Page 335 - The Tribology Handbook

Rolling bearing lubrication C8

roo 000 Method of lubrication

50 ooal Rolling bearings may be lubricated with grease by a
lubrication system as described in other sections of the
handbook or may be packed with grease on assembly.
z
W- 20000
Lb
d
g 10000
2
a
8 5000
W
: Packing ball and roller bearings with grease
r
:: 2000 (a) The grease should not occupy more than one-half
OI to three-quarters of the total available free space in the
a
1300 covers with the bearing packed full.
1000 (b) One or more bearings mounted horizontally-com-
pletely fill bearings and space between, if more than one,
but fill only two-thirds to three-quarters of space in covers.
500 (c) Vertically-mounted bearings-completely fill bear-
20 40 60 80 100 120 140 ing but fill only half of top cover and three-quarters of
TEMPERATURE, 'C bottom cover.
Figure 8.2 \/ariation of operating life of a high- (d) Low/medium speed bearings in dirty environments
quality grade 3 /ithiurn hydroxystearate grease with -comP1etely bearing and covers.
speed and temperature

Calculation of relubrication interval

The relubrication period for ball and roller bearings may
be estimated using Figures 8.1 and 8.2. The following is an
example in terms of a typical application: Relubrication of ball and roller bearings
Required to know : Approximate relubrication period Relubrication may be carried out in two ways, depending
for the following: on the circumstances :
(a) Replenishment, by which is meant the addition of
Bearing type : Medium series bearing 60 mm bore. fresh grease to the original charge.
Cage : Pressed cage centred on balls. (b) Repacking, which normally signifies that the bearing
Speed : 950 rev/nnin. is dismounted and all grease removed and discarded, the
Temperature : 120°C [The bearing temperature bearing then being cleaned and refilled with fresh grease.
(not merely the local ambient tem- An alternative, if design permits, is to flush the bearing
perature) i.e. either measured or with fresh grease in situ. (Grease relief valves have been
estimated as closely as possible.] developed for this purpose.)
The quantity required per shot is an arbitrary amount.
Position : Vertical shaft. Requirement is only that sufficient grease is injected to
Grease : Lithium grade 3, disturb the charge in the bearing and to displace same
Duty : Continuous. through the seals, or grease relief valves.
From Figure 8.1 : 60 mm bore position on the lower A guide can be obtained from
edge of the graph intersects the Dxw
medium series curve at approxi- W=-
200
mately 3100 revlmin.
where W is quantity (g)
Factor for pressed cages on balls is about 1.5; thus D is outside diameter (rnm)
3100 x 1.5 = 4650 revlmin. and w is width (mm)
Factor for vertical mounting is 0.75. Thus 4650~0.75
= 3488 sev/min. If grease relief valves are not fitted, the replenishment
This is the rnaximum speed rating ( 100%). charge should not exceed 5% of the original charge. After
Now actual speed = 950 rev/min; therefore percentage grease has been added to a bearing, the housing vent plug
-
(if fitted) should be left out for a few minutes after start-up
9 50
of maximum = -x 100 = 27% (say 25% approxi- in order to allow excess grease to escape. A better method,
348% if conditions allow, is to push some of the static grease in the
mately). cover back into the bearing to redistribute the grease
In Figure 8.2 the 12D"C vertical line intersects the 25% throughout the assembly. This method is likely to be
speed rating curve for the grade 3 lithium grease at unsatisfactory when operating temperatures exceed about
approximately 1300 hours, which is the required answer. 100°C.
C8.3