Workpiece support trays for furnances

Details Workpiece support trays for furnances Workpiece support trays for furnances

2001-06-14

2002-09-24

Lu, Jiping (Department: 3749)

Heating

Accessory means for holding, shielding or supporting work...

Openwork tray, basket, or grid structure for heat treating...

Reexamination Certificate

active

06454564

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a workpiece support tray for use in furnaces. More particularly, the present invention relates to a workpiece support tray assembly having a replaceable push/pull bar removably coupled to removable guide shoes and an articulating connector for facilitating smooth motion through modem furnaces while avoiding warping and jamming during use.
2. Description of the Related Art
Workpiece support trays are commonly used for the batch heat treatment of metal articles in high temperature furnaces. Such metal articles include gears, bearings, connecting rods, and other articles that are subject to frequent high loads, where heat treatment of the outside surface is desired to prevent wear, but where the interior of the teeth or other load bearing component retains its flexible and ductility for improved fatigue resistance.
Furnaces commonly use support trays, loaded with the workpiece to be hardened, that are simply lowered onto and lifted off furnace piers or supports with forklifts. Other furnaces have been developed having roller rail and chain-driven systems for feeding the workpiece support tray into the heating chamber of the furnace. Such furnaces, so-called “IQ,” or integral quench furnaces, use such roller rail systems to guide the workpiece support tray into the heating chamber vestibule having a plenum and a heat box. The heating chamber vestibule is further integrally combined to another section of the furnace, the quench tank.
As is known, the quench bath can be of either oil or water, depending on the speed and extent of quenching desired. After quenching, the workpiece support tray and workpiece articles are lifted out of the quench bath and then typically removed from the furnace by the roller rail and chain drive systems into a wash tank that also contains the same roller rail system. The workpiece support tray and the workpiece articles are there washed. After the wash tank, the workpiece support tray is typically transported to a draw furnace, where the draw furnace heats the workpiece support tray and the workpiece articles to obtain the desired case depth or hardness, based on the specification to which the workpiece articles are to be treated.
The furnaces that typically use the roller rail system for moving the workpiece support tray and the workpiece articles placed thereon often heat the workpieces to between 1700 and 1900° F., followed by a quenching to 150 to 200° F. in a matter of two minutes or less. Thus, the workpiece support tray must be capable of cycling many times through this extreme thermal cycle, and the expansion and contractions resulting therefrom. In sum, the workpiece support tray must be a very robust and durable product.
However, prior art workpiece support trays were typically formed from a solid cast tray or grid, some of which included integrally cast shoes, or a pair of rails, that rest upon the rollers and roller rails of the furnace. But, when subjected to the heating and quenching cycles, all metals will reach a useful thermal cycle life, including the alloys used for workpiece support trays. Not only are solid cast trays damaged by impacts from forklifts and other handling, after a number of heating and cooling cycles, a precipitation of carbides occurs, changing the grain boundaries in the metal. As this occurs, the metal simply does not hold together as strongly and as resiliently and becomes quite brittle and subject to fracture. At some point, the alloy's useful life is simply over. However, the entire trays of the solid cast type typically never reach that point, due to breakage of only a portion. If a corner is broken, the entire tray usually must be scrapped or repaired. While the corner can be welded back in place, such welds typically do not work very well. Moreover, as the alloy becomes carburized, carbon diffuses the metal and must be ground out before it can be welded. Even in the best cases, the weld will only be 80 or 90% as strong as the original cast using traditional welding processes. Thus, once a section is broken off, unless it can be reliably re-welded on, the solid cast tray may be unusable.
Also, if a corner of the tray breaks off and cannot be replaced and the loading of articles on the tray remains the same, the local stress, determined in pounds per square inch, will also increase, depending on how much of the tray is missing. As the tray is heated, this will cause the tray to sag. When attempts are made to remove the tray from the furnace, it may not track on the roller rail system properly and it may catch or snag on objects, This can be very detrimental to the furnace.
If the chain guide and roller rail systems of the furnace become obstructed, and the tray will not pull out or it becomes snagged, or if a temporary snag causes any of the workpiece articles to fall from the tray into the furnace, the furnace must be shut down. It must be allowed to cool down, after which a person must enter the furnace and remove all the parts and repair the damage. The furnace may be down for days, as it takes several days just to cool. Some furnaces also have radiant tubes on the sidewalls and/or use electric heating elements, which can be broken, cracked or shorted out by a jammed tray or fallen workpiece article. Moreover, persons that enter such furnaces must be certified and experienced in order to do the work. Thus, a reliable support tray is highly desirable.
Moreover, the stiffness of solid cast trays of the prior art created difficulties in the case of worn furnace rollers. The rollers typically have a flange on each side. They sit relatively close to each other within mountings and are typically not bolted in. With time and use, they become distorted and not necessarily flat. In some cases, the difference in height between rollers could be a quarter of an inch over a 10 inch span. As the tray heats, it thus warps and sags. As the rails of the sagging portions of the cart trays are pulled over the roller rails, they hit the face of the roller, causing the tray to lift up. As the tray rises and lifts up, the load can shift and the tray can be pushed off the track.
Also, cast trays are occasionally connected one to the other with fasteners to improve the throughput of the furnace. However, if bolted and not put together properly, the nuts and bolts can come loose. Worse yet, one tray can rise or fall relative its neighboring tray(s), causing too high of a gap and also allowing the tray to fall down off the roller.
Further, solid cast trays are formed with an inherent draft in order to facilitate their removal from the core pattern within which the trays are molded. When such cast trays are bolted together, the cast trays are thinner at the top than the bottom because of this draft. But if bolted together tightly to avoid jamming on the rollers, the angle of the draft is sometimes imposed into the assembly, especially after heating, forming the tray assembly into a “V.” In turn, this formation can lift the trays up off of the rollers, also causing a jam in the furnace.
While there are non-solid cast tray systems, none have been able to fully overcome the shortcomings of the prior art solid cast trays. For example, U.S. Pat. Nos. 2,765,159, 3,025,045, 3,044,755, 3,156,456, and 4,308,009 all teach non-solid heat treating grids or trays. However, none are considered appropriate for IQ furnaces, as is the tray of the present invention, and none are apparently capable of being reliably drawn through a furnace on roller rail and chain-driven systems now currently in use.
SUMMARY OF THE INVENTION
The solution to these shortcomings of the prior art, and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. According to the present invention, an articulating, flexible, relatively lightweight workpiece support tray includes a plurality of spacers juxtaposed between a plurality of lateral I-

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