1. Field of the Invention
This invention pertains to fasteners, and more particularly to threaded fasteners in which the threads are replaceable on a shank.
2. Description of the Prior Art Threaded fasteners in the form of bolts and screws are well known and are in widespread use. Such fasteners are usually in one piece, and they invariably include a head and a threaded shank. In many instances, the shank is threaded for its full length. In other cases, only a portion of the shank is threaded.
The industrial process of rotational molding is a particularly severe application for threaded fasteners. Large screws, which typically have a 0.75 inch diameter and an eight-inch length, are used to releasably join two large mold supporting spiders to each other. The screw shanks pass through clearance holes in a first spider. The screw threads, which are usually hardened, engage receivers in the other spider. At the start of a molding cycle, the screws are tightened into their associated receivers to draw the spiders toward each other and close the molds. At the end of the cycle, the screws are reversed to completely disengage from the receivers. The spiders can then be separated from each other and the molds opened. After the workpieces have been removed from the molds, the process is repeated. The repeated turning of the screws causes their threads to wear relatively quickly, even though they are hardened. Consequently, the screws, which are expensive, must periodically be replaced. In addition, debris and coolant from the machinery can enter the receivers. As a result, the screw threads occasionally seize to the receivers. When that occurs, the screw shank must be flame cut in two. The entire screw, as well as the receiver, are discarded. That remedy is expensive, not only from the standpoint of material loss but also from the associated unproductive downtime.
A further problem with prior screws used in the rotational molding process is that they tend to fall out of the first spider when the spiders are separated at the end of a molding cycle. The worker must then either search for the screw or obtain a new one.
In some other applications of threaded fasteners, it is desirable or even necessary to employ multi-component fasteners. Such applications include railroad tracks; the bolt of U.S. Pat. No. 1,208,550 is an example. Another application of multi-part fasteners are self-tapping screws such as are shown in U.S. Pat. No. 4,022,099 and 4,900,207. U.S. patent 4,126,338 shows a housing end and replaceable threaded sleeve that are useful for coupling a fluid conduit. Canada patent 715,793 describes a shear bolt that includes a stud that threads into a sleeve.
None of the fasteners of the foregoing patents is suitable for use with rotational molding equipment. Therefore, a need exists for improvements in the spider joining screws that are used in rotational molding.