Patent Application
20130269900
1. Technical Field
The present embodiments relate generally to the field of railroad couplers, and more specifically, to the manufacturing of railway couplers and their various parts through the use of no-bake or air-set casting.
2. Related Art
Sand casting is one of the earliest forms of casting. Its popular use is due to its low cost and the simplicity of materials involved. A sand casting or a sand molded casting is a cast part produced through the following process: (1) placing a pattern in sand to create a mold, which incorporates a gating system; (2) removing the pattern; (3) filling the mold cavity with molten metal; (4) allowing the metal to cool; (5) breaking away the sand mold and removing the casting; and (6) finishing the casting, which may include weld repair, grinding, machining, and/or heat treatment operations. This process is now explained in more detail.
In sand casting, the primary piece of equipment is the mold, which contains several components. The mold is divided into two halves—the cope (upper half) and the drag (bottom half), which meet along a parting line. The sand mixture is packed around a master “pattern” forming a mold cavity, which is an impression of the shape being cast. The sand is usually housed in what casters refer to as flasks, which are boxes without a bottom or lid, used to contain the sand. The sand mixture can be tamped down as it is added and/or the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. The pattern is removed, leaving the mold cavity. Cores are added as required, and the cope is placed on top of the drag.
Cores are additional pieces that form the internal openings, recesses, and passages of the casting. Cores are typically comprised of sand so that they can be shaken out of the casting, rather than requiring the necessary geometry to slide out. As a result, sand cores allow for the creation of many complex internal features. Each core is positioned in the mold before the molten metal is poured. Recesses in the pattern called core prints anchor each core in place. The core may still shift, however, due to poor fit up between core and core prints, the flow of the metal around the core, or due to buoyancy in the molten metal.
Small metal pieces called chaplets are fastened between the cores and the cavity surface to provide further support for the cores. Chaplets are small metal pieces that are fastened between the core and the cavity surface. Chaplets consist of a metal with a higher melting temperature than that of the metal being cast in order to maintain their structure to support the core. After solidification, the chaplets are cast inside the casting and the excess material of the chaplets that protrudes is cut off.
In addition to the external and internal features of the casting, other features must be incorporated into the mold to accommodate the flow of molten metal. The molten metal is poured into a pouring basin, which is a large depression in the top of the sand mold. The molten metal funnels out of the bottom of this basin and down the main channel, called the sprue. The sprue connects to a series of channels, called runners that carry the molten metal into the cavity. At the end of each runner, the molten metal enters the cavity through a gate that controls the flow rate and minimizes turbulence.
Chambers called risers that fill with molten metal are often connected to the runner system. Risers provide an additional source of metal during solidification. When the casting cools, the molten metal shrinks and the additional material in the gate and risers acts to back fill into the cavities as needed. Open risers also aid in reducing shrinkage. When open risers are utilized, the first material to enter the cavity is allowed to pass completely through the cavity and enter the open riser. This strategy prevents early solidification of the molten metal and provides a source of material to compensate for shrinkage. Lastly, small channels are included running from the cavity to the exterior of the mold. These channels act as venting holes to allow gases to escape the cavity. The porosity of the sand also allows some air to escape, but additional vents are sometimes needed. The molten metal that flows through all of the channels (sprue, runners, and risers) will solidify attached to the casting and must be separated from the part after it is removed. Molten metal is poured into the mold cavity, and after it cools and solidifies, the casting is separated from the sand mold.
The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture on the surface of the casting, making them easy to distinguish from castings made by other processes. Air-set, or no-bake, molds can produce castings with much smoother surfaces. The benefit to providing a smoother surface is discussed in more detail below but is not insignificant in improving the performance of castings made utilizing the air-set casting process. After molding, the casting is covered in a residue of oxides, silicates, and other compounds. This residue can be removed by various means, such as grinding or shot blasting. Several other surface condition benefits result from the use of the air-set process compared to the green sand process. These include benefits with regards to surface inclusions, surface porosity, laps, and scabs. Details of a comparison between required surface conditions and what can be obtained using the air-set process are provided below.
During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since 1950, partially-automated casting processes have been developed for production lines, some including hydraulics to compact the sand.
Green sand is an aggregate of sand (about 90%), bentonite clay or binder (about 7%), which includes pulverized coal, and water (about 3%). It is termed “green” because like a green tree branch, it contains water. The largest portion of the aggregate is always sand, which can be either silica or olivine. There are many recipes for the proportion of clay, but they all strike different balances between moldability, surface finish, and ability of the hot molten metal to degas. The coal, typically referred to in foundries as sea-coal, is present at a ratio of less than 5% and partially combusts in the presence of the molten metal leading to off-gassing of organic vapors. Also, the presence of 2-3% water results in increased occurrence of gas defects in the casting after reacting with the molten steel. Rough surface discontinuities can form as a result of the off gassing or vapors and can result in lower fatigue life for couplers and coupler parts. Given the cyclic loading to which coupler assemblies are subjected, it is important to provide as long a fatigue life as possible.
Another type of mold is a skin-dried mold. A skin-dried mold begins like a green sand mold, but additional bonding materials are added and the cavity surface is dried by a torch or heating lamp to increase mold strength. This improves the dimensional accuracy and surface finish, but lowers the collapsibility. Dry skin molds are more expensive and require more time, thus lowering the production rate.
Another type of sand that may be used in sand casting is dry sand. In a dry sand mold, sometimes called a cold box mold, the sand is mixed only with an organic binder. The mold is strengthened by baking it in an oven. The resulting mold has a high dimensional accuracy, but is expensive and results in a lower production rate.
The casting process for the manufacture of couplers has historically employed the green sand process. While this process has served the railroad industry well, there are disadvantages associated with the green sand process, such as poor material strength, porosity, and poor surface finish, resulting in shorter fatigue life, large tolerance variation, and secondary grinding/machining is often required after the casting process. Additionally, a large number of weld repairs may be required at finishing time to fix either surface or subsurface defects. Production rates are also low and include high finishing labor costs. For reasons that will become more apparent below, these disadvantages can require earlier replacement of couplers and/or knuckles, and create additional manufacturing costs that can be avoided. It would be beneficial, therefore, to use another casting process in the manufacture of railroad coupler assemblies to overcome, or at least ameliorate, these disadvantages.
The system may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like-referenced numerals designate corresponding parts throughout the different views.
In some cases, well known structures, materials, or operations are not shown or described in detail. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. It will also be readily understood that the components of the embodiments as generally described and illustrated in the Figures herein could be arranged and designed in a wide variety of different configurations.
Many of the disadvantages of using the green sand process discussed above can be overcome, or at least ameliorated, by using a no-bake, or air-set, casting process. “No-bake” and “air-set” refer to the same type of process and are considered interchangeable throughout this disclosure. The Association of American Railroads (AAR) coupler 100, shown in
The coupler also transmits the longitudinal forces pulling and pushing a railcar in service operations. These forces can be of significant magnitude—often many hundreds of thousands of pounds—and require that the load path of force through the coupler assembly be precisely controlled. Design loads per the AAR Specification M-211 reach 650,000 pounds for the knuckle and 900,000 pounds for the coupler body. Uniform loading helps ensure uniform wear patterns and in turn more uniform load distribution. Finally, the strength of the coupler and its fatigue life is important in order to prevent premature failure of parts, which is directly influenced by dimensional consistency and consequently the level of uniform load distribution.
Surface finish or texture of the coupler has a definite effect in maintaining the required coupler strength and fatigue life. The no-bake casting process provides better dimensional control, improved load path for operating forces, more uniform wear patterns, castings with fewer weld repairs, and improved surface texture for improved strength and fatigue life compared to the green sand process.
In no-bake mold casting, molten metal is poured into a non-reusable mold made from a mixture of sand, quick-setting resin, and catalyst, and the mold is held together until solidification occurs. No-bake sand molding produces a sand mold of considerable strength, which can be free standing without the need for a traditional, steel flask and therefore unlimited in size and shape. The traditional flask is heavy and rigid, which limits green sand operations by the molding efficiencies that result from the limitations of metal flasks.
The no-bake casting process involves the use of chemically-bonded sand systems. The use of the chemical bonding agents typically makes the no-bake process somewhat more expensive than the green sand process. As part of the no-bake casting process, a resin and catalyst are mixed together. Examples of gas catalysts used for curing include sodium silicate (CO2), amine, SO2, and ester cured phenolic systems. Examples of a liquid catalyst can include the air-set system. Through a chemical reaction, the resin hardens into a very strong bond. Sometimes an accelerator may be added to speed the hardening process. The no-bake casting process can also be less sensitive to the air temperature and moisture as compared to green sand operations.
The no-bake process uses graded kiln dried sand, which is mechanically mixed with a resin (or binder) to bind the sand together. Most binder systems are variations on a few basic chemicals, such as furan, phenolic urethane, and sodium silicate. Usually the no-bake method of forming the mold is accomplished at room temperature. Therefore, unlike the green sand process that requires curing the sand, water, and clay mixture at elevated temperature, the no-bake process derives its name by eliminating the baking process required when using the green sand method.
A chemical hardener is then added to the sand mix, which reacts with the binder and begins to set the sand into a solid form. At this point, the fluid sand is poured into a mold around a pattern or multiple patterns. Once poured, the sand is left to set. The binder causes the sand particles to bond together forming a very stable and accurate shape for the cavity that will be used to pour the final casting. Setting time depends on the type of hardener used. The sand sets into a solid block from which the pattern equipment is drawn. Cores are then added to the mold and the mold is closed and ready for casting.
Refractory coatings can be applied to resin-bonded cores and molds. These coatings are sometimes referred to as a wash. Coatings can be used for several reasons, including: (1) to improve surface finish; (2) to control the heat transfer characteristics and microstructure in the steel casting; (3) improve venting of a core; and (4) to prevent certain types of defects in the casting.
In contrast to the green sand process, as discussed, this hardened mold does not require the use of a traditional fabricated metal flask. Flask size limitations can be a detriment for the green sand process by preventing the manufacturer from varying the number of multiple parts in a single flask or by limiting the size of a single casting that can fit in a given flask because of the pre-existing fabricated metal flask sizes. Heavy, steel flasks cannot be cost-effectively modified to accommodate new customer parts sizes, if different than presently-used flasks. Purchasing various single flasks can become costly. Typical sizes can range up to four feet wide, six feet long, and depths from 18-24 inches deep for both cope and drag. Accordingly, an air-set mold is well-suited to larger, heavier castings as the mold strength allows casting of greater weights of metal. A solid sand structure allows a mold to be formed of various sizes, producing the best yields available for each solid sand structure. Also, sand use for a mold can be kept to a minimum without compromising quality so production costs are reduced. The chemical bonding of the sand particles for the no-bake process provides for a better surface condition compared to the green sand process where water and clay are used as the bonding agents.
State-of-the-art foundry equipment on modern-day air-set lines allows up to 100% reclamation of the primary raw material, sand. This reclaimed sand is broken down, cooled and filtered, to be used repeatedly. To maintain sand quality and mold strength, the reclaimed sand is mixed with new sand at a ratio of 75%:25%. This process keeps production costs to a minimum without compromising quality. Note that the new sand to reclaim sand ratio varies depending on the typical casting geometry and weight; the ratio of 75%:25% is only a typical value. Industry values range from 95%:5% to 40%:60%.
Some characteristics and advantages that distinguish the no-bake mold process from that of other sand molding processes, such as the green sand process, include: molds are chemically cured at room temperature; the process produces precise and repeatable dimensions; and finishing labor costs and scrap are reduced while obtaining high casting yields.
As a measure of the dimensional stability of the no-bake process compared to the green sand process, the Steel Founders' Society of America publishes values for dimensional tolerances in Supplement 3 of their Steel Casting Handbook. Base tolerances for castings made by the no-bake process are listed as plus or minus 0.020 inches compared to plus or minus 0.030 inches for castings made by the green sand process. While these are both small dimensions, the ability to have tolerances with a reduction in range of one third is significant when it comes to assuring proper load paths and operational characteristics of coupler assemblies as explained above. The tolerances of the coupler parts depends on casting weight and dimension as will be discussed below, so the tolerance achievable with the no-bake process when compared to the green sand process varies across different parts and dimensions of the railroad coupler. In all cases, however, the tolerances achievable with the no-bake process are smaller than the tolerances required by the AAR Specification M-211.
The no-bake process also allows for smaller draft angles than the green sand process. A draft angle refers to the small slope included for the vertical surfaces of the casting pattern, as oriented in the mold box, so that the pattern can be drawn away from the mold. The draft angle must be included both in the top of the cope and the drag portions of the patterns. Where the green sand process requires a draft angle of 1.5 degrees or more for typical shapes, the no-bake process requires only a 1.0 degree draft angle. Where the green sand (manual) process requires a draft angle of 2.0 degrees or more for deep pockets, the no-bake process requires only a 1.5 degree draft angle for deep pockets. The required draft angle of the green sand process results in a significantly greater deviation from the nominal dimension at cast points that are farther from the pointing line of the entire casting than a casting produced by the no-bake process. Smaller draft angles can promote better part loading and increase bearing area. This small difference is significant when accounting for the interfacing of the complicated shapes that make up the parts in a coupler assembly, and when combined with the reduced tolerance range.
During the locking and unlocking operations, the knuckle 208 rotates about the axis of the knuckle pin 212. The knuckle tail 228 must pass under the knuckle shelf seat 232 on the lock 220 during the locking and unlocking operations. The lock must also move downward and upward in a lock chamber 236 of the body 204 during the locking and unlocking operations. Also, during lockset, the lock 220 must move upward in the lock chamber 236 of the body such that a lockset seat 240 on a lock leg 244 sits with precision on a leg-lock seat 248 of the thrower 216.
The parts of the coupler assembly 200 should have accurate dimensional characteristics to ensure successful operation. The better the dimensional characteristics, the smoother the operation. The larger the dimensional variation, the rougher the operation, and if large enough, the parts will jam and the coupler may become inoperable. Smooth surface finishes also aid in successful operation, which will be discussed in more detail below. If the tolerances of the parts are too large, interference can occur when the knuckle 208 is rotating relative to the body 204 and the lock 220. This interference can result in sticking conditions making difficult the operations of locking and unlocking the coupler. In some cases, extremes of tolerances in relative part dimensions have resulted in coupler inoperability and/or an inability to interchange parts.
The coupler load path for draft (pull) and buff (push) forces generated during train operations is also dependent on precise control of dimensional tolerances of the coupler parts. For draft forces, the coupler is designed to receive the pulling forces at the pulling faces 252 of the knuckles 208 (shown in
Buff forces are maxed during switching operations when freight cars impact each other. The coupler assembly is designed to react to the buff forces at the buffing shoulders 260 of the coupler body 204 and at the buffing shoulders 261 of the knuckle 208. If tolerances of the coupler parts are not controlled accurately, buffing forces can be transmitted at the pin protector bosses 256, 286 (see
While the green sand casting process has been used successfully for many years to produce coupler parts, the no-bake casting process results in a better surface finish and therefore can reduce cracking and associated issues that are created when surface conditions are less than optimal. The normally higher costs associated with the no-bake process have been minimized or offset by: reducing casting finishing (gauging) time, spending less capital investment on items such as special flasks that are not required, requiring less casting defect weld repair, reducing processing time, and yielding more dimensionally-consistent and higher-quality parts with improved part life.
The creation of good surface finish or texture has been established as a priority by the American Association of Railroads (AAR) through action taken by their Coupling Systems and Truck Castings Committee. In the past, certain surface conditions, such as sand inclusions and seams, have been found in critical areas of coupler parts. In some cases, surface conditions can result in cracks which result in reduced fatigue life of the coupler or knuckle. For instance, a radius area 281 between the coupler horn 264 and the shank 268 has received the attention of the Federal Railroad Administration. See Code of Federal Regulations, Title 49, 215.123. Cracks in this area now require replacement of the coupler. Furthermore, a smoother surface achievable through use of the no-bake process adds to tighter tolerances, which will be further discussed below. A part made with tighter tolerances has better fit and functions better with their mating parts, which also increases fatigue life.
In the effort to make sure surface conditions do not result in premature coupler failure, the Coupling Systems and Truck Castings Committee has included specific surface finish criteria as a part of the AAR Specification M-211. Foundry and Product Approval Requirements for the Manufacture of Couplers, Coupler Yokes, Knuckles, Follower Blocks, and Coupler Parts, Specification M-211, Last Adopted October 2009. Section 11.2 of the AAR Specification M-211 reviews specific surface acceptance levels, which are defined utilizing Steel Castings Research and Trade Association (SCRATA) Comparators for the Definition of Surface Quality of Steel Castings. The SCRATA comparators are nine categories, each with five quality levels, decreasing from 1 to 5, in which level 1 is the highest quality and level 5 is the poorest:
A. Surface Roughness—the natural surface of the casting after shot blasting.
B. Surface Inclusions—non-metallic material trapped on the casting surface.
C. Gas Porosity—indications of gas at the casting surface.
D. Laps and Cold Shuts—surface irregularities giving a wrinkled appearance.
E. Scabs—slightly raised surface irregularities.
F. Chaplets—indications of chaplets or internal chills.
G. Surface Finish—Thermal Dressing—surface remaining after using oxy-gas or air-carbon arc processes for metal removal.
H. Surface Finish—Mechanical Dressing—surface remaining after using a mechanical means of dressing a cast surface or a previously thermally dressed surface.
J. Welds—indications of welds fully or partially removed by thermal or mechanical dressing.
The following Tables 1 and 2 are comparison charts respectively for the coupler 204 and knuckle 208 that show the minimum surface conditions required by the AAR Specification M-211 and the improved surface conditions achievable using the no-bake process. Figure A.11 referred to in Table 1 is a three-page figure shown in Appendix A of the AAR Specification M-211 in which the shaded areas are the critical areas and the non-shaded areas are the non-critical areas. One of ordinary skill in railroad couplers would know to refer to Figure A.11 to determine which areas are currently considered critical as distinguished from non-critical areas by the AAR. In general, however, the critical areas are those areas that take on more load force with regards to the draft and buff forces discussed above and also those areas that interface or wear with other parts.
The data in Table 1 was obtained through visual comparison of a number of coupler bodies 204 produced by the no-bake process with SCRATA plates representing 1 through 5 in each of the above categories. With reference to categories D through J in Table 1, no laps, scabs, chaplets, or welding were observed. Also, surface conditions of Thermal Dressing and Mechanical Dressing do not depend on the casting process, but result from an individual performing surface conditioning after the casting process has been completed. The frequency with which Thermal and Mechanical Dressing operations must be performed is, however, a result of the casting process, so a comparison with the green sand process is still helpful. As indicated, the surface quality of a coupler produced by the no-bake process is superior in just about every category, and at least equal to the minimum requirements under the AAR Specification M-211.
The data in Table 2 was obtained through visual comparison of a number of knuckles 208 produced by the no-bake process with SCRATA plates representing 1 through 5 in each of the above categories. With reference to categories D through J in Table 1, no laps, scabs, chaplets, or welding were observed. As with the coupler, the surface quality of the knuckles was superior in almost every category, or at least equal to the minimum requirements under the AAR Specification M-211.
The no-bake process may be used to manufacture the coupler body 204, the knuckle 208, the lock 220, the thrower 216, and the locklift 224 in such a way that better (smaller) tolerances for various relative dimensions are achieved due to the no-bake process. As discussed above, tolerances for the no-bake process is plus or minus 0.020 inches and the draft angle is about one (1.0) degree or less for typical features. Actual tolerances, however, vary with weight and dimension of the casted parts according to the Steel Founder's Society of America (SFSA) Tolerance Tables. Table 3 below shows the T3 tolerances used for the no-bake process used by the manufacturers. For comparison, Table 4 shows the T5 tolerances that correspond to the green sand process typical of conventional railroad couplers.
By way of a simple example, suppose a casted part is made by both the no-bake and the green sand processes, both that weigh about 100 pounds. Suppose a dimension of interest is about 2.0 inches. The tolerance achievable by the no-bake process is about 0.051 inches while the tolerance of the part made by the green sand process is about 0.102, which is about twice that achievable by the no-bake process.
Because one must round the weight up to 500 pounds and the length up to 4 inches in the example of
Using the same 500 pound estimate, and rounding to 6 inches, the 5¾ inch dimension in Table 3 indicates a tolerance of about plus or minus 0.080 inches. Because of rounding, this tolerance is probably closer to plus or minus 0.075 inches. The corresponding tolerance from Table 4 using the green sand process is about plus or minus 0.167 inches, again about twice that which is achievable using the no-bake process.
The 5⅞ inch dimension between the knuckle pulling lug 258 and the knuckle pin hole 282 results in a tolerance of about plus or minus 0.061 inches for the no-bake process compared to about plus or minus 0.108 inches for the green sand process, not quite a two-fold improvement. The 1⅝ inch dimension between the knuckle pin hole 282 and the pin protector bosses 286 results in a tolerance of about plus or minus 0.049 inches for the no-bake process compared to about plus or minus 0.095 inches for the green sand process, again about a two-fold improvement.
The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations can be made to the details of the above-described embodiments without departing from the underlying principles of the disclosed embodiments. For example, the steps of the methods need not be executed in a certain order, unless specified, although they may have been presented in that order in the disclosure. The scope of the invention should, therefore, be determined only by the following claims (and their equivalents) in which all terms are to be understood in their broadest reasonable sense unless otherwise indicated.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12685346 | Jan 2010 | US |
| Child | 13916114 | US |
