Article
citation information:
Wieczorek, A.N.
Ensuring
appropriate conditions for lubrication of gear transmissions as a priority
for maintenance services in industrial transport. Scientific Journal of Silesian University of Technology. Series
Transport. 2021, 111, 193-204. ISSN:
0209-3324. DOI: https://doi.org/10.20858/sjsutst.2021.111.17.
Andrzej N. WIECZOREK[1]
ENSURING
APPROPRIATE CONDITIONS FOR LUBRICATION OF GEAR TRANSMISSIONS AS A PRIORITY FOR
MAINTENANCE SERVICES IN INDUSTRIAL TRANSPORT
Summary. This paper presents
the problem of the operation of gear
transmissions in the context
of ensuring appropriate lubrication conditions. Damage caused by improper lubrication was discussed. Attention was drawn to the issue of oil contamination. Furthermore, a method for determining the thickness of the oil film of gear wheel teeth was described. There was also performed an analysis of the lubrication conditions of sample gear transmissions
of the load, manufacturing technology and surface roughness.
Keywords: gear, wear, lubrication
1. INTRODUCTION
Root Cause Analysis (RCA) is often
used by industrial plant maintenance services to identify causes of problems or
incidents to eliminate them. Due to the use of the analysis of the causes of
failures/damages, it is possible to effectively counteract their consequences,
thus, significantly affect the economics of the actions taken. This remark
applies in particular to the operation of drive units, the service life of
which is significantly limited by various forms of wear [1]. Excessive wear [2]
of components may, on the one hand, affect the level of safety of the people
operating these machines, and on the other hand, cause additional dynamic
excitations negatively affecting their service life [3-6] or generating vibroacoustic effects
[7-11].
One of the most important components
of the drive units is gear transmission [12, 13]. In the case of these
components, teeth fractures at the base usually occur due to fatigue or surface
chipping. However, if proper lubrication conditions are not ensured, that is,
the layer separating the mating surfaces does not have a sufficient thickness,
the wear of these surfaces may be intensified.
This paper presents a method of
determining the lubrication conditions of teeth in gear transmissions, which
can be used by maintenance services to predict possible damage to gear teeth
caused by improper lubrication conditions, as an element of the RCA analysis
aimed at ensuring proper operating conditions. Knowledge of the parameters characterising the conditions of lubrication of the teeth
allows taking preventive actions, for example, using appropriate lubricants or
reducing the permissible loads.
2. DAMAGE TO TEETH CAUSED BY
INADEQUATE LUBRICATION
In the case of gear transmissions,
the elements particularly vulnerable to damage caused by improper lubrication
[1] are primarily gear teeth. These elements are required to meet demanding
requirements concerning strength and service life. However, they are exposed
to:
- surface seizure (adhesive wear) caused, for example,
by the improper choice of lubricant,
- surface scratches caused by the presence of single
hard grains, for example, sand (Figure 1A),
- local overheating of tooth surfaces due to
insufficient amount of lubricant (Figure 1B),
- abrasive wear caused by improper lubrication conditions
(Figure 1C),
- fatigue chipping of the surface (Figure 1D).
Lubricants, similarly to components
of machines and other equipment, are subject to degradation processes; this
occurs due to the action of high temperatures and oxygen. During the operation
of industrial gear transmissions, the content of solid impurities in oils and
greases usually increases, significantly affecting the intensity of wear of
mating surfaces of teeth and rolling bearings. Figure 2 shows the results of
the analysis of the content of solids and iron as a function of the operating
time for gear transmissions used in armoured face
conveyors and belt conveyors in the mining industry [2]. It can be noticed in
the Figures that the content of solids and iron increases along with the
increase in the operating time and that the differentiation in the content
depends on the type of machine. However, it should be mentioned that the
durability of surfaces of friction pairs depends not only on the conditions of
their operation but also on the correct design and selection of parameters characterising the load, as well as on the material and
lubricating properties.
3. PARAMETERS CHARACTERISING
THE LUBRICATING PROPERTIES OF OILS
The main purpose of lubrication [3]
is to reduce the friction of mating surfaces by creating a layer of lubricant.
It is advisable to form an oil film layer with a thickness that allows
separating the protrusions in the mating surfaces.
The parameter characterising
the separation layer is the minimum thickness of the lubricant layer (hmin),
which is formed in conditions of the electrohydraulic lubrication (the diagram
showing the elastohydrodynamic contact zone is
presented in Figure 3). The thickness of the oil film depends on some design
and operational factors, as well as on the parameters of the lubricant (this
issue is discussed in detail in [14]). In practice, the method specified in
ISO/TR 15144-1:2014 (E) standard is often used to
calculate the minimum thickness of the lubricant layer hmin in gear teeth [15].
In this standard, calculations of the relative thickness of oil film λ are usually performed only for 5
points of the engagement section (points A, B, C, D and E shown in Figure 4).
However, it is possible to calculate the parameter λ also for other points of the engagement section.
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A |
B |
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C |
D |
Fig. 1. Examples of damage to teeth
in industrial gear transmissions: A – surface scratches,
B – signs of overheating, C – abrasive wear, D – pitting
Lubrication conditions depend
primarily on the roughness of the mating surfaces because the thickness of the
oil film may be smaller than the height of the protrusions. In such a
situation, the type of friction will be unfavourable
(mixed or boundary friction). The parameter characterising
the type of friction between the mating surfaces is the relative thickness of
oil film (parameter λ),
determined by the following relationship [16]:
(1)
where:
Rq1,2 – surface roughness.
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A |
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B |
Fig. 2. Plots of the content of
impurities (A) and iron (B) in oil as a function of operating time, determined
for gear transmissions of belt conveyors and armoured
scrap conveyors
In the range of values λ=(0–1), there occurs mixed boundary friction (for the parameter Ra, this range will be λ=(0–0.8)). In the range of the relative value of oil film thickness λ=(1–3), the predominant type of friction is mixed friction (for the parameter Ra, this range will be λ=(0.8–2.4), whereas in the range of λ=<3–10> the conditions of EHD lubrication are created (for the parameter Ra, this range will be λ=(2.4–8)). These conditions are characterised by the formation of an oil film layer with a thickness exceeding the height of surfaces irregularities. Above the relative value of the oil film thickness λ>10, there occurs liquid hydrodynamic friction (for the parameter Ra, this range will be λ>8) [16, 17].
Fig. 3. Pressure distribution pEHD and the shape of the lubrication gap h in
the conditions of elastohydrodynamic contact (EHD) between mating surfaces [16]
Fig. 4. Geometry of a gear wheel
with an external involute profile
The thickness of the oil film, and thus, the lubrication conditions of gear transmissions, is affected, inter alia, by the following factors:
- location of the contact point in the engagement
section of the mating teeth,
- variable number of pairs of teeth in meshing,
- nominal and instantaneous loads,
- surface roughness,
- rotational speed of gears,
- oil viscosity
grade.
Figure 5 shows results of calculations (performed according to ISO/TR 15144-1:2014(E)) of the relative thickness of oil film λ as a function of the position of gear wheels of a sample gear transmission in the engagement section (for the needs of the analysis, the parameter λ was determined for 106 points of the engagement section) for a fixed (nominal) load. In the analysis, there were used parameters characterising the teeth of the second stage of the right angle spur planetary gear transmission with the gear ratio i=39. Gear wheels of this gear transmission were made of alloy steels that were subjected to thermal and thermochemical treatment and then ground (accuracy class 6 according to PN-ISO 1328 and roughness Ra=0.8 μm were obtained). In Figure 5, there can be easily noticed, a stepwise change in the thickness of the oil film, associated with the change in the number of pairs of teeth being engaged. The lowest values of the oil film thickness are observed for the inner single-tooth engagement point (point B in Figure 5).
Fig. 5. The plot of the relative
thickness of oil film λ as a
function of the position in the engagement section for the second stage of the
right angle spur planetary gear transmission
Another important factor that affects lubrication conditions is the nominal load of a given stage of the gear transmission. Figure 6 shows plots of the relative thickness of oil film λ as a function of the power of the drive motor of the second stage of the right angle spur planetary gear transmission (calculations considered the value of the torque and the gear ratio for a given stage) for different viscosity grades of the lubricating oil. In this Figure, it is easy to note that satisfactory lubrication conditions can be obtained only in a fairly narrow range of values of the transmitted power and oil viscosity.
During the operation of drives of machines, there are frequent changes in the instantaneous load (sample plot showing the variability of the load of the gear transmission of an armoured face conveyor is presented in Figure 7). The variability of the load causes some fluctuations in the thickness of the oil film, and the value of parameter λ take place during the operation. These changes are relatively large because, as shown in Figure 8, they can lead to variations in the value of the parameter λ up to 0.15.
Typically, problems with lubrication relate to the highest stages of a gear transmission, which have a higher
gear ratio and load torque than the lower stages. Figure
9 shows the results of the calculations of the relative thickness of oil film λ for a 3-stage
spur gear transmission
at point B of the engagement section. This Figure clearly shows that
lubrication conditions deteriorate along with an increase in the gear ratio.
Lubrication
conditions can be improved primarily by reducing the roughness of surfaces of
gear wheel teeth through process changes. This can be seen in Figure 10, which
shows the results of calculations for the relative thickness of oil film λ for the previously analysed second stage of the right angle spur planetary
gear transmission. This Figure shows plots of the relative thickness of oil
film λ depending on the
parameter Ra for various viscosity grades, determined for the second stage of
the right angle planetary gear transmission operating at the assumed drive
motor power of 450 kW.
Fig. 6. Plots of the relative
thickness of oil film λ at point
B of the engagement section as a function of the power of the drive motor and
the viscosity grade (the results obtained for surface roughness Ra = 0.6 μm)
Fig. 7. A sample plot of
the values of the instantaneous load in relation to the nominal load of the
right angle planetary gear transmission of an armoured
face conveyor
Fig. 8. Instantaneous relative
values of the thickness of oil film λ
determined at point B of the engagement section of the right angle planetary
gear transmission
Similarly to Figure 6, the mixed
lubrication conditions shown in Figure 10 are only within a fairly narrow range
of values of the parameter Ra and kinematic viscosity. For a typical roughness
of surfaces of the teeth subjected to profile grinding, the analysed
pair of teeth operate under boundary lubrication conditions.
Fig. 9. Plots of the relative
thickness of oil film λ as a function of the gear ratio, determined for a
3-stage spur gear transmission used in belt conveyors
When analysing
lubrication conditions, the direction of measuring the parameter Ra should also be considered. As can be
seen in Figure 11, there is a variation in the roughness depending on whether
the measurement is carried out in the direction of the pitch line or perpendicular
to it (along the tooth profile). In the case of the planetary gear transmission
subject to the analysis, the value of the parameter Ra measured in the
direction of the pitch line was 0.21 μm, while in the direction perpendicular to it
– 0.81 μm.
Figure 12 shows the impact of the direction of measuring the roughness on the
plots of the relative thickness of oil film λ
at point B as a function of the position in the engagement section for the
second stage of the right angle spur planetary gear transmission.
4. CONCLUSION
As a part of this study, there was
presented, a possibility of using a modified calculation method based on the
ISO/TR 15144-1:2014 (E) standard for determining the
lubrication conditions of typical industrial gear transmissions used in drives
of conveyors in industrial transport. The analysis covered primarily the second
spur gear stage of the right angle spur planetary gear transmission with the
gear ratio i=39, as well as a 3-stage spur gear
transmission. The selected method can be used by maintenance services in RCA to
determine the optimal lubrication conditions for given operating conditions of
the gear transmission. The decision regarding the choice of operating
conditions, type of lubricant and requirements concerning the roughness of the
surface of gear wheels should be taken only after the results of these
calculations have been obtained. Such an approach can improve the service life
and the time of failure-free operation of drives of industrial transport
machines.
Fig. 10. Plots of the relative
thickness of oil film λ at point
B of the engagement section as a function of the roughness and the viscosity
grade for a right angle planetary gear transmission (the results obtained for
drive motor power P=450 kW)
Fig. 11. View of the profilogram of the tooth surface subjected to profile
grinding
Fig. 12. Plots of the relative
thickness of oil film λ as a
function of the position in the engagement section for the second stage of the
right angle spur planetary gear transmission, determined for the roughness Ra = 0.21 μm
(direction consistent with the pitch line) and for the roughness Ra = 0.81 μm
(direction perpendicular to the pitch line)
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Received 11.03.2021; accepted in revised form 09.05.2021
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