Material or article handling – Chamber of a type utilized for a heating function and... – Including driven device and/or inclined flow path to carry...
Reexamination Certificate
2000-06-12
2001-08-07
Bratlie, Steven A. (Department: 3652)
Material or article handling
Chamber of a type utilized for a heating function and...
Including driven device and/or inclined flow path to carry...
C198S841000, C432S153000, C432S239000
Reexamination Certificate
active
06270305
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A
BACKGROUND OF THE INVENTION
In a known type of furnace, product is conveyed through a heating chamber of a furnace on a conveyor belt. The belt travels over the surface of a hearth, which is typically formed by shallow metal pans called hearth plates. A furnace muffle surrounds the hearth plates and the belt to define the heating chamber. See
FIG. 1
, in which the belt is omitted and a portion of the muffle is shown cut away to better illustrate the hearth plates.
This type of conveyor furnace is used for a variety of applications, including stainless steel brazing. Stainless steel brazing requires a high temperature and a low dewpoint inside the furnace. In this application, copper wire is wrapped around a joint. The product with the joint is passed through the furnace, melting the copper and brazing the joint. The melting temperature of copper is approximately 1080° C., so the temperature within the muffle must be in excess of 1100° C. and is typically in the range of 1120 to 1130° C. The oxygen level must also be very low, typically measured by a dewpoint of −70° F. or less in a hydrogen atmosphere. Because oxygen in a hydrogen atmosphere reacts with the hydrogen to form water, the oxygen level in such a furnace is typically determined by measuring the dewpoint in the furnace.
In this type of furnace, there is continuous friction between the belt and the hearth. Hearth materials vary, but the overwhelming choice of hearth material is metal and for higher temperature applications is typically a nickel-chrome (Ni—Ch) alloy. This material is also used for the belt. This material is able to withstand the temperatures in the furnace while retaining sufficient strength to support the product without failure. The coefficient of friction between the belt and the hearth ranges from 0.3 to nearly 0.6 when the temperature is above 1100° C. A high coefficient of friction causes serious drag and wear life problems.
Also, in furnace atmospheres where oxygen is present, slight oxidation occurs on the belt and hearth plates, providing a protective coating against wear and grinding. In furnace atmospheres with very low dewpoints and at high temperatures, however, no or minimal oxidation occurs, and metal is often transferred from the belt onto the hearth. These metal deposits on the hearth create bumps and burrs that cause severe drag and wear problems against the traveling belt.
SUMMARY OF THE INVENTION
The present invention provides a high-temperature conveyor furnace with a low friction conveyor travel surface. The furnace includes a muffle defining a heating chamber, hearth plates providing a hearth surface within the heating chamber, and a conveyor belt to convey product through the heating chamber. The conveyor belt is formed of a first material, such as a nickel-chrome alloy. A plurality of low-friction inserts are supported by the hearth plates, preferably in openings in the hearth plates, to provide a surface upraised from the hearth surface upon which the conveyor belt travels. The inserts are formed of a second material, such as an electronics grade ceramic, that is different from the first material of the conveyor, thereby minimizing friction between the inserts and the conveyor belt. The low-friction surface provided by the inserts is particularly suitable for high temperature, low dewpoint furnace applications and H
2
atmosphere applications, such as stainless steel brazing.
In the presently preferred embodiment, the inserts include a base portion and an upraised portion having an overhang or shoulder that extends radially beyond the base portion. The base portion fits within an appropriately-sized opening formed in the hearth plate. The overhang or shoulder of the upraised portion rests on the upper surface of the hearth plate.
The size and spacing of the inserts are chosen to adequately support the conveyor belt and product while minimizing the amount of material in each insert. Minimizing the amount of material in the insert minimizes breakage, thermal shock due to temperature differentials across the insert, and cost. The underside of each insert preferably includes a recess formed therein, which additionally aids in minimizing the amount of material.
REFERENCES:
patent: 3582054 (1971-06-01), Beck
patent: 3955668 (1976-05-01), Buschbora et al.
patent: 4412816 (1983-11-01), Schlumberger et al.
patent: 4569661 (1986-02-01), Kramer
patent: 5136616 (1992-08-01), Heuss
patent: 0479516 (1992-04-01), None
Hutcheson Robert
Orbeck Gary
Bratlie Steven A.
BTU International Inc.
Weingarten, Schurgin Gagnebin & Hayes LLP
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