Universal coupling

Details Universal coupling Universal coupling

2000-04-07

2002-03-19

Kastler, Scott (Department: 1742)

Metallurgical apparatus

Means for melting or vaporizing metal or treating liquefied...

By stirring or mixing molten metal

C222S602000

Reexamination Certificate

active

06358467

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the art of processing and treating molten metal. More particularly, this invention relates to a new and improved coupling design for a molten metal processing system.
2. Discussion of the Art
Molten metal processing systems can usually be classified into several different types of systems. For example, degassing/flux injection, submergence and pumps are frequently used general categories.
Systems which fall into the degassing/flux injection category generally operate to remove impurities from molten metal. More specifically, these systems remove dissolved metals, such as magnesium, release dissolved gases, such as hydrogen, from molten metal, and through floatation remove suspended solid impurites. In order to achieve these functions, gases or fluxes are introduced into a molten metal bath which chemically react with the impurities to convert them to a form (such as a precipitate or a dross) that can be separated readily from the remainder of the molten metal.
Systems which fall into the submergence category generally operate to melt scrap metal, such as by-products of metal processing operations and aluminum beverage cans, in order to recover the scrap metal for productive use. In a typical submergence system, the scrap metal is introduced onto the surface of the molten metal and drawn downward or submerged within the molten metal where it is melted. In its melted form, the scrap metal is substantially ready for productive use.
The pump category can be further classified into three different types of systems including transfer pumps, discharge pumps, and gas-injection pumps. A transfer pump typically transfers molten metal from one furnace to another furnace. A discharge pump transfers molten metal from one bath chamber to another bath chamber. A gas-injection pump circulates molten metal and adds a gas into the flow of molten metal. Although the present invention is particularly well suited for use with a gas-injection pump or degassing system, it must be appreciated that this invention may be used with any rotor/shaft system, including but not limited to the systems mentioned above.
Known molten metal processing apparatus of the foregoing types typically include the common feature of a motor carried by a motor mount, a shaft connected to the motor at an upper end, and an impeller or rotor connected at a lower end of the shaft. A coupling mechanism is used to connect the upper end of the shaft to the motor. The components are usually manufactured from a refractory material, such as graphite or ceramic. In operation, the motor drives the shaft which rotates the impeller about its central vertical axis. The rotating impeller may serve any number of functions. For example, in a submergence system the impeller may draw molten metal downwardly to assist in the submergence of scrap materials deposited on the surface of the melt. In a pump system, the impeller may be contained within a housing to effect a pumping action on the metal. In a degassing/flux injection system, the impeller may introduce gas or flux into the molten metal via a passage located in the impeller body. Furthermore, the impeller may serve other conventional functions.
An important feature of impeller/shaft systems is the coupling mechanism which connects the upper end of the shaft to the motor. With reference to
FIGS. 1A-1C
, a series of shafts for known coupling designs are shown. Connecting an upper end of a shaft to a motor is most commonly achieved via a straight thread design as shown in FIG.
1
A. The straight thread design includes an upper end
10
′ having a plurality of external threads
12
′. The threaded upper end is threaded into a coupling (not shown) extending down from a drive system (not shown). Like any conventional threading mechanism, the shaft is screwed into the coupling by turning it several times until it is tight and secure.
The straight thread design suffers from several shortcomings. During operation, the shaft of a rotor/shaft system is exposed to a number of forces, particularly shear forces resulting from cantilever loading. The straight thread design is a relatively weak coupling because the machining of the coupling causes stress risers in a ceramic or graphite shaft. This results in an increased potential for shaft failure which is obviously undesirable. Furthermore, when a shaft breaks, it typically breaks just below the coupling leaving little if any shaft extending from the coupling. Thus, there is little material to work with in order to unscrew the stub. In addition, because the resistance of the straight thread design is equal in both directions, it is extremely difficult to unscrew. In other words, a significant amount of torque is required to remove the stub. A chisel and hammer are generally required to accomplish removal.
Removing the stub with a chisel and hammer causes additional problems. The use of a chisel to remove the graphite stub may accidentally deform the threads in the coupling. Thus, the threads will have to be re-formed to their original dimensions. Such re-forming operations are time consuming and often result in shaft run-out. Moreover, because graphite is a soft material, the normal replacement of the shaft in a straight thread design may lead to graphite deposit in the coupling threads, resulting in binding and shaft run-out.
Additional problems arise when the straight thread design is used in connection with a degassing system. When used for such applications, the straight thread does not operate with optimal sealing properties which is an important characteristic for degassing systems to prevent leakage of the purge gas.
Two other known coupling designs have been introduced in order to overcome some of the problems associated with the straight thread design. The first is an electrode thread design, as shown in FIG.
1
B. The electrode thread design includes a recess
14
′ in the upper axial end of the shaft having a series of internal axial threads
16
′. A male mating member (not shown) threads into the recess thereby connecting the drive system to the shaft. The second coupling is a tapered design which is shown in FIG.
1
C. In this design, the upper end of the shaft is tapered and is configured to frictionally fit into a coupling (not shown). A male threaded shaft (not shown) extends from the coupling and fastens into a tapped bore
20
′ extending through the central axis of the shaft.
The tapered design provides marginally increased strength to resist the lateral forces applied to the shaft. When the shaft does break for the tapered design, it is tedious to remove the portion of the shaft which still remains connected to the motor. The resistive force or required torque to remove the remainder of the shaft is so great that removal of a broken shaft can be done only with a significant amount of time and effort and a risk of damaging the coupling.
The electrode thread design also provides marginally increased strength to resist the lateral forces applied to the shaft. However, when the electrode thread design is used in connection with degassing equipment, it suffers from poor sealing properties which is an undesirable characteristic in such an application. Such a system does not seal well because of the large threads which are used. Additionally, because the threads are of a relatively soft material, they experience deformation which makes removal or backing off of the shaft extremely difficult.
Accordingly, a need exists in the art of processing molten metal to provide a coupling design for rotor/shaft systems which has optimal sealing properties, low run-out potential, relatively high strength to resist transverse forces, and can be easily removed at the end of its life or upon shaft failure. The present invention achieves such advantages and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a coupling mechanism for a molten metal processing system includes an elongated shaft having a f

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