that are difficult to access or need support at various parts such as with intermediate shafts.
D ) Linear Bearing
Linear bearings or adjustable slide bearing are those designed to provide free motion in one direction.
They are quite simple in construction and tend to have large load carrying capabilities; thanks to its large contact area.
In total, they consist of just two parts; a movable linear bearing and a guide rail. However, the rails can be of any length or profile.
The key advantage of these bearing types is its high positioning accuracy. In addition, they have provided to adjust wear.
The bearing is placed inside the tapered hole of the main housing assembly. The housing is generally made of aluminum alloy and features; flat mounting face and four tapped bolt holes.
2 ) Antifriction Bearing
Unlike their name, they are not free from friction but possess much lower friction when compared to other bearing types.
An antifriction bearing is preferred over journal bearing when; much less friction is needed for very low differential surface speed.
They are essential to mechanized equipment; thanks to their better capabilities at guiding or holding moving parts.
Above all, they require less maintenance and minimize friction and wear.
These are interference fitted to the shaft utilizing a rolling element bounded by both inner and outer rings.
Therefore in a way they are better with radial forces as most of them are absorbed by these two rings.
In its simplest form, they are better at cleanliness, low cost of maintenance, no lubrication, reliability, and small overall dimension.
An antifriction bearing can be classified into two main categories; ball bearing and roller bearing with its subtypes.
A ) Ball Bearing
A ball bearing consists of rolling spherical elements or ball; used to maintain the separation between the bearing races.
Such an arrangement allows shaft motion with little rolling resistance and sliding.
A ball bearing use at least two races to contain the balls. Thus, reducing rotational friction and supports radial and axial load.
Now since one of such races to rotates it causes the ball to roll. And we all know a rolling body always has a lower coefficient of friction.
These are required for applications where low friction, high speed and low to medium load is required.
Because of the smaller contact area between the balls and races. These bearings simply cannot support large loads in general. However, they are much better than another tolerating misalignment.
Based on their design capabilities these can be divided into the following types:
( i ) Single Row Ball Bearing:
A single row ball bearing is the most simple yet versatile solution; offering very good performance to price ratio. They are characterized by having a single row of balls; within the groove cut on the inner and outer rings of the bearings.
Such a bearing is used in all the fields of mechanical engineering thanks to its low cost; great radial load capacity, good axial load capacity, high speed, low noise and need for less maintenance.
( ii ) Double Row Ball Bearing:
A double row ball bearing corresponds in design to single row ball bearings. The only change is, here they have two rows of balls going through separate inner and outer groove cut into the bearings.
Here, the acting force direction between balls and grooves diverges at the bearing axis. Thus forming an angle of 30° to the radial plane allowing strong radial and axial loads in both directions.
This makes these bearing type better at handling the bi-directional axial load even with tilting effects. In return, they require a better coaxiality between the shaft and housing. This bearing type is used typically in pumps and motor.
( iii ) Double Row Self-aligning Ball Bearing:
This type of bearing has a common outer spherical race for both the rows of the ball. Thus have a spherical bore on the outer race to add self-aligning properties against; angular misalignment of the shaft with respect to the housing.
A typical self-aligning ball bearing can compensate for misalignment as much as three to five degrees ( 3-5o ).
While these have the same external dimensions as a non-adjusting bearing; it can take extreme radial forces and journal load.
Furthermore, it aligns correctly itself with the shaft; without adding unwanted stress on the bearing housing.
Since this bearing type features the lowest friction of all rolling bearings; it’s best used for very high-speed operation or where considerable shaft deflection/misalignment is expected.
( iv ) Angular Contact Ball Bearing:
Due to their internal design with raceways in and out of rings; displacing them in the direction of the bearing axis. They can withstand high radial and axial loads while still reaching for high speeds.
That is possible because of the nonstop contact between the balls and raceways.
These bearings are generally preferred in settings that require high accuracy and reliability. For example, in gearboxes, clutches, pumps, electric motors, and other high-speed applications.
The axial load carrying capacity of these bearing types depends much on the contact angle. In general, it increases with the contact angle varying between 15° to 40°.
The contact angle is the angle between the line joining contact point for the ball and the radial raceway.
B ) Roller Bearing
A roller or rolling element bearing is similar to the ball bearings in design. Here, a cylindrically shaped object is placed; between the two bearing rings or races in the form of the rolling element.
Because of its shape featuring a greater area of contact; these are better at carrying large loads without any deformation.
Roller bearings are thus used mainly for rotating shafts with requirements for heavier loads.
They are called as roller bearings because it carries the load by a cylindrical rolling element between the two race surfaces.
While roller bearings can handle higher loads, these are generally limited only to high load low-speed operations.
It is due to their higher radial load capacity but lower axial load capacity with increasing friction under axial loads.
The key advantage of using roller bearing over ball bearings are; low starting friction, better shaft alignment, can withstand shock loads, easy to mount, small overall dimensions, reliable and has a lower maintenance cost.
These are used mainly in applications such as; rolling mills, machine tools, wind turbines, gear reduction, power generation, etc.
Based on their design capabilities these can be divided into the following types:
( i ) Cylindrical Roller Bearing
Made of hollow cylindrical rollers these are designed to carry heavy radial loads. In different series, it has single, double and multi-row rolling elements.
In a typical cylindrical roller, the rolling element with its cage is guided within the internal flange of the outer race.
It is one of the most popular bearing types thanks to its abilities like; interchangeability, simple construction, high load capacity, and better speed ratings.
Based on its technical specifications it can be further classified into NU, RNU, N, NNU, NJ or NUP subtypes.
These can be found mainly in machine brushes, electric motors, alternators, etc.
( ii ) Tapered Roller Bearing
A tapered roller bearing has both its inner and outer rings tapered; along with the tapered rolling element.
Thanks to its unique design it can withstand both axial and radial high loads. But in contrast, this bearing can take only a few degrees of misalignment.
With increased or decreased contact angle, here the total load capacity can be increased or decreased.
They can be found mostly in cars and to some extent in wagon wheels.
Because of its unique design with tapered race and rolling element; there is no roller slippage off the tracks while the movement.
( iii ) Spherical Roller Bearing
The spherical roller bearing is designed to permit angular misalignment. This is made possible thanks to its two rows of symmetrical rollers.
Here, the energy transfer between its inner and outer races can happen more efficiently. It is so because of its common spherical outer ring; with two inner rings inclined to the bearing axis.
They can be found commonly in Industrial Gearboxes, Cement Grinding Rolls, Conveyors, and a typical Sugarcane crusher.
This bearing type is best known for its robust design; long service life, high load carrying capacity, low friction and automatic adjustment of alignment.
( iv ) Needle Roller Bearing
A needle roller bearing is a bearing type that uses cylindrical rolling elements; which are smaller in dia but longer in length.
A typical design for a needle roller bearing includes an inner race or shaft, outer race, needle rolling element.
This unique design characteristic allows them to have; a significant load-bearing capacity but a small cross-sectional height.
It can be explained based on its design characteristic; which features a large surface area in contact with the bearing raceways.
This enables it to support radial loads at high rotational speed; not to mention its exceptional axial load capacity.
These are preferred where the outside diameter of the bearing is generally restricted. For example, gear pumps, transmissions, pumps, rocker arm, compressors, etc.
3 ) Fluid Bearings
It is a type of bearing where the rotating and stationary parts are never under direct contact. Here the shaft is supported by a thin layer of lubricating fluid or gas.
Because of their simple nature, they were the first type of rotary bearing to appear before other bearing types.
One of the simplest examples of fluid bearing with hydrodynamic lubrication is the journal bearing. Here the fluid film is used to create separation between the two surfaces; using the shaft speed to pressurize fluid and generate lift.
Due to its hydrodynamic nature to support large loads using a thin film of oil; it is often called as the hydrodynamic bearing.
The other type is called hydro-static bearings. A hydrostatic bearing does not rely on rotating of the shaft to develop a fluid film; instead, it uses externally pressurized lubricant for the purpose.
A hydrostatic bearing features two bearing surface and an orifice plate, from which the pressurized fluid is pumped in.
While the hydrodynamic bearing is simple in construction and easy to maintain. They can only work under slow speed conditions.
On another hand since hydrostatic bearing relly on external pressure pump; they work well under varying load and speed conditions. In contrast, they have higher operational costs and require frequent maintenance.
A ) Journal Bearing
A journal bearing is a typical example of Hydrodynamic fluid film bearing in action. In these bearings, the shaft rotates freely within the shell separated by a thick layer of lubricants or oil.
Here metal-to-metal contact occurs only during the starting and stopping operation. This is when most of the bearing damage took place.
When the journal ( part of the shaft within the bearing ) moves; it generates pressure on the fluid film. Thus forcing them to form a wedge-shaped zone.
Under sufficient speed and fluid pressure, it creates a hydrodynamic lift that sustains shaft load.
Here, even when the bearing clearance area is filled with oil. At rest because of the downward force ( weight ) of the shaft; oil film is squeezed out of the bottom thus forming metal to metal contact.
As the journal starts to rotate slowly it forces the oil upwards along with itself; under the influence of friction force.
Here the journal and the bearing are mostly protected by a phenomenon known as boundary lubrication. But as the speed increase more and more oil kept trapped between the two surfaces.
Once sufficient speed is obtained there is sufficient pressure created to sustain the shaft load. However, it also causes the journal to displaces from its concentric position.
5 ) Thrust Bearing
This is one of such bearing designed specifically to support axial or thrust load. Different types of rotary thrust bearings are being used for different axial/thrust load conditions.
Thrust bearings are used typically in automotive and aerospace applications; where there are high-speed operations involving lubrication.
They also used in applications such as rolling mill, rock crushing, coal pulverizing and other heavy-duty industrial applications.
Due to their specific construction and high load-carrying capacity. They are well suited for application involving higher load and stiffness; while lower speed or centrifugal force.
In general, a ball-type thrust bearing is used for lower thrust applications. While a roller type thrust bearing has higher load carrying capacities. A single-acting thrust bearing can carry load only in one direction; while a double-acting can load both ways.
Here, when the axial forces are applied on the shaft it is distributed evenly across all elements of the thrust bearing.
A thrust bearing can be classified mainly based on its geometry; fixed vs tilting pad types. The fixed type has its thrust collar and pads parallel to each other.
Thus it lacks the converging wedge negatively affecting its thrust load carrying capacity.
So an additional converging wedge is added in the tilting pad type to improve its load capacity. Because they have pivot it is easier for them to tilt; in the direction of the force created by oil wedge.
In short in not just makes it better at carrying thrust load but also makes it self-orienting. Although the degree of self-aligning change based on; bearing’s dimension and shape of the barrel roller.
6 ) Composite Bearing
It is self-lubricating, high quality dry sliding bearings in the application. These bearings are made from a combination of materials.
For example, resin, polyester, and other fibers including lubricating ingredients.
Some of the key characteristics of a composite bearing are; no lubrication, minimum space requirement, good sliding properties, a wide range of operating temperatures, wear-resistant, and no maintenance requirement.
Composite plain bearings are primarily used where a heavy load is to be supported in less space or with lower rotational motion.
A typical PTFE composite bearing can work between temperature –200 and +250 °C. On another hand, POM type composite bearings can work for just –40 to +110 °C.
These bearings are typically used for the solar tracking system, Offshore drilling rigs, inland vessels, hydraulic systems, construction work, etc.
7 ) Magnetic Bearings
A magnetic bearing is a device that is used to support load or shaft using a magnetic force. Here, it supports the rotating shaft without any physical contact; thus permits low friction and wear.
While a wide variety of magnetic bearings have been developed over time; only a handful of them was practically feasible to become active magnetic bearing.
The most commonly used magnetic bearing type is RMB’s “Radial magnetic bearing”. They are used widely in very high-speed operations, precise tools, bearingless motors, flywheels, and artificial heart pumps.
One of its key features is to operate under vacuum conditions at high speed. That is why it is the only bearing type used for aerospace applications.
The basic operating principle of magnetic bearing is electromagnetism. The effect of this force on shaft depends on current and the gap between the objects.
An electronic control unit is thus set up to maintain just the right amount of magnetic force to counter and balance the shaft weight.
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