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What SORT of bearing should you choose?
Use this chart data to initiate the bearing
selection process. See the link "Bearing
Engineering and Knowledge Application Menu" for
additional bearings knowledge information.
Sizing of bearing is usually done
through supplier's catalogues or application experts. Typical
selection procedures are given in the notes.
Legend:
A =
Excellent
B = Good
C
= Satisfactory
F = Poor
NR =
Inconvenient or not recommended
- =
Not applicable
Bearing
Type |
Pure
Radial Load |
Pure
Axial load |
Combined
Load |
Moment
Load |
High
Speed |
High
Running Accuracy |
High
Stiffness |
Quiet
Running |
Low
Friction |
Comp. for
Errors of Alignment During Operation |
Comp. for
Errors of Alignment (Initial) |
Location
Bearing Installation |
Non-location Bearing Installation |
Axial
Disp. Possible in Bearing |
Ball
Single - Row |
C |
C |
C |
F |
A |
A |
C |
A |
A |
F |
F |
B |
C |
NR |
C Ball
Single Double - Row |
C |
C |
C |
C |
C |
C |
C |
C |
B |
NR |
NR |
C |
C |
NR |
Ball Self
- Aligning |
C |
F |
F |
NR |
B |
B |
F |
B |
B |
A |
B |
C |
C |
NR |
Ball
Angular Contact |
C |
C |
B |
F |
B |
A |
C |
B |
B |
F |
F |
B |
NR |
NR |
Ball
Angular Contact Back - to - Back |
B |
C |
B |
C |
C |
B |
B |
C |
C |
NR |
NR |
B |
C |
NR |
Ball Four
- Point Contact |
F |
C |
C |
C |
B |
C |
C |
C |
C |
NR |
NR |
B |
F |
NR |
Cylindrical Roller N, NU |
B |
NR |
NR |
NR |
A |
B |
B |
B |
B |
F |
F |
NR |
A |
A |
Cylindrical Roller NJ, NUP |
B |
C |
C |
NR |
A |
B |
B |
C |
B |
F |
F |
C |
C |
C |
Cylindrical Roller Double Row |
A |
NR |
NR |
C |
A |
A |
A |
B |
B |
NR |
NR |
NR |
A |
A |
Full
Complement Cylinder Roller |
A |
C |
F |
NR |
F |
C |
A |
F |
F |
F |
F |
C |
C |
C |
Full
Cylinder Roller Double -
Row |
A |
C |
F |
C |
F |
C |
A |
F |
F |
NR |
NR |
C |
C |
C |
Needle
Roller |
B |
NR |
NR |
NR |
C |
C |
B |
C |
F |
NR |
NR |
NR |
A |
A |
Spherical
Roller |
A |
C |
A |
NR |
C |
C |
B |
C |
C |
A |
B |
B |
C |
NR |
Taper
Roller |
B |
B |
A |
NR |
C |
B |
B |
C |
C |
F |
F |
B |
NR |
NR |
Taper
Roller
( Face - to - Face) |
A |
B |
A |
F |
C |
C |
A |
C |
C |
F |
F |
A |
C |
NR |
Thrust
Ball |
NR |
C |
NR |
NR |
C |
B |
C |
F |
C |
NR |
NR |
C |
NR |
NR |
Thrust Bal
with Spherical Housing Washer |
NR |
C |
NR |
NR |
C |
C |
C |
F |
C |
NR |
- |
C |
NR |
NR |
Thrust
Cylinder Roller |
NR |
B |
NR |
NR |
F |
B |
B |
F |
F |
NR |
NR |
C |
NR |
NR |
Thrust
Needle Roller |
NR |
B |
NR |
NR |
F |
C |
B |
F |
F |
NR |
NR |
C |
NR |
NR |
Thrust
Spherical Roller |
NR |
A |
C |
NR |
C |
C |
B |
F |
C |
A |
B |
B |
NR |
NR |
(From Engineers
Edge)
There are many types of rolling-element bearings, each tuned for a
specific kind of load and with specific advantages and disadvantages.
Ball bearings
Ball bearings use spheres instead of cylinders. Clever use of surface
tension allows balls of high accuracy to be made much more cheaply than
comparable cylinders. Ball bearings can support both radial
(perpendicular to the shaft) and axial loads (parallel to the shaft).
For lightly-loaded bearings, balls offer lower friction than rollers.
Ball bearings can operate when the bearing races are misaligned.
Roller bearings
Common roller bearings use cylinders of slightly greater length than
diameter. Roller bearings typically have higher radial load capacity
than ball bearings, but a low axial capacity and higher friction under
axial loads. If the inner and outer races are misaligned, the bearing
capacity often drops quickly compared to either a ball bearing or a
spherical roller bearing.
Needle bearing
Needle roller bearings use very long and thin cylinders. Since the
rollers are thin, the outside diameter of the bearing is only slightly
larger than the hole in the middle. However, the small-diameter rollers
must bend sharply where they contact the races, and thus the bearing
fatigues relatively quickly.
Tapered roller bearing
Tapered roller bearings use conical rollers that run on conical races.
Most roller bearings only take radial loads, but tapered roller
bearings support both radial and axial loads, and generally can carry
higher loads than ball bearings due to greater contact area. Taper
roller bearings are used, for example, as the wheel bearings of most
cars, trucks, buses, and so on. The downsides to this bearing is that
due to manufacturing complexities, tapered roller bearings are usually
more expensive than ball bearings; and additionally under heavy loads
the tapered roller is like a wedge and bearing loads tend to try to
eject the roller; the force from the collar which keeps the roller in
the bearing adds to bearing friction compared to ball bearings.
Spherical roller bearings
Spherical roller bearings use rollers that are thicker in the middle
and thinner at the ends; the race is shaped to match. Spherical roller
bearings can thus adjust to support misaligned loads. However,
spherical rollers are difficult to produce and thus expensive, and the
bearings have higher friction than a comparable ball bearing since
different parts of the spherical rollers run at different speeds on the
rounded race and thus there are opposing forces along the bearing/race
contact.
Thrust bearing
An axial load is supported by this type, typically to support a
vertical shaft against gravitational loads. Spherical, conical or
cylindrical rollers are used; and non rolling element bearings such as
hydrostatic or magnetic bearings see some use where particularly heavy
loads or low friction is needed.
Sizing a Rolling Bearing - Examples
Problem 1.
Determine a suitable deep groove ball bearing with 65mm bore to carry
radial load of 5.15kN at 1000rpm for a nominal life of 20000 hours.
Solution. (Using the ISO L10 method - Outlined on page 8)
Data:
bore = 65mm
N = 1000 RPM
h or Lh= 20000 hours
Fr = 5.15kN
1. Determine the number of hours. (It tells you... h = 20000)
2. Convert to design life in millions of revs;
3. Determine radial load, Fr, and axial load, Fa,
for the bearing.
Fr=5.15kN
Fa=0kN
4. Calculate Fa/Fr.
Fa/Fr=0
5. Select bearing from tables p 26,27 for a shaft size of d = 65mm:
Range for dynamic load rating C: 11900 to 119000
Range for static load rating Co: 9650 to 78000
6. Ratio Fa/Co = 0.
Now read graph on p22; (Note: The vertical axis is Fa/Co)
If Fa/Co = 0 then e = 0.2 (estimated from graph)
7. Equivalent dynamic bearing load P:
Since Fa/Fr = 0, then Fa/Fr < e, so P = Fr + Y*Fa = Fr + 0 = 5.15 kN
8. Bearing life equation:
L10 = (C / P ) ^3
Re-arrange to solve for C...
9. Select bearing from catalogue with C greater than 54.7kN..(See p27).
6213
bearing:, C=55.9kN, Co=34000
ID=65mm, OD=140mm, Breadth = 33mm
10. Calculate equivalent static bearing load, Po.
Po=0.6*Fr+0.5*Fa so Po = Fr + 0 = 5.15kN
So Po=5.15kN is much less than Co=34kN. So bearing is OK.
11. Check minimum radial load on bearing. (There is none- OK)
12. Check maximum speeds.
Limiting speeds: 5300rpm with grease, 6300rpm with oil. This is fine.
Problem 2.
A deep groove ball bearing is required to carry a radial load of 2.18kN
and a thrust load of 450N at a speed of 1600rpm. Select a
suitable bearing for a working life of 10000 hours.
Solution. (Using the ISO L10 method - Outlined on page 8)
Data:
Fr = 2.18kN
Fa = 450N
RPM = 1600
Lh = 10000
ID = 28mm (minimum shaft size)
1. 10000 hours
2. Determine the required basic rating life, L10, of
the bearing in millions of revolutions.
3. Determine radial load, Fr, and axial load, Fa, for the bearing.
Fr = 2.18kN
Fa = 450N
4. Calculate Fa/Fr.
Fa/Fr = 450 / 2180 = 0.206
5. Select bearing from p26.
D=28mm does not exist, so go up to 30mm
Range of load rating: From (C=4.49kN, Co=2.9kN) to (C=43.6kN, Co=23.6kN)
6. Use ratio to get e from graph on p22.
Fa/Co = 0.450/2.9 = 0.155 (min size)
Fa/Co = 0.450/23.6 = 0.019 (max size)
Now read graph on p22; (Note: The vertical axis is Fa/Co)
If Fa/Co = 0.155 then e = 0.325 (from graph)
If Fa/Co = 0.019 then e = 0.22 (estimated from graph)
7. Equivalent dynamic bearing load P:
In all bearing cases above, Fa/Fr < e, so P = Fr = 2.18 kN
8. Bearing life equation:
L10 = (C / P ) ^3
Re-arrange to solve for C...
9. Select bearing from catalogue with C greater than 21.5kN..(See p26).
6306 bearing: C=28100, Co=14600
ID=30mm, OD=62mm, B=19mm
10. Calculate equivalent static bearing load, Po.
Po=0.6*Fr+0.5*Fa = 0.6*2.18 + 0.5*0.45 = 1.533kN
So Po=1.533kN is much less than Co=14.6kN. So bearing is OK.
11. Check minimum radial load on bearing.
Is Fr>0.01C? Where Fr=2.18, 0.01C = 0.01*28.1 = 0.281
Yes! (Bearing axial loading is OK)
12. Check maximum speeds.
Limiting speeds: 9000rpm with grease, 11000rpm with oil. This is fine.
Problem 3.
A deep groove ball bearing with minimum bore of 60mm is required to
carry a radial load of 4kN and a thrust load of 2.2kN at 1000rpm.
Select a suitable bearing for a working life of 10000 hours.
Solution.
Data:
bore*60mm
Fr=4kN
Fa=2.2kN
RPM=1000
Lh=10000 hours
1. Determine the required basic rating life, L10, of
the bearing in millions of revolutions.
2. Determine radial load, Fr, and axial load, Fa,
for the bearing.
Fr=4kN
Fa=2.2N
3. Calculate Fa/Fr.
Fa/Fr=2200/4000=0.55
4. Estimate factors X and Y.
Fa/Fr*0.44
estimate X=0.56, Y=1.5
5. Calculate the Equivalent dynamic bearing load, P.
P=X*Fr+Y*Fa=(0.56*4)+(1.5*2.2)
P=5.54kN
6. Calculate the Basic dynamic load rating, C.
7. Select bearing from catalogue with C greater than step 6.
6212 bearing:
C=47500
Co=28000
ID=60mm
OD=110mm
8. Calculate Fa/Co.
Fa/Co=2200/28000=0.079
9. Check X and Y.
e = 0.28
so Fa/Fr greater than e
so Xactual=0.56,Yactual=1.6
Repeat step 5
to step 8
5. Calculate the Equivalent dynamic bearing load, P.
P=X*Fr+Y*Fa=(0.56*4)+(1.6*2.2)
P=5.76kN
6. Calculate the Basic dynamic load rating, C.
7. Select bearing from catalogue with C greater than step 6.
we require next largest bearing
6312 bearing:
C=81900
Co=48000
ID=60mm
OD=130mm
10. Calculate Equivalent static bearing load, Po.
Po=0.6*Fr+0.5*Fa
Po=3.5kN
but, Fr=4kN
therefore use Po=4kN
bearing is OK.
11. Check maximum speeds.
Limiting speeds:
5000rpm with grease
6000rpm with oil.
Problem 4.
The vertical shaft shown is supported by two deep groove ball
bearings. Bearing A at the top is allowed to float whereas bearing B at the bottom carries
all the axial thrust. The shaft is driven by vee belts through a
pulley. The sum of the belt tensions is 5.4kN and the estimated
weight of the pulley assembly is 900N. The shaft has a minimum
diameter, based on strength, of 24mm and rotates at 300rpm. Use
the SKF catalogue for selection of two bearings based on a life of 9000
hours and a shock factor of 2 for vee belt forces.
Solution:
Data:
F1+F2=5.4kN
FaA=0
FaB=900N
RPM=300
Shock factor=2
Lh=9000 hours
IDmin=24mm
1. Determine the required basic rating life, L10, of
the bearing in millions of revolutions.
2. Determine radial load, Fr, and axial load, Fa,
for the bearings.
Shock factor, f=2, meaning all radial
loads must be doubled.
Using moments,
3. Calculate Fa/Fr.
Bearing A: Fa/Fr=0
Bearing B: Fa/Fr=900/3600=0.25
4. Estimate factors X and Y.
Bearing A: Fa/Fr=0,
X=1, Y=0
Bearing B: Fa/Fr<0.44, X=1, Y=0
5. Calculate the Equivalent dynamic bearing load, P.
Bearing A: P=Fr=7.2kN
Bearing B: P=Fr=3.6kN
6. Calculate the Basic dynamic load rating, C.
Bearing A
Bearing B
7. Select bearings from catalogue with C greater than step 6.
Bearing A
|
Bearing B |
6305 bearing |
6205 bearing |
ID=25mm |
ID=25mm |
OD=62mm |
OD=52mm |
C=22500 |
C=14000 |
Co=11400 |
Co=6950 |
8. Calculate Fa/Co.
Bearing A
Fa/Co=0
Bearing B
Fa/Co=900/6950=0.1295
9. Check X and Y.
Bearing B
e = 0.31
e greater than Fa/Fr
therefore X=1,Y=0 is used.
10. Calculate Equivalent static bearing load, Po.
Bearing A
Po=Fr=7.2kN
Bearing B
Po=0.6*Fr+0.5*Fa
Po=0.6*3.6+0.5*0.9
Po=2.61kN
but,
Fr=3.6kN
therefore Po=Fr=3.6kN
in both cases Po>Co
11. Check maximum speeds.
Limiting speeds:
Bearing A
11000rpm with grease
14000rpm with oil.
Bearing B
12000rpm with grease
15000rpm with oil.
Assignments
1. FLYWHEEL BEARING ASSEMBLY. A flywheel is held by 2 deep groove ball bearings.
(a) Draw a FBD, determine the loads on the shaft and complete the statics (reactions)
(b) Determine the minimum size of the shaft (using simple beam bending
equations) . Note: This is not a complete shaft design according to
Shaft Design Standards - we will do this later. Assume a simple working
stress of 100MPa.
(c) Determine the loads on the bearings
(d) Select bearings. Assume 25000 hour life.
(e) Explain the bearing arrangement in terms of assembly issues, load
sharing, radial and axial loads, preload and temperature changes
causing expansion, maintenance.