1. Field of the Invention
The present invention relates to roller mills for pulverizing, or grinding, various mineral materials. The invention relates more particularly to the construction of pulverizing rolls for a roller mill of the type in which the pulverizing rolls are mounted on a tapered threaded shaft and secured by a nut.
2. Description of the Prior Art
Pulverizing rolls are typically made of a very hard, wear-resistant cast metal. Such metal is extremely difficult to machine. However, in pulverizing rolls of the type mounted on tapered threaded shafts, a precision tapered bore is often required, or at least desired, to fit accurately on the shaft.
In the past the required precision tapered bore has been provided by either hand grinding or precision machine grinding the inner bore walls of a monolithic roll member made of the aforementioned hard cast metal, as shown in FIG. 4 of the drawings. However, this method of manufacture is extremely expensive because of the difficulty of hand grinding or precision machine grinding the bore. Also, when the roll has wornout, it has to be discarded and replaced with a similar roll having the same disadvantage.
In another prior roll construction and method of manufacture, shown in FIG. 5 of the drawings, the hard metal outer roll material has been cast about an inner sleeve made of a machineable metal, and the inner sleeve has been precision machined to the required taper. While this method of manufacture is less expensive than the aforementioned method because the precision machining of the correct taper on the machineable inner sleeve is easier than grinding or machining such taper on the hard cast metal roll material itself, the method still leaves much to be desired. When the roll wears out and is in need of replacement, the entire roll, including the inner sleeve, must be discarded. Also, a roll of this type is still considerably more expensive than a roll which requires little or no machining or grinding of the taper.
Roller mills of the aforementioned type have journal bearings which permit the tapered, threaded shafts to rotate relative to their supporting arms. Since the rolls on such shafts rotate with the shafts and are used to grind minerals and other substances to a fine powder, they operate in a dusty environment. The powder tends to invade the journal bearings and increase friction therein. As the friction increases, the torque tending to cause the roll to rotate about the shaft also increases. When the resistance to rotation between the journal bearings and the shaft becomes greater than the resistance between the roll and shaft, the roll begins to rotate about the shaft. Because the rolls are cast of a very hard metal and the shafts are usually made of a relatively soft machineable steel, the rotation of the roll about the shaft can quickly result in destruction of the shaft's machined surface. The expensive shaft must then be discarded, and further cost incurred in disassembling and reassembling the components. Also, once the roll becomes loose on the shaft, the nut tends to loosen. In extreme cases, the nut unscrews from the shaft, thereby allowing the roll to drop off and cause extensive damage to the roller mill assembly.
Many attempts have been made to avoid these consequences, such as by tightening the nut as tightly as possible, tack-welding the roll to the shaft, tack-welding the nut to the shaft, or placing a bar across the end of the shaft and welding it to the shaft and nut. All of these measures are undesirable because they tend to damage the shaft and reduce its useful life, and make it difficult to remove the roll from the shaft. They are also time-consuming and thus costly.
Accordingly, there is a need for a simpler, less expensive pulverizing roll which will mount on a tapered threaded shaft, will not slip or rotate about the shaft and can be quickly and easily replaced when worn-out.