Introduction
Correct
and efficient lubrication is essential for the satisfactory function of rolling
element bearings and to attain maximum service life. It is advisable for both
design engineers and maintenance technicians thoroughly understand the basics
of tribology and its effects on the proper function of rolling element
bearings.
Each
rolling element bearing represents a “tribological system” consisting of the
bearing and an adequate lubricant. In the system – namely, in a rolling element
bearing – the lubricant primarily separates the metallic surfaces and thereby
reduces friction, as well as preserving the steel parts and preventing or
neutralizing the effects of contamination.
In
many cases, the bearing life is determined by the service life of the
lubricant: the failure of the lubricant will definitely terminate the life of
the bearing. In the case of oil lubrication, the lubricant may also act as a
coolant, actively taking part in the heat dissipation and thereby supporting
the thermal equilibrium in bearings operating at high speeds.
Lubrication
types
Generally,
three types of lubrication are used in rolling element bearings – grease, oil
and solid or dry lubricants. The selection of a suitable lubrication method
should be made in the early stages of design. The operating conditions – such
as operating speeds, temperature and environment – have to be carefully
evaluated.
The
latest state-of-the-art lubricants feature special agents – additives – to
enhance specific lubricant properties, i.e high pressure characteristics,
temperature stabilization, etc.
Grease
lubrication
Grease
is by the most common form of lubricants, in fact, about 90% of all rolling
element bearings are grease lubricated. Simple application, minimal
maintenance, easy sealing, a wide spectrum of lubricants and the availability
of pre-greased and greased-for-life bearings are some of the main advantages of
grease lubrication.
Grease
consists of a soap structure, which in most cases is a metal soap based on
lithium soap with a base oil and additives.
Oil lubrication
Oil
is usually used for bearing lubrication, if an oil supply system is already
incorporated into the design of a machine for other purposes (e.g gearbox
lubrication, etc.)
Oil
lubrication is also used for special conditions that require effective heat
dissipation. In certain high-speed applications, the use of oil allows for a
highly accurate and easily dosed application, for example via oil jet or oil
mist.
Solid and dry
lubrication
Where
application do not allow the use of oil or grease, for example, due to heat,
several other materials can be used, such as graphite powder or pressed
graphite cages, polytetrafluomethylene (PFTE) powder, metallic coverings,
sliding varnish and various other surface treatments.
Speed ability
Different
lubricants and lubrication methods have different abilities for operating under
given speeds. The most important factor for evaluating the “speed ability” of a
certain lubrication method is the speed characteristics (n.dm); where n is the
bearing operating speed (rpm) and dm the bearing pitch diameter (mm):
dm = (d + D)/2
where,
d = bearing bore diameter and D = outer diameter.
Table 1. Typical
n.dm values of different lubricant types
|
||
S/N
|
Lubrication method
|
n.dm
|
1
|
Grease
lubrication:
|
|
(a) Standard
bearing greases
|
500,000
|
|
(b) Special
greases
|
1,000,000
|
|
2
|
Oil
lubrication:
|
|
(a) Oil bath
lubrication
|
500,000
|
|
(b) Circulating
oil lubrication
|
750,000
|
|
(c) Splash oil
lubrication
|
800,000
|
|
(d) Oil mist
lubrication
|
1,500,000
|
|
(e) Minimum
quantity lubrication
|
3,000,000
|
|
Note: For n.dm values of ˃1,000,000 practical experience is of major importance as special appliances such as oil coolers, additional pumps or a separate compressed air system for oil and air lubrication, may become necessary.
Table
1 shows some typical n.dm values of various lubricating methods. All values are
for guidance only. The lubricant supplier should be contacted for detailed and
accurate values for a specific lubricant.
Characteristic
parameters
Viscosity
This
is one of the most important features for selecting an oil or grease. The term
viscosity describes the ability of oil to flow under given conditions and
temperatures. A low figure indicates thin oil; higher figure denote thicker
oils. In the case of grease, the viscosity of the respective base oil is
indicated.
Since
the viscosity of a lubricant depends considerably on its temperature, the
nominal viscosity of a lubricant is typically indicated with a defined
reference temperature, usually 40˚C.
Consistency
The consistency indicates the "stiffness" of a grease.
It is classified by the NLGI (National Lubrication Grease Institute) scale
according to DIN 51818 where the penetration depth of a standard test cone into
a grease specimen at a standard test temperature and after a standard time is
measured. The deeper the test cone penetrates, the softer the grease is and
hence the lower the NLGI grade. (see Table 2)
Table 2. NGLI consistency grades
of greases
|
||
S/N
|
NGLI consistency grades (DIN51818)
|
Worked penetration (0.1mm)
|
1
|
000
|
445 to 475
|
2
|
00
|
400 to 430
|
3
|
0
|
355 to 385
|
4
|
1
|
310 to 340
|
5
|
2
|
265 to 295
|
6
|
3
|
220 to 250
|
7
|
4
|
175 to 205
|
8
|
5
|
130 to 160
|
9
|
6
|
85 to 115
|
Stiffer lubricants have
higher consistency and therefore higher NLGI grades. For lubricating rolling
bearings, grease lubricants of NLGI grades 2 and 3 are widely used. For certain
high-speed applications, for example spindle bearings, grade 0 and grade 1
greases may be used.
Soft greases are
optimal for small and miniature bearings, low temperatures, high-speeds or when
central lubrication system is installed. Stiffer greases are suitable for large
bearings running at low speeds or for high-temperature applications. Stiffer
greases also provide a better sealing effect.
Grease
volume
The lubricating grease
fill volume in bearing housings is determined mainly by the bearing design and
its actual operating speed. The free space within the bearing itself has to be
fully filled with grease in all cases. The grease fill volume for bearing
housing cavities should be chosen according to Table 3.
Table 3. Relationship between speed ratios and
grease filling volumes
|
|||
S/N
|
Speed ratio
(%) ͦ
|
Grease filling
(%) ͦ ͦ
|
|
˃
|
≤
|
||
1
|
-
|
20
|
80
to 90
|
2
|
20
|
75
|
30
to 50
|
3
|
75
|
25
|
|
˚ In % of the
speed ratings with grease lubrication given in the bearings product tables
˚˚ In % of
bearing housing cavity volume
| |||
Grease
volume
The lubricating grease
fill volume in bearing housings is determined mainly by the bearing design and
its actual operating speed. The free space within the bearing itself has to be
fully filled with grease in all cases. The grease fill volume for bearing
housing cavities should be chosen according to Table 3.
Table 3. Relationship between speed ratios and
grease filling volumes
|
|||
S/N
|
Speed ratio
(%) ͦ
|
Grease filling
(%) ͦ ͦ
|
|
˃
|
≤
|
||
1
|
-
|
20
|
80
to 90
|
2
|
20
|
75
|
30
to 50
|
3
|
75
|
25
|
|
˚ In % of the
speed ratings with grease lubrication given in the bearings product tables
˚˚ In % of
bearing housing cavity volume
|
|||
Grease
service life
Bearing lubricants
undergo permanent mechanical stress as they are over-rolled continuously by the
rolling elements. Additional lubricants change their characteristics, particularly
when opening at high temperatures, in the presence of humidity, pollution and
chemical agents. The service life of lubricants is therefore limited; its length
depends on the individual operating conditions, particularly the operating
temperatures and the speed.
In the case of
grease-for-life bearings, mainly with shields or seals on both sides, the
service life of the lubricant is expected to be longer than the required
bearing life rating. When considering the maintenance of bearings, it is
essential to estimate the service life of the lubricant realistically and
schedule regular re-lubrication.
Re-lubrication
intervals
Re-lubrication
intervals are very much related to the actual operating conditions and the
individual characteristics of the lubricants in question. Modern
high-performance lubricants allow for much longer service life due to the
progress in lubricants research, development and manufacturing. For accurate
figures on specific lubricants the
grease vendor should be consulted; for standard lubricants the realistic
service life in hours, tn, can be roughly estimated with the following
equation:
tn = (a.106/n.√d) – b.d
where,
a and b = bearing type and series coefficient (see Table 4 below), n = bearing
operating speed (in rpm); d = bearing bore diameter (mm)
|
Table 4. Bearing
type and series coefficients for calculating lubricants service life
|
|||
|
S/N
|
Bearing types and series
|
Coefficient
|
|
|
|
|
a
|
b
|
|
A
|
Deep groove ball bearings
|
|
|
|
1
|
160,
60, 62
|
75
|
18
|
|
2
|
63
|
65
|
18
|
|
3
|
64
|
55
|
18
|
|
B
|
Angular contact ball bearings
|
|
|
|
1
|
72
B
|
65
|
18
|
|
2
|
73
B
|
55
|
18
|
|
3
|
32
|
55
|
18
|
|
4
|
33
|
55
|
18
|
|
C
|
Four-point contact bearings
|
|
|
|
1
|
QJ
2
|
65
|
18
|
|
2
|
QJ
3
|
55
|
18
|
|
D
|
Self-aligning ball bearings
|
|
|
|
1
|
12,
22
|
75
|
18
|
|
2
|
13,
23
|
65
|
18
|
|
E
|
Cylindrical roller bearings
|
|
|
|
1
|
N.
10, N. 2, N. 2..E
|
75
|
18
|
|
2
|
N.
3, N. 3..E
|
65
|
18
|
|
3
|
N.
4
|
55
|
18
|
|
F
|
Taper roller bearings
|
|
|
|
1
|
302..,320..,322..,
|
20
|
7
|
|
2
|
303..,313
|
18
|
7
|
|
3
|
323..,
|
15
|
7
|
|
G
|
Spherical roller bearings
|
|
|
|
1
|
222..
|
20
|
7
|
|
2
|
223..,
|
15
|
7
|
For safety reasons, the re-lubrication
intervals of new machines or plants where no practical experience exists should
not exceed 50-60% of the initial calculated service life of the lubricant.
Additional and more precise information
on specific characteristics of lubricants, their chemical reactions with some
elements and the anticipated service life of the lubricant under certain
operating conditions are available from the lubricant manufacturers.
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