Solid anti-friction devices – materials therefor – lubricant or se – Solid anti-friction device – article or material therefor – Elemental or alloyed metal
Reexamination Certificate
2000-11-13
2002-09-24
Howard, Jacqueline V. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Solid anti-friction device, article or material therefor
Elemental or alloyed metal
C508S100000, C428S673000
Reexamination Certificate
active
06455475
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sliding member usable under sliding conditions, for example, in vacuum, high temperature, or ultraclean environment, where none of liquids, viscous materials, and solid lubricants can be used, and a process for producing the same.
2. Description of the Related Art
For example, when sliding members and rolling members, such as sliding bearings, seal mechanisms, bolts, nuts, pistons, crankshaft fulcrums, pins, cams, links, jack shafts, gears, shaft seal parts, and rolling element bearings, are used at high temperatures or under vacuum, two planes in pair cause relative movement while they are in contact with each other. This creates friction or microscopic collision between the surface of one of the members and the surface of the other member. When the pressure of contact between two planes (plane pressure) is high and the relative movement speed is high, local heat generation or scratch of the counter material by projections, damage by abrasion and the like occur. In extreme cases, seizing or adhesion occurs, often leading to suspension of operation.
In order to prevent these unfavorable phenomena, when sliding members are used under severe sliding conditions, a suitable level of lubricity should be required to act on a portion between two planes facing each other.
Here, in the case of sliding members to be used under ordinary environment, a liquid or a viscous lubricating oil (lubricant) may be provided between two planes in pair to acquire satisfactory lubricity. Thus, the two planes in pair are placed in a fluid film lubrication state or a boundary lubrication state. This can avoid adhesion between the two planes.
On the other hand, when a lubricating oil is used as lubrication between the sliding members, which should be unavoidably used under vacuum, high temperature, or ultraclean environment, the presence of the lubricating oil itself becomes an obstacle to maintain a desired environment, or otherwise the lubricating oil easily undergoes a change of properties which results in the disappearance of the desired lubricity. For this reason, lubricating oils cannot be used under vacuum, high temperature, or ultraclean environment. This makes it necessary to ensure the lubricity by materials other than liquid or viscous lubricating materials.
Various solid lubricants have hitherto been used as alternative to liquid or viscous lubricants. Representative examples thereof include {circle around (1)} graphite, {circle around (2)} compounds such as molybdenum disulfide, phthalocyanine, lead oxide, and boron nitride, and {circle around (3)} plastics. A suitable material is selected among these materials by taking into consideration the environment (for example, temperature, degree of vacuum, corrosiveness, chemical activity, pressure of the plane of contact, and sliding speed) and used. Since, these alternative materials have both advantageous and disadvantageous properties respectively, the use of them under severe conditions in such required environments is limited naturally.
For example, graphite can be used in the air without causing any problem at a temperature of about 400° C., which is a higher temperature than where conventional lubricating oils can be used. Further, graphite is suitable for use where electrical conductivity is required. However, graphite is almost useless under vacuum environments. On the other hand, molybdenum disulfide can be used at a temperature of 1100° or above under vacuum environment, and can be used in the air at a temperature of 300° C. or below without undergoing oxidation. Molybdenum disulfide, however, has a low durability against repeated use for a long period of time. Phthalocyanine can withstand a temperature up to 600° C., and lead oxide can withstand a temperature up to 500° C. That is, these substances have features of high resistance to high temperatures. Plastics have a low coefficient of friction even in bulk. In particular, fluororesins have a very low coefficient of friction of 0.04, but on the other hand, due to the nature of the resins, the maximum temperature which they can withstand is 250° C. (starting temperature of decomposition), which is very inferior compared to inorganic materials and metals.
Thus, there are a wide variety of coating-type lubricants for use in sliding members, ranging from liquids to solids, each of which have both advantages and disadvantages.
Accordingly, for sliding members used under severe conditions, instead of using lubricants, various attempts have been made to ensure the lubricity by thinly coating a soft metal on the sliding surface of hard metal.
FIG. 4
shows a general construction of this type of conventional sliding member, wherein one sliding surface
10
a
in a pair of sliding members
10
,
12
, is thinly coated with a soft metal layer
14
formed of a soft material such as lead, indium, gold, or silver. The sliding members
10
,
12
are formed of a hard metal moved each other while they are in contact with the soft metal layer
14
. This concept is based on such an idea that lubricating action is caused by the soft metal, in which low shearing stress of the soft metal is utilized.
That is, in this case, a true contact area A between the sliding members
10
,
12
is determined by the hardness of a harder metal member (sliding member
12
). When the contact surfaces once come into true contact with each other and followed by relative movement (friction) to cause shear separation of the two surfaces from each other, a breakage occurs in the thin soft metal layer
14
. In particular, when any lubricating oil is absent between the two surfaces, a piece of soft metal
16
separated by the breakage is fast moved around suitably between the two contacting surfaces. Even though a part of the soft metal layer
14
has been breached, it has been found that a piece of soft metal
16
is transferred from other part onto this breached portion, thereby the breached portion is self-repaired. In this case, when lubricating oil is used therewith, it becomes difficult to transfer the fluid soft metal
16
to the breached portion. Therefore, self-repairing is hardly expected, and on the contrary, this sometimes causes inconvenience.
Here, silver as a soft metal is said to be the best material for use in high load slide bearings, because silver is highly compatible with the hard metal member, and at the same time, has good thermal conductivity and toughness.
Electroplating is generally used for coating a soft metal on a sliding member formed of a hard metal. For example, when a sliding member is plated with silver on a commercial scale, a commonly used method is such that plating is carried out using a plating solution composed mainly of silver potassium cyanide at a current density of not less than 20 A/dm
2
by taking advantage of high stability of the cyan complex.
The electroplating of the sliding member on its sliding surface, however, suffers from the following drawbacks: (1) a special power supply unit should be generally provided; and (2) when the objects have complicated shapes or members which are very small, the current distribution is likely to be uneven, resulting in a variation in plating thickness or plating quality from portion to portion.
Accordingly, the electroplating can be satisfactorily brought its advantage only for mass-produced objects having a simple shape. By contrast, the electroplating cannot be satisfactorily used for members having a complicate shape and deformed members having a labyrinth contour. Also, the electroplating cannot be satisfactorily used for objects, which are required to execute the plating on customer's site (in particular, for example, in the case of assembling, installation, and repair), but not within the production plant. Thus, the development of a method, other than electroplating, which can surely and easily form a soft metal layer on a sliding member in its sliding surface, has been strongly desired. Further, harmful effects of highly poisonous cyanogenic comp
Browdy and Neimark
Ebara Corporation
Howard Jacqueline V.
LandOfFree
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