SECTION 10 - BALL AND ROLLER BEARINGS
601. The radial reaction on a bearing is 1500 lb.; it also
carries a thrust of 1000 lb.; shaft rotates 1500 rpm; outer ring
stationary; smooth load, 8-hr./day service, say 15,000 hr. (a)
Select a deep-groove ball bearing. (b) What is the rated 90 % life
of the selected bearing? (c) For , compute the probability of the
selected bearing surviving 15,000 hr.
Solution:
, outer ring stationary
assume
(a) Table 12.3
use 321,
To check:
Table 12.2, ,
3.2 % higher than 31,800 lb. Safe.
Therefore use Bearing 321, Deep-Groove Ball Bearing.
(b)
(c)
602. A certain bearing is to carry a radial load of 500 lb. and
a thrust of 300 lb. The load imposes light shock; the desired 90 %
life is 10 hr./day for 5 years at . (a) Select a deep-groove ball
bearing. What is its bore? Consider all bearings that may serve.
(b) What is the computed rated 90 % life of the selected bearing?
(c) What is the computed probability of the bearing surviving the
specified life? (d) If the loads were changed to 400 and 240 lb.,
respectively, determine the probability of the bearing surviving
the specified life, and the 90 % life under the new load.
Solution:
Assume
Table 12.2,
(a)
Assume
For light shock, service factor ~ 1.1
Table 12.3,
Bearing No.
, lb
, lb
Bore
217
14,400
12,000
85 mm
312
14,100
10,900
60 mm
Select, Bearing No. 312
(b) Table 12.2
(c)
use
(d)
Table 12.2
Life:
603. The smooth loading on a bearing is 500-lb radial, 100 lb.
thrust; . An electric motor drives through gears; 8 hr./day, fully
utilized. (a) Considering deep-groove ball bearings that may serve,
choose one end specify its bore. For the bearing chosen, determine
(b) the rated 90 % life and (c) the probability of survival for the
design lufe.
Solution:
Table 12.1, 8 hr./day fully utilized, assume 25,000 hr
(a) assume
consider
Table 12.3
Bearing No.
, lb
, lb
207
4440
3070
306
4850
3340
305
3660
2390
Select 305, ,
Bore (Table 12.4) = 25 mm
(a)
Table 12.2,
Rated Life:
(c)
605. A No. 311, single-row, deep-groove ball bearing is used to
carry a radial load of 1500 lb. at a speed of 500 rpm. (a) What is
the 90 % life of the bearing in hours? What is the approximate
median life? What is the probability of survival if the actual life
is desired to be (b) 105 hr., (c) 104 hr.?
Solution:
Table 12.3, No. 311
assume
(a)
For median life = 5( 90 % life) =
(b)
(c) 104 hr
606. The load on an electric-motor bearing is 350 lb., radial;
24 hr. service, ; compressor drive; outer race stationary. (a)
Decide upon a deep-groove ball bearing, giving its significant
dimensions. Then compute the selected bearings 90 % life, and the
probable percentage of failures that would occur during the design
life. What is the approximate median life of this bearing? (b) The
same as (a), except that a 200 series roller bearing is to be
selected.
Solution:
outer race stationary,
Table 12.1
90 % Life, hrs = 50,000 hrs
(a)
Table AT 12.3
earing No.
, lb
, lb
208
5040
3520
209
5660
4010
306
4850
3340
307
5750
4020
Use No. 209
Table 12.4, Dimension
Bore = 45 mm
O.D. = 85 mm
Width of Races = 19 mm
Max. Fillet r = 0.039 mm
90 % Life:
Probability.
% failures = 1 0.916 = 0.084 = 8.4 %
Median Life = 5(58,740) = 293,700 hrs
(b) Table 12.3,
use No. 207,
Bore = 35 mm
O.D. = 72 mm
Width of Races = 17 mm
90 % life:
Probability.
% failures = 1 0.926 = 0.074 = 7.4 %
Median Life = 5(66,530) = 332,650 hrs
608. A deep-groove ball bearing on a missile, supporting a
radial load of 200 lb., is to have a design life of 20 hr.; with
only a 0.5 % probability of failure while . Using a service factor
of , choose a bearing. ( A 5- or 6- place log table is
desirable.)
Solution: No need to use log table.
assume
Table 12.3
Select No. 201,
VARIABLE LOADS
610. A certain bearing is to carry a radial load of 10 kip at a
speed of 10 rpm for 20 % of the time, a load of 8 kips at a speed
of 50 rpm for 50 % of the time, and a load of 5 kips at 100 rpm
during 30 % of the time, with a desired life of 3000 hr.; no
thrust. (a) What is the cubic mean load? (b) What ball bearings may
be used? What roller bearings?
Solution:
(a)
For 1 min.
(b)
assume
1 min = 57 rev
Table 12.3, Ball Bearing
Use Bearing No. 217,
(c) Table 12.3 (Roller Bearing)
Use Bearing No. 213,
612. A deep-groove ball bearing No. 215 is to operate 30 % of
the time at 500 rpm with and , 55 % of the time at 800 rpm with and
, and 15 % of the time at 1200 rpm with and . Determine (a) the
cubic mean load; (b) the 90 % life of this bearing in hours, (c)
the average life in hours.
Solution:
Bearing No. 215, ,
Table 12.2,
At 30 % of the time, 500 rpm
At 55 % of the time, 800 rpm
At 15 % of the time, 1200 rpm
(a)
For 1 min.
(b)
For 1 min = 770 rev
(c) Average life = 5(11,000) = 55,000 hrs
MANUFACTURERS CATALOG NEEDED
614. A shaft for the general-purpose gear-reduction unit
described in 489 has radial bearing reactions of and ; . Assume
that the unit will be fully utilized for at least 8 hr./day, with
the likelihood of the same uses involving minor shock. (a) Select
ball bearings for this shaft. (b) Select roller bearings. (c) What
is the probability of both bearings C and D surviving for the
design life?
Solution:
Problem 489,
Ref: Design of Machine Members, Doughtie and Vallance
at C.
Table 12.1, 8 hr/day, fully utilized, Text
assume for 90 % reliability
Table 9-7, Doughtie and Vallance,
Two-row spherical Type, No. 207
Bore = 1.3780 in,
At D.
Table 9-7, Doughtie and Vallance,
Two-row spherical Type, No. 407
Bore = 1.3780 in,
(b) at C,
Table 9.8, Doughtie and Vallance
Use No. 207, Bore = 1.3780 in,
at C,
Table 9.8, Doughtie and Vallance
Use No. 307, Bore = 1.3780 in,
(c) For probability:
(c.1) at C, Bearing No. 207, Two-row spherical bearing,
Table 9-3, Reference
Probability = 95.8 %
at D, Bearing No. 407, Deep-groove bearing,
Table 9-3, Reference
Probability = 93.3 %
(c.2) at C, Roller Bearing No. 207,
Table 9-3, Reference
Probability = 98.8 %
at D, Roller Bearing No. 407,
Table 9-3, Reference
Probability = 95.7 %
615. A shaft similar to that in 478 has the following radial
loads on the bearings, left to right: 803 lb, 988 lb, 84 lb, and
307 lb; no thrust. The minimum shaft diameter at the bearings are
1.250 in, 1.125 in, 1.000 in, and 1.0625 in. Assume that the
service will not be particularly gentle; intermittently used, with
. (a) Select ball bearing for this shaft. (b) Select roller
bearings.
Solution:
Ref: Design of Machine Members by Doughtie and Vallance
Table 12.1, Text, (intermittent)
90 % reliability,
assumed
(a) Ball Bearing
(a.1) 803 lb,
Table 9-7, Ref.
Two-row spherical type, 207
Bore = 1.3780 in
(a.2) 988 lb,
Table 9-7, Ref.
Two-row spherical type, 306
Bore = 1.1811 in
(a.3) 84 lb,
Table 9-7, Ref.
Deep-groove type, 106
Bore = 1.1811 in
(a.4) 307 lb,
Table 9-7, Ref.
Deep-groove type, 106
Bore = 1.1811 in
(b) Roller Bearing
(b.1) 803 lb,
, Bore = 1.3780 in
use No. 207,
(b.2) 988 lb,
, Bore = 1.1811 in
use No. 206,
(b.3) 84 lb,
, Bore = 1.1811 in
use No. 206,
(b.4) 307 lb,
, Bore = 1.1811 in
use No. 206,
- end -
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