Lubricants and their method of preparation

A lubricant, and a method for its preparation, essentially comprising colloidal particles of a metal oxide hydrate over the surface of which has been reacted fatty acid molecules having long Carbon chains. The metal used to form the metal oxide hydrate is chosen from the first transition period of the periodic table, and is preferably either Chromium, Manganese, Iron, or Cobalt. The fatty acid preferably has a Carbon chain length in the region of 16 to 24 inclusive and is present in an amount not greater than 20% by weight of the metal oxide hydrate.

This invention relates to lubricants for reducing wear of the bearing 
surfaces of relatively movable members. 
The lubricant to which this invention relates is particularly, though not 
exclusively, of use in gas bearing gyroscopes. In a gas bearing gyroscope 
the gyroscope rotor is supported by means of a gas bearing. These bearings 
are usually of the type wherein the rotation of the various bearing 
components generates the required gas pressure to maintain a working 
clearance between the bearing surfaces. At low rotational speeds therefore 
the gas pressure is insufficient to maintain the working clearance and 
rubbing contact occurs between the bearing surfaces. 
Hitherto at least one of the bearing surfaces has been coated with a known 
lubricant film in order to reduce wear during rubbing. It has been found 
that after a number of stops and starts of the gyroscope the known 
lubricant film breaks down and allows rubbing contact with resulting 
removal of material from the contacting areas of the bearing which 
accumulates and leads to its evenutal seizure. 
An object of the invention is to provide a more durable lubricant, and a 
method of preparation, which may be employed in a gas bearing. 
According to the invention a lubricant includes a colloidal form of a metal 
oxide hydrate, the metal being chosen from those elements in the first 
transition period of the periodic table, and a fatty acid having a chain 
length in the region of 16 to 24 inclusive carbon molecules, the fatty 
acid being combined with surface regions of the colloidal particles of the 
metal oxide hydrate. 
Preferably the said lubricant is prepared by forming a suspension, 
comprising a quantity of a colloidal form of the metal oxide hydrate and a 
suspending medium, adding to the suspension the fatty acid in a proportion 
of not more than 20% by weight of the mass of suspended colloid, 
dispersing the fatty acid with respect to the colloidal particles in 
suspension such that the fatty acid combines with a surface region of the 
colloidal particles to form a lubricant layer on said particles, and 
subsequently removing the suspending medium.

The invention will be better understood from the following description of 
the composition of a lubricant, the preferred method of preparing the 
lubricant and examples of its use. 
The basic component of a lubricant according to the invention is a 
colloidal form of a metal oxide hydrate compound of an element chosen from 
the group of elements comprising the first transition period of the 
periodic table, i.e. Chromium, Manganese, Iron, and Cobalt. In the example 
being described the chosen element is Iron and the colloid is therefore 
Iron III oxide hydrate. In its colloidal form such a compound forms 
particles comprising a large number of molecules. 
The Iron atoms may carry an ionic charge of up to +3, i.e. Fe.sup.3+ ions, 
and during the formation of the oxide hydrate these positive ionic charges 
are not completely cancelled. The fatty acid added to form the lubricant 
comprises a basic acid ring molecule which has an attached Carbon chain, 
the basic acid molecule having a single unbalanced negative ionic charge. 
The fatty acid molecules react with the metal oxide hydrate molecules to 
form an ionic bond balancing the opposite ion charges. Sufficient, but not 
excess, fatty acid is added to restrict the reaction of those metal oxide 
hydrate molecules on the surface of colloidal particles, thus forming a 
substantially stable surface layer surrounding an inner core of unreacted 
metal oxide hydrate molecules. The carbon chains of fatty acid molecules 
remain unadsorbed and outside the surface of the particles. 
The fatty acid component of the lubricant, preferably has a chain length in 
the region of Carbon 16 to 24 inclusive, in the present example it is 
stearic acid having a chain length of 18 Carbon molecules. The possible 
range of Carbon chain lengths is much greater, but it has been found that 
shorter Carbon chains, i.e. much less than 16, do not produce the desired 
lubricating properties, whilst longer chains tend to be unstable. It has 
also been found that a proportion by weight of fatty acid to metal oxide 
hydrate of progressively greater than approximately 20% causes the 
colloidal particles of the metal oxide hydrate to break up into 
progressively smaller particles with a concomitant loss of lubricant 
properties. 
The preferred method of preparing the lubricant is as follows: 
The Iron III oxide hydrate is precipitated from an aqueous solution of Iron 
.sup.3+ ions, by the addition of a suitable alkali precipitant, in this 
example Ferric Chloride, whilst the pH value of the solution is maintained 
within the range 6.0 to 8.0. 
The precipitated solid product is then washed in water until free from 
chloride ions, or other anions. 
The precipitant is then transferred to a non-aqueous medium by washing with 
a suitable organic solvent such as acetone until free of water followed by 
further washing with a volatile hydrocarbon, such as heptane, until free 
of acetone. 
The Iron III oxide hydrate is dispersed in the heptane at this stage and 
can be stored in this form as a suspension if necessary. 
The metal oxide hydrate content of the suspension is subsequently 
determined by analysis so that the required quantity of fatty acid, up to 
20% by weight as previously mentioned, can be added to the suspension. The 
resulting mixture is dispersed to a stable colloidal suspension by use of 
high frequency vibrations. 
Finally the stability of the final product, i.e. the dispersed lubricant, 
is established by analysis of the infrared spectrum of the dispersion. 
The lubricant is applied to the surfaces to be lubricated in the heptane 
dispersed form or it may be further diluted by a halogenated solvent such 
as chloroform. In either case the solvents are allowed to evaporate 
depositing a lubricant residue on the surfaces. 
The lubricant is useful in a gas bearing gyroscope in which the gyroscope 
rotor is supported by means of a gas bearing on a steel shaft. At normal 
rotor speeds sufficient gas pressure is produced between the bearing 
surfaces, i.e. between the inner surface of the rotor and the steel shaft, 
to support the rotor. At lower speeds the pressure is correspondingly 
lower and at very low speeds the bearing surfaces are in rubbing contact 
resulting in wear of these surfaces. To reduce this wear hitherto at least 
one of the surfaces is normally coated with a known lubricant film, but 
after a number of stops and starts the film is found to break down and 
allow material to be rubbed from the bearing surfaces, this material 
accumulating and leading to eventual seizure of the bearing. 
To overcome this problem described the lubricant is applied in small 
quantities on both bearing surfaces where it is held within irregularities 
on the bearing surfaces to form reservoirs of lubricant. When the 
gyroscope is started and run down the bearing surfaces rub against each 
other removing the small amount of material from the surfaces which 
contains a quantity of the lubricant. The lubricant thus removed forms 
thin layers of easily sheared lamellae in the areas of contact between the 
bearing surfaces thus acting as a lubricating film. 
The irregularities in the bearing surfaces in which the reservoirs of 
lubricant are formed may be produced artificially by abrasion, or even by 
grinding or sintering during manufacture. Alternatively the naturally 
occurring irregularities in any surface may be employed. The lubricant is 
introduced in these irregularities by polishing the bearing surfaces using 
the lubricant as a polishing compound. 
Another example of an application of the lubricant is in the lubrication of 
a ball or roller bearing. The lubricant is introduced to the bearing 
during or after assembly, merely by spreading it over the ball or roller 
surfaces. Then when the bearing is first turned the lubricant is spread 
roughly evenly over the bearing contact surfaces and the easily sheared 
lubricating lamellae referred to earlier are formed. 
It will be appreciated that the lubricant has other possible applications, 
it is for example, envisaged that it may be used as a replacement for, or 
supplement to the normal solid lubricant such as graphite in air 
compressor cylinders.