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REPORT 18792
First edition
2008-12-15
Lubrication of industrial gear drives
Lubrification des entraînements par engrenages industriels
Reference number
ISO/TR 18792:2008(E)
©
ISO 2008
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ISO/TR 18792:2008(E)
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ISO/TR 18792:2008(E)
Contents Page
Foreword. v
Introduction . vi
1 Scope . 1
2 Terms and definitions. 1
3 Basics of gear lubrication and failure modes.3
3.1 Tribo-technical parameters of gears. 3
3.2 Gear lubricants. 5
3.3 Base fluid components . 6
3.4 Thickeners . 8
3.5 Chemical properties of additives . 9
3.6 Solid lubricants . 10
3.7 Friction and temperature . 10
3.8 Lubricating regime. 11
3.9 Lubricant influence on gear failure. 11
4 Test methods for lubricants . 15
4.1 Gear tests . 15
4.2 Other functional tests. 16
5 Lubricant viscosity selection . 19
5.1 Guideline for lubricant selection for parallel and bevel gears (not hypoid). 19
5.2 Guideline for lubricant selection for worm gears. 24
5.3 Guideline for lubricant selection for open girth gears. 24
6 Lubrication principles for gear units . 26
6.1 Enclosed gear units. 27
6.2 Open gearing. 34
7 Gearbox service information . 39
7.1 Initial lubricant fill and initial lubricant change period . 39
7.2 Subsequent lubricant change interval. 39
7.3 Recommendations for best practice for lubricant changes. 40
7.4 Used gear lubricant sample analysis. 41
Bibliography . 52
Figures
Figure 1 — Load and speed distribution along the path of contact. 4
Figure 2 — Scraping edge at the ingoing mesh . 5
Figure 3 — Schematic diagram of shear effects on thickeners. 9
Figure 4 — Mechanisms of surface protection for oils with additives. 11
Figure 5 — Examples of gear oil wear test results. 15
Figure 6 — Immersion of gear wheels . 27
Figure 7 — Immersion depth for different inclinations of the gearbox. 29
Figure 8 — Immersion of gear wheels in a multistage gearbox . 30
Figure 9 — Examples of circuit design, combination of filtration and cooling systems . 34
Figure 10 — Immersion lubrication. 37
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ISO/TR 18792:2008(E)
Figure 11 — Transfer lubrication. 37
Figure 12 — Circulation lubrication . 38
Figure 13 — Automatic spraying lubrication . 38
Tables
Table 1 — Symbols, indices and units. 1
Table 1 (continued) . 2
Table 2 — General characteristics of base fluids. 6
Table 3 — Example of influence factors on wear . 12
Table 4 — Example of influence factors on scuffing load (transmittable torque). 13
Table 5 — Example of influence factors on micropitting (transmittable torque) . 14
Table 6 — Example of influence factors on pitting (transmittable torque) . 14
1)
Table 7 — ISO Viscosity grade at bulk oil operating temperature for oils having a viscosity
2)
index of 90 . 20
1)
Table 8 — ISO Viscosity grade at bulk oil operating temperature for oils having a viscosity
2)
index of 120 . 21
1)
Table 9 — ISO Viscosity grade at bulk oil operating temperature for oils having a viscosity
2)
index of 160 . 22
1)
Table 10 — ISO Viscosity grade at bulk oil operating temperature for oils having a viscosity
2)
index of 240 . 23
Table 11 — ISO viscosity grade guidelines for enclosed cylindrical worm gear drives . 24
Table 12 — Advantages and disadvantages of various open girth gears lubricants . 25
2
Table 13 — Minimum Viscosity recommendation for continuous lubrication [mm /s at 40 °C]. 26
2
Table 14 — Minimum base oil viscosity recommendation for intermittent lubrication [mm /s at
40 °C] . 26
Table 15 — Typical maximum oil flow velocities. 33
Table 16 — Advantages and disadvantages of greases .35
Table 17 — Advantages and disadvantages of oils. 35
Table 18 — Advantages and disadvantages of lubricating compounds. 36
Table 19 — Lubrication system selection based on pitch line velocity . 39
Table 20 — Lubrication system selection based on the type of lubricant. 39
Table 21 — Typical recommended lubricant service . 40
Table 22 — Examples for an on-line oil condition-monitoring system . 40
Table 23 — Sources of metallic elements . 47
Table 24 — What the ISO codes mean. 49
Table 25 — Example of particle size and counts. 49
Table 26 — Characteristics of particles. 51
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ISO/TR 18792:2008(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 18792 was prepared by Technical Committee ISO/TC 60, Gears, Subcommittee SC 2, Gear capacity
calculation.
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ISO/TR 18792:2008(E)
Introduction
Gear lubrication is important in all types of gear applications. Through adequate lubrication, gear design and
selection of gear lubricant, the gear life can be extended and the gearbox efficiency improved. In order to
focus on the available knowledge of gear lubrication, ISO/TC 60 decided to produce this Technical Report
combining primary information about the design and use of lubricants for gearboxes.
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TECHNICAL REPORT ISO/TR 18792:2008(E)
Lubrication of industrial gear drives
1 Scope
This Technical Report is designed to provide currently available technical information with respect to the
lubrication of industrial gear drives up to pitch line velocities of 30 m/s. It is intended to serve as a general
guideline and source of information about the different types of gear, and lubricants, and their selection for
gearbox design and service conditions. This Technical Report is addressed to gear manufacturers, gearbox
users and gearbox service personnel, inclusive of manufacturers and distributors of lubricants.
This Technical Report is not applicable to gear drives for automotive transmissions.
2 Terms and definitions
For the purposes of this document, the following terms, definitions, symbols, indices and units apply.
Table 1 — Symbols, indices and units
Symbol, index Term Unit
A, B, C, D, E points on the path of contact —
b face width mm
3
C cubic capacity of the oil pump
cm
d diameter mm
d
outside diameter pinion, wheel mm
a1, 2
d
base circle diameter pinion, wheel mm
b1, 2
d
operating pitch diameter pinion, wheel mm
w1, 2
0,5 1,5
f
curvature factor N /mm
H
f
load factor —
L
F
circumferential load at base circle N
bt
n
rotational speed of the oil pump driving shaft rpm
shaft
p pressure bar
2
p
hertzian stress
N/mm
H
P
gear power kW
P
gear power loss kW
vz
P
total gearbox power loss kW
vzsum
s slip —
t time sec
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ISO/TR 18792:2008(E)
Table 1 (continued)
Symbol, index Term Unit
V
oil quantity l
Q
oil flow l/min
e
Q
oil flow through the bearings l/min
bearings
Q
oil flow through the gear mesh l/min
gears
Q
oil pump flow l/min
pump
Q
oil flow through the seals l/min
seals
v pitch line velocity m/s
v
surface velocity pinion, wheel m/s
1, 2
v
sliding velocity m/s
g
v
pitch line velocity m/s
t
v
sum velocity m/s
Σ
V
oil tank volume l
tank
z
number of pinion teeth —
1
β helix angle degree
relation between the calculated film thickness and the
λ —
effective surface roughness
2.1
intermittent lubrication
intermittent common lubrication of gears which are not enclosed
NOTE Gears that are not enclosed are referred to as open gears.
2.2
manual lubrication
hand application
periodical application of lubricant by a user with a brush or spout can
2.3
centralized lubrication
intermittent lubrication of gears by means of a mechanical applicator in a centralized system
2.4
continuous lubrication
continuous application of lubricant to the gear mesh in service
2.5
splash lubrication
bath lubrication
immersion lubrication
dip lubrication
process, in an enclosed system, by which a rotating gear or an idler in mesh with one gear is allowed to dip
into the lubricant and carry it to the mesh
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ISO/TR 18792:2008(E)
2.6
oil stream lubrication
pressure-circulating lubrication
forced-circulation lubrication
continuous lubrication of gears and bearings using a pump system which collects the oil in a sump and
recirculates it
2.7
drop lubrication
use of oil pump to siphon the lubricant directly onto the contact portion of the gears via a delivery pipe
2.8
spray lubrication
process in oil stream lubrication by which the oil is pumped under pressure to nozzles that deliver a stream or
spray onto the gear tooth contact, and the excess oil is collected in the sump and then returned to the pump
via a reservoir
2.9
spray lubrication for open gearing
continuous or intermittent application of lubricant using compressed air
2.10
oil mist lubrication
process by which oil mist, formed from the mixing of lubricant with compressed air, is sprayed against the
contact region of the gears
NOTE It is especially suitable for high-speed gearing.
2.11
brush lubrication
process by which lubricant is continuously brushed onto the active tooth flanks of one gear
2.12
transfer lubrication
continuous transferral of lubricant onto the active tooth flanks of a gear by means of a special transfer pinion
immersed in the lubricant or lubricated by a centralized lubrication system
3 Basics of gear lubrication and failure modes
3.1 Tribo-technical parameters of gears
3.1.1 Gear types
There are different types of gear such as cylindrical, bevel and worm. The type of gear used depends on the
application necessary. Cylindrical gears with parallel axes are manufactured as spur and helical gears. They
typically have a line contact and sliding only in profile direction. Cylindrical gears with skewed axes have a
point contact and additional sliding in the axial direction. Bevel gears with an arbitrary angle between their
axes without gear offset have a point contact and sliding in profile direction. They generally have
perpendicular axes and are manufactured as straight, helical or spiral bevel gears. Bevel gears with gear
offset are called hypoid gears with point contact and sliding in profile and axial directions. Worm gears have
crossed axes, line contact and sliding in profile and mainly axial direction.
3.1.2 Load and speed conditions
The main tribological parameters of a gear contact are load, pressure, and rolling and sliding speed. A static
load distribution along the path of contact as shown in Figure 1 can be assumed for spur gears without profile
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ISO/TR 18792:2008(E)
modification. In the zone of single tooth contact the full load is transmitted by one tooth pair, in the zone of
double tooth contact the load is shared between two tooth pairs in contact.
1
Key
1 spur gear without profile correction
Figure 1 — Load and speed distribution along the path of contact
The static load distribution along the path of contact can be modified through elasticity and profile
modifications. Due to the vibrational system of the gear contact, dynamic loads occur as a function of the
dynamic and natural frequency of the system. A local Hertzian stress for the unlubricated contact can be
derived from the local load and the local radius of curvature (see Figure 1). When a separating lubricating film
is present, the Hertzian pressure distribution in the contact is modified to an elastohydrodynamic pressure
distribution with an inlet ramp, a region of Hertzian pressure distribution, possibly a pressure spike at the
outlet and a steep decrease from the pressure maximum to the ambient.
The surface speed of the flanks changes continuously along the path of contact (see Figure 1). The sum of
the surface speeds of pinion and wheel represents the hydrodynamically effective sum velocity; half of this
value is known as entraining velocity. The difference of the flank speeds is the sliding velocity, which together
with the frictional force results in a local power loss and contact heating. Rolling without sliding can only be
found in the pitch point with its most favourable lubricating conditions. Unsteady conditions with changing
pressure, sum and sliding velocity along the path of contact are the result. In addition, with each new tooth
coming into contact, the elastohydrodynamic film must be formed anew under often unfavourable conditions of
the scraping edge of the driven tooth (see Figure 2).
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2
3
1
Key
1 pinion (driving)
2 wheel (driven)
3 first contact point
Figure 2 — Scraping edge at the ingoing mesh
3.2 Gear lubricants
3.2.1 Overview of lubrication
Regarding gear lubrication, the primary concern is usually the gears. In addition to the gears there are many
other components that must also be served by the fluid in the gearbox. Consideration should also be given to
the bearings, seals, and other ancillary equipment, e.g., pumps and heat exchangers, that can be affected by
the choice of lubricant. In many open gear drives the bearings are lubricated independently of the gears, thus
allowing for special fluid requirements should the need arise. However, most enclosed and semi-enclosed
gear drives utilize a single lubricant and lubricant source of supply for the gears, bearings, seals, pumps, etc.
Therefore, selecting the correct lubricant for a gear drive system includes addressing the lubrication needs of
not only the gears but all other associated components in the system.
A lubricant is used in gear applications to control friction and wear between the intersecting surfaces, and in
enclosed gear drive applications to transfer heat away from the contact area. They also serve as a medium to
carry the additives that can be required for special functions. There are many different lubricants available to
accomplish these tasks. The choice of an appropriate lubricant depends in part on matching its properties to
the particular application. Lubricant properties can be quite varied depending on the source of the base
stock(s), the type of additive(s), and any thickeners that might be used. The base stock and thickener
components generally provide the foundation for the physical properties that define the lubricant, while the
additives provide the chemical properties that are critical for certain performance needs. The overall
performance of the lubricant is dependent on both the physical and chemical properties being in the correct
balance for the application. The following clauses describe the more common types of base stocks, thickeners
and additive chemicals used in gear lubricant formulations today.
3.2.2 Physical properties
The physical properties of a lubricant, such as viscosity and pour point, are largely derived from the base
stock(s) from which they are produced. For example, the crude source, the fraction or cut, and the amount of
refining, such as dewaxing, of a given mineral oil can significantly alter the way it will perform in service. While
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ISO/TR 18792:2008(E)
viscosity is the most common property associated with a lubricant, there are many other properties that
contribute to the makeup and character of the finished product. The properties of finished gear lubricants are
the result of a combination of base stock selection and additive technology.
3.3 Base fluid components
A key element of the finished fluid is the base oil. The base oil comes from two general sources: mineral; or,
synthetic. The term mineral usually refers to base oils that have been refined from a crude oil source, whereas
synthetics are usually the product of a chemical reaction of one or more selected starting materials. The
finished fluids can also contain mixtures of one or more base oil types. Partial synthetic fluids contain mixtures
of mineral and synthetic base oils. Full synthetic fluids can also be mixtures of two or more synthetic base oils.
As a current example, mixtures of polyalphaolefins (PAO) and esters are commonly used in synthetic
formulations. Mixtures are generally used to tailor the properties of the finished fluid to a specific application or
need. An overview of the general characteristics of different base fluids is shown in Table 2. Additional
information regarding base fluid characteristics is shown in the following sections.
Table 2 — General characteristics of base fluids
Mineral Polyalpha– Poly-alkylene- Phosphate
Characteristic Ester
paraffinic olefins (PAO) glycol (PAG) esters
Viscosity – temperature
relationship (typical viscosity 90 – 130 130 – 150 50 – 140 200 – 240 <100
index)
Specific heat
1,0 1,3 – 1,5 1,1 – 1,3 1,1 – 1,3 1,0 – 1,2
(relative)
Pressure-viscosity at 1 GPa
1,0 0,8 0,5 ~1,0 1,0 – 1,1
(relative)
Comparability solvency with
Excellent Good Excellent Poor Good
mineral fluids
Comparability solvency with PAO
Good Excellent Excellent Poor Good
fluids
Good to
Additive solvency Good Excellent Limited Good
Excellent
3.3.1 Mineral-based fluids
Mineral-based gear oils have been successfully used for several years in many industrial gear drive systems.
Mineral oil lubricants are petroleum-based fluids produced from crude oil through petroleum refining
technology. Paraffinic mineral-based gear oils have viscosity indices (VI) that are commonly lower than most,
but not all synthetic-based gear oils. This usually means that the low temperature properties of these
mineral-based lubricants will not be as good as for a comparable grade synthetic fluid. If low ambient
temperatures are involved with the operation of the equipment, this should be factored into the decision
process. At high temperatures, mineral-based lubricants are more prone to oxidation than synthetics due in
part to the amount of residual polar and unsaturated compounds in the base component. Mineral-based
lubricants will generally provide a higher viscosity under pressure than most synthetics and therefore provide
a thicker film at moderate temperatures. On the other hand, at higher temperatures, usually around 80 °C to
100 °C or more, the higher VI of synthetic fluids generally overcomes the disadvantage of having a lower
pressure-viscosity coefficient. At these higher temperatures, the film thickness can be higher for PAOs
compared to mineral oils. Probably the primary advantages of mineral-based oils over synthetic-based oils are
their lower initial purchase cost and greater availability worldwide. If a mineral oil is preferred, some of the
weaker properties, compared to a synthetic fluid, can be improved through the thickener and additive systems
available today.
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3.3.2 Synthetic-based fluids
Synthetic oils differ from petroleum-based oils in that they are not found in nature, but are manufactured
chemically and have special properties that enhance performance or accommodate severe operating
conditions. Because they are manufactured, many of their properties can be tailored to meet specific needs
through the choice of starting materials and reaction processing. Many synthetic oils are stable at high
operating temperatures, have high VI, i.e. smaller viscosity changes with temperature variations, and low pour
points. This means that equipment filled with most commercially available synthetic gear oils can be started
without difficulty at lower bulk oil temperatures than those using mineral oils. Another key advantage is that
they are inherently more stable at higher temperatures against oxidative degradation than their mineral
counterparts, again owing this advantageous property to the uniformity and composition of the fluid structure.
Each typ
...