This invention concerns lightweight fasteners such as for aerospace applications. A bolt with a roll formed thread and better than usual effective thread run-out is provided. The bolt may be used with a shortened nut with a truncated thread for clearance from the bolt thread run-out. Because of such improvements the nut and bolt can each be up to one pitch length shorter than a conventional nut and bolt. Alternatively, the bolt may be used with a conventional length nut or collar, and the tensile strength of the fastener increased.
Further, such a bolt may be used with a swaged collar rather than a nut, and the enhanced strength or shortened length can be advantageous. The bolt may have some of the thread on its tip deleted to form longitudinal flutes which lock a swaged collar or nut onto the bolt. In such an embodiment, the decreased strength due to deleting some of the thread may be offset by increased strength provided by the improved bolt.
Weight of the fasteners is of great concern in airplanes and other aerospace applications. The nuts, bolts, rivets and the like employed for securing the structural elements of an airplane contribute a substantial portion to the total weight of the airplane since very large numbers of such fasteners are used. It is not unusual to use over 100,000 fasteners on one airplane. Thus, there has been a long effort to reduce the weight of fasteners without decreasing the strength, or preferably decreasing weight while increasing strength. Even an apparently small decrease in weight on an individual fastener can have a large impact on the total weight of an airplane.
Nuts and bolts are ubiquitous fasteners on aircraft. The vast majority of aircraft bolts have roll formed threads because of the superior fatigue properties of these threads as compared with machined or ground threads. To make such bolts, a machined metal blank is rolled between a pair of thread forming dies for placing a thread on the blank. Metal is not removed from the blank in this process, but instead the metal is deformed as the thread forming dies press in to form the root and flanks of the thread. The displaced metal flows outwardly to form the crest of the thread.
A conventional thread rolling die has uniform thread forming ridges and grooves over most of its width for making a uniform thread. The edges of the dies are, however, chamfered or rounded somewhat to avoid damage to the die. This chamfer results in a short run-out zone between the cylindrical shank of the bolt and the end or tip on which the thread is fully formed. The run-out zone is also advantageous on the bolt since it avoids a sudden change in cross section of the bolt which could weaken the bolt, particularly in fatigue.
In conventional threaded aircraft fasteners the run-out zone has a length of up to two times the pitch of the thread. Within the run-out zone the root of the thread is not fully developed because of the chamfer on the roll forming die. That is, the thread is shallower than in the portion of the bolt where the thread is fully developed. Concomitantly, the crest of the thread in the run-out may not be fully developed since less metal is displaced from the root. Thus, in the run-out the outside diameter of the crest of the thread is less than the major diameter in the fully threaded portion. The flanks of the bolt thread, which carry the tensile load on the nut and bolt combination, may or may not be fully developed in the run-out. Thus, in the run-out the thread is referred to as imperfect and is nonfunctional.
The maximum length of the run-out under the specifications used in the aerospace industry is 2P, where P is the pitch of the thread. The actual length of the run-out due to normal manufacturing variations is in the range of from 1.5P to 2P.
Nuts used with conventional aerospace fasteners typically have a counterbore collar concentric with the threaded hole through the nut. The length of the counterbore is such that when fully tightened, the threaded portion of the nut does not extend into the run-out on the bolt. If it were to extend into the run-out there would be thread interference between the crest of the nut thread and the incompletely formed root of the bolt thread, and the nut could not be properly tightened on the parts being secured.
The length of the cylindrical shank of a bolt from its head to the beginning of the thread run-out is referred to as the "maximum grip." If the fastener has a flush head, the grip is from the top of the head to the end of the shank. With a raised head not countersunk into one of the parts being secured, the grip is the cylindrical, unthreaded length of the shank.
The plane corresponding to the maximum thickness of parts to be held by a bolt is sometimes also referred to as the "maximum grip," or "maximum grip plane".
Terminology referring to the form and size of threads as used throughout this specification are those commonly applied t screw threads such as set forth at pages 45-1 through 45-8 of Tool Engineers Handbook, 2nd Ed., American Society of Tool Engineers, 1959. High strength aerospace fasteners are designed so that the maximum grip corresponds to the maximum thickness of the parts being secured together. The minimum grip is typically one-sixteenth inch (1.6 mm) less than the maximum grip. For example, an aircraft fastener may have a nominal length of one quarter inch (6.35 mm) for the maximum grip. The fastener would be used for securing together parts having a total thickness in the range of from three-sixteenths inch (4.76 mm) to onequarter inch (6.35 mm). If the thickness of parts being secured together is exactly one-quarter inch, for example, the installer has the option of selecting either of two conventional fasteners, one to fit at its minimum grip length or the other at its maximum grip length.
The depth of the nut counterbore in a conventional aerospace fastener is the difference between the maximum grip and minimum grip plus about 1.5P. A small amount may be added to account for accumulated manufacturing tolerances. This means when the nut is secured against parts having the minimum grip, the end of the thread in the nut is at the end of the run-out, a distance of about 1.5P from the end of the cylindrical shank of the bolt. When the nut is secured against parts having the maximum grip, the thread in the nut stops about one-sixteenth inch (1.6 mm) plus 1.5P from the end of the run-out. The same concept is present in metric bolts, and the difference between minimum grip and maximum grip is typically one millimeter or two millimeters. Thus, the depth of a nut counterbore is 1 mm or 2 mm plus 1.5P.
It has been recognized that if the thread run-out were reduced to 1P or less instead of 2P, the length of the counterbore on the nut could be reduced, thereby reducing the total length of the nut. This also permits a shortening of the bolt thread and overall length by 1P.
The weight savings in an airplane by reducing the length of both the nut and bolt by as little as 1P can be quite substantial.
Aerospace bolts have been developed with an effective run-out of only about 1P. In one such design, for example, a special roll forming die is used. Instead of tapering the root of the thread and producing an imperfect thread, a full thread is carried to within 1P of the cylindrical shank. The root of the thread then increases in a very short distance toward the shank diameter. This has permitted reduction of the total length of the nut and bolt by 1P without significantly reducing the tensile strength of the nut and bolt combination.
This type of short run-out bolt has drastically reduced fatigue properties as compared with a conventional bolt having a run-out length of up to 2P. An exemplary tensile fatigue test simulates an application where the parts secured together do not have parallel faces. For example, two parts may be secured together with the face engaged by the nut being out of perpendicular with the axis of the bolt shank by three degrees.
In a three degree tensile fatigue test there is comparable off-axis loading which induces some bending in the bolt. In an exemplary fatigue test an assembly of a nut and bolt is cycled between an upper tensile load of 50% of the rated capacity of the combination and a minimum load of 5% of the rated load bearing capacity of the combination. The number of cycles to failure is measured.
In such a test the bolt with a modified thread having only an effective 1P run-out has only about 20% of the fatigue life of a conventional bolt with up to 2P run-out. For example, if a conventional bolt has fatigue life of 100,000 cycles, the modified lightweight bolt may have a fatigue life as low as 20,000 cycles in the three degree off-axis fatigue test. The adverse impact on tensile fatigue is apparently due to having a rather deep thread root close to the maximum grip plane at the end of the cylindrical shank. Such a short run-out bolt is also substantially poorer in a lap shear test than a bolt with a run-out of 2P.
Thus, there are two types of shortcomings in existing aerospace fasteners. In one type of fastener there is adequate tensile strength and tensile fatigue strength, but the fastener is relatively long and therefore relatively heavy. On the other hand, a fastener has been produced which is shorter and hence lighter, but the tensile fatigue properties of that fastener are significantly adversely affected.
Thus, it is desirable to provide an aerospace fastener with a roll formed thread where the length of the nut and bolt can be reduced without reducing either the tension capability of the combination or the off-axis tensile fatigue strength of the combination. It is desirable to provide the option of reducing length, and hence, weight, or maintaining a standard length and increasing strength of the fastener. It is particularly desirable to provide such a combination of a nut and bolt that has increased strength as compared with conventional aerospace fasteners. It is also desirable to make such a fastener using roll forming dies relatively unchanged from conventional roll forming dies.
When designing aerospace fasteners for high volume production one must take into account manufacturing tolerances. The tolerances are quite small but the parts themselves are small and these tolerances should be accounted for in the thread run-out.
The thread on a fastener has a specified pitch diameter D.sub.P. The blank on which the thread is rolled is typically machined to have a tip diameter substantially the same as the pitch diameter of the finished thread. There is a certain tolerance, however, on the machined diameter and the actual size may be slightly larger or slightly smaller than the nominal. In the parlance of the art when the blank is oversize at the tolerance limit, it is referred to as the maximum material condition, and when it is undersize at the tolerance limit it is referred to as the minimum material condition.
When fasteners are roll formed the actual thread dimensions differ depending on the variation of the blank diameter from the nominal diameter. Thus, the major diameter of the thread for the maximum material condition is larger than the major diameter for the minimum material condition. These may be referred to as the maximum material major diameter D.sub.MM and the minimum material major diameter D.sub.mM. Concomitantly, the minor diameter of the thread at its root also has a maximum material minor diameter D.sub.Mm and a minimum minor diameter D.sub.mm.
The nuts used with such bolts also have maximum and minimum material conditions. The maximum material condition for the nut is essentially one with the smallest hole which is within tolerance. Fasteners are designed so that when both the nut and bolt are in the maximum material condition, there is virtually no clearance between the mating threads. When either or both of the nut and bolt are at less than the maximum material condition, there is some clearance, and the maximum clearance occurs when both a nut and bolt are in the minimum material condition.
There is also a length tolerance on the shank. Thus, the location of the maximum grip plane has a small tolerance. In typical aerospace fasteners, the tolerance on the grip is -0.000 and +0.010 (0.25 mm). The bolt designer must be cognizant of this tolerance as well.
As an example, the bolt must be made so that when it has the maximum material condition and the nut with which it is used has the maximum material condition and the fastener is used on minimum grip parts, the nut can be tightened against the parts without interference by the nut thread against the root of the thread in the bolt runout. Concomitantly, the thread root and the bolt run-out should not be so small as to significantly decrease the strength properties of the bolt.
In a very high proportion of the holes where aerospace fasteners are used, the shank has an interference fit in the hole. The maximum material major diameter of the thread is smaller than the shank diameter so that the tip of the bolt can be readily inserted in the hole. It is common to provide a lead-in at the end of the shank for a smooth transition between the shank diameter and the major diameter of the thread to assist in guiding the shank into the hole. Typically, the lead-in takes the form, in a longitudinal cross section of the bolt, of a convex arc which is tangent to the shank diameter a short distance from the maximum grip plane and decreases toward the major diameter on the other side of the grip plane. Thus, at the grip plane, the bolt diameter is slightly smaller than the shank diameter. The lead-in continues to a diameter that is at least smaller than the maximum material major diameter. The lead-in is formed on the machined blank rather than being developed during the roll forming step.
A smooth continuous curve has been defined for the run-out of the thread root for maximizing strength and minimizing weight of fasteners. There are acceptable deviations from the defined curve which are within tolerance. As long as the thread root run-out falls within a carefully defined envelope, the strength of the fastener is maintained and there is no interference between the crest of the nut thread and the root of the bolt thread in the run-out.