Mechanical component

A mechanical component consisting of solid bodies which are mobile relative to one another under a variable frictional force and are separated from one another by a fluid organic mass, characterized in that the change in frictional force is induced by phase transitions between different thermotropic liquid-crystalline phases or between a thermotropic liquid-crystalline phase and an isotropic phase.

BACKGROUND OF THE INVENTION 
The invention relates to a mechanical component consisting of solid bodies 
or parts of bodies, which are mobile relative to one another and are 
separated from one another by a fluid organic mass, wherein it is possible 
to vary the frictional forces acting between the bodies by changing the 
molecular order in the organic mass. 
The motion of solid bodies within machines and the motion of a machine 
relative to a fixed base are, inter alia, determined by the friction which 
is applicable in each case between the bodies concerned. The fundamental 
distinction must here be made as to whether the bodies slide on one 
another or roll on one another. The magnitude of the sliding friction 
depends on whether the bodies slide directly on one another or are 
completely separated from one another by a lubricant. This is called dry 
friction in the first case and hydraulic friction in the second. So-called 
semi-hydraulic friction occurs if the lubricating film is incompletely 
formed. Sliding friction also takes place always in rolling bearings 
between the rolling elements and their guide elements. 
As is known, the lubrication processes in machines can be classified in two 
groups. In hydrodynamic lubrication, the load-bearing capacity in a 
lubricating film is produced in the form of a gap of relatively large 
dimensions. The friction is then primarily determined by the temperature 
dependence of the viscosity of the lubricant. In the case of 
elasto-hydrodynamic lubrication, a very small lubrication gap is formed 
from an initially linear or punctiform contact of two elastic bodies. The 
flattening in the so-called Hertz contact region, together with the 
increase in the viscosity of the lubricant with pressure, has the result 
that the solid bodies moving relative to one another hardly touch directly 
or not at all. In existing practice, very high pressures, in the range of 
1 to 40 kbar, are necessary to produce sufficiently high viscosities. By 
comparison, the pressures applied in hydraulic devices are much too low to 
enable a substantial change in the viscosity of the hydraulic oil and 
hence also the motion sequences. The pressure conditions in lubricants and 
in hydraulic oils have been described (Dubbel, Taschenbuch fur den 
Maschinenbau, [Pocketbook for Machine Engineering], Springer-Verlag, 
Berlin). 
It is elementary to minimize the frictional losses in bearings by selecting 
a suitable lubricant. A large number of liquid lubricants--these are 
organic compounds in most cases--are nowadays in use. It is likewise 
elementary to ensure a high frictional force by selection of the 
materials, if a clutch effect or braking effect is to be achieved. The 
possibility, in principle, of changing the function of a mechanical 
component by varying the viscosity--for instance, changing the function 
from that of a slide bearing to that of a clutch--is provided by 
electro-rheology. In this case, the viscosity in layers of colloidal 
solutions is varied by means of an applied electric field (J. E. 
Stangroom, Electro-rheological Fluids, Phys. Technol., volume 14, pages 
290-296 (1983)). 
Some organic compounds do not pass directly from the crystalline phase into 
the isotropic-liquid phase on heating, but pass through one or more 
additional phases within clearly defined temperature ranges. These phases 
have anisotropic physical properties, as are observed in crystals, but are 
at the same time fluid like ordinary isotropic liquids. The phases formed 
by molecules of elongate shape are also described as a rod-like or 
calamitic phase. As distinct from the completely disordered isotropic 
phase, a long-range order of the orientation applies in this case. In the 
nematic phases (abbreviated as N) of hitherto known low-molecular 
compounds, the molecules can freely rotate about their longitudinal axis. 
Closely related to the nematic phase is the cholesteric phase which is 
formed by optically active elongate molecules or is obtained by addition 
of optically active compounds to nematic compounds. For purposes of the 
present invention, cholesteric phases are included in the term nematic 
phase. As a result of intermolecular interaction, parallel-aligned 
rod-like molecules can be assembled into layers and the latter can be 
arranged in space at always identical spacings. This layer structure is 
typical of the smectic phases. Different smectic phases can arise which 
differ by the arrangement of their components within the layers. The 
centres of gravity of the molecules within one layer can be arranged at 
random (for example in the S.sub.A phase and the S.sub.C phase) or 
regularly (for example in the S.sub.B phase). The phases have been 
designated approximately in the order in which they were discovered. 
Nowadays, smectic phases S.sub.A to S.sub.K are known. The features of 
such calamitic phases have been described (for example G. W. Gray, J. W. 
Goodby, Smectic Liquid Crystals, Leonard Hill, Glasgow (1984)). 
Liquid-crystalline phases can also be formed by disc-shaped compounds 
(so-called discoid phases). The discoidnematic phase here has the molecule 
arrangement which is the easiest to describe. In the so-called 
discoid-columnar phases, such molecules are combined in column-like 
arrangements as the result of intermolecular interactions. The features of 
discoitectic phases have been described, for example, in Mol. Cryst. Liq. 
Cryst. 106, 121 (1984). More recently, compounds have been disclosed which 
form so-called phasmidic phases, which are likewise thermotropic 
liquid-crystalline phases (for example J. Malthete et al., J. Phys. 
(Paris) Lett., volume 46, 875 (1985)). Thermotropic liquid-crystalline 
phases are also formed by polymers and their mixtures with low-molecular 
compounds (for example H. Finkelmann in Thermotropic Liquid Crystals, John 
Wiley & Sons, New York (1987) pages 145-170). The exploitation of the 
favourable viscosity within a single thermotropic liquid-crystalline phase 
for clock movements has already been described (European Patent 
0,092,389). 
It is known that the transitions between the Liquid-crystalline phases are 
pressure-dependent (for example G. M. Schneider et al., Physica 139 & 140 
B, 616, (1986)). The dependence of the transition temperatures between the 
various phases are subject to the known Clausius-Clapeyron rules. In 
general, the existence ranges of the liquid-crystalline phases are shifted 
to higher temperatures by an increase in pressure. The order of the 
appearance of the phases remains unchanged in most cases, but it is 
possible that an additional pressure-induced phase arises. Thus, the 
transition temperatures of, for example the compound 
##STR1## 
for normal pressure are S.sub.F -S.sub.C 160.degree. C., S.sub.C -S.sub.A 
195.degree. C., S.sub.A -I 204.degree. C., for 250 bar S.sub.F -S.sub.C 
171.degree. C., S.sub.C -S.sub.A 204.degree. C., S.sub.A -N 206.degree. C. 
and N-I 208.degree. C. It is also known that the intermolecular 
interactions which determine the viscosity change during such transitions. 
The term phase transition includes, within the meaning of the mechanical 
component according to the invention, so-called pretransitional phenomena 
(described in: G. Vertogen, W. H. de Jeu, Thermotropic Liquid Crystals, 
Fundamentals, Springer-Verlag, Berlin 1988). These are changes in the 
molecular order and hence physical properties in the event of changes in 
pressure or temperature even before the phase transition is reached, for 
instance in the case of a pressure increase before the transition from 
nematic to S.sub.B is reached. The invention thus also comprises a 
mechanical component which changes its frictional force as a result of an 
induced change in the temperature or pressure in a fluid organic mass, if 
this mass has at least one enantiotropic thermotropic liquid-crystalline 
phase. In particular, it comprises such a component which is operated at 
temperatures which are 0.1.degree.-30.degree. C. above an enantiotropic 
transition from a liquid-crystalline phase into another liquid-crystalline 
phase or from the isotropic phase into a liquid-crystalline phase. 
It is a requirement of the component according to the invention that a 
temperature or pressure difference is induced. The existence ranges of the 
phases concerned depend, in addition to the pressure, also on the 
temperature, so that changes in temperature during operation must be 
allowed for. 
Conventional lubricants change their viscosity as a function of the 
temperature or pressure to such a small extent that very large temperature 
differences or pressures are required in order to obtain a favourable 
change in the frictional properties. For this reason, the applicability of 
this dependence to mechanical components--such as rolling bearings, 
rolling couplings and toothed gearings--with relatively small contact 
areas is limited in the case of high contact pressures and a high 
elasticity of the materials of the solid bodies concerned. Moreover, it 
has hitherto not been possible to use the electro-rheological liquids, 
because they tend to undergo sedimentation. In addition, the moving 
machine parts are subject to wear. In general, it is not easy to 
accomplish high electric field strengths between mobile, electrically 
conductive machine parts. 
SUMMARY OF THE INVENTION 
It is the object of the invention to make it possible in a simple manner, 
to change the frictional force acting between two solid bodies moving 
relative to one another. 
According to the invention, the object is achieved by introducing a 
thermotropic liquid-crystalline mass, or a mass capable of forming one or 
more thermotropic liquid-crystalline phases, between the bodies of a 
mechanical component, which are mobile relative to one another, and 
inducing a phase transition between different thermotropic 
liquid-crystalline phases or between a thermotropic liquid-crystalline 
phase and the isotropic phase (abbreviated as I) by changing the 
temperature conditions or pressure conditions within this mass. The object 
is made more difficult by the fact that the frictional forces, which are 
established in a complicated manner in flowing, anisotropic media, cannot 
be precalculated and that it is impossible to state which of the numerous 
viscosity coefficients of the particular phases concerned are decisive for 
this effect.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It has been found, surprisingly, that devices according to the invention 
can be realized by means of compounds or mixtures of compounds having a 
phase transition from an isotropic phase to a smectic phase, or from a 
discoid-columnar phase to a discoid-nematic phase. The I-S.sub.B, 
I-S.sub.A, I-S.sub.C and I-S.sub.E transitions are preferred here. The 
transition I-S.sub.B is particularly preferred. 
It has also been found that devices according to the invention can be 
realized by means of compounds and mixtures of compounds having a phase 
transition from the nematic phase into a smectic phase. N-S.sub.B and 
N-S.sub.A phase transitions are preferred. Compounds or mixtures having an 
N-S.sub.B phase transition are particularly preferred. 
It has been found, surprisingly, that devices according to the invention 
can be prepared by means of compounds or mixtures of compounds having 
phase transitions between smectic phases of different order. The S.sub.B 
-S.sub.A and S.sub.C -S.sub.A transitions are preferred. The S.sub.B 
-S.sub.A phase transition is particularly preferred. 
The present invention also includes components which exploit the multi-step 
change in the frictional force, when the fluid layer passes successively 
through a plurality of phase transitions with rising or falling 
temperature or rising or falling pressure. By the choice of the compounds 
used and the selection of their proportions in mixtures, the phase 
sequence and the temperature intervals or pressure intervals of the phases 
can be adjusted within wide ranges. Preferred phase sequences with rising 
pressure are N-S.sub.A -S.sub.B, I-S.sub.A -S.sub.B, N-S.sub.C -S.sub.B, 
I-S.sub.C -S.sub.B, I-S.sub.A -S.sub.C -S.sub.B, N-S.sub.A -S.sub.C 
-S.sub.B and, N-S.sub.A -S.sub.C -S.sub.G. 
It has also been found that devices according to the invention can be 
produced, if the organic masses (sic) located between two bodies mobile 
relative to one another consists of compounds or mixtures of compounds 
which have a phase transition from an isotropic phase or discoidnematic 
phase to a discoid-columnar phase. In this case, compounds or mixtures 
with transitions from an isotropic phase to a discoid-columnar phase are 
preferred. 
The present invention also includes devices whose fluid organic masses 
consist of compounds or mixtures of compounds which form so-called 
calamitic or discoid reentrant phases, that is to say which pass, for 
example, through the phase sequence N-S.sub.A -N or N-S.sub.A -I with a 
rise in temperature or pressure, the range of existence of, for example, 
the S.sub.A - phase being located above the pressure interval of a nematic 
phase. The existence of such liquid crystals has been described (for 
example, L. Longa, and W. H. de Jeu, Phys. Rev. A26 1632 (1982)). 
It has also been found that mechanical components according to the 
invention can be produced by means of compounds of the formula I 
EQU R.sup.1 --A--(Z.sup.1 A.sup.1).sub.1 --(Z.sup.2 A.sup.2).sub.m --(Z.sup.3 
A.sup.3).sub.n --R.sup.2 
wherein 
R.sup.1 and R.sup.2 each are an unsubstituted or substituted alkyl group 
having 1 to 18 C atoms, in which one or two adjacent CH.sub.2 groups can 
also be replaced by --O--, --CO--, --CHOH--, --CHCN--, --OOC--, --COO--, 
--CH.dbd.CH-- and/or --C.dbd.C--, a perfluoralkyl group having 1 to 12 C 
atoms, in which one or two CF groups can also be replaced by --O--, 
--CHF--, --CH.sub.2 -- and/or --CHOH-- and/or one CF.sub.3 group can be 
replaced by CF.sub.2 H or --CH.sub.2 OH, and 
R.sup.2 can also be H, F, Cl, --CN or --COOH, 
A, A.sup.1, A.sup.2, A.sup.3 are each 1,4-phenylene which is unsubstituted 
or mono- to di-substituted by CN groups or F or Cl atoms and in which one 
or two CH groups can also be replaced by N atoms, 1,4-cyclohexylene in 
which one or two CH.sub.2 groups can also be replaced by an O atom or CHF 
or CF.sub.2 groups, and 1,4-bicyclo[2.2.2]octylene, 
Z.sup.1, Z.sup.2, Z.sup.3 are each --CH.sub.2 CH.sub.2 --, --CH.sub.2 O--, 
--OCH.sub.2, --COO--, --OOC--, --CH.sub.2 CF.sub.2 --, --CF.sub.2 CF.sub.2 
--, --CH.dbd.N--, --CH.dbd.CH--, --C.dbd.C-- or the single bond and 
l, m, n are each 0 or 1, 
and also by means of mixtures of compounds of the formula I or mixtures 
which predominantly consist of compounds of the formula I. 
The meaning of the symbols R.sup.1, R.sup.2, A, A.sup.1, A.sup.2, A.sup.3, 
Z.sup.1, Z.sup.2, m and n is as indicated above. The cyclic structural 
elements A, A.sup.1, A.sup.2 and A.sup.3 in the formula I are simplified 
as follows: PE represents a 1,4-phenylene group, CY represents a 
1,4-cyclohexylene group and BO represents a 1,4-bicyclo[2.2.2.]octylene 
group. 
The compounds of the formula I thus comprise compounds of the part formulae 
Ia, Ib, Ic and Id 
EQU R.sup.1 --A--R.sup.2 Ia 
EQU R.sup.1 --A--Z.sup.1 --A.sup.1 --R.sup.2 Ib 
EQU R.sup.1 --A--Z.sup.1 --A.sup.1 --Z.sup.2 --A.sup.2 --R.sup.2Ic 
EQU R.sup.1 --A--Z.sup.1 --A.sup.1 --Z.sup.2 --A.sup.2 --Z.sup.3 --A.sup.3 
--R.sup.2 Id 
Compounds of the formula Ib are particularly preferred. The groups Z.sup.1 
and Z.sup.2 in the formulae Ib to Id are preferably single bonds or 
--CH.sub.2 CH.sub.2 --. 
The compounds of the formula Ia comprise the preferred part formulae 
Ia.alpha. and Ia.beta.: 
EQU R.sup.1 --PE--R.sup.2 Ia.alpha. 
EQU R.sup.1 --CY--R.sup.2 Ia.beta.. 
Formula Ib comprises the preferred part formulae Ib.alpha. and Ib.gamma.: 
EQU R.sup.1 --PE--Z.sup.1 --PE--R.sup.2 Ib.alpha. 
EQU R.sup.1 --CY--Z.sup.1 --CY--R.sup.2 Ib.beta. 
EQU R.sup.1 BO--Z.sup.1 --BO--R.sup.2 Ib.gamma.. 
Among these, the compounds of the part formula Ib.beta. are particularly 
preferred. 
The compounds of the formula Ic comprise those of the preferred part 
formulae Ic.alpha. to Ic.differential. 
EQU R.sup.1 --CY--Z.sup.1 --PE--Z.sup.2 --PE--R.sup.2 Ic.alpha. 
EQU R.sup.1 --PE--Z.sup.1 --CY--Z.sup.2 --PE--R.sup.2 Ic.beta. 
EQU R.sup.1 --CY--Z.sup.1 --PE--Z.sup.2 --CY--R.sup.2 Ic.gamma. 
EQU R.sup.1 --PE--Z.sup.1 --PE--Z.sup.2 --PE--R.sup.2 Ic.delta. 
EQU R.sup.1 --CY--Z.sup.1 --CY--Z.sup.2 --PE--R.sup.2 Ic.epsilon. 
EQU R.sup.1 --CY--Z.sup.1 --CY--Z.sup.2 
--CY--R.sup.2 Ic.differential.. 
Amongst these, the compounds of the part formulae Ic.alpha., Ic.beta. and 
Ic.gamma. are particularly preferred. 
The compounds of the formula Id comprise the preferred part formulae 
Id.alpha. to I.delta.: 
EQU R.sup.1 --PE--Z.sup.1 --PE--Z.sup.2 --PE--Z.sup.3 --PE--R.sup.2Id.alpha. 
EQU R.sup.1 --CY--Z.sup.1 --PE--Z.sup.2 --PE--Z.sup.3 --PE--R.sup.2Id.beta. 
EQU R.sup.1 --CY--Z.sup.1 --PE--Z.sup.2 --PE--Z.sup.3 --CY--R.sup.2Id.gamma. 
EQU R.sup.1 --CY--Z.sup.1 --CY--Z.sup.2 --CY--Z.sup.3 --CY--R.sup.2Id.delta.. 
Amongst these, the compounds of the part formula Id.beta. are particularly 
preferred. 
The radicals R.sup.1 and R.sup.2 are preferably alkyl and alkoxy and, in 
addition, the carbonitrile group the carboxylic acid group are preferred 
for R.sup.2. 
A, A.sup.1, A.sup.2 and A.sup.3 are preferably CY, PE or BO. The alkyl 
radicals in the radicals R.sup.1 and R.sup.2 can be unbranched or 
branched. Preferably, they are unbranched and are methyl, ethyl, propyl, 
butyl, pentyl, hexyl, heptyl, oxtyl, nonyl, decyl, undecyl, dodecyl, 
tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl or octadecyl. 
Some of the compounds of the formula 1 are known and some are still novel. 
They are prepared by generally known methods or analagously to these 
methods. They can be found, for example, in the Houben-Weyl series, 
Methoden der Organischen Chemie, [Methods of Organic Chemistry], 
Georg-Thieme-Verlag, Stuttgart. 
Compounds of the formula I can contain one or more asymmetric C atoms. In 
this case, the formula I also comprises enantiomer mixtures and racemates 
in addition to optically active enantiomers. 
It has also been found that a mechanical component with variable frictional 
force can be produced by introducing, between the mobile bodies, a mass 
which contains at least one compound of the formula II 
##STR2## 
wherein R.sub.3 to R.sub.8 independently of one another are H, F, Cl or a 
substituted or unsubstituted alkyl group having 1 to 18 C atoms, in which 
one or two non-adjacent CH.sub.2 groups can also be replaced by --O--, 
--CO--, --COO-- or --CF.sub.2 --. 
The radicals R.sub.3 to R.sub.8 in the formula II are preferably 
alkanoyloxy, alkoxy or alkyl. 
An advantageous route to the truxene derivatives of the formula II leads 
via suitable indan-1-ones (C. R. Acad. Sci. Ser. C, 287, 545 (1978)). Some 
of the compounds of the formula II have not yet been described. They are 
prepared by this synthetic route and, in other respects, generally known 
methods. These can be found, for example, in the Houben-Weyl series 
Methoden der Organischen Chemie, [Method of Organic Chemistry], 
Georg-Thieme-Verlag, Stuttgart. 
The compounds of the formulae I and II, which are stable per se, can 
contain up to 1% by weight of an antioxidant, such as hydroquinone or 
2,6-di-tert.-butyl-4-methylphenol, in order to retard autooxidation 
reactions which take place in mechanical components operated at high 
temperatures with access of air. 
The compounds of the formula I or II are used as individual substances or 
as mixtures of compounds of the formulae I and II. 
Suitable mixtures contain 2 to 10, preferably 2 to 5, compounds of the 
formula I and/or II. Suitable mixtures can also contain further components 
in addition to one or more compounds of the formula I or II. Such mixtures 
contain 5 to 99.9% by weight, preferably 30 to 95% by weight, of one or 
more compounds of the formula I or II. The additional components of such 
mixtures can be cyclic and acyclic hydrocarbons, which contain one to five 
identical or different radicals from the group comprising 
EQU --OH, 
EQU --F, 
EQU --Cl, 
EQU --CN, 
or are unsubstituted, liquid-crystalline main chain polymers or side chain 
polymers, or the above-mentioned antioxidants. 
Compounds are preferred which form enantiotropically or monotropically 
discotic phases. These include hexa-substituted derivatives of benzene, 
hexa-substituted derivatives of triphenylene, hexa-substituted derivatives 
of cyclohexene and phthalocyanine derivatives. 
The use of compounds of the formula I or II and mixtures thereof in the 
machine component according to the invention is favoured by the fact that 
the transitions between different liquid-crystalline phases as well as 
between liquid-crystalline phases and the isotropic phase takes place with 
only slight delays in time. Metastable phases due to subcooling, such as 
arise in the transition from the liquid phase to the crystalline phase of 
non-mesomorphous compounds, can thus be excluded if the arrangement is 
operated above the crystallization temperature. This temperature can be 
adjusted to a sufficiently low level by the use of eutectic or 
non-eutectic mixtures of compounds having a liquid-crystalline phase. A 
further favourable aspect is that only low enthalpies of transition, as 
compared with the enthalpies of fusion, apply to the 
isotropic/liquid-crystalline or liquid-crystalline/ liquid-crystalline 
phase transitions. 
The choice of heat supply or heat removal depends on the function of the 
device according to the invention within a machine. The transfer of heat 
energy from the surface of the metallic or non-metallic bodies, mobile 
relative to one another, to the fluid layer can take place by convection 
by means of a liquid or a gas. Heat supply is also possible by electric 
resistance heating, by electromagnetic waves if at least one of the 
bodies, mobile relative to one another, is transparent, or by an 
alternating electric field between two electrically conductive bodies. The 
removal of heat energy from the fluid layer can be effected by a cooling 
liquid or a gas. Removal by heat radiation alone is also possible. 
Furthermore, heat energy can be removed in special cases by means of 
Peltier elements. 
If sufficient frictional heat is generated in the layer, which is below the 
particular transformation temperature, and poor heat removal is ensured at 
the same time, the phase transformation takes place without heating. 
The frictional forces which arise in the liquid-crystalline mass of a 
machine component according to the invention depend on the alignment of 
the molecules relative to the direction of motion of the solid bodies of 
the component. It is possible to influence this alignment by pretreating 
the body surfaces by rubbing in a preferential direction or by applying a 
thin layer of an organic or inorganic material. 
The machine component according to the invention can be used as a clutch, 
as a brake, as a mechanical overload preventer, as a hydraulic damping 
element or as an element for hydraulic force transmission (the two latter 
machine components can be combined under the term hydraulic device). The 
use as a slide bearing with a dependence, adjustable for the intended 
application, of the frictional loss on the temperature is also possible. 
This is of particular advantage with a view to saving mechanical energy 
because, at low rotational frequencies in a radial bearing or thrust 
bearing, a high effective viscosity and hence a low rotational transition 
frequency n.sub.u from semi-fluid friction to fluid friction can be 
selected. When the rotational frequency is increased, the temperature can 
be raised by the frictional heat alone or by additional supply of heat 
energy to such an extent that a transition to a phase of lower effective 
viscosity takes place. The relationships between frictional loss, 
viscosity and n.sub.u are known (R. Stribeck, VDI-Zeitschr. volume 46, 
page 1341 (1902)). 
In a machine component according to the invention, a braking action can be 
achieved by a drastic increase in viscosity--for example in the case of a 
phase transition from nematic to S.sub.B --when the static pressure in the 
lubrication gap of a slide bearing or of two metal discs mobile relative 
to one another, is increased. In this case, the static pressure in the 
organic mass can be varied from the outside by means of an additional 
hydraulic piston. A prerequisite is sufficient sealing of the space taken 
up by the organic mass. The braking performance of such a component is 
additionally determined by the heat removal from the lubrication gap. 
The use of the mechanical component according to the invention as a clutch 
can be realized in the same way. 
The invention includes a mechanical component in which motion sequences are 
additionally determined by the pressure dependence of the viscoity of the 
medium which is to be transported. In this case, the use of an organic 
medium with one or more pressure-dependent transitions between 
liquid-crystalline phases or between a liquid-crystalline phase and the 
isotropic liquid phase allows a hydraulic damper to be produced which can 
absorb loads from widely different ranges of orders of magnitude, in which 
case different damping constants have to be applied depending on the phase 
relationship. The temperature due to the compression work on the fluid 
organic mass is of subordinate importance in practice. 
A mechanical component according to the invention allows wear to be reduced 
in the frictional transmission of forces. In this case, the fact is 
exploited that, even at relatively small contact pressures, an increase in 
viscosity and hence a transition from mixed friction to fluid friction 
takes place, if a phase transition from nematic or isotropic to S.sub.B or 
S.sub.E occurs as a result of an increase in pressure. The use of 
compounds of the formula I and mixtures thereof in the mechanical 
component according to the invention represents an improvement of 
gearboxes which use so-called traction fluids (compare O. Dittrich, 
VDI-Berichte 680, pages 201-219 (1988)). 
The compounds of the formula I or II and mixtures thereof show excellent 
wetting of metal surfaces and ceramic surfaces. 
A mechanical component according to the invention can be used as a Visko 
clutch (W. Peschke, SAE Technical Paper Series, No. 860 386, Detroit 
(1986)) if, as a result of an increase in pressure during engagement of 
the clutch, in spite of the resulting increase in temperature, an increase 
in viscosity or even only a small increase in viscosity occurs as compared 
with the declutched state. The disadvantages resulting from the use of the 
poorly wetting silicone oils can also be avoided in this way. 
The present invention makes it possible, in an advantageous manner, to 
induce a change in the frictional force acting between two solid bodies 
moving relative to one another. A machine component according to the 
invention is distinguished by a structure which is particularly simple and 
not susceptible to faults, and does not require any colloidal solutions 
which promote wear. Moreover, a considerable saving in energy can be 
achieved thereby. The compound of the formula I or II and the mixtures 
prepared from them are outstandingly suitable for this purpose, because of 
their stability and the scope for using the compounds for adjusting 
suitable phase transitions within fairly wide temperature ranges. 
The examples which follow are intended to explain the invention without 
limiting it. The friction moment (friction force.times.lever arm) is that 
torque in N.times.m which must be applied in order to maintain an existing 
rotational movement. It is determined by a measurement of the energy 
dissipation in a manner known to those skilled in the art. 
EXAMPLE 1 
A mechanical component (FIG. 1), the function of which can selectively be 
that of a slide bearing or that of an effective brake, consists of a 
sheathed and temperature-controllable Duran glass sleeve (1) with a ground 
joint and of a shaft (2) guided for a length of 70 mm and having a 
diameter of 10 mm, likewise of Duran glass (produced by Schott Glaswerke, 
Mainz). The shaft is connected via a piece of thick-walled rubber hose (3) 
to a drive unit (not shown in FIG. 1) which is fitted with a measuring 
device which allows the energy dissipation, attributable to the machine 
component, to be measured as a function of the rotational frequency of the 
shaft. (4) is a retaining device. Before starting, the separate ground 
joint elements are heated with hot air and coated with a sufficient 
quantity of 
##STR3## 
so that, on assembly, a film free of air bubbles is formed on the entire 
friction surface. Paraffin at a temperature of 56.degree. C. is passed 
through the shell and the shaft is rotated at 1 Hz. The fraction moment of 
the sliding device is found to be 4.times.10.sup.-4 N.times.m. Paraffin 
oil at a temperature of 44.degree. C. is then fed in at a flow velocity of 
5 ml.times.s.sup.-1. After 30 seconds, a friction moment of 
2.times.10.sup.-1 N.times.m is measured. After the oil temperature has 
been raised to 56.degree. C., the original value of the friction moment is 
re-established. The following are used analagously (the temperatures are 
stated in degrees C., through which the system must pass for a change in 
the friction moment): 
______________________________________ 
trans,trans-4-methoxy-4' -pentyl-bicyclohexyl 
29 
trans,trans-4,4' -dipentyl-bicyclohexyl 
110 
trans,trans-4-(2-cyanoethyl-4' -pentyl- 
30/109 
bicyclohexyl 
trans,trans-4-(2-cyanobutyl)-4' -pentyl- 
80 
bicyclohexyl 
##STR4## 
4,4'-dipentyl-biphenyl 52 
4-heptyl-4'-propyl-biphenyl 51 
4-hexyl-4'-hexyloxy-biphenyl 
68/84 
4'-octyloxy-biphenyl-4-carboxylic acid 
88/96/ 
ethylester 112 
4-pentyl-4'-(propynyl)-1-biphenyl 
83 
4'-decyl-biphenyl-4-carboxylic acid 
247 
##STR5## 
1,2-bis[trans-4-pentylcyclohexyl]ethane 
109 
1,2-bis[trans-4-ethylcyclohexyl]ethane 
29 
##STR6## 
4,4'-diethyl-bicyclo[2.2.2]octane 
209 
##STR7## 
4-trans-(4-pentylcyclohexyl)-2-hydroxyethyl- 
59/73 
benzene 
##STR8## 
4-hexylphenyl 4-trifluoromethoxbenzoate 
110 
4-heptadecafluoro-octylphenyl 4-cyanophenyl- 
145 
benzoate 
##STR9## 
1,2-bis[4-butylphenyl]cyclohexane 
107 
1,2-bis[4-dodecylphenyl]cyclohexane 
109 
##STR10## 
trans-1,4-bis[4-pentylphenoxymethyl]cyclohexane 
##STR11## 
4,4'-bis[trans-4-pentylcyclohexyl]biphenyl 
247/275 
______________________________________ 
EXAMPLE 2 
A mixture of 64.5% by weight of 4,4'-dipentylbicyclo [2.2.2]octane and 
35.5% by weight of 1,2-bis[trans-4-pentylcyclohexyl]-ethane has a 
transition temperature of 168.degree. C. between an S.sub.B phase and a I 
phase and can be utilized in the manner described in Example 1 for a 
thermal change in the friction moment. 
EXAMPLE 3 
A mixture of 42.2% by weight of 4'-hexyloxy-biphenyl-4-carbonitrile and 
57.8% by weight of 4-(trans-4-octylcyclohexyl)-1-(2-cyanoethenyl)-benzene 
passes on cooling through the phase sequence N-S.sub.A -N. In a mechanical 
component according to Example 1, a smaller friction moment is measured 
below 19.degree. C. than above this temperature. 
EXAMPLE 4 
A hydraulic damper, consisting of a cylinder (1), bounded at the top and 
bottom in the manner shown in FIG. 2 and having an internal diameter of 10 
mm, and of a ring-shaped cover (2) screwed on and of an 8 mm long piston 
(3) which is guided along the cylinder wall and has two bores (4) of 0.5 
mm diameter each, connecting the two cylinder spaces, is almost completely 
filled with the compound 
##STR12## 
which has a transition temperature of 53.5.degree. C. from the S.sub.A 
phase to the isotropic phase, and placed into a water bath which holds the 
cylinder with its contents at a temperature of 51.degree. C. In the 
position of the upper stop, the piston is moved with a total force of 
2.0N, including the weight of the piston and of the guide rod. A time of 
175 seconds is required for the piston stroke of 30 mm length. When the 
temperature of the arrangement is raised to 56.degree. C., 4.2 seconds are 
measured. 
The damping constants of the device accordingly have the following values: 
1.2.times.10.sup.4 Ns/m at 51.degree. C. 
2.8.times.10.sup.2 Ns/m at 56.degree. C. 
EXAMPLE 5 
A sleeve (1) with a conical ground joint and a ground core (2) fitting into 
it, both consisting of Duran glass (standard designation NS 29/32, DIN 12 
249), are the essential parts of a clutch according to the invention (FIG. 
3). The sleeve is rigidly joined to a lifting device (3) (not shown in 
more detail in FIG. 3), the rotating parts of which have a moment of 
inertia of 2.5.times.10.sup.-4 kgm.sup.2 and a torque, which is to be 
overcome, of 0.32 Nm. The core is part of a horizontally arranged hollow 
shaft (4), closed on one side, of 2 mm thick of Duran glass, which is 
guided in two bearings (5) and can be driven via a transmission belt (6) 
by a motor (6) of sufficient power. Before starting, the separate ground 
joint elements are heated with hot air to about 80.degree. C., and 0.10 g 
of the compound 
##STR13## 
having a phase transition from smectic B to nematic at 24.degree. C., is 
uniformly distributed over the surface of the core so that, when pushed 
into the sleeve by application of a small contact pressure (0.5 to 1N), an 
air bubble-free layer of the organic compound is formed. The motor is then 
switched on and the shaft is rotated at 10 Hz. The phase transition from 
nematic to smectic is induced by briefly feeding cooling water at a 
sufficient flow velocity through a lateral feeder (8) sketched in FIG. 3, 
from a reservoir (9) at a temperature which, for the purpose of rapid heat 
removal, is at least 6.degree. C. below the transition temperature. The 
rotary motion of the lifting device at a frequency of 10 Hz takes place 
after a dead time of less than 10 seconds within an acceleration time of 
less than 5 seconds. Disengagement is effected by feeding water from a 
reservoir (10) at a temperature above the transition temperature, via a 
three-way valve (11). (12) is a collecting channel. 
EXAMPLE 6 
The arrangement of Example 5 is operated, in the manner described therein, 
with the compound 
##STR14## 
which has a transition temperature of 49.degree. C. from smectic B to 
smectic A. Here again, a selection is possible between engagement and 
idling by means of altering the fluid layer and an associated phase 
transition. 
EXAMPLE 7 
A radial bearing consists of a polished brass shaft (diameter 15 mm) and a 
temperature-controllable, undivided bearing shell of brass (load-bearing 
width 100 mm). The bearing clearance is 0.01 mm, and the load on the 
bearing during operation is 5.0N. A little lecithin is put onto the 
sliding surfaces and rubbed with wool to form a thin uniform layer. The 
warmed bearing is then embedded into a coherent layer of the compound 
##STR15## 
having a transition from a smectic A phase to a nematic phase at 
78.degree. C. At a rotational frequency of 10 Hz and a temperature of the 
bearing of 73.degree. C., a coefficient of friction .mu. of 0.15 results, 
and a .mu. of 0.007 is measured at 81.degree. C. and the same rotational 
frequency. In the manner generally known, .mu. is determined by the torque 
which must be applied to maintain the motion at a defined rotational 
frequency. 
EXAMPLE 8 
The arrangement of example 5 is operated, in the manner described therein, 
with the compound 
##STR16## 
which has a transition from a discoid-columnar phase to an isotropic phase 
at 66.degree. C. Here again, a choice is possible between engagement and 
idling by changing the temperature of the fluid layer and by an associated 
phase transition. 
EXAMPLE 9 
A mechanical component (FIG. 1) with variable friction moment consists of a 
sheathed and a temperature-controllable Duran glass sleeve (1) with a 
ground joint and a shaft (2) guided for a length of 70 mm and having a 
diameter of 10 mm, likewise of Duran glass (produced by Schott Glaswerke, 
Mainz). The shaft is connected via a piece of thick-walled rubber hose (3) 
to a drive unit (not shown in FIG. 1) which is fitted with a measuring 
device which allows the energy dissipation attributable to the machine 
component to be measured as a function of the rotational frequency of the 
shaft. Before starting, the separate ground joint components are heated 
with hot air and coated with a sufficient quantity of 
2,3,7,8,12,13-hexakis[decanoyloxy]truxene having a transition from 
discoid-nematic to columaric-discotic at 84.degree. C., in such a way 
that, on assembly, a film free of air bubbles is formed on the entire 
friction surface. Paraffin oil at a temperature of 82.degree. C. is passed 
through the shell, and the shaft is rotated at 1 Hz. The friction moment 
of the sliding device is found to be 3.times.10.sup.-3 N.times.m. Paraffin 
oil at a temperature of 95.degree. C. is then fed at a flow velocity of 5 
ml.times.s.sup.-1. After 10 seconds, a friction moment of 
9.times.10.sup.-2 N.times.m is measured. After the oil temperature has 
been lowered to 82.degree. C., the original value of the friction moment 
is re-established. 
EXAMPLE 10 
A hydraulic damper (FIG. 4) for absorbing forces from two load ranges of 
widely different orders of magnitude consists of a steel cylinder (1) 
having an internal diameter of 28 mm, a hydraulic piston (2) mobile 
therein and having a sketched--generally known--sealing system, and a pipe 
(3) of 250 mm total length and 2.0 mm internal diameter, which extends 
from the piston space into an open container (4). A coherent, air 
bubble-free volume filled by the compound of the formula 
##STR17## 
is present in the piston space, pipe and stock container. This compound 
has a transition temperature of 32.5.degree. C. between the smectic A 
phase and the nematic phase. The system, at 35.degree. C., is then charged 
with a load of 12,000N (Newton). To cover a length of 65 mm, the piston 
takes 3.0 seconds. Under otherwise the same starting conditions, this time 
is 3.5 seconds when a load of only 185N is charged. 
EXAMPLE 11 
At normal pressure (1 bar), trans, trans-4-methoxy-4'-pentyl-bicyclohexyl 
has a transition temperature from S.sub.B to I of 29.degree. C. In the 
arrangement shown in Example 10 and under the same conditions, a 
substantially higher damping constant than at normal pressure can be 
achieved with this compound at high pressures.