This document provides information on main bearing greases for Tunnel Boring Machines (TBMs) that use the Earth Pressure Balance (EPB) method for tunnel excavation. It discusses: 1) The importance of properly sealing and lubricating the main bearings of the TBM cutting head, which supports the cutting tools, to prevent damage from water and debris. 2) Various types of greases used for sealing the main bearings, including those suitable for different temperature ranges and environments. 3) Properties greases must have to provide effective sealing, such as resistance to water and wear. 4) Factors to consider in grease selection like thickener type, base oil, and

1. Title of the article:
Main bearing greases for TBM EPB
by LAMANNA Luigi Franco (*)
Premise
Allow me to make an in-depth analysis, my previous article on the same topic, on the importance of sealing and
lubricating the base bearings of the rotating cutting head of an EPB-TBM, which is the external part on which the tools
(cutters) are placed [1], also equipped with special bearings, with the function of breaking up the rock [2] during the
excavation of a tunnel.
Photo 01 - Implemented disc cutters
The rotating cutting head of a TBM (Tunnel Boring Machines), is equipped with a rotary motion, controlled by electric
motors, located in the rear area of the TBM, whose base and filling bearing lubrication system is made with a system
of sealant, which is pumped, through a geometric configuration called a “labyrinth”, capable of guaranteeing high
levels of reliability and availability without aggravating the extremely difficult working conditions associated with these
types of excavation work by means of a mechanized cutter.
This sealing system ensures that no other foreign material, such as water present in the ground or particular products
of contaminating mineral aggregates, can penetrate inside the space, between the rotating and fixed part, and damage
the bearings which, in case of failure, pointing out that they cannot be easily replaced, these can potentially cause
long months of downtime of a TBM, as well as having accelerated wear of the bearings themselves, with large cost
overruns that can easily reach several million dollars.
Photo 02 - TBM mechanized cutters manufactured by CREG, China
(China Railway Engineering Equipment Group Co., Ltd)
Tunneling specializes in research, design and manufacture
of a full range of TBMs excavation
Therefore, to protect these bearings from the hostile environment in which the TBM is located, during the tunneling
phase, there is another fundamental component that deserves attention, which are the seals used for protection and
sealing. Therefore, as already mentioned above, the seal of the base bearings is essential because they must achieve
multiple objectives: to retain the sealing lubricant inside the bearing assembly, to prevent the entry of water, mud and
debris and to absorb any e significant misalignment that can occur, due to the strong loads impressed, during the
operation of the cutter.

2. The seal must be able to fulfill these tasks throughout the entire duration of the excavation project, even if all of this
is subjected to continuous abrasion by rock particles.
However today, the new TBMs are all equipped with auxiliary equipment which, together with sophisticated
centralized lubrication systems, can inject sealing oil or grease directly into the bearings in precise quantities to ensure
optimal lubrication with minimal waste and without environmental contamination, also maximizing in lower costs.
Therefore, correct lubrication management allows multiplication both in terms of economic and operational benefits.
Main products for bearing lubrication-sealing
Friction reduction is an important concept for achieving the maximum efficiency of an EPB TBM during its operation.
In addition, the working conditions of an EPB TBM require lubrication to prevent water from entering the work area
inside the closed EPB TBM and underground.
Lubrication products are classified into two groups according to their purpose of use as basic main bearing lubrication
[3] and end shield sealing of EPB TBM (called tail).
However, the lubricating grease that is used is intended to reduce the friction between the main drive and the main
basic bearing, thus increasing the life of the main bearing and optimizing the overall efficiency.
In addition, the purpose of using the sealant “grease” for the basic main bearings is also to prevent the entry of dust,
dirt particles and water into the working chamber.
Photo 03 - Samples of greases used for the Tests
Sealing of the labyrinth
The EPB TBM end mill is usually equipped with two sets of base main bearing waterproof sealing systems, the first by
means of a head main bearing gasket. The second, sealed, more internal, towards the side of the excavation chamber,
here too there is a system for the lubrication of the transmission group seals. In fact, this second external seal has the
purpose of separating the main bearing with the excavation chamber. This type of seal is able to form 4 separate areas
from each other.
The internal one is a special axial seal that can withstand the pressure of the chamber where the gears are present.
The special combination of these two sealing systems is able to withstand a water pressure of up to 10 bar.
The whole sealing system is guaranteed by a particular lubrication / sealing by injection of a particular sealing and
waterproofing "grease" and is also equipped with a monitoring system for possible leaks, monitoring the quantity of
"grease" injected into the sealing system through the monitoring of pressure and flow.
Photo 04 - Illustration of the seal of the head bearing of a TBM-EPB
SKF Group headquarters, SE-415 50, Gothenburg, Sweden

3. Paste sealant (grease) for the main bearing base
We have seen that every main bearing base must be protected and lubricated as this is the most important part of the
EPB TBM cutter. This protection is ensured by the sealing and lubricating grease.
There are different types of sealing greases present on international markets that have the high environmental
standards required today by the most important manufacturers of EPB TBM mechanical cutters:
- the main sealing greases for bearings must have strong adhesion to metal surfaces and greater resistance to washing
than normal EP greases, even if they have less lubricating properties;
- a sealing grease of the EP type is a low consistency grease with mineral oil and lithium soap containing EP additives
(this type of additives give the mineral oil a lubricating capacity and a strong resistance to breaking the film thanks to
the so-called polar molecules [4]). It is extremely suitable for the lubrication of bearings that operate in the presence
of both radial and axial loads with good pumpability even at low working temperatures from - 30° to + 110° C.
Penetration DIN-ISO-2137 equal to 260-275 dmm;
Photo 05 - Illustration of the main roller bearing for heavy-duty applications,
designed to transfer all head loads of a TBM-EPB to the rear.
- a LGWM2 type sealing grease is a synthetic-mineral oil based grease using the latest complex calcium sulfonate thickening
technology. It is suitable for those EPB TBM cutters that are used to dig tunnels that cross rivers or for excavations in sea water.
It has a resistance to water DIN 51 807/1, 3 hours at 90° C. This particular sealing grease also has the characteristic of being used
with a working temperature from - 40° to + 110° C.
- LGHP2 type sealing grease is a mineral oil based grease using a polyurea based thickener. It is suitable for those EPB TBM cutters
that are used to excavate tunnels that have long operating durations at high temperatures.
This particular sealing grease has the characteristic of being used with a working temperature from - 40° to + 150° C, a water
resistance according to DIN-51 807/1 for 3 hours at 90° C and a consistency class NLGI equal to 2-3, penetration DIN-ISO-2137
equal to 245-275 dmm.
-a sealing grease of the LGET2 type is a grease based on fluorinated synthetic oil that uses a PTFE thickener and therefore
particularly suitable for those TBM EPB cutters that are used to excavate tunnels that have extremely high operating temperatures
from 200° up to 260° C.
This particular sealing grease has the characteristic of having a density of about 1.9 g.cm3, double the average density of a normal
sealing grease for head bearings.
It has an NLGI consistency class of 2 and DIN-ISO-2137 penetration of 265-295 dmm.
Indications of some properties of sealing greases
The most important typical properties that sealing greases must possess, based on tests carried out on real or simulated operating
mechanisms in the laboratory (where from personal experience I can confirm that they never exactly reproduce the real operating
conditions), are:
- a prevention of wear according to the method of the 4 balls DIN 51350-T5 and ASTM-D-2266 which must be about 0.5 mm.

4. To reduce friction and wear, solid products alone or in suspension, in a base oil or directly in the grease, are used, as we have
illustrated above indicating some of the most important formulations of sealing greases. Some of the most common materials
used are:
graphite, molybdenum disulfide, polytetrafluoroethylene (PTFE, Teflon), mica, zinc and copper oxide.
- a resistance to the spray of water under pressure directly on the sealing grease according to the ASTM-D-4049 method at 25° C
which must be less than 15 %.
- and the Water Washout Resistance test according to the ASTM-D-1264 method at 20° C which must be < 6%.
The operating environment of a sealing grease is important, but often direct users do not consider when making the choice. A
humid environment, whether it is simply air humidity or direct washing action or in the presence of water, greatly affects the
sealing grease and represents an important factor in the right selection of the lubricant to be adopted for an EPB TBM. When
water enters the labyrinth, the sealing grease can soften (even become semi-fluid) or harden, emulsify or repel water, change the
adhesive properties, making the surface protection of the metal inadequate for the corrosive action of water (rust ).
Resistance to washing with water is a characteristic that affects both head and tail sealing greases. The test involves a 38° C [100
° F] or 80° C [175° F] spray of water directly onto a test bearing loaded with the sample sealing grease. The trend is to test under
very harsh conditions at 80° C (175° F).
The < 6% value reported is the percentage change in weight at the end of the test.
Plant-based sealing greases
In terms of performance, vegetable-based lubricating greases, based on my current knowledge, are superior to EP and petroleum-
based lubricating greases.
In addition to the great availability on the market today, I can confirm with certainty that they have a disadvantage. These
lubricating greases based on vegetable oils are prone to forming waxes at low temperatures due to their tendency. Furthermore,
they have not as good oxidative stability as that of petroleum-based lubricating greases.
Today, all over the world, efforts are made to reduce the environmental impact of industrial and non-industrial risks. I think that
soon there will be further new "natural" lubricating greases on the international market capable of offering good performance as
well as a favorable cost compared to that of synthetic greases.
Plant-based lubricating greases are essentially non-hazardous to the environment and can be disposed of at cheaper costs than
their oil-based counterparts. In some states, governments have already imposed that only completely biodegradable products are
used, and in addition these lubricating greases, to be used for EPB TBMs, must ensure harmony with the ecosystem.
However, plant-based lubricating greases, as a start, it will be more useful to use them in those areas at risk where their release
into the environment could cause a serious problem for human health or a serious environmental hazard.
NLGI Grade (National Lubricating Grease Institute)
The National Lubricating Grease Institute (NLGI) has indicated over the years the standardization in numerical terms of
classification of TESTs that must describe the properties or performance of sealing greases.
I repeat, the NLGI has standardized a numerical scale for the consistency of greases on the basis of the penetration range
processed according to ASTM-D-217. This scale provides for a numbering from 000 for semi-fluid greases to 6 for solid greases.
The most common grease grade is NLGI grade 2. Greases with this grade have a soft and buttery texture. It should be noted that
the consistency of the fat is linked to the content of the thickener but not to the viscosity of the base oil.
In particular, the main function of sealing greases is to effectively protect the main bearing by preventing water and soil from
entering the sealing system.
Also remember that the basic main bearing is the most expensive part of a TBM, and therefore must be well maintained to avoid
premature failures. For this reason, the highest quality of raw materials is required in the production of fats. That is why a MERKEL
certification is required.
I would like to point out that, in addition to standard greases, for the main bearing, highly biodegradable greases produced based
on renewable raw materials are present on international markets.
Penetration with the cone
The consistency of the sealing grease is measured at 25° C after the sample has been subjected to 60 strokes in the sealing grease
manipulator according to ASTM-D-217. After the sample has been prepared, the cone of a penetrometer is released and made to
penetrate the sealing grease under its own weight for 5 seconds. Then the depth reached by the cone is read in tenths of a
millimeter. The deeper the cone reaches into the sealing grease, the higher the penetration result and the softer the sealing
grease.

5. Degree of
Consistency NLGI
Penetration Range
(1/10 mm)
Description
000 445 – 475 Fluid
00 400 – 430 Semi-fluid
0 355 – 385 Very soft
1 310 – 340 Soft
2 265 – 295 Medium
3 220 – 250 Medium-hard
4 175 – 205 Hard
5 130 – 160 Very hard
6 85 - 115 Solid
Table 01 - Definition of NLGI grades
Temperature
I want to point out that controlling the operating temperature of the mechanism to be lubricated produces heat caused by friction,
and this is very important for two reasons:
- the first is the viscosity that a lubricating grease must have, which will tend to lower at increasing temperatures. It is very
important that the chosen lubricating grease must have the ability to keep a protective film sufficient to ensure protection from
friction.
- the second reason is that there are temperature limits that must not be exceeded, because if they exceed certain limits they can
actually create big problems.
Generally, petroleum-based lubricating greases cannot be used above 150° C (300° F) on a regular basis.
While, ester-based synthetic lubricating greases can also be used at 230° C (450° F) and polyol esters at 270° C (520° F). Instead
silicones can be used at 205° C (400° F).
Other special synthetic greases such as fluorosilicones can be used up to 230° C (450° F).
The determination of the apparent viscosity at low temperatures (ASTM-D-1092 method) allows to define the flow and
pumpability properties of a grease at low temperatures. Greases designed for operation at extremely low temperatures (below
zero) must not stiffen or oppose excessive resistance to the rotation of the bearings after exposure to low temperatures. A correct
viscosity is able to ensure adequate separation between surfaces without causing too much friction.
The methods reported (IP-186 and ASTM-D-1478) allow to detect the starting and running torques of lightly loaded ball bearings
at temperatures as low as - 54° C.
Thickening agents
After considering the reasons for using a grease, I suggest users to carefully examine the actual selection of the most suitable type
of sealing grease for certain lubrication and consumption requirements bearing in mind that the vast majority of companies, those
who market EPB TBM sealing greases do not actually produce them.
The thickener combined with the base oil will allow the structure to pass from semi-fluid to "paste" (sealing grease).
Allow me to quickly recall the correlation that sealing greases available today on the international market can have based on their
chemical formulation (i.e. thickener + base oil). The type of thickener and the concentration are the dominant factors of a good
sealing grease.
The water resistance and heat resistance of a good base sealant grease are mainly determined by the thickener. There are two
broad categories of thickeners used in the preparation of sealing greases. Soap-based thickeners (fatty acid metal salts) and non-
soap-based thickeners (hydrocarbons, inorganic and organic).
These can range from simple greases, containing mineral oils thickened with metal soap, to complex soaps, containing two or
more thickeners, as well as organic and mineral thickeners combined with synthetic and / or mineral based oils.
In most sealing greases currently on sale, end users use greases based on refined petroleum oils (paraffinic and / or naphthenic),
for a low price / performance ratio.
I remind these users that the physical properties important for the selection of a sealing grease containing base oil must include:
- lubricating properties;
- excellent viscosity;
- excellent resistance to oxidation (even at high temperatures);
- pour point (excellent behavior at low temperatures);

6. - miscibility with additives;
- low volatility (resistance to evaporation and the ability to operate at higher temperatures).
Finally, it must be remembered that a sealing grease is not a thick oil, as it is erroneously thought by users of EPB TBM cutters,
but a thickener.
Since the thickener used specifically affects the final properties of the sealing grease, especially the one used for TBM-EPBs, I
suggest identifying the various types, asking your supplier for the type of thickener [5].
I would like to point out that the final properties of a sealing grease will vary depending on the thickeners used and additives used
in the base oil.
To better understand the importance of the quality of a sealing grease for EPB TBMs, I would like to point out some small
considerations on the formulation of the sealing grease. Those formulated with a metallic soap base can be divided into
conventional soaps (lithium, calcium, aluminum, mixed lithium / calcium soaps) and those formulated with complex soaps (lithium
complex, aluminum complex and calcium complex).
The best known and most used thickener in the formulation of sealing greases is the superbasic calcium sulfonate complex, that
is, a super complex soap.
Similarly, the formulations of greases with thickeners, not based on soap, are identified on the basis of the thickener, such as:
clay, silica and polyurea.
However, many manufacturers of sealing greases for EPB TBMs report slightly different values and properties from each other.
Conclusions
The analysis carried out so far has allowed us to interpret one of the most important problems to be taken into due consideration,
when we use a head sealing grease, that is to maintain the seal constant to prevent subsidence of the ground at the front of the
excavation, in front of the head of the EPB TBM itself.
In general, butt seal grease formulations contain between 70 - 90% base oil, 5 - 15% thickeners agents and up to 10% additives.
Oils and additives are essentially products that are used throughout the sealing grease and oil industry in general.
However, thickeners have a significant influence on the working temperature and water resistance, a fundamental property of a
good quality sealing grease.
The thickener in the sealing grease formulation usually, as written above, is a metallic soap, but thickeners are not soaps. The
soaps are made from an acid-neutralized saponification process of animal grease.
While the soap-thickened sealant greases comprise approximately 85% of all greases in use. They are inexpensive and also offer
good lubrication. The non-soap in thickeners are less frequently used. While they usually offer high temperature melting, some
(such as bentonite, clays and silica gel) can leave a small residue which can be very abrasive to the wear of the metal parts of an
EPB TBM, so we will have a grease. poor quality sealant.
Polyurea-based sealing greases are a newer technology and are used in thickening agents. These sealing greases offer good
resistance to washout and thus prevent subsidence of the soil at the face of the excavation.
Photo 06 - Subsidence
The use of soap-based sealant greases represent a wide range of metal soaps. Soaps, in this chemical sector, are often identified
by metal because it is the metal that determines the characteristics of the soap.
Below is a summary of the list of soaps:
-Lithium stearate;
- Lithium 12-hydroxystearate;

7. - Lithium complex;
- Hydrated calcium;
- Anhydrous calcium;
- Calcium complex;
- Aluminum;
- Sodium;
- Barium.
So in conclusion, sodium-based butt sealing grease are often incompatible sealing greases with other thickeners.
In addition, as discussed above, sodium sealing greases offer less water resistance than others types of sealing greases.
However, the consumption of the head sealing grease depends on the type of EPB TBM used based on the geological-geotechnical
situations and other conditioning factors such as: the pressure inside the excavation chamber and the presence of water.
Many times, these factors are not due to consolidation problems or even to the very nature of the land affected by the excavation
process but to their poor quality of opposing the strong pressure exerted by the water, and its entry into the shield, while a failure
is more likely to occur at the end of the TBM. This subsidence contributes to the deformation of the ground above the tunnel and
is called subsidence.
I want to conclude by pointing out that the subsidence phenomenon, due to the lack of tightness of the tail fat, generates a
complexity of phenomena, all aimed at causing a lowering of the ground on the surface. In particular, this causes:
- damage to the infrastructures and buildings present on the surface;
- a lack of worker safety during the work phases;
- an increase in the stresses in the temporary coating with a decline in the resistance parameters of the soils due to their
heterogeneity;
- a trigger of the instability of the whole slope.
But this (subsidence of the ground in the tail of the TBM) [6] due to its complexity, will be the subject of a subsequent study, very
detailed, not only for the characterization of the problem but also for a series of soil conditioning products that come used.
(*) LAMANNA LUIGI FRANCO
Independent Consultant Tunneling
Mining and oil specialized in mechanized tunneling with Hard Rock TBM and soft soil EPB Shields expert and consultant in
structural reinforcement.
LAMANNA Luigi Franco, performs for 45 years professional consultingand technical direction inthe various sectors of civil, industrial,
military, hydraulics, railway, highway and recent years in the sector of “tunneling” and “mining” on the correct use of special “resin”
and related technologies for consolidation, repair and maintenance of masonry, concrete, iron and wood.
Is the author of numerous scientific publications and is always engaged in the study and development of “innovative materials” and
related techniques and technologies used.
Note:
[[1] - Tools, among which we consider the 17 "cutters (single and double), which detach the material from the front that they attack, the rippers
(knives), equipped with a special cutting edge with hard metal, and the scapers (scrapers) Buckets, on the other hand, provide for the removal
of the detached material, also protecting the disc cutters from damage.
[2] - Along the TBM excavation route the ground is very heterogeneous and can appear as compact, degraded or weak rock, even loose ground,
especially in fault zones.
[3] - Usually the main bearing diameter varies up to 12 - 15 meters and has a weight that can exceed 200 tons. The level of consumption of both
greases depends on the recommendations of the EPB TBM suppliers.
[4] - the novelty consists in the fact that synthetic esters are already equipped with this type of molecules and therefore, unlike mineral oils or
hydrocarbon-based oils, which do not have them, do not require the addition of additives.
A mineral-based oil, with increasing workloads, has points of discontinuity where the lubricant film breaks, causing friction peaks unlike with
lubricant with 3% EP additives. With lubrication performed with synthetic esters there are no friction peaks thanks to the polar molecules
naturally present in it.
[5] - To better understand, purely as an indication to anticipate your future needs, some typical representative properties of the thickeners most
used in the formulation of fats and which have the greatest influence on the final properties:
I. calcium soap fats. They have great mechanical stability, excellent water resistance and have poor oxidation stability.
II. 12-hydroxystearate lithium fats. They have a very soft and thermostable structure, good water resistance.

8. III. complex aluminum greases. They have good pumpability and are easy to use at low temperatures with excellent adhesion to the
metal parts. These sealing greases can be usable up to 150-160 ° C. They also have shorter life times than most polyurea or lithium
complex sealing greases.
IV. calcium complex fats. These greases are very complex in the production phase because they tend to harden when stored for a long
time in the warehouse or at high pressures.
V. calcium sulphonate complex fats. There are very few companies capable of producing such sealing greases. They are technologically
very advanced products but with poor performance properties.
VI. lithium and calcium fats. They are excellent products and offer excellent water resistance.
VII. complex lithium fats (lithium / calcium). They are excellent products like the previous one, especially if used at high temperatures.
VIII. polyurea fats. These sealing greases are very soft and can be used at temperature and speed and are used on any type of bearing.
They are used where long lubrication is required.
IX. Bentonite grease. The thickener in this type of fat does not melt until the temperature at which the oil contained evaporates.
[6] - Tail grease is a waterproofing sealant used in the tail of the shield. It is a lubrication product used in EPB TBMs to fill the brushes present in
the tail shield of the cutter. The purpose of use is to prevent the flow of water, grouting material and earth inside the TBM-EPB.
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