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An asymmetric wrenching system with high torque transfer capability for aerospace pins.

ABSTRACT

The installation of common threaded aerospace fasteners by the application of a torque to a nut or collar is made possible by an internal wrenching element or recess feature adapted to the threaded end of a pin, which accepts a mating anti-rotation key designed to partially balance the applied torque. In applications such as the mechanical joining of composite structures accomplished by wet clearance fit installations of permanent fasteners, high nut or collar seating torques not adequately opposed by frictional resistance at the contact surfaces of the fastener and joint members effectively shift a greater proportion of the torque reaction requirement onto the recess and mating anti-rotation key which in turn can experience high torsional stresses exceeding their design capability and result in frequent service failures. In particular, the industry standard hexagonal recess and key have been shown to be highly susceptible to such severe conditions with failure rates as high as 20% in field applications. A superior asymmetric recess design was developed for use in aerospace pins to enhance the reaction torque capability of the fastener and mating key over the current baseline, without compromising established functional requirements, enabling fastener use in more demanding service applications. In this paper, the installation performance capability of threaded pins with the asymmetric recess is compared to equivalent substitutes with alternative recess designs through a series of standard industry tests conducted across a range of part sizes. Numerical simulations and experimental test results demonstrate that the asymmetric recess and key exceed the torsional strength and fatigue life of the current industry standard and other commercially available designs, offering the potential to increase productivity and reduce the cost of structural assembly and maintenance operations.

CITATION: Pinheiro, R., Gurrola, R., and Dzebo, S., "An Asymmetric Wrenching System with High Torque Transfer Capability for Aerospace Pins," SAE Int. J. Aerosp. 9(1):2016.

INTRODUCTION

Unlike bolts and screws that have wrenching features or recesses located on their heads, aerospace pins employ an anti-rotation recess located in their threaded extremities as wrenching feature, as depicted in figure 1. This characteristic allows for a smooth, axisymmetric head that provides several advantages in terms of weight and aerodynamic performance.

The anti-rotation recess located on the threaded side of the pin also provides benefts for the assembling and fastening of structures, as a single tool can be used to provide torque and anti-rotation simultaneously [5].

Aerospace pins are normally intended for a single installation (no reusability) and can be made out of diverse materials, such as Titanium 6AL-4V, Nickel alloys and stainless steels, with different head configurations, thread lengths, finishes, and can be installed with various types of prevailing torque nuts and frangible collars [7].

Fastener Installation

Aerospace pins are usually installed to carefully prescribed torque values in order to provide the intended preload [4]. For installing nuts, the torque control is provided by the installation tool, whereas frangible collars have a built-in break-off torque. Example of recommended seating torques for different sizes and nut materials are listed on table 1.

These fasteners are often installed with special torque tools that have driving sockets with coaxial anti-rotation keys [6], as shown in figure 3.

During installation, as the driving socket engages the nut or collar and the anti-rotation key engages the pin recess, the torque acting upon the pin is shown in figure 4.

The torque balance for the pin can be written as follows [3][4]:

[T.sub.T]+[T.sub.P]=[T.sub.H]+[T.sub.S]+[T.sub.K]

Where [T.sub.T] is the thread torque applied by thread engagement, [T.sub.p] is the prevailing torque from the nut or collar, [T.sub.H] is the reaction torque from head friction, [T.sub.S] is the reaction torque from shank friction, and [T.sub.K] is the anti-rotation key reaction torque.

Common aerospace pins system were designed and configured for use in metallic structures in interference fit conditions [1]. In such applications, most of the applied torque is absorbed in the pin shank and through head friction, with little or no torque being transferred to the anti-rotation key. For these applications, the widely adopted hex key has been successfully used for decades. However, as new structural materials, such as composites, and different installation conditions are becoming more common [2], problems have been observed in many aircraft assembly lines around the world.

TYPICAL INSTALLATION PROBLEMS

For clearance fit applications commonly found in composite structures, in the absence of shank friction, the reaction torque is shared between head friction and the anti-rotation key:

In this case, the combination of different factors can lead to key overloading and installation failures. The four main contributing factors for these installation problems are high prevailing torque, galling, low friction sealant, and improper tool operation.

High Prevailing Torque

Nuts and collars used with aerospace pins are fitted with some form of anti-vibration feature that prevents them from loosening in service, especially in the absence of preload. The most common form of anti-vibration mechanism used is crimping, when a portion of the nut or collar threads is deformed around the circumference, generating impedance to rotation, in the form of prevailing torque.

When high strength materials are used in combination with finishes that produce significant friction, and the fastener threads are close to their maximum material conditions, a considerable amount of impedance and prevailing torque can be generated. The problem with high prevailing torque is that it can be generated in the absence of preload, before the nut or collar reaches the structure. In this case, there is no head friction and all torque applied to the system is equilibrated solely by the anti-rotation key. This condition can produce excessive key stresses, especially for commonly used configurations such as hex key, reducing fatigue life or even leading to sudden fracture.

Galling

Certain combinations of fastener materials and finishes can be susceptible to galling during installation. If severe galling and complete seizing of a nut or collar happens before any preload is generated, all the applied torque is transferred directly to the anti-rotation key. Since the target installation torque is usually based on the desired seating torque for a nut or collar, these torque values are much higher than the capability of typical anti-rotation keys, and a galling condition can lead to sudden key failure

Low Friction Sealant

Some types of lightweight sealant used in composite applications, including the ones filled with hollow microspheres, have very low coefficients of friction. This condition affects the torque balance in clearance ft, as the capability of reacting torque through head friction becomes reduced by the low friction. In dry installations, without sealant, the fastener head tends to absorb about 70% of the applied torque through friction, leaving the remaining 30% for the anti-rotation key. In wet, low friction conditions, this balance can be reverted, leading to key overloading, as shown in figure 6.

Depending on the types of fasteners being installed, repeated and continuous overloading can lead to a very low endurance life for the anti-rotation keys, with some manufacturers reporting key failures in up to 20% of installations.

Improper Tool Operation

Another source of anti-rotation key overload is through improper tool operation. In the situation where the nut or collar is approaching its fully seated position, just before collar break off or tool clutch activation, all the torque generated from the tool has to be equilibrated by the operator's hands. At this moment, if the operator lets the tool rotate freely but keeps the tool active, the nut will stop spinning around the pin, and the tool will start spinning around the driving socket. When that happens, the totality of tool torque generated has to be equilibrated by the key. In other words, it is possible to overload the anti-rotation key with the full seating torque of the system in every installation, by just letting the tool turn freely. As the labor force in aerospace assembly lines tends to be very skilled, this condition is not very common, but considering operator fatigue and tight production schedules as factors, it is not difficult to imagine that this form of overloading can be observed throughout the industry.

Typical Key/Recess Failure Modes

When overloading occurs to the anti-rotation key/recess, the most common failure modes observed are: twisted key, fractured key, round-off key, and cam-out recess. These failure modes can be associated to two basic key/recess characteristics, torsional and bearing strength.

Twisted key, as depicted in figure 7, occurs when the material of the key is ductile enough to endure large plastic deformation in torsion.

Depending on the severity, this deformation may prevent proper key engagement.

Key fracture is observed when the overloading produces a sudden failure or rupture. This failure mode is usually combined with fatigue, when a crack is propagated during regular use, gradually reducing the torsional strength, ultimately fracturing during an overload condition. This failure mode is also observed when the key is made out of brittle, low toughness materials. Figure 8 shows an example of hex key fracture.

Key rounding-off is a gradual process that particularly occurs when there is clearance in the engagement between a key and its mating recesses. As the offset between key and recess profiles increases, the bearing area between them becomes very small, degenerating line contact. This is particularly prominent in keys with hexagonal configuration, where the bearing stresses are concentrated on edges at the hex vertices, deforming and rounding them. A hex key with round-off edges is shown in picture 9.

Similarly to what happens to a key, high bearing stresses can deform or cam-out the recess, distorting its surfaces, eliminating its torque transfer capability. This is commonly observed in aerospace pins made out of relatively soft materials and fitted with hex recesses that are prone to cam-out during torque overloading.

THE ASYMMETRIC KEY/RECESS SYSTEM

As most of the installations problems involving anti-rotation keys are related to overloading, a new key/recess systems was created such that its torsional and bearing strength can repeatedly withstand the full seating torque required to install high strength aerospace nuts and collars, making it capable of handling the maximum torque the installation tool can apply during an overloading condition

Taking advantage of the single use characteristic of typical aerospace pins, this key/recess system was developed with the notion of a preferential direction for transmitting torque during installation. As shown in figure 10, the system has an asymmetric configuration that minimizes bearing and torsional stresses, improving torque transfer capability and endurance life.

Features

The asymmetric recess can have between 6 to 8 grooves, depending on the fastener's size. The large amount of grooves makes for an easy indexing as the 8 groove configuration can be engaged every 45 degrees. This is particularly helpful in areas with difficult access.

The asymmetric key/recess system is configured for an easy probing and engagement, and its improved strength allows for the fastener to be engaged without stopping the installation tool.

When used in the reverse (non-preferential) direction the asymmetric key/recess system also provides superior torque transfer capability, as shown on table 4.

Enhanced Torsional Strength

In order to withstand the seating torque of high strength nuts and collars, the asymmetric recess key/recess system was designed with a larger diameter when compared to existing commercially available systems, such as the hex, as shown in figure 12. This larger diameter provides for a much increased torsional strength, as refected by polar moment of inertia values, shown on table 2.

Enlarged Bearing Surface

Two of the main failure modes observed with hex keys in the field, key round-off and recess cam-out, are associated with high bearing stresses. The asymmetric system was designed to maximize the bearing area between key and recess by increasing the number of faces in contact and also increasing the size of the contact faces. This enhanced contact area provides much lower operating bearing stresses and higher durability than typical keys/recess systems, such as the hex.

Figure 13 shows the contact interface between an asymmetric key. This interface is engineered in such a way that it always provides a face-to-face contact condition, regardless of tolerances and components variation, assuring a proper engagement and bearing in all installations.

Figure 14 Red area highlighting the bearing for a single lobe of an asymmetric key.

Improved Bearing Load Direction

One of the difficulties of fitting a large recess in the extremity of an aerospace pin resides on the fact that there is little room between the thread minor diameter and the recess major diameter. To overcome this limitation, the asymmetric profile is constructed in such a way that when torque is applied, the key slightly pulls the recess radially inward, eliminating its radial dilation. This is achieve by employing a negative or undercut angle A, as depicted in figure 15. When recess and key are in contact, the radial component of the bearing force is directed towards the center of rotation.

Figure 16 shows the recess plastic strain distribution around the bearing contact area with the key.

Shallow Recess

As the bearing area is greatly improved with the asymmetric system, it requires a much shallower engagement when compared with other systems, such as the hex, as shown in figure 17.

The shallow recess helps the pin manufacturing and also the overall tensile properties.

RECESS INFLUENCE ON MECHANICAL PROPERTIES

The geometry of the asymmetric recess and its optimized torque effectiveness enable the mechanical properties of the fastener to either be unaffected or to be enhanced. The shallower recess results in a greater cross sectional area in the portion of the threads where the highest loading occurs. This moves the reduced cross sectional area to the lighter loaded area, resulting in lower principal stresses at the base of the recess (See figure 19 and figure 20). This enables a larger recess area, improving torque effectiveness while maintaining or even increasing tensile strength capability. Pins with conventional hex recesses tend to have the highest stress at the base of the recess. This shift of the location of the highest stress away from the recess is evidenced by the difference in failure modes for asymmetric recess pins compared to hex recess pins as shown in Figure 21. In addition to static strength enhancement, this recess also results in fatigue life comparable to or better than that of hex recess pins, with either all tests going to the endurance limit or failing in a portion of the pin away from the recess (See Table 3).

TORQUE TRANSFER CAPABILITY

With enhanced cross section and large bearing area, the asymmetric key/recess system has exceptional torque transfer characteristics as demonstrated by static torque tests, shown in table 4.

In these tests, pins are held stationary by their heads while torque is applied directly to the mating keys, on a fixture that prevents any spurious loading, as shown in figure 22.

When compared to other anti-rotation systems, the asymmetric recess shows noticeable improvement and is the only system that can withstand the seating torque of high strength nuts and collars, for all tested fastener sizes, as illustrated on figure 23.

DURABILITY TEST

As there is no industry standard covering durability or reusability of anti-rotation keys, these characteristics are often obtained by installing a large number of fasteners under controlled conditions. But this approach can be very inconvenient as it is sensitive to installation conditions and influenced by many factors that are difficult to control in practical conditions. In order to overcome these problems, an expedite test method that eliminate all external factors and allows for direct comparison between different anti-rotation key systems was devised.

This test consist in driving a nut on a pin, through a small, low weight test button, with all the torque applied by the socket being transmitted directly to the anti-rotation key, as shown in figure 24. This configuration eliminates the dependence that the torque transmitted to the anti-rotation key has on any external factors, such as thread friction, head friction, structure stiffiness, interference condition, tool operator, fastener wear, galling, sealant, etc.

The procedure is simple:

1. Locate fastener components (pin and nut) on test button

2. Engage on tool socket and key

3. Hold key engagement with hand (figure 25)

4. Apply prescribed torque

5. Remove fastener

In this test, the torque repeatability is controlled by the installation tool, and the same fasteners can be re-used as long as there is no noticeable recess damage or wear.

Since the principal objective of the asymmetric key/recess design is to develop enough torsional strength to withstand the seating torque of high strength fasteners in all conditions, the durability tests were conducted with target torques as listed on table 1. These are very demanding conditions and every single installation can be considered an overloading condition, as the key is experiencing the full seating torque of the fastener.

The asymmetric key/recess systems performed exceedingly well, easily achieving 1000 installations with a single key, for all sizes tested. Table 5 shows the results for durability tests performed in Titanium 6AL-4V pins with stainless steel nuts, for different sizes.

Minimal wear was observed in the asymmetric keys tested, with only witness marks present on the edge of bearing surfaces, as shown in figure 26.

SUMMARY/CONCLUSIONS

The asymmetric recess/key system presented in this paper was designed and engineered specifically to be used with aerospace pins, fitted into thread extremities. The asymmetric system has a superior torque transfer capability and can repeatedly withstand the seating torque of high strength fasteners, virtually eliminating the common installation problems observed with other anti-rotation systems. In addition, the asymmetric recess is configured in such a way that it does not influence the fastener's mechanical properties, enabling its use with existing types of aerospace pins with a seamless direct replacement of existing key/recess systems.

REFERENCES

[1.] Aviation Maintenance Technician Handbook - Airframe Volume 1, "FAA Airframe Handbook V1", Federal Aviation Administration, 2012.

[2.] Campbell Flake C Jr., "Manufacturing Processes for Advanced Composites", Elsevier, 2003.

[3.] Reiff John D., "A Procedure for Calculation of Torque Specifications for Bolted Joints with Prevailing Torque", Journal of ASTM International, March 2005, Vol. 2, No. 3

[4.] Bickford John H., "Introduction to the Design and Behavior of Bolted Joints", CRC Press, 2007

[5.] Pinheiro, R., Frias, E., Lantow, R., Olkowski, J. et al., "A New Generation Cordless Electric Tooling," SAE Technical Paper 2009-01-3268, 2009, doi:10.4271/2009-01-3268.

[6.] Pinheiro, R., Dibley, C., Olkowski, J., Lantow, R. et al., "A Quick Change System for Portable Fastening Tooling Systems," SAE Technical Paper 2009-01-3269, 2009, doi:10.4271/2009-01-3269.

[7.] Pin Fastening Systems, Alcoa Fastening Systems & Rings Aerospace, 2016. Web. 15 Mar. 2016

CONTACT INFORMATION

Rodrigo Pinheiro is a New Product Development Manager at Alcoa Fastening Systems & Rings. He can be reached at 310 997 5487.

Rodrigo Pinheiro, Robert Gurrola, and Sead Dzebo Alcoa Fastening Systems & Rings

Table 1. Example of seating torque values (lbf-in) for##
installing aerospace pins
with nuts made of different materials

Size   Al 7075  Ti 6AL-4V  A286

.1640  15-25    15-25      30-40
.1900  25-35    30-40      40-50
.2500  60-80    80-95      110-130
.3125  130-160  150-200    200-250

Table 2. Polar moment of inertia for different key systems

Fastener  Polar     Moment of Inertia([in.sup.4])
Size      Hex       Spline-Lok[R]                  Asymmetric

.1624     4.45E-06  5.72E-06                       6.06E-06
.1900     4.45E-06  1.22E-05                       1.23E-05
.2500     9.39E-06  3.07E-05                       3.35E-05
.3125     2.94E-05  6.41E-05                       6.45E-05

Table 3. Tension-Tension fatigue results for Titanium 6AL-4V pins with
Asymmetric recess, in different sizes

Size   MAX Load (lbf)  MIN Load (lbf)  (**) Avg. Fatigue Life (cycles)

.1640  715             71.5            130,000
.1900  1,050           105.0           130,000
.2500  1,950           195.0           120,331
.3125  3,140           314.0           111,339

Size   Typical failure Mode

.1640  No failure
.1900  No failure
.2500  Thread minor cross section
.3125  Thread minor cross section

Table 4. Max Torque for Asymmetric key/recess system with Titanium
6AL-4V pins with, in different sizes. Both installation##
(preferential) and
removal (reverse) directions.

                               Max Torque(lbf-in)

Fastener Size           .1640  .1900               .2500  .3125
Installation Direction  47.4   76.4                155.3  252.8
Removal Direction       37.5   53.1                115.8  202.2

Table 5. Installation and removal durability test##
results for Titanium 6AL-4V
pins with asymmetric recess, in various sizes

Pin Size  Requirement (# Cycles)  Seating Torque (in-lbt)

.1640     1000                    25
.1900     1000                    35
.2500     1000                    95
.3125     1000                    175

Pin Size  Installation Speed
          (PPM, est.)         Failure Description

.1640     10-20               No failure. Test discontinued.
.1900     10-20               No failure. Test discontinued.
.2500     10-20               No failure. Test discontinued.
.3125     10-20               No failure. Test discontinued.
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Author:Pinheiro, Rodrigo; Gurrola, Robert; Dzebo, Sead
Publication:SAE International Journal of Aerospace
Article Type:Report
Date:Sep 1, 2016
Words:3431
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