Patent Application
20130150270
The present invention relates lubricants, principally stick or solid lubricants such as those used in the railway industry to control friction between the contact points of a track and railway wheel.
Rail transportation imposes a unique environment in that the common means of transit involves the use of steel wheels on a steel track. Generally the wheels of a train comprise a tread surface for contacting the crown of a track and a flange extending downward that rides along the inner face of the rails, keeping the train on the track. These two conjoined points of contact provide for unique lubricating challenges. There is both a need for reduced friction, particularly as to the flanges to permit less wear on wheels and flanges and more efficient travel, and at the same time there is the need for friction between the wheel tread and the rail crown to permit traction and the application of motive force to drive a train. This environment is further complicated by the nature of railway systems being prone to accumulations of dust and debris and general exposure to the elements.
Early developments of this technology to address the issue of wheel to rail lubrication involved the use of grease or oil applied to the flange of a wheel, often sprayed. This was unsatisfactory in part due to over-spray and the spread of a spray lubricant to the wheel tread. Later developments involved the use of solid stick lubricants which use applicators to apply the solid lubricant directly to the wheel flange. Many of the later solid lubricants focused on the combination of a dry lubricating powder with a carrier. Laboratory testing has revealed that the use of materials such as molybdenum disulfide and even graphite have nominal value when used in combination with other lubricants and in field conditions such as with railway track where nominal contamination negates their value. Molybdenum disulfide, the most common lubricant of choice for this use, yields excellent coefficient of friction values with high persistence in a steel to steel application where extreme pressures are found, but to gain the potential benefits of this lubricant it must be driven into clean steel surfaces in pure form. If the steel surfaces are contaminated or the molybdenum disulfide particles are contaminated by having been suspended in oil or polymer carriers the particles cannot bond readily with the steel and are mostly swept away without achieving significant results. In laboratory tests comparing the results of using pure molybdenum disulfide powder against those found using molybdenum disulfide suspended in oil or polymer carriers, it appears that over 90% of the lubricating value of the molybdenum disulfide is lost. When combined with oil or polymer carriers the coefficient of friction values obtained and persistence of the molybdenum disulfide lubricating film revert to values closer to those of the oil or polymer carrier alone.
As such a clear need exists for a solid form lubricant which employs modified traditional liquid based lubricants which is more economical and does not suffer adversely from being applied under conditions found in the field.
Prior art such as U.S. Pat. No. 5,126,219 to Howard, et al. discloses a method of introducing mineral particles and oil into a polymer matrix to create microporous structure that can be further fashioned into a fiber or sheet. However, in this Howard process the oil is then extracted to leave a microporous membrane and is not used for lubrication.
U.S. Pat. No. 3,729,415 to Davis discloses a method of dispersing ultra high molecular weight polyethylene into oil to create rigid state gel lubricants; however the Davis '415 gel based lubricants lack the ability to compensate for variable coefficient of friction requirements found in this application.
U.S. Pat. No. 4,486,319 to Jamison discloses a similar lubricant preparation however the Jamison '319 is designed to be thermally responsive and will reabsorb the lubricant when heat and pressure forces are removed, negating its utility in the wider range of environments addressed with the present invention.
U.S. Pat. No. 5,435,925 to Jamison also has a similar lubricant incorporating “bleed control agents” which control the application of lubrication, but is similarly unsuitable for use in the wide range of environments addressed with the present invention.
U.S. Pat. No. 4,915,856 to Jamison teaches a railway wheel based lubricant designed to be applied by abrasive deposition. The intention is to reduce the coefficient of friction to a minimum possible value. Also shown is a two part co-extruded product that will contain liquid elements inside an outer jacket. However the Jamison '856 patent also fails to anticipate the need for varied coefficient of friction applications.
U.S. Pat. Nos. 6,649,573 and 7,683,014 to Mitrovich disclose a two part lubricant stick formula and method for railway wheel application composed of polymer, lubricants, oil, and an optical brightener. However the Mitrovich patents also focus on the use of molybdenum disulfide as the primary lubricant and fails need of a higher coefficient of friction product for the rail crown point of application.
U.S. Pat. Nos. 5,308,516 and 5,173,204 to Chiddick et al. are principally friction modifiers and disclose railway friction modification sticks composed of polymer, grease, lubricant powder, and mineral powders for friction enhancement.
U.S. Pat. No. 7,709,426 to Eadie et al. discloses railway lubricant sticks composed of a thermoplastic resin and grease but is without an absorbent which would permit the use of oils and liquid lubricants.
None of the disclosed prior art provides for a solid lubricant which is both economical and relatively environmentally friendly and which can be manufactured with a singular system for use in a wide range of environments.
It is an objective of this invention to improve on the prior art by providing a solid lubricant that with variations in components will allow a single means and manner of manufacture to produce a lubrication stick to satisfy the distinctly different lubrication requirements presented by the different zones of the wheel to rail contact areas. The first zone is found where the wheel flange contacts the rail gauge face or inner sides of the two opposing rails. This area requires a lubricant that will reduce the frictional forces to a minimum value. Reduced friction in this area results in reduced wear and reduced power requirements to propel the train. The consequence is less fuel consumption and maintenance expense. The other critical area of the wheel to rail contact zone is between the wheel tread and the rail crown. In dry conditions the steel to steel coefficient of friction is 0.5 to 0.7. Values this high result in high wear rates and significant noise when relative motion is experienced between the wheel and rail. It has been found that maintaining a coefficient of friction (COF) value in this zone of 0.3 to 0.4 results in significant rail wear and noise reductions without significant loss of motive traction or braking performance. Given all the variabilities of weather, climate, and rail conditions found in the field, reaching and maintaining this critical window is very difficult. Importantly, all prior art that has attempted to satisfy the stringent lubrication requirements of this zone have focused on utilizing lubricants with intrinsically low COF properties and by metering product very carefully onto the rail crown incrementally lowering the COF until the target of 0.3 to 0.4 is reached. Misapplication and over application can cause window overshoot and result in compromised braking, traction, and safety issues. The disclosed invention avoids this problem by utilizing lubricants with COF properties intrinsically in the target range and as such over application has no deleterious consequences. With the disclosed invention measured COF values tend to oscillate within the target range instead of outside it as found with the prior art.
A solid lubricant stick of the proposed formulation can be produced through a single production means and can have the individual ingredients varied in such a way as to produce a lubricant for the wheel flange where a low COF is needed or a lubricant composition suitable for the wheel tread with a medium COF value. With alternate ingredients the stick can be further adjusted to compensate for variations of lubricant application rates due to ambient temperature variation from seasonal changes or climatic conditions. Adjustment of the ratio of the abrasion resistant olefin in the polymer matrix phase can result in a very rapid wearing, high application rate stick or a very slow wearing, low deposition rate stick.
The proposed solid lubricant stick is comprised principally of select elements from each of four differing Groups. Each Group holds elements that can contribute a similar necessary property to the final blend. The First Group consists of a liquid or combination of liquids such as mineral oil, petroleum oil, synthetic oil, vegetable oil, silicone oil such as polydimethyl siloxane, and propylene glycol, that will yield the desired lubricating properties of a precise COF value for the total blend.
The Second Group is comprised of one or more particulate mineral absorbents such as silica, calcium carbonate, talc, and magnesium carbonate that absorbs the liquid lubricants from the First Group. This serves to permit the mixture to consist of a great deal of liquid without becoming excessively wet to the point of bleeding into the product packaging or onto the personnel handling the product. Some absorbents such as silica can hold four times their weight in absorbed liquid. This Group also serves a critical function as an extrusion process modifier in that it increases the viscosity of the compound in its molten state permitting it to be molded by the extrusion process. At high levels of liquid content, without the absorber, the extrudate is so fluid as to be difficult to extrude into a cooling fixture, and must be molded by more expensive alternative injection or compression process.
The Third Group is comprised of polymers that will form a matrix that holds the other Group elements in a useful shape that can be applied to the railway wheels. In the preferred embodiment the polymeric matrix comprises two olifinic polymers. The first is a high density polyethylene (HDPE) with density of 0.920 to 0.965 grams per cubic centimeter. This resin is extrudable on a single or twin screw extruder as is commonly found in the art. The second is an ultra-high molecular weight polyethylene (UHMWPE), that in pure state cannot be processed on a screw type extruder. UHMWPE is often selected for its very high abrasion resistance character and is usually processed on a ram extruder as the resin does not plasticate in the usual fashion of thermoplastic resins. However, UHMWPE when combined with lower density polyethylene or HDPE in the presence of oils develops the desirable characteristic of swelling and separating its polymer chain to create microporous crevices that can hold the absorbent element and oils in suspension when mixed in a properly designed twinscrew extruder. UHMWPE also acts as an extrusion process aid in that it contributes significantly to increase of the melt viscosity of the mixture in the molten state, just as the absorbent does. In the preferred embodiment, the UHMWPE does not actually melt at the extrusion process temperatures as the other polyethylene components do. In the preferred embodiment the UHMWPE particles become sticky rather than flowable and weld to the other olefinic particles so as to create a structurally sound matrix. Varying the UHMWPE content and percentage creates a finished stick with different wear and deposition rate profiles as may be needed for different applications with a higher proportion of UHMWPE wearing slowly and a lower concentration of UHMWPE creating a stick which wears more quickly depositing lubricant at a higher rate.
The Fourth Group is optional and is comprised of a color pigment additive to allow quick identification of different product formulations for the different points of application, or for identification of formulas for high deposition rate or seasonal use. For example, titanium dioxide is a common white pigment, carbon black yields black and shades of grey, iron oxide produces reds and yellows, cobalt aluminate or titanate gives blues and greens, or organic substitutes are available for any of these.
In the preferred method of fabrication the chosen components are mixed in a single stage in a twin screw extruder of the parallel high screw speed type, with the raw components jointly introduced. The components are heated between 275 and 475 degrees in the twin screw extruder and then extruded through a die to create the preferred profile form.
In the preferred method of manufacture, the compound is extruded directly into a sealed water bath cooling fixture kept under vacuum for gas extraction and to hold the cooling form in close contact with the fixture. The formed lubrication sticks, when cooled, may then be cut to any usable length.
The preferred profile form is a round or rectangular shape which is adapted to be placed in an application carriage.
As a finished product in use the end of the stick is pressed into the wheel of a train at the appropriate location by an application carriage with a set or variable pressure. The rotating wheel abrades the polymer matrix away at the desired rate to release the amount of lubricant necessary. The newly exposed pores containing the lubricant and liquid then transfer their contents to the wheel, which is then further transferred to the rail surfaces, and then on to the following railcars which benefit from the applied lubrication.
To illustrate how a low COF lubricant is constructed in the proposed method; (for use at the rail gauge-face to wheel-flange point of contact), an options is to select a medium viscosity mineral oil from Group One with 15% of a liquid extreme pressure additive. This is mixed with a blend of absorbents from Group Two, plus matrix polymers from Group Three, and red oxide pigment from Group Four. In practice this will provide a red lubrication stick which when in use will result in a COF of approximately 0.1. To construct a higher COF (0.35) lubrication stick a silicone oil may be selected from Group One in the previous blend, which will yield a product suited for service on the top-of-rail to wheel-tread zone.
Accordingly, the scope of the invention should be determined not by the embodiments discussed, but by the appended claims and their legal equivalents.
