Expanded – threaded – driven – headed – tool-deformed – or locked-thr – Having structure to restrict rotation of threaded – mating... – Comprising a thread lock
Reexamination Certificate
2002-08-13
2004-11-16
Sandy, Robert J. (Department: 3677)
Expanded, threaded, driven, headed, tool-deformed, or locked-thr
Having structure to restrict rotation of threaded, mating...
Comprising a thread lock
Reexamination Certificate
active
06817816
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fasteners and self-tapping fasteners that form internal threads using a swaging or roll forming process. More particularly the invention relates to a fastening element and method capable of forming a fastener assembly by engagement with a self-tapping fastener that reduces the required end load to start the tapping process and assists in the proper alignment of the self-tapping fastener.
BACKGROUND OF THE INVENTION
Self-tapping fasteners such as self-tapping screws or bolts fall into two broad classes. The first are those which are provided with cutting edges at the work entering end. The second and most common type are those which are so designed to form uniform load carrying internal threads into untapped fasteners or pilot holes with a swaging operation. Fasteners of the first type have numerous disadvantages and one of the most significant being that they all form chips which are cut from the body to which they are driven. As a result, self-tapping fasteners that form threads by deforming a thread pattern within a pilot hole have become the most popular design. Such fasteners are available from a variety of sources and are marketed under the trademark TAPTITE® in connection with a trilobular or three-lobe thread forming blank design.
FIGS. 1-2
illustrate a conventional three-lobed fastener according to the prior art. All threads have a characteristic pitch and diameter because of the lobulation of the threads, the radial offset from the axis will vary about the circumference. In general, standard thread diameters and pitches are provided to lobular fasteners, but the lobes tend to have a slightly larger diameter than a standard thread diameter. This enable the lobes to positively form corresponding internal threads as the fastener is driven into an appropriately sized pilot hole into the shape of conforming internal threads.
As the fastener is rotated the lobes engage the inner wall of the pilot hole (not shown) and begin to displace material within the pilot hole. In a typical self-tapping fastener, the threaded fastener is provided with a stabilizing zone having stabilizing threads at the end of a fastener shaft and a thread forming zone with corresponding thread forming threads along the shaft of the fastener adjacent the stabilizing zone. The stabilizing zone as illustrated in
FIGS. 1-2
often has a reduced diameter enabling it to fit within an initial untapped hole in a relatively perpendicular fashion. The thread forming zone often has a sloped or tapered shape with a diameter that increases linearly between the stabilizing zone and the full diameter main body of the fastener.
Prior known constructions have often provided the thread stabilizing zone and the thread forming zone with a higher out of round than the full diameter main body. In one example, the out of round of the thread forming zone gradually tapers back from the highest out of round adjacent to the stabilizing zone toward the lower out of round that defines the full diameter main body. In another often preferred example, the thread forming zone can define an approximately constant profile high out of round along its entire axial length that transitions step wise at the main body into the characteristic lower out of round. In connection with either example, there is a difference between the high out of round at the stabilizing section and at the main body cross section.
As a self-tapping fastener is driven into an untapped pilot hole the thread forming threads encounter the sidewalls of the hole initially. These threads often exhibit an increasing outer diameter and higher out of round. As such, the lobes are able to gradually apply increasing thread forming pressure to the pilot hole until each formed internal thread is contacted by the first full diameter thread. This first full diameter thread often has the out of round profile of the rest of the main body. It provides final formation of each thread in the pilot hole to the desired shape.
Self-tapping threaded fasteners are frequently preferred in applications where it is possible to use a metal screw which is harder than the material of a mating element such as a blank or nut through which a threadless bore for the screw has been made. In general, properly forming internal threads in a bore requires several swaging blows from the underlying lobes of the fastener. This process, in essence, forms a shape in the ductile metal of the untapped pilot hole or fastener corresponding to the threads of the self-tapping fastener. A sufficient number of forming threads is necessary to complete the process. Depending upon the nature and hardness of the metal into which a self-tapping fastener is driven, a relatively high driving torque is usually required, particularly in metal having an appreciable thickness. This often results in a stripping torque to driving torque ratio that is relatively low. The requirement of high driving torque not only creates problems with respect to drivability but a low driving torque to stripping torque ratio can restrict the usage of automated power drivers in assembly lines.
It is well known that the driving torque of individual fasteners can vary considerably due to the presence of any lubricant, slight variations in the material hardness into which the fastener is driven, in the hole size, in the fastener diameter, as well as dullness of cutting edges or from misformed or damaged threads (especially the lead threads) from handling or processing such as plating. Similarly, failure torque, including stripping torque of the mating threads as well as the failure torque of the fasteners themselves can vary somewhat considerably from one fastener to the next. The clutch or related mechanisms of the power drivers cannot be relied upon to disengage at precisely the same torque value each time. If the driver is set just above the normal driving torque, and any of these variations causes an increase driving torque, conventional tapping fasteners will not be driven in fully and loose assemblies could result. If the driver clutch is so adjusted to give a greater driving torque so as to overcome any such difficulty, a conventional tapping fastener can then be overdriven, resulting in stripped threads or broken fasteners, either of which will result in costly delays of the assembly line while repair or replacement is made.
It is also known, that in many cases the efficiency and thus the usefulness of self-tapping operation can be problematic, particularly because at the beginning of each operation considerable pressure or end load must be applied by means of a conventionally used power driven tool to cause the self-tapping screw to properly start winding itself into the material adjacent the cylindrical surface defining the threadless bore. Such forces can make proper alignment difficult. Difficulties may be encountered when the bore is originally, or thereafter becomes oriented at an angle relative to a driven self-tapping fastener such that the fastener is not in perfect alignment with the axis of the bore. As a result, the fastener may become permanently askew and not seat properly. This can be where the lead thread of the fastener is initially slightly misformed or thereafter becomes distorted.
Such problems have been acute where for example, the bore axis extends horizontally and the self-tapping fastener is driven from a position relatively higher than or relatively lower than the axis. In many such instances, the threads of the self-tapping fastener which are designed to form threads within the bore upon proper engagement then are mangled or otherwise distorted. If the resulting assembly is formed at all, it may have significantly impaired holding characteristics since the underside of the fastener itself may be damaged and thus weakened. Additionally, the entire fastening assembly may be weakened and put in jeopardy. Moreover, the cocked or askew fastener head may have roughened the surface of the structural element containing the bores such that it would not hold paint, or such that
Liniak Berenato & White
ND Industries, Inc.
Rodriguez Ruth C.
Sandy Robert J.
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