Lubricants and Lubrication Part 13 pot

677 16.8 Grease Performance16.8.1Test Methods Many test methods are used today; all are meant to judge the single or combinedand more or less complex properties of greases.. re-16.9.3 St

677 16.8 Grease Performance16.8.1

Test Methods

Many test methods are used today; all are meant to judge the single or combinedand more or less complex properties of greases The last summary containingdetailed descriptions of ASTM and DIN methods was written by Schultze in 1962[16.196]; another emphasizing on Russian GOST and former East German TGLmethods was published in 1984 [16.197] The most important ASTM and DINmethods are described in Tables 16.10 and 16.11 Complete collections are pub-lished regularly [16.198, 16.199], the same is true for the French AFNOR, theEnglish IP, the Japanese JIS, and some other national collections The develop-ment of international standards (EN and ISO) is slowly proceeding

Tab 16.10 Important ASTM test methods for lubricating greases.

ASTM D 0128-95 Analysis

ASTM D 1092-93 Apparent viscosity

ASTM D 1263-94 Leakage tendencies of automotive wheel bearing greases

ASTM D 1264-96 Water washout characteristics

ASTM D 1478-91 Low-temperature torque of ball bearing greases

ASTM D 1742-88 Oil separation during storage (air pressure method)

ASTM D 1743-94 Corrosion preventive properties

ASTM D 1831-88 Roll stability

ASTM D 2509-93 Load carrying capacity, Timken method

ASTM D 3337-94 Life and torque in small bearings

ASTM D 3527-95 Life performance of automotive wheel bearing greases

ASTM D 4049-86 Resistance to water spray

Tab 16.11 Important DIN (ASTM) test methods for lubricating greases.

DIN 51804-1 (ASTM D 217-97), replaced by DIN ISO 2137

Determination of cone penetration of greases with hollow cone and solid cone

DIN 51804-2 (ASTM D 1403-97), replaced by DIN ISO 2137

Determination of cone penetration of greases with one-quarter cone

Test using the FAG roller bearing grease testing apparatus FE 9

Because of the large number of publications about the different test methods,their improvement, and their comparison, only some surveys concerned with EP[16.200], wear [16.201] and with standard tests [16.202, 16.203, 16.384] or perfor-mance tests using real components [16.204–16.208] are cited One remark about thelast group – the FAG FE 8 test, which has become an increasingly important repla-cement for the SKF R2F and Timken tests in Europe, has found its way to the USAwhere it is used for the development of greases for all kinds of heavy-duty applica-tion [16.385] The methods themselves are described in Chapters 18 and 19

One point in dealing with standards and test methods according to Jnemann[16.212] cannot be taken serious enough – critical analysis

16.8.2

Analytical Methods

The time of chemical separation procedures as described by ASTM D 128 hasnearly gone, elemental analysis of greases is nowadays performed by spectro-scopic methods, e.g X-ray fluorescence spectrometry (XRF), inductively coupledplasma atomic emission (ICP), or atomic absorption spectrometry (AAS) [16.213],with attention being directed mostly to methods of preparation [16.214, 16.215].Infrared spectroscopy was introduced as a means of identifying greases and their com-ponents ca 45 years ago [16.216–16.218] Its use has steadily grown since and beenextended to questions of structure, development, and manufacture also [16.219].Nuclear magnetic resonance spectroscopy (NMR) has also been used to investigatestructural questions [16.220] and electron microscopy has enabled not only the scrutiny

of soap fibers [16.221] but also study of thermal changes in greases [16.222]

The use of thermogravimetry (TG) is usually limited to investigations of base oils[16.223], but the other thermoanalytical method, differential scanning calorimetry(DSC) [16.224, 16.225, 16.386, 16.387], or a combination of both, is expected tobecome a valuable tool in the analysis of grease and antioxidant analysis

Chromatographic methods, e.g gas (GC) and high-pressure liquid phy (HPLC), are mainly used to identify the components of liquid or liquefiedgrease [16.226, 16.227]

chromatogra-The automation of analytical methods has led to a demand for tools that not only itate the documentation of data but also aid their interpretation This can be achieved

facil-679 16.9 Applications of Greaseswith suitable laboratory information systems (LIMS) [16.228, 16.229] The use of such asystem must be accompanied by intensified concern with the efficiency of tests [16.230]and critical analysis of possible errors in the resulting data [16.231, 16.232] Suitabledatabases [16.233] or internet resources [16.234] can be also used.

16.9

Applications of Greases

Increased knowledge of base oils and thickener systems enables the selectionand naming of greases on the basis of these chemical and physical insights Theselection of a grease is always a compromise between the demands of a customerand the circumstances the grease must face during its operational life – tempera-ture, speed, load including centrifugal forces and vibrations, re-lubrication inter-vals based on a knowledge of the lubrication points, for applications which can

be roller bearings, plain bearings, chassis, joints, 5th wheels, door locks, switchesand seals of different design [16.235–16.237] It has already been mentioned thatthe desired 24 grease properties described by 12 characteristics sometimes con-tradict each other

16.9.1

Rolling Bearings

The rolling bearing industry, one of the key industries for greases, supports all kinds

of manufacturer not only with bearings, which can be standard or tailor-made ucts, but also with consulting and service concerning the design of new equipmentand the maintenance of that already in existence The lifetime of rolling bearings isconnected with that of the grease used, especially under extreme conditions[16.238] Sophisticated test equipment has been developed, mainly to ensure betterlifetime predictability for the selected bearing together with the selected grease[16.239, 16.240] The test rigs SKF R2F and later FAG FE 9 have found great accept-ance in Germany [16.241, 16.242]

prod-Clean production and low-noise greases are meeting reliability and lifetimerequirements of rolling bearings The stringent requirements of high-precisionbearings, for small bearings in video and audio applications, and bearings for mili-tary use have been established for years [16.243, 16.244]

On the basis that ca 80 % of all bearings are grease-lubricated, in 1992 theAmerican Society of Tribologists and Lubrication Engineers published a bookabout the life factors of rolling bearings [16.245]; the German Society for Tribol-ogy published another in 1994 [16.246] These handbooks describe calculation ofthe lifetime of bearings, taking into consideration the effects of the tribological

influence of a grease taking into account bearing size, speed, viscosity, and perature Current knowledge of the factors affecting calculation of the lifetime ofthese bearings has been released in the new handbook of the German Society for

tem-680 16 Lubricating Greases

Tribology, the second edition of which has been published in September 2006[16.392] In Ref [16.393] it is shown that the lifetime can be prolonged substan-tially by changing specific influencing factors which do not reduce the lifetimebut, on the contrary, increase it Design of a grease reservoir for the bearingscontaining a greater volume of grease will also increase the lifetime of rollingbearings [16.394]

In general greases used in rolling bearings such as ball-, deep groove ball-, thrustball-, spherical-, taper-, cylindrical- or needle roller bearings must have good workingstability This can be checked by the prolonged penetration test and the Shell-Rollertest (ASTM D 1831) [16.247] Conventional lithium greases in the NLGI class 2 arerecommended for most types of bearing at working temperatures up to 120 C.Greases of the NLGI class 1 are preferred for needle bearings For bearings exposed

to temperatures above 120 C complex soap or polyurea thickened greases are able For bearings that must operate under high load and/or low speed the base oil

aerospace or military use have to ensure performance down to below –70 C temperature performance can be checked with the low-temperature torque (ASTM

Low-D 1263), the low-temperature penetration (AFNOR NF T 60-171) and the flow sure (DIN 51805) tests [16.248–16.250] Greases of that kind need base oils with suf-ficiently low pour points In many military applications long-life properties are alsorequired; these can be fulfilled by use of synthetic base oils only [16.251]

pres-Because plain bearings are often exposed to moisture or water, calcium thickened greases are recommended When open housings are used in a dustyatmosphere frequent re-lubrication makes it possible to wash out the contaminatedgrease

a bearing) as a means of selecting the right base oil viscosity for a grease can beaccepted only as a rule of thumb; its dependence on base oil viscosity has not yetbeen established and a commonly accepted test method is not yet available [16.252].16.9.1.1 Re-lubrication Intervals

DIN 51 825 [16.253] under normal environmental conditions with temperatures up

to 70 C and a mean bearing load of P/C < 0.1 Figure 16.8 and Table 16.12 take intoconsideration the type of bearing and the speed For each 15 K temperature increasethe re-lubrication interval is said to be reduced by 50 % Severe working conditions

681 16.9 Applications of Greases100000

Fig 16.8 Re-lubrication intervals.

Tab 16.12 Relationship between bearing type and correction factors for re-lubrication intervals (GFT Worksheet 3).

Angular contact thrust ball bearings Double row 1.4

Full complement 25

Spherical roller bearings without lips (E-design) 7–9

a) k f = 2 for radial load or increasing thrust load and

k = 3 for constant thrust load.

682 16 Lubricating Greases

The reduction factors are shown in Table 16.13

Tab 16.13 Reduction factors for re-lubrication cycles Reduction factors f 1 to f 5 for poor operating and environmental conditions (GfT Worksheet 3).

Effect of dust and moisture on the bearing contact surfaces

In re-lubrication it is usually impossible to remove the used grease Consequently

amount of grease necessary for re-lubrication is given in Table 16.14

Tab 16.14 Amount of grease necessary for re-lubrication Amounts of grease lubrication (GfT Work sheet 3).

Re-lubrication quantity m1for weekly to yearly re-lubrication

683 16.9 Applications of GreasesRe-lubrication quantity m3before restarting after several year of standstill

m 3 = D  B  0.01 [g]

V = free space in the bearing

(p/4)  B  (D2– d2)  10–9– (G/7800) [m3] or

(p/4)  B  (D 2 – d 2 )  10 –9– ([G¢  0.4536]/7800) [m3 ]

d = bearing bore diameter [mm]

D = bearing outside diameter [mm]

B = bearing width [mm]

G = bearing weight [kg]

G¢ = bearing weight [lb]

16.9.2

Cars, Trucks, Construction Vehicles

Most modern cars do not need re-lubrication, with the exception of door hinges,lock mechanisms, and battery poles But among the approximately 30 hiddengreases (Fig 16.9 and Table 16.15) in a modern car [16.254, 16.255] only the con-stant-velocity (CV) joint greases are required in substantial quantities Althoughimproved conventional lithium greases containing molybdenum disulfide are still

in use [16.256], lithium complex or polyurea greases are already preferred in somemodern cars and this usage will increase in the future [16.257] Most of the greasesused in cars, for example the greases for CV joints, hub units, starters, alternators,seat adjustments, clutches release bearings, belt–pulley bearings, window levers andwindshield wiper gears, are specified and approved by the large motor companiesand developed in close cooperation with the grease manufacturers For the sameapplication, however, different motor companies have different grease specifications

Fig 16.9 Greases hidden in a modern car.

684 16 Lubricating Greases

Tab 16.15 Thirty hidden greases – a partial list of grease applications.

Belt–pulley bearing ABS follower/motor bearings

Clutch bearing/spline Caliper pin

Cruise control module

Fan clutch bearing

Turbo/supercharger Electrical contacts

Horn contact Body hardware

Heating/cooling Door check arms/hinges

Electric antenna gear/clutch Temperature sensor

Remote control mirror assembly Visco fan bearing

Seat adjustment gears Water pump bearing

Sunroof rails

Windshield wiper gear

Wheel bearing U-Jointand approvals; for example, most European and US motor companies prefer lithiumcomplex greases in the front wheel bearings, whereas Japanese manufacturers pre-fer polyurea greases

ASTM D 4950 (Tables 16.16 and 16.17) describe the minimum requirements ofcurrent greases in automotive service–fill applications for passenger cars, trucks,and other vehicles operating under various service conditions [16.258] Grease packsfulfilling these minimum requirements can have the NLGI certification marks asshown in Fig 16.10, these labels are provided by the NLGI on request

Tab 16.16 ASTM D 4950 Specifications LA and LB.

Chassis grease classifications – intended use of chassis (L) classified greases:

Chassis components and universal joints under mild to severe duty

– Prolonged re-lubrication intervals

– High loads

– Severe vibration

– Exposure to water or other contaminants

685 16.9 Applications of Greases

Tab 16.17 ASTM D 4950 Specifications GA, GB and GC.

Intended use of wheel bearing (G) classified greases:

Service typical of wheel bearings operating under mild duty/moderate duty

– Normal urban, highway and off-highway service

GC Classification

Service typical of wheel bearings operating under mild duty/severe duty

– High bearing temperatures

– Frequent stop and go service (buses, taxis, police)

– Severe braking service (trailer towing, heavy towing, mountain driving)

Lithium-based multipurpose greases have replaced several other greases for the lubrication of trucks and construction equipment Conventional lithium soap basedgreases that require frequent re-lubrication are still in use for the wheel bearings oftrucks and trailers Modern trucks and trailers with prolonged oil-drain intervals requirelithium complex greases with semi-synthetic or fully synthetic base oils Lithiumgreases containing black solid lubricants are recommended for 5th-wheel applications,for chassis points, and for plain bearings of construction equipment Many trucks andbuses and much construction equipment uses centralized lubricating systems, de-signed for semi-fluid greases of NLGI class 00 or 000, for onboard re-lubrication Othersystems require greases of NLGI class 2 Lithium greases optimized for low temperatureapplications, good pumpability, and low oil separation are recommended

re-16.9.3

Steel Mills

In Europe calcium complex and sometimes polyurea greases based on mineral oilare used for lubricating continuous casting equipment The US market prefers alu-Fig 16.10 NLGI certification marks.

686 16 Lubricating Greases

minum complex and polyurea greases and Japanese equipment manufacturersmainly recommend polyurea greases Calcium sulfonate complex greases areachieving increasing commercial acceptance in Europe and the USA, because oftheir high EP values and excellent corrosion-protection properties Some rollingbearing manufacturers equip continuous castings with double- or triple-sealed bear-ings that are greased for life, preferably with synthetic polyurea greases When thesealing of the bearings is not perfect re-lubrication has a cleaning function Con-taminated grease that could result in limited lifetime is squeezed out

Conventional EP lithium greases, calcium complex greases, calcium sulfonatecomplex greases, lithium complex greases, and aluminum complex greases, allbased on mineral oils are used in hot rolling equipment In India a successful trialhas been conducted with a locally produced titanium complex grease [16.259] Most

The rotating shift system is typically used for hot-rolling lubrication In general thebearings are not re-greased during operation, only during maintenance

Many different greases, e.g conventional EP lithium greases, lithium complexgreases, calcium complex greases, and calcium sulfonate complex greases, all based

on mineral oil, are used in cold rolling equipment Modern greases use a lithium–calcium mixed soap for improved water resistance Some calcium complex greaseshave helped to prolong the lifetime of bearings in the pickling section significantly.16.9.4

Mining

Open pit mines which use hydraulic excavators and dumper trucks and/or wheelbucket excavators and belt systems to ship the spoil and coal are especially largegrease consumers Lithium, lithium–calcium mixed-soap-based or lithium complex

additive package are used Black solid lubricants are also recommended Because theequipment is exposed to dust or water and mud, the sealing efficiency of the bearingsdetermines their lifetime and must be supported by a grease

Kilometers of wire rope are used on the excavators or drag lines and in the ground mines During production of the wire ropes lubricants are applied to ensurecorrosion protection and to minimize friction of the single wires when the ropestretches under load The amount of lubricant in the core should be ca 25 % w/w ofthe core

under-Lay-up lubricants based on wax-resin are applied during manufacture to ensurelubrication of the individual wires and stands After production the wires must havecorrosion protection; this can be achieved by painting or by applying a bitumen-based lubricant with a solvent

During operation in some countries the wire ropes are maintained by cleaningthe surface and re-lubrication In many countries bitumen-based products are still

in use, but are being replaced by bitumen-free greases or even biodegradable cants It is essential to apply the lubricant on top of the sheave-wheel or shortly after

lubri-687 16.9 Applications of Greasesbending over the winder drum when a large twisting motion or unloading opensthe wires and sucks in the lubricant.

Winch greases, preferable lithium complex greases with improved water tance, ensure prolonged winch lifetimes up to several years

resis-16.9.5

Railroad, Railway

Depending on the design of the driving system of the locomotive a gear oil or a geargrease is required The poor sealing properties of these kind of gears lead to leakage;this is usually minimized by use of bitumen-based products Modern greases arethickened with lithium or sodium soap and based on mineral oils of up to

consistency; the apparent viscosity is usually checked with a Brookfield viscometer,for example with a number 3 spindle at 93 C and at 4 rpm and giving results of

5000 to 10 000 cP [16.261]

The axle bearings are greased with conventional lithium EP greases The opment of ready-to-build-in axle boxes and the increasing speed of modern trainshave led to improvement of the high-temperature performance and lifetime ofgreases

devel-Switch lubrication, wheel flange lubrication (mainly used in Europe), and railtrack-side lubrication in curves, used mainly in the USA and Canada, cause environ-mental problems Biodegradable greases based on esters have better wear-protectionand consumption performance [16.262–16.264] These greases must be sprayablebecause of the means of application

16.9.6

Gears

Fibrous sodium soap greases of NLGI-class 0, 00, or even 000 consistency are used.Although the load-carrying capacity, anti wear properties, and adhesion of thesegreases is especially good, lithium-based greases and greases based on synthetichydrocarbons or polyglycols are gaining in importance

Rather sophisticated greases are used in the gears of do-it-yourself tools Greases

in modern drilling machines must fulfil several tasks – lubricate the gears, the ings, and the piston responsible for the hammer drilling operation Power tools forprofessional use have even more stringent requirements with regard to low temper-ature torque, high temperature performance, and lifetime

bear-Girth gear drives are a type of drive widely used for many large systems in theprimary industries [16.265] These open gears require an adhesive grease based onhigh-viscosity oils Such greases are usually sprayed on the teeth by means of a cen-tralized lubricating system Precise adjustment is required, because these girth geardrives occupy a key position in manufacturing processes Most manufacturers ofadhesive greases have, therefore, developed a maintenance scheme with a tribologi-cal emphasis and offer this as a service to their clients

The FDA (Food and Drug Administration, USA) lists ingredients permitted foruse in food-grade grease formulations (21 CFR §178.3570); white oils are listed in 21CFR §178.3620 The USDA (United States Department of Agriculture) has approvedthe finished products USDA H-1-approved products can be used in food processing

in which incidental and unavoidable contact between food and lubricant can occur

food is allowed Since USDA stopped its activities in 1999 The NSF (National tation Foundation, Ann Arbor, Michigan, USA) has taken over the role of USDA bydeveloping standards and certifying products being used in the food industry or indrinking-water systems

Sani-16.9.8

Textile Machines

Because greases could contaminate textiles, they are formulated with white oils andthickened with water-soluble sodium soaps, and are thus removable by washing.16.9.9

Application Techniques

Greases are applied by hand and brush, with gloves, grease dispensers, grease guns,and pumps from all kinds of container (e.g tins, pails, drums, or even bulk)[16.267] The pumps can be driven manually, by use of a power supply, or by com-pressed air Most pump systems use a follower plate fitted into the container Theplate will follow the grease level The rubber lip must follow the inner surface of thegrease container, to avoid any sucking in of air

Trucks, buses, construction and forestry equipment, continuous casting and rolling equipment, paper mills, printing machines, presses, excavators for open pitmines, and many more machines used industrially have centralized lubricating sys-tems These systems can be designed for greases in NLGI-class 2 as parallel type forsingle or dual line systems usually using valves or manifolds, or as series typeusually using progressive piston-type metering manifolds For NLGI-classes 00 and

cold-000 single-line systems with valves, of similar design to oil-centralized lubricationsystems, are used The amount of grease and the re-lubrication time is mechanically

or electronically adjustable The greases are selected for the lubrication point, buthave to fulfil specific requirements, e.g pumpability at low temperature, reduced oilseparation, and no tendency to hardening in progressive plunger metering devices

689 16.9 Applications of GreasesThe greases must be free from air bubbles, because transport of the grease to thelubricating points will be affected.

Greases should be kept clean during storage and application, because any tamination will increase the risk of reduced lifetime Because oil-wet surfaces catchany dust, grease containers must be stored closed [16.268]

con-16.9.10

Special and Lifetime Applications

Fire-resistant greases as described by the British Coal Specification [16.269] are stillbased on phosphate esters or their mixtures with other base oils Since Spengler andWunsch described the applications of greases in precision instruments in 1970[16.270] nothing essential has changed in this area The same is true for applications

of greases in nuclear power plants [16.271] In spacecraft or high vacuum tions new findings have been reported [16.272] Greases based on perfluoropolyalk-ylethers are still doing their job [16.273, 16.274], but greases based on multiply alky-lated cyclopentane have also generated interest [16.275, 16.276] These applicationsare mainly meant for lifetimes up to decades and the limiting factors are mainlywear [16.277] and high operating temperature [16.278]

applica-16.9.11

Applications with Polymeric Materials

There are three ways in which greases and polymeric materials can be present gether: polymers can be contained in greases as thickeners, as solid lubricants, and

to-as additives; they can act to-as sealants [16.279]; and they can be one or even both ofthe solid partners in friction couples Although base oils are the primary considera-tion [16.280], when the interaction of greases with sealants is important [16.281,16.282] – the performance of oils is described in Chapter 11 – the influence of addi-tives, and among these especially EP additives, must always be considered [16.283].Even migration of soap molecules into a polymer can occur [16.284] An enormousnumber of polymers is known and used as components of machinery [16.285] Al-though the tribological fundamentals for use of polymeric solids have been describ-

ed in detail by Bartenev and Lavrentev in 1981 [16.286], by Uetz in 1985 [16.287],and by Yamaguchi in 1990 [16.288], and a brief overview over the tribology of plasticswas published in 1994 [16.289], the lubrication of polymers has been described indetail only by Spengler and Wunsch [16.290]

An early test method for the action of lubricants on polymeric materials was thependulum according to Barker [16.291] During the automation of this process amore meaningful friction and wear tester was developed [16.292, 16.293] It provedalso to be necessary to test the tendency of a polymer to form cracks under the influ-ence of its lubricant and tension [16.294]

The lubrication of polymers or polymers and metals as friction couples is onefield in which silicone greases and, better, fluorinated silicone greases, or, even bet-ter, greases based on perfluoropolyalkylethers are of some advantage Although sili-

690 16 Lubricating Greases

cone greases made with highly dispersed silicic acid should be used as general cants and as sealants [16.295], silicone greases made with the usual thickeners aresometimes preferable to other synthetic greases With calcium stearate or 12-hydro-xystearate, for example, they are permitted in food applications [16.296], althougheven here compatibility problems can occur, for example with POM (polyoxymethy-lene) and dimethylsilicone oils [16.297] and – this is primarily necessary for the baseoils – creeping should be avoided with the aid of an epilamization agent, i.e a fluori-nated polymer applied to the polymer in high dilution and which forms a kind ofnetwork on the solid surface, thus dramatically reducing surface tension [16.298]

lubri-16.10

Grease Market

In 2004 36.1 million tons of lubricants were used worldwide Of these 1.2 milliontons were greases [16.299] The NLGI (the National Lubricating Grease Institute)has reported that 828,442 tons of grease were produced in 2004 [16.300] and it must

be considered that only approx 67 % of the world’s grease manufacturers ted to this figure

contribu-Conventional lithium greases (56.9 %) are the most used Lithium complexgreases account for 15.1 % and are followed by conventional calcium greases with8.5 % (Table 16.18)

Tab 16.18 Global share of thickeners (2004).

Conventional lithium soap 56.9

Lithium complex soap 15.1

Conventional calcium soap 7.4

Aluminium complex soap 4.8

Calcium complex soap 3.1

Organophilic clay thickeners 2.5

Anhydrous calcium soap 1.1

Other metallic soap 0.7

Although lithium soap based greases are the most used products worldwide,there are significant differences among local markets with regard to other grades ofgrease In the US and Canadian market lithium complex greases (33.3 %) are themost used Aluminum complex greases (9.3 %) are also above the global average InEurope the calcium complex greases (6.2 %) are above the global average, but behind

of lithium complex grades (10.4 %) In Japan the polyurea greases (21.4 %) arebehind conventional lithium greases (58.2 %); both figures are above the global fig-

691 16.11 Ecology and the Environmentures (Table 16.19) Among the calcium complex soap type the calcium sulfonatecomplex soaps hold 40 %, steadily increasing.

Tab 16.19 Local share (%) of high-temperature thickeners (2004).

The main consumer market is the automotive market, with ca 50 % Depending

on the specific structure of a country it is less than 50 % in highly industrializedcountries and more than 50 % in less industrialized countries The grease used areconventional lithium greases, conventional calcium greases, and often sodiumgreases for re-lubrication of trucks, construction, farming, and forestry equipment,and old-fashioned cars

The industrial grease market is divided into a wide range of different and made greases Big consumers are steel mills, mines, especially open-pit mines, rail-road companies, motor companies, and manufacturers of all kind of machinery;they rely on many suppliers Often these suppliers emphasize the performance of awide variety of greases Among industrial grease customers the roller bearing indus-try is very important; in addition to high-quality conventional lithium greases theyincreasingly need specific greases to fulfil their customers’ requirements

tailor-In the field of biodegradable greases in Germany products used in switches havebeen completely replaced by ester based biodegradable greases Trials with biode-gradable wheel-flange greases in Austria, The Netherlands, and Germany [16.301]have been successfully completed Testing in Canada of a biodegradable grease forrail track-side lubrication proved its superior load-carrying and wear-protection per-formance [16.302]

16.11

Ecology and the Environment

Pre-industrial greases consisted of tallow or vegetable oils and their reaction ucts with lime; they were therefore, centuries ago, not merely ecologically compat-ible lubricants, but even rapidly biodegradable When mineral oils took over, most ofthe thickeners and some of the additives remained ecologically acceptable The eco-logical and environmental aspects of modern base oils have been described in detail

prod-in Chapter 7 Of all the vegetable oils that could be chosen as base oils for greases,

in Central Europe only rape seed oil has been introduced on a technical scale[16.303] and only for loss-lubrication applications Initially calcium 12-hydroxystea-

692 16 Lubricating Greases

rate was chosen as the most suitable thickener [16.304] Greases of that kind provedsuperior to lithium- and mineral oil-based general-purpose grades (Fig 16.11) Clay-based greases were also tried, but did not come up to expectations [16.305] The lim-itations of vegetable oil-based greases soon led to the introduction of transesterifiedvegetable oils [16.306] and synthetic esters The latter have been described in detail

in Chapter 5 Because of industrial demands greases based on calcium stearate were usually soon replaced by lithium and lithium–calcium greases[16.307–16.309] Polyurea greases [16.310], aluminum complex greases [16.311], andtitanium complex greases [16.312] followed

12-hydroxy-The biodegradability of greases essentially reflects the biodegradability of their baseoils, although the CEC L-33 test had to be modified for them [16.313] and in GermanyDIN 51828 was introduced [16.314] In round robin tests greases based on rape seed oiland calcium 12-hydroxystearate achieved biodegradability close to 100 % Methodsenabling the use of emulsifiers led to significantly lower results [16.315]

Biodegradability is one aspect of the necessary reduction of the toxic potential ofgreases The elimination of toxic components such as polycyclic hydrocarbons frombase oils is almost complete This is also true for chlorine substitution The substitution

of antimony and lead cations and nitrite anions in additives is not yet complete, because

it is not easy to maintain the same performance level, but the work by the suppliers ofadditives remains focused on minimizing ecotoxicity [16.316–16.319] In Germany thedemand for environmentally harmless products [16.320] became an additional aspect ofthe performance of greases; changes in the demands for water pollution class (WGK) 0,that came into force in 1999 aggravated the situation The most suitable antioxidants areonly accepted in concentrations which are too low, hydrocarbons including polyalphao-lefins, white oils, and low-viscosity polybutenes are not accepted and the most suitablesoaps are also excluded In Sweden a project, called Ren smrja i Gteborg, is gainingattention Here the thickeners are regarded as a separate group [16.321]

Although experiences with biodegradable greases are promising [16.322], and though customers should be encouraged to try these in bearing applications that

Lithium multipurpose grease

Fig 16.11 The friction–load behavior

of rapeseed oil-based calcium 12-hydroxystearate grease.

693 16.12 Grease Tribologyused to be the domain of mineral oil-based greases [16.323], some problems, mainlyrelating to legal aspects of waste management [16.324] and the balance between eco-logical requirements and economical possibilities [16.325], remain unsolved There-fore the third and possibly most important aspect of applying greases in a mannerthat is both environmentally responsible and economical is to minimize the amount

of lubricant necessary and to maximize lifetime by using the most inert materials[16.326, 16.327] Growing awareness of a rising “peak oil” problem is expected tocontribute to acceleration of corresponding efforts [16.388]

16.12

Grease Tribology

General aspects of friction and wear [16.328], and of lubricating oils and theirviscosity [16.329], have been described in detail in Chapters 2 and 3 Recent tribolo-gical research on greases has dealt with the time-dependence of their behaviorunder stress as a fourth factor in addition to temperature, pressure, and thickenerproperties [16.330] Behavior in elastohydrodynamic (EHL) contacts, where thethickness of grease films has been reported to exceed that of their base oil undercomparable conditions [16.331, 16.332], but which has also been reported todecrease to some extent during operation [16.333], and the behavior under starvedconditions, when the load is supposed to be carried by thickener particles deposited

in the center of the contact [16.334], has also been of concern Soap deterioration isanother important subject [16.335, 16.336, 16.389] The field of research is wideningevery day, from more practical problems, for example electroerosion in rolling bear-ings [16.390], to more theoretical problems, for example the flow of magnetorheolo-gical and electrorheological fluids (and the application of a simplified Herschel–Bulkley model) [16.391] A combination of simple numerical and experimentalmethods has been used as a grease design tool [16.337] A few published reflections,focusing on problems concerning the critical energy levels [16.338] and the chaoticbehavior [16.339] of greases and the peculiarities of tribometry, the collective termfor the measuring techniques and test methods used in tribology, have attractedinterest [16.340]

Lubricants are used between surfaces which are in tribological contact, and moverelative to one another, to achieve a particular value of the coefficient of friction or toreduce the wear of rubbing surfaces, or both Solid lubricants are required for lubri-cation under extreme conditions where the bearing surfaces in tribological contactmust still be effectively separated The life of lubricated machine parts depends onthe functional and tribological design and optimization of lubricants as calculablefunctional elements To make a systematic choice of a suitable lubricant it is abso-lutely necessary to understand the relationship between friction, wear, and lubrica-tion and the interaction between the elements of the tribological system and the spe-cific properties of each element

Special attention must be given to the different stress factors and the structure ofthe tribological system (see Chapter 2)

Products containing solid lubricants are often used to solve problems, particularly

in boundary and mixed frictional states when high specific loads are applied to ing surfaces and at very low hydrodynamically effective speeds but also in criticalapplications where, e.g., the lubricant must perform over a wide temperature range

slid-or under extreme temperature conditions, fslid-or example in aviation and in rockettechnology

Dry lubrication with solid lubricants is also required in nuclear reactors, in highvacuum applications, in aggressive environments, and in applications where con-tamination by lubricating oils or greases cannot be tolerated

17.1

Classification of Solid Lubricants

A solid lubricant is often defined as any solid material which reduces friction and/orwear of contacting surfaces in relative motion A vast range of materials and coat-ings could be judged to behave as solid lubricants on the basis of this definition.Various systems are used to classify the different types of solid lubricant An arbi-trary, but useful, classification is into structural lubricants, mechanical lubricants,soaps, and chemically active lubricants (Sections 17.1.1 to 17.1.4, respectively)

17

Solid Lubrication

Christian Busch

Lubricants and Lubrication 2nd Ed Edited by Th Mang and W Dresel

Copyright
2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim

695 17.1 Classification of Solid LubricantsThe main purpose of all these substances is to build up a continuous adherenthard or soft film on the rubbing surfaces These films can be applied by mechanical,chemical, electrochemical or physical processes, for example dipping, lapping,painting, immersion, electrolysis, electrophoresis, spraying, plating, welding, bak-ing, cupping, sintering, or ionic plating in vacuum.

17.1.1

Class 1: Structural Lubricants

The most widely used solid lubricants are graphite and molybdenum disulfide.Their satisfactory lubricating properties are assumed to result from their layered lat-tice structures In addition to these two substances are other solids, for examplemetal halides and sulfides, which have, in the main, inherent lubricant properties, alamellar hexagonal crystal structure, and are usually anisotropic (Table 17.1)

Tab 17.1 Structural lubricants.

Graphite fluoride (CF x ) n

Molybdenum disulfide MoS 2

Molybdenum diselenide MoSe 2

Tungsten disulfide WS 2

Tungsten diselenide WSe 2

Niobium disulfide NbS 2

Niobium diselenide NbSe 2

Tantalum disulfide TaS 2

Tantalum diselenide TaSe 2

Titanium disulfide TiS 2

Titanium diselenide TiSe 2

Titanium telluride TiTe 2

Barium hydroxide Ba(OH) 2

Other than graphite and molybdenum disulfide these are not yet extensively used

in industry

696 17 Solid Lubrication

17.1.2

Class 2: Mechanical Lubricants

There are different types of substances within this class whose lubricating effect isbased on different physical and mechanical properties or special conditions Acommon classification divides these lubricants into self-lubricating substances,substances that need a supporting medium to create lubricating properties, andsubstances with indirect lubricating properties based on their hardness

17.1.2.1 Self-Lubricating Substances

These can be classified as organic compounds (Table 17.2), metal films (Table 17.3),chemical surface layers, and glasses

Organic Compounds

Tab 17.2 Self-lubricating organic substances.

Linear polymers (thermoplastic types)

poly-mers and increases their hardness Graphite also increases the elasticity module ofPTFE

697 17.1 Classification of Solid LubricantsMetal Films

Tab 17.3 Self-lubricating metal films.

Nickel Ni and Ni–Cr alloys

Copper Cu and Cu alloys

Friction can be reduced by the coating of the body material surfaces with a thinfilm of a soft metal, because the friction depends on the shear strength of the softmetal film The durability depends on the film hardness, homogeneity, and adhe-sion The lubricating effect of soft metal layers is limited by their melting point

Chemical Surface Layers (Conversion Films)

In addition to the naturally occurring oxide films present on the surface of mostbase materials exposed to air, other solid lubricant films can be formed by chemical

or electrochemical action on the metal surface

Chemical surface coatings such as zinc, iron, or manganese phosphates behavesimilarly to soft metal coatings but consist not of a metal but of metal salts Bonder-ization (phosphate treatment) creates a thin, microcrystalline, strongly adheringphosphate layer on the metal surface; this reduces the coefficient of friction and thedanger of seizure during the running-in period The lubricating efficiency of thelayer, which is normally 2–5 lm thick, is based on its lower shear strength in com-parison with the metal

Glasses

The structure of glasses consists of random three-dimensional networks in which theformation of chains or sheets are possible The constituents of glass are divided into net-

PbO, etc.) The strongest bonds in these glasses are the Si–O bonds with an averagebond distance of 1.62  There is no absolute Si–O bond distance in a glass because ofthe absence of symmetry and this means that glass softens and has no fixed meltingpoint

The lubrication properties of glass depend on the composition The coefficient offriction at a given temperature is a function of the viscosity, the thermal conductivity,the rate of shear, the area of shear, the capacity to dissolve different amounts ofoxide from the surface of the material to be lubricated, and the contact anglebetween the glass and the material, because this determines the capacity of the glass

to wet the material

Tab 17.4 Inorganic compounds needing a supporting medium.

Metal sulfides ZnS, SnS 2 , FeS, …

Metal fluorides CaF 2 , LiF, …

Metal phosphates Zn 2 P 2 O 7 , Ca 3 (PO 4 ) 2 , Fe 2 P 2 O 7 , …

Metal hydroxides Ca(OH) 2 , Mg(OH) 2 , Zn(OH) 2 , …

Metal oxides PbO, ZnO, FeO, Fe 2 O 3, …

Natural oxide films on metals, which are usually approx 100  thick, have beeninvestigated by Whitehead with regard to their influence on the coefficient of fric-tion He found that their action depends primarily on the relative mechanical prop-erties of metal and oxide It is generally accepted that the oxide film reduces surfacedamage, makes sliding smooth, and often reduces friction

Sulfides, fluorides, phosphates and hydroxides are claimed to act as a supportingagent or a catalyst by producing friction- and wear-reducing layers Calcium hydroxide,

sur-face of steel This oxide has better tribological properties than the more common

struc-ture compared with the corundum-like lattice strucstruc-ture of ferric oxide

The process of formation of these layers depends on the chemical composition ofsteel and, in particular, on its surface chemistry

Phosphate layers can also be applied by galvanic techniques Such procedures aremainly used to create phosphate layers as precoatings for dry-film application and aslubricant carriers in cold metal-forming processes In addition to acting as a lubri-cant carrier, the phosphate coating can be plastically deformed with the steel slugand, therefore, in conjunction with the lubricant, prevents metal–metal contact andthus reduces surface friction and wear

The three main types of phosphating solutions contain zinc, iron, and manganesephosphates, and of these the zinc phosphate is probably the most widely used.Metal Powders

In contrast with structural lubricants and self-lubricating mechanical lubricants, thelubricating properties of the other mechanical lubricants are mainly based on thesupporting effect of a carrier substance or a binder The main purpose of these sub-stances, which include Pb, Sn, Zn, Cu, Ag, and In, is to improve the adhesion andcohesion properties of the non-self-lubricating mechanical lubricants

699 17.1 Classification of Solid Lubricants17.1.2.3 Substances with Indirect Lubricating Properties Based on their Hardness

(Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD),

and Diamond-like Carbon (DLC) layers)

Vapor-deposited (VD) coatings of oxides, borides, nitrides, or silicates, for ple TiN, TiNC, CrN, ZrN, and AlTiN, usually have quite high coefficients of fric-tion but can prevent seizure even at high temperature and offer excellent wearresistance

exam-Another relatively new form of thin-film layer with proven good tribologicalproperties is diamond-like amorphous carbon, referred to as DLC Hardness val-ues for DLC range from 500 to 13 000 HV depending on the amount of hydroge-nation, from 50 % to zero Hydrogenated DLC is shown as (a-C:H) and hydrogen-free DLC as (a-C) In addition to hard-coatings there are also soft physical vapor-

normally deposited by various sputtering technologies like IBAD assisted deposition) or CFMS (closed-field-magnetron sputtering), afford excel-lent tribological properties in terms of wear-resistance and low coefficient of fric-tion They are used for cutting and forming tools, ball bearings, and sliding con-tacts with steel and ceramics

(ion-beam-17.1.3

Class 3: Soaps

Soaps are the metal salts of the higher saturated and unsaturated fatty acids and ofresin acids and they are sometimes understood to include salts of naphthenic acidsand synthetic fatty’ acids The most effective of these are polar compounds withactive groups in a long-chain molecule, presumably because the reactive groupattaches itself to the surface being lubricated and resists removal They often givethe lowest coefficients of friction obtainable with solid lubricants but in general can-not be used above their melting points or at high loads

The main function of soaps in lubrication technology is in the preparation ofgreases The subject is discussed in more detail in Chapter 6 The main use of soaps

as lubricants in their own right depends on their formation in situ on a metal face, by the chemical attack of a fatty acid on the metal

sur-17.1.4

Class 4: Chemically Active Lubricants

This category includes Extreme Pressure (EP) and Anti Wear (AW) additives and avariety of chemicals which interact with the metal surface to produce a lubricating

or protecting layer (see Chapter 6)

Graphite has a resonance structure which enables binding of the carbon atoms asfirmly with three bonds as with four Each carbon atom is linked to its nearest neigh-

single covalent C–C bond by the interaction of non-localized electrons associatedwith the 2pz orbital of the planar carbon atoms It is this extra bonding which givesgraphite its thermal stability; the p electrons are responsible for the magnetic andelectric properties of graphite The planes themselves are held together largely byvan der Waals forces which are much weaker than the planar bonding forces andthe strength of bonding to a substrate

The low friction of graphite is not based solely on its crystal structure It alsodepends on adsorbed films, particularly of water vapor, which provide surfaces withlow cohesion Thus, graphite develops its most favorable lubricating properties inthe presence of moisture

17.2.1.2 Molybdenum Disulfide

The crystal of molybdenum disulfide has a hexagonal structure with trigonal

chemically very stable It is resistant to most acids and is insensitive to radioactiveradiation The temperature limit in air, above which there is increasingly rapid oxi-dation to molybdenum trioxide, is 370 C Moist air reduces the onset of oxidation to

350 C In vacuum it decomposes at 1100 C to molybdenum and sulfur When ciently pure, it is a semiconductor and diamagnetic

bonds (van der Waals forces) between the sulfur layers enable easy movement of thelamellae over each other, resulting in a low friction between the sliding parts Onthe other hand, the ionic bond between Mo and S gives the layers a high strength sothat they can resist the penetration of surface asperities

701 17.2 Characteristics17.2.2

Heat Stability

Heat stability is required if thermal decomposition of the solid lubricants is to beavoided, especially in the presence of oxygen-, carbon dioxide-, or water-containingatmospheres Very often the decomposition products are either corrosive vapors orabrasive solids

When heated in air, graphite forms carbon dioxide above 450 C; this escapes

20 C In air, appreciable oxidation to molybdenum trioxide occurs above 400 C and

monoxide is an excellent lubricant up to 700 C Metal-free phthalocyanines havebeen established as very efficient lubricants up to temperatures of 800–900 C Glasshas proved to be an efficient lubricant at extreme loads and temperatures up to

1200 C

Other solids, for example metal oxides, sulfides or phosphates, also afford lent heat stability Without a supporting liquid, however, these solids provide nolubricating properties but build up separating layers only The lubrication effect is,therefore, limited by the heat stability of the carrying substance

excel-17.2.3

Melting Point

The melting point of a solid lubricant can be regarded as a physical aspect of heatstability If, as a result of rising temperature, the elastic vibrations in a crystalbecome so violent that two component particles become separated by a distancegreater than a given critical distance then the lattice yields at this point, because thestress which has forced it apart is capable of tearing the lattice open Under thismetastable condition the crystal melts and the cohesive energy of the crystal whichgives it mechanical strength is destroyed

17.2.4

Thermal Conductivity

Frictional heat, developed by rubbing of material surfaces at the high spots, is alsotransmitted to the solid lubricant, which must be capable of dissipating this heat asquickly as possible – or local welding of the material might occur, despite the pres-ence of the solid This is particularly true of plastic solid lubricants, which all havepoor thermal conductivity

of the solid can be:

of heat;

The chemisorbed molecules within the particles can affect:

17.2.6

Mechanical Properties

The mechanical properties of a given solid are much less important than themechanical properties of the solid relative to the metal it is to lubricate Possibly themost important properties relate to hardness, compressibility, and tensile strength.17.2.7

Chemical Stability

Chemical stability is closely linked with heat stability When solid lubricants are to

be used intermittently over long periods in relatively inaccessible places, tendency tocorrosion can be serious Corrosion can occur in three ways:

products, which then react with the metal surface; and

solid lubricant producing material which has a corrosive action on the metal

17.2.8

Purity

The importance of purity should never be overlooked Contamination can arise fromthree sources:

703 17.3 Products Containing Solid Lubricants

17.2.9

Particle Size

Particle size is a geometrical term which does not take into account the internalstructure of a particle or, indeed, very fine details of particle-surface topography.With this note of caution, it can be assumed that the smaller a non-porous particle,the more active it is Although such particle activity is required in some applicationsthere are other fields, for example many metal-forming operations, for which it isdesirable to have a relatively large particle size range to give the required film prop-erties With graphite and molybdenum disulfide, relatively coarse particle sizes areadvantageous because this results in optimum resistance to oxidation

Three requirements result from this:

(i) The level of adhesion between the lubricant film and the surface of the rial must be great enough to ensure that this lubricant film adheres to thissurface when it is subjected to friction

mate-(ii) The internal cohesion of the film must be sufficiently large that the film doesnot split when subjected to friction

(iii) The adhesion between the particles and layers in the shearing directionshould be as small as possible to keep the resistance to friction low

These main requirements can be met only by self-lubricating dry lubricants

bonding of a metal (Mo) and strong polarizing effect of a non-metal (S) with a highdegree of polarization), meets these requirements as well as can be achieved; it is, as

a result, the most commonly used Other solid lubricants which are applied in der form are organic compounds such as PTFE and graphite, although these fulfilthe requirements listed above to a limited extent only

pow-Before the solid lubricant powder can be applied, the surface of the material must

be thoroughly cleaned Roughing the surface mechanically or with phosphatesimproves adhesion and therefore the lifetime The lubricant can be effectivelyapplied by simple rubbing with cloths, sponges, brushes, or polishing pads or pol-

704 17 Solid Lubrication

ishing buffs, by applying it using suitable carriers, and coating by cathode tion in an ultra-high vacuum

evapora-17.3.1.1 Solid Lubricants in Carrying Media

As already mentioned, very few solid lubricants have the appropriate adhesive andcohesive properties which enable them to create an effective lubricating layer withlow friction coefficients and give them a sufficient lifetime

Most substances used as solid lubricants require a carrying medium, a bondingagent and/or pretreatment of the material surface, to help create, or improve, theiradhesive and film-creating properties The substances used as bonding agents are:

It is also advantageous to pretreat the surfaces by:

17.3.2

Dispersions and Suspensions

Dispersions and suspensions in carrying liquids with low volatility are mainly used

in areas in which, for tribological reasons, a dry lubricating film should be created,but where effective application of a powder is not possible for technical reasons Thesame types of solid lubricant are used here in the same way as for powders

Dispersions and suspensions of solid lubricants in water are usually used to coatmass elements for cold and hot forming The most commonly used substances hereare salts, special white solid lubricants, and graphite

Dispersions and suspensions in oils also act as aids in forming techniques, andthey are also used as additions in gear- and oil-lubricating systems The solid lubri-cants used here in forming techniques are the same types as those used as disper-

lubri-cating gears and for general use in oil lubrilubri-cating systems

17.3.3

Greases and Grease Pastes

Addition of solid lubricants to greases is primarily intended to have a positive effect ontheir capacity to absorb pressure, and the ability to withstand wear and tear, and friction.The specific advantages of solid lubricants in comparison with oil-soluble, chemicallyreactive additives is that they react neutrally to many types of plastic and elastomer, per-form well at high temperatures and have good safety reserves with regard to emergencyrunning properties, which come into action when the layer of grease collapses The use

705 17.3 Products Containing Solid Lubricants

run-ning-in processes and the capacity to withstand wear and tear in boundary and mixedfriction areas Where oscillating movements or vibration is involved, white solid lubri-cants have the advantage These effects are illustrated in Figs 17.1 and 17.2

The exceptional aging stability shown by solid lubricants is particularly important

in terms of the lifetime of the lubricant, and can be used to advantage in the priate systems and structures Generally speaking, solid lubricants in greases are

appro-10 mµ

250 mµ

Surfacecontour( wearprofile)≈ Fig 17.1molybdenum disulfide, showingLithium grease with

the irregular pattern of the wear profile, the chipping of the material, and serious wear The maximum wear depth exceeds

no chipping of material.

Scanning electron photomicrographs (50
magnification)

706 17 Solid Lubrication

used at concentrations of 1–3 % When this proportion rises to over 10 % they areknown as grease pastes, because the solid lubricant at this concentration has anoticeable thickening effect

High-performance lubricating greases do not usually contain one single ing element, but instead are made up of combinations of two or more substances,which have a synergistic effect This applies both to the combination of graphite and

exam-ple calcium hydroxide, zinc phosphate, or iron sulfide

17.3.4

Pastes

Pastes are solid lubricants in a carrying oil To achieve the correct consistency, theproportion of solid lubricant must be at least 40 % To reduce the undesirable bleed-ing’ effect, modern formulations often include a small amount of soap, althoughtribologically this plays a minor role only

types of solid lubricant Because of their high load-bearing capacity, these pastes areparticularly useful when movement with very slow relative speeds is involved, e.g.,assembly processes, and running-in processes

Pastes prepared from combinations of white solid lubricants are especially able for use where oscillating movements or vibration is involved White pastes areparticularly good at preventing the wear and tear which results from vibration andare proven to afford excellent protection against fretting corrosion

suit-These special white solid lubricants create thin adhesive layers on the surfaceswhich are subject to friction These layers principally adhere to the surfaces because

of tribochemical (e.g zinc diphosphate) and/or physical adhesive forces (e.g cium hydroxide) Another special feature of these layers is that they regeneratethemselves This, and their innate high degree of resistance to wear and tear, meansthat longer operating periods can be achieved than when normal greases are used,without the need for additional re-lubrication

cal-Another area where pastes are traditionally used is for lubrication of screw tions Special screws made of steels which are resistant to high temperatures, based onchrome/nickel alloys, are prone to seizure because an oxide layer is not created.The pastes used in this particular application area are usually made up of combi-nations of solid lubricants and special metal powders More recent developmentsshow that the use of formulations which do not contain metal, and are based onwhite solid lubricants, perform well when used at high temperatures