Basics of Anti-friction Bearings And their selection from ......•To ensure free rotation of shaft...
Transcript of Basics of Anti-friction Bearings And their selection from ......•To ensure free rotation of shaft...
Basics of Anti-friction BearingsAnd their selection from Manufacturer’s catalogue
Dr. Chandan SharmaDepartment of Mechanical Engineering
Engineering College Ajmer
Functions of a Bearing
• To ensure free rotation of shaft or axle with minimum friction
• To support the shaft and hold it in the correct position
• To take up the forces that acts on the shaft and transmits them to frame
or foundation
Types of Bearing
• Based on direction of force (Radial bearing and thrust bearing)
• Based on friction between shaft and bearing surface (Sliding contactbearing and Rolling contact bearing)
Types of Bearing
• Based on friction between shaft and bearing surface (Sliding contactbearing and Rolling contact bearing)
Applications of Rolling Contact Bearings
Rolling contact bearings are used in following applications:
• Machine tool spindles
• Automobile front and rear axles
• Gear boxes
• Small size electric motors
• Crane hook and hoisting drums
Types of Rolling Contact Bearings
A Rolling contact bearing consist of four parts:
• Inner and outer races
• Rolling element (balls/rollers/needles)
• Cage (for holding rolling element together and spacing them evenly)
Deep groove ball bearings
• Most frequently used bearing
• Point contact between balls and races
Advantages:
High load carrying capacity
Takes load in radial as well as axial directions
Excellent performance in hi-speed applications
Generates less noise due to point contact
Wide range of bore diameter (few mm to 400 mm)
Disadvantages:
Accurate alignment is required
Poor rigidity as compared to Roller bearings
Cylindrical Roller Bearing
• Used for high load carrying capacity
• Relatively short rollers are positioned and guided by cage
Advantages:
High radial load carrying capacity (line contact)
More rigid than DGBB
Low coefficient of friction in high speed applications
Disadvantages:
In general cannot take thrust load
Requires precise alignment
Generates relatively more noise
Needle Bearing (Quill bearing)
• Length to diameter ratio > 4
• Can be used without inner and outer races
• Suitable where limited radial space is available
Advantages:
Compact and light weight
Large load carrying capacity compared with their size
Large load carrying capacity particularly at low speeds
Disadvantages:
Cannot be manufactured with high degree of accuracy
This results in high friction in needle bearings
Coefficient of friction can be up to 4 times than that of cylindrical roller bearing
Angular Contact Bearing
• Line of reaction makes an angle with the axis of bearing
• Often used in pairs to take thrust load from both directions
Advantages:
Can take radial as well as thrust load
More load carrying capacity than DGBB
Disadvantages:
Two bearings are required to take thrust loads
Must be mounted without axial play
Requires initial pre-loading
Self aligning bearings
• Consist of two rows of balls or rollers
• The inner race and balls can freely roll and adjust to the angular misalignment ofthe shaft
Advantages:
Can take high radial as well as thrust load
More suitable where misalignment can arise due to deflection
Disadvantages:
Comparatively costlier
Used in agriculture machinery, railway axle boxes
Can be self aligning ball bearing or spherical roller bearing
Principle of Self aligning bearings
Taper Roller Bearing
• Rolling elements in the form of frustum of cone
• Often used in pairs to balance the thrust component
Advantages:
Can take heavy radial as well as thrust load
These bearings has more rigidity
Can be assembled and disassembled
Disadvantages:
Two bearings are required to take thrust loads
Comparatively costlier
Cannot tolerate misalignment
Thrust ball bearing
• The use of large number of balls results in high thrust load carrying capacity
Disadvantages:
Cannot take radial load
Performance satisfactory at low and medium speeds only
Cannot tolerate misalignment
Does not operate well on horizontal shafts
Continuous pressure is required to hold shaft ring and housing ring
Material of Rolling Contact Bearings
Balls, inner and outer races – high carbon chromium steel (1% C and 1.5 % Cr)
Balls and races are through hardened
Cages are made from stampings of low carbon steel
The rollers are made of case hardened steels
Rollers are case carburized
Balls are through hardened whereas rollers are case hardened
Selection of Rolling Contact Bearings
For low and medium radial loads, ball bearings are used
For heavy loads and large shaft diameters, roller bearings are used
Thrust ball bearings are used for medium thrust loads
For high speed applications, DGBB, ACB, and CRB can be used
For rigidity considerations, roller bearings are preferred over ball bearings
For noise considerations, ball bearings are preferred over roller bearings
Fa/Fr < 0.7 SRDGBB
0.7 < Fa/Fr < 1.5 DRDGBB
Fa/Fr > 1.5 Rolling contact bearings
Factors affecting selection of Rolling Contact Bearings
Static load carrying capacity
Life of a bearing
Dynamic load carrying capacity
Equivalent dynamic load
Load-life relationship
Static load carrying capacity (C0)
It is the load acting on the bearing when the shaft is stationary
It produces permanent deformation in balls and races which increases with increasing load
It is defined as static load which corresponds to a total permanent deformation of
balls and races at the most heavily stressed point of contact, equal to 0.0001 of the
ball diameter
Stirbeck’s equation d is ball diameter, z no. of balls and k
is factor that depends on curvature at
the point of contact
5
zdk C
2
0
Life of a bearing
The life of an individual ball bearing is defined as the number of revolutions (or hours of service at some given constant speed) which the bearings run before the first evidence of fatigue crack in balls or races.
The life of a single bearing is difficult to predict
Hence life is defined in terms of average performance of a group of bearings
L10 life (also called as rated life) is the life which 90% of the bearings will reach or exceed before fatigue failure
Average life or L50 is the life which 50% of the bearings will reach or exceed before fatigue failure
1050 L5 L 6
hmr
10
N L 60 L
Dynamic load carrying capacity (C)
It is based on the fatigue life of the bearing
It is based on the assumption that inner race is rotating and outer race is stationary
It is defined as radial load in radial bearings (or thrust load in thrust bearings) that
can be carried for a minimum life of one million revolutions
The minimum life in this definition is the L10 life, which 90% of the bearings will
reach or exceed before fatigue failure
Equivalent Dynamic load (P)
In actual applications, the force acting on the bearings has two components –radial and axial
It is therefore necessary to convert the two components acting on the bearing into a single hypothetical load fulfilling the conditions applied to the dynamic load carrying capacity
It is defined as constant radial load in radial bearings (or thrust load in thrust
bearings) which if applied to the bearing would give same life as that which the
bearing will attain under actual condition of forces
P is equivalent dynamic load, X and Y are radial and
Thrust load factors and V is race rotation factor
V=1.0 when inner race rotates and 1.2 when outer race rotates
ar F Y FV X P
Load-life relationship
The relationship between the dynamic load carrying capacity, the equivalentdynamic load and the rated life of the bearing is given by:
Where:
L10 is the rated life (in million revolutions)
C is dynamic load carrying capacity
P is equivalent dynamic load
p=3 (for ball bearings) and p=10/3 (for roller bearings)
p
10P
C L
Selection of bearing life
Bearing life for wheel applications (speed of rotation not constant)
Selection of bearing life
Bearing life for industrial applications (speed of rotation constant)
Load factor
Designation of Rolling Contact Bearings
The rolling contact bearing is usually designated by 4 digits:
The last 2 digits indicate the bore diameter in mm (bore diameter divided by 5).For ex. XX15 indicates a bearing of bore diameter 75 mm.
The third digit from right indicates load series of the bearing:
• Extra light series – 0 or 1
• Light series – 2
• Medium series – 3
• Heavy series – 4
The fourth digit (and sometimes 5th digit)
from right specifies the type of rolling
contact bearing.
For example, digit 6 indicates deep groove ball bearing.
Selection of Rolling Contact Bearings from Catalogue
a. Calculate radial and axial loads and shaft diameter (if not given).
b. Select type of bearing for the given application. (or ratio of Fa and Fr)
c. Determine X and Y from the tables.
For that we require Fa / C0 and C0 has to be taken from catalogueHence selection is done by trail and error
d. Calculate equivalent dynamic load P = X Fr + Y Fa
e. Make decision about the expected bearing life and express L10 in million revolution.
f. Calculate dynamic load capacity from load life relationship.
g. Check whether the selected bearing of series has required dynamic capacity or
not. If not, select the bearing of the next series and go back to step (c) and
repeat the procedure until C (calculated) < C(tabulated).
h. Calculate revised life of the bearing for the tabulated value of C using load life
relationship.
Selection of Rolling Contact Bearings from Catalogue
a. Calculate radial and axial loads and shaft diameter (if not given).
b. Select type of bearing for the given application. (or ratio of Fa and Fr)
c. Determine X and Y from the tables.For that we require Fa / C0 and C0 has to be taken from catalogue
Hence selection is done by trail and error
Selection of Rolling Contact Bearings from Catalogue
Selection of X and Y:
Selection of Bearings for Cyclic loads and Speeds
In certain applications, bearings are subjected to cyclic loads and speeds.
For example, (i) radial load of 2500 N at 700 rpm for 25% of time
(ii) radial load of 5000 N at 900 rpm for 50% of time
(iii) radial load of 1000 N at 750 rpm for remaining 25% of time
3
321
3
33
3
22
3
11e
........... N N N
.............. PN PN PN P
Reliability of Bearings
In certain applications, where there is risk to human life, it becomes necessary to selecta bearing having a reliability of more than 90%.
The relationship between life and reliability is given by a statistical curve known as‘Weibull distribution’.
For Weibull distribution:
For L50 = 5 L10, a=6.84 and b = 1.17
under test bearings of no. Total
mr Lcompletedly successful bearings of No. y Reliabilit
b
a
L
e R
Reliability of Bearings
If desired reliability is other than 90%, following formula may be used:
In a system, if there are N bearings, each having a reliability of R, then the completeReliability of system is given by:
b
1
90
e
e
10
R
1log
R
1log
L
L
NS R R
Bearing failure - Causes and remedies
The failure of anti-friction bearings occurs not due to breakage of parts but due todamage of working surfaces of their parts. The principle types of surface wear are asfollows:
Abrasive wear
Corrosive wear
Pitting
Scoring
Abrasive Wear
Causes:
When bearing is made to operate in environment contaminated with dust, foreignparticles or rust
Remedies:
Provision of oil seals
Use of high viscosity oils enables fine particles to pass without scratches
Corrosive Wear
Causes:
When water or moisture enters the bearing
Also caused by corrosive element present in additives in the lubricating oil
Remedies:
Providing complete enclosure free from external contamination
Selecting proper additives
Replacing lubricating oil at regular intervals
Pitting failure
Causes:
When load on the bearing exceeds surface endurance strength
Characterized by pits which continue to grow resulting in complete destruction
Also caused by corrosive element present in additives in the lubricating oil
Remedies:
Improving surface hardness improves surface endurance strength
Scoring failure
Causes:
Excessive surface pressure, high surface speed and inadequate supply of lubricantresults in breakdown of the lubricating film
This results in excessive frictional heat and overheating at the contacting surfaces
Remedies:
Proper selection of parameters such as surface speed, surface pressure and flow oflubricating oil such that resulting temperature at the contacting surfaces is withinpermissible limits
Bearing failure
Lubrication of Rolling Contact Bearing
Objective of lubrication:
Reduce friction between balls/rollers and races
Dissipation of frictional heat
Prevention of corrosion
Protection of bearing from dirt and foreign particles
Oil as lubricant:
More effective in carrying away heat
Feeds more easily into contact areas
More effective in flushing out dirt, corrosive and foreign particles from the bearings
Grease as lubricant:
Simple housing design, less possibility of leakage
Less maintenance cost
Better sealing against rust
Guidelines for selection of lubricant
Temperature less than 100°C, grease is preferred and above 100°C, oil should be used
When bore (in mm) x speed (in rpm) is below 2000000, grease is suitable. For highervalues, oil should be used
Grease is suitable for low and moderate loads while oils are used for heavy dutyapplications
If central lubrication system exist (for lubrication of other parts), oil can be used forlubricating bearings also
Though in majority of applications, grease offers simplest and cheapest mode of lubrication
Mountings of Rolling Contact Bearing
Mountings of Rolling Contact Bearing
For the bearing to function satisfactorily and attain required life, correct method ofmounting and cleanliness should be observed.
Following precautions should be taken:
Mounting should be carried out in dust free and dry environment
Before assembly, the burrs on the shaft and shoulders should be removed
The bearing should not be taken out from its packing before it is assembled
While mounting bearing, direct blows should never be applied to the bearing surfaceotherwise race or cage may get damaged
Large bearings are mounted by heating them up to 80°C to 90°C above the ambienttemperature by induction heating and then shrinking them on the shaft. These shouldnot be heated by direct flame
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