In this article we will discuss about the definition and classification of journal bearing.
Definition of Journal Bearing:
The portion of the shaft which is in actual contact with the bearing is known as journal. For every machine and engine, it is necessary to have a provision for the support of rotating shaft. Such support is called bearing. In other words, the shaft must be supported through bearing. Simple bearing has been shown in Figs. 8.11(a) and 8.11(b).
Figure 8.11 shows a simple bearing with a rectangular base and above it there is a cylindrical block with a hole through it to fit a shaft. It not only acts as a support to the shaft but also helps in smooth running of the shaft. Thus, bearing provides a support such that the shaft should be allowed to rotate freely.
To avoid frictional resistance which opposes rotation between the journal and the inner face of the bearing, proper designing of the bearing is required and also there should be a provision for the lubrication. Proper lubrication keeps the smooth running of shaft which causes less power loss, less heat production, and also less noise production. There are several engines or machines which have bearing such as crank shaft of IC engine, spindle of lathe, central drill, milling and grinding machines, etc.
For better performance, the shaft bearing should be properly fitted and there should be a provision for the replacement of damaged parts.
Classification of Journal Bearings:
There are many types of bearings that serve different purposes and are shaped differently.
(1) Sliding contact bearing:
(a) Journal bearing
(b) Thrust bearing
(c) Collar bearing
(2) Rolling contact bearing:
(a) Ball bearing
(b) Roller bearing
(1) Sliding Contact Bearing:
Sliding contact bearing is categorized by the nature of motion between the shaft and the bearing surface. In this case, the motion of the shaft is purely a sliding motion and because of the surface contact, the frictional force is more and hence it requires more lubrication. The sliding contact bearings are of three types as shown in Figs. 8.12(a)-8.12(d).
(a) Journal Bearing:
Journal bearing is one in which the load acts perpendicular to the axis of the shaft as shown in Fig. 8.12(a). The load on the bearing acting perpendicular to the shaft axis is known as journal bearing.
Journal bearing or sliding contact bearing may be classified as:
(i) Solid journal bearing
(ii) Bushed journal bearing
(iii)Plummer block/Pedestal bearing
(i) Solid Journal Bearing:
Solid journal bearing has been shown in Figs. 8.11(a) and 8.11(b). It is the simplest form of journal bearing and is also known as sliding contact bearing. Frictional force magnitude is more for this bearing and hence more lubricating oil is required. The constructional details of this bearing are simple and made of cast iron rectangular base surmounted with a cylindrical block.
The cast iron base has two holes to fix it on ground or elsewhere. A hole is made in cylindrical block to fit a shaft. A small hole is provided at the top of the cylindrical block to supply lubricating oil into the bearing. The entire block supports the shaft.
(ii) Bushed Journal Bearing:
Figure 8.13 shows a bushed bearing in sectional view. It is the modified form of solid journal bearing and is made up of cast iron rectangular base with two holes to be fixed up on ground or on a plate. A cylindrical block is surmounted at the top of the base.
A bush made up of ductile metal such as gun metal, brass, or bronze metal is inserted in the cylindrical block. The bush has internal diameter equal to the external diameter of the shaft (Fig. 8.14). The bush is pressed tightly inside the bore such that it is prevented from rotation or movement. The bush can be replaced periodically when it is spoiled due to friction. A small hole is made up to the shaft at the top of the cylindrical block to provide lubrication.
(iii) Plummer Block/Pedestal Bearing:
Figure 8.15 shows a left half sectional view of a Plummer block. Plummer block bearing is a split type of journal bearing. The entire assembly as shown in Fig. 8.15 is used for intermediate support of a long line shaft. Regarding constructional details, it is actually classified into two parts. The lower bottom part is made up of cast iron known as pedestal and the upper part is known as cap. Both parts are attached with each other by means of two square-headed bolts.
If both the parts are joined together, it forms a hole in the central region equal to the external diameter of the shaft. A gun metal bush which is made in two halves is fitted in the hole. A snug is fitted at the bottom of the bush which prevents shifting of the bearing from rotation along with the rotation of shaft. The pedestal base has two holes by which it is grounded on support.
(b) Thrust Bearing:
Thrust bearing is one in which the load acts along the axis of the shaft as shown in Fig. 8.12(b).
Foot Step Bearing/Pivoted Bearing:
Figure 8.16(a) shows a cross section of the assembly of bearing. A pictorial drawing of foot step bearing (quarter portion removed) has been shown in Fig. 8.16(b).
Foot step bearing is a type of thrust bearing. It is used to support a vertical shaft subjected to an axial load. The lower end of the shaft is supported at the bearing. The bearing is placed inside a hole lined with a ductile metal liner and is supported on a disc/pad made up of gun metal or brass. When the shaft rotates, the pad and bush have the tendency to rotate, but the rotation of the disc is prevented by a snug.
There is also a provision of supplying lubricating oil to have a smooth running of the shaft. Foot step bearing is rarely used and is said to be technically uncommon. But in certain equipments, it becomes necessary to use foot step bearing.
(c) Collar Bearing:
Collar bearing is one in which the load acts along the axis of the shaft with a provision of one or more than one collars on it. The collar prevents the movement of the shaft in axial direction as shown in Figs. 8.12(c) and 8.12(d).
Advantages of Sliding Contact Bearing:
(a) Operation can be done at high speed.
(b) Heavy radial loads can be carried.
(c) Ability to take up shock and vibration.
(d) Operation is noiseless.
Disadvantages of Sliding Contact Bearing:
(a) High friction losses at the starting point.
(b) Length of bearing is more.
(c) More lubrication is required.
(d) More maintenance is required.
(2) Rolling Contact Bearing/Anti-Friction Bearing:
A bearing which has a rolling contact present in between two surfaces is known as rolling contact bearing. It is a well-established fact that smooth rolling surface such as that of a ball can roll freely than a sliding surface. This basic principle has been used to fabricate such low friction bearings.
In this case, the motion between the shaft surface and the bearing surface, either ball or roller, is purely rolling. The rolling friction is much less than sliding and hence such bearings are called anti-friction bearings.
Rolling contact bearings consist of four parts:
(a) Inner race of steel
(b) Outer race of steel
(c) Ball or roller of hardened steel
(d) Cage or retainer of gun metal
Following are the two types of anti-friction bearings:
(a) Ball bearing
(b) Roller bearing
(a) Ball Bearing:
A ball bearing assembly is shown in Fig. 8.17. It consists of two metallic rings known as inner ring and outer ring. The inner ring is mounted on the shaft with its lower surface and the outer surface of it has the groove made centrally all over the circumference known as inner race. The outer ring has the outer surface fitted with housing of the bearing and the inner surface is grooved all around circumference centrally known as outer race.
In between the inner race and the outer race, there are a number of steel balls placed. The number of steel balls is decided by the size of the shaft. All steel balls are set equally in the spaces made inside a special cage or retainer which is made by pressed steel. All steel balls equally spaced in between the inner and outer race bounded by cage make the complete assembly of the bearing shown in Figs. 8.17(b) and 8.17(c).
If the shaft rotates, the inner ring along with balls also rotates with it. The outer ring fits tightly in the housing of the bearing and does not rotate. The steel balls have a point contact with the bearing surface which is suitable for high loads. Grease is used as a lubricant in the ball bearing assembly.
(b) Roller Bearing:
A cylindrical roller bearing assembly is shown in Fig. 8.18. The constructional detail is similar to that of ball bearing. Cylindrical roller bearing consists of inner and outer race and a number of hardened steel cylindrical rollers in place of ball and a set of retainers. All cylindrical rollers are free to roll in between the inner and outer race. The inner race is also rotated along with shaft. Outer ring is fixed with the housing of the bearing.
The cylindrical rollers are in contact with the surface and hence they are capable of taking heavy load compared to spherical ball bearing.
Grease is used as a lubricant in this bearing. All rollers equally spaced in between the inner and outer race bounded by cage makes the complete assembly of the bearing shown in Figs. 8.18(b)-8.18(d).
Advantages of Ball/Roller Bearings:
There is a smooth running of the bearing and hence low starting and low friction are present.
(a) It is best to take up radial as well as thrush loads.
(b) Alignment of the shaft is more accurate.
(c) Failures can be detected easily by loud noise.
(d) Sizes of the bearings are standardized and hence the selection of bearings can be easily done.
Disadvantages of Ball/Roller Bearings:
(a) At high speed, more noise is detected.
(b) Manufacturing cost of ball bearings is high.
(c) Low resistance in case of shock load.
(d) Housing of the bearing is complicated.
(e) Entry of dust, dirt, or metal chips spoils the bearings early.