Daniel P. Hess

Modelling nonlinear dynamics of bolted assemblies

Abstract The dynamics of threaded fastened assemblies represent a highly nonlinear constrained vibration problem that is nontrivial, but of considerable practical importance. In this paper, a dynamic model of a fastened assembly is developed which incorporates a dynamic fastener model with dynamic structural component models. Simulations from the model reveal that fastener placement in assemblies can be optimized to reduce vibration-induced loosening. This result is fundamentally significant since the current practice for placement and orientation of fasteners in assemblies is generally based on convenience or static load and strength considerations, and not dynamic considerations.

Direct Verification of Threaded Fastener Locking Compounds and Adhesives

This article explores the feasibility of a method to directly verify that thread lock compound or adhesive has cured sufficiently to provide secondary locking. The application of a torque after assembly and cure time was found to provide a viable verification test. Fastener material and coatings were found to significantly affect cure. For example, medium-strength anaerobic compound used with fasteners made of inactive material such as stainless steel had curing issues which prevented determination of a useful verification test torque. However, modifications such as higher-strength compound or primer can be introduced making this method viable even for fasteners made with inactive materials and coatings. This article outlines a process to implement this method in practice. This requires sample tests with representative product to determine a test verification torque. This process is particularly useful in identifying curing and locking performance issues, and provides guidance for modification so that the method can be successfully implemented in practice. Test results show that application of verification test torque with or without standard vibration test exposure does not degrade the locking performance.

Axial rotation mechanics in a cadaveric lumbar spine model: a biomechanical analysis

BACKGROUND CONTEXT Postoperative patient motions are difficult to directly control. Very slow quasistatic motions are intuitively believed to be safer for patients, compared with fast dynamic motions, because the torque on the spine is reduced. Therefore, the outcomes of varying axial rotation (AR) angular loading rate during in vitro testing could expand the understanding of the dynamic behavior and spine response. PURPOSE To observe the effects of the loading rate in AR mechanics of lumbar cadaveric spines via in vitro biomechanical testing. STUDY DESIGN An in vitro biomechanical study in lumbar cadaveric spines. METHODS Fifteen lumbar cadaveric segments (L1-S1) were tested with varying loading frequencies of AR. Five different frequencies were normalized with the base line frequency (0.125 Hz n=15) in this analysis: 0.05 Hz (n=6), 0.166 Hz (n=6), 0.2 Hz (n=10), 0.25 Hz (n=10), and 0.4 Hz (n=8). RESULTS The lowest frequency (0.05 Hz) revealed significant differences (p<.05) for all parameters (torque, passive angular velocity, axial velocity [AV], axial reaction force [RF], and energy loss [EL]) with respect to all other frequencies. Significant differences (p<.05) were observed in the following: torque (0.4 Hz with respect to 0.2 Hz and 0.25 Hz), passive sagittal angular velocity (SAV) (0.4 Hz with respect to all other frequencies; 0.166 Hz with respect to 0.25 Hz), axial linear velocity (0.4 Hz with respect to all other frequencies), and RF (0.4 Hz with respect to 0.2 Hz and 0.25 Hz). Strong correlations (R2>0.75, p<.05) were observed between RF with intradiscal pressure (IDP) and AR angular displacement with IDP. Intradiscal pressure (p<.05) was significantly larger in 0.2 Hz in comparison with 0.125 Hz. CONCLUSIONS Evidences suggest that measurements at very small frequencies (0.05 Hz) of torque, SAV, AV, RF, and EL are significantly reduced when compared with higher frequencies (0.166 Hz, 0.2 Hz, 0.25 Hz, and 0.4 Hz). Higher frequencies increase torque, RF, passive SAV, and AV. Higher frequencies induce a greater IDP in comparison with lower frequencies.

Tests on Loosening of Aviation Threaded Fasteners with Different Washer Configurations

This paper presents results from an experimental investigation on loosening of aviation threaded fasteners with internal teeth lock washers and plain washers. The tests are performed on a transverse test machine which provides dynamic shear. Five different configurations are tested: (1) only lock washer, (2) lock washer on plain washer, (3) no washers, (4) only plain washer, and (5) lock washer on hardened plain washer. The resistance to loosening of these configurations is assessed by comparing loss of preload versus cycles of dynamic shear loading. Internal teeth washers used alone under a plain nut without additional plain washer are found to provide the worst case with respect to loosening resistance. Use of a plain washer under the lock washer provides significant improvement in resistance to loosening. In addition to providing protection to clamped part surfaces and better load distribution in a joint, the data presented reveals the use of plain washers under a plain nut can provide improved resistance to loosening. Relative hardness of the components, e.g., plain washer compared to lock washer, influence the performance.

Threaded Fastener Secondary Locking Requirements

Secondary locking features in threaded fasteners are in widespread use in machinery, structures, and systems. In this paper, the mechanism for loosening of threaded fasteners is explained qualitatively and defined quantitatively in terms of the self-loosening moment inherent to threaded fasteners and external load-induced loosening moments. Equations for loosening moments are defined. This paper provides analysis which quantifies the locking action required to prevent loosening due to the inherent self-loosening moment and external load loosening moments in threaded fastener joints. This paper provides the basis and method for engineers to properly design or specify secondary locking features in threaded fasteners to provide sufficient locking and prevent loosening. Requirements for secondary locking feature moments are developed in terms of loosening moments and factor of safety. Test data and calculations are provided to substantiate the requirements. Examples are provided for common secondary locking features including prevailing torque and adhesive.

Ductility of Titanium Alloy and Stainless Steel Aerospace Fasteners

This paper presents results from tests aimed to assess the relative ductility of titanium alloy Ti 6Al-4V and stainless steel A286 aerospace fasteners of comparable size and tensile strength. A test procedure is developed, and tensile tests are performed on test fasteners. All test fasteners fracture in the threaded region. Elastic and plastic deformation at rupture are extracted from the resulting load versus displacement curves and used to compute the ductility index for each test fastener. The ductility index quantifies the relative ductility between the different fastener materials. The average ductility index for the titanium alloy fasteners is about one-tenth the average value for the A286 fasteners. In addition, the fracture surfaces of the titanium alloy test fasteners fracture perpendicular to the axis of tensile loading, whereas the A286 test fasteners fracture across three or four threads, which corresponds to about a 45° angle. Both the relative ductility index values and fracture surface characteristics indicate much less ductility in the titanium alloy fasteners. These results are not intended to discourage the use of titanium alloy fasteners but rather to provide additional data for use in proper joint design when the benefits of lower weight or extreme temperature use are required.

Effect of Reuse of Locknuts with Prevailing Torque Locking Feature

This paper presents results from tests on the reuse of locknuts with a prevailing torque locking feature. Aerospace and non-aerospace products are tested. Many qualification specifications provide reuse requirements for prevailing torque locking features in terms of unseated reuses. This paper provides data for comparison of locknut reuse for unseated and seated to preload conditions. All locknuts tested, except one, maintain required prevailing torque levels over 15 unseated reuses as typically required. The unseated reuse tests exhibit a moderate decrease in prevailing torque with reuse with the largest decrease usually occurring during the first reuse. The effect of seating to preload is a higher loss in prevailing torque compared with the unseated results. Reuse in seated conditions does not generally meet unseated requirements. Between 10 and 25% of the assembly prevailing torque is lost during initial seating to preload. This loss continues with reuse but at a lower rate. However, inconsistent fluctuations in prevailing torque are observed for tests with high preload of 85% yield strength of the bolt. For all the seated to preload reuse tests, the tightening torque to achieve a set preload increased with reuse, despite the corresponding reduction in prevailing torque. This phenomenon results from the breakdown of the solid film or as-received lubricant provided with the locknuts.

Engineering outreach web page

This paper describes an engineering outreach web page that was developed to aid high school students and others interested in engineering as a possible career. The page draws on input from engineers, engineering students and nonengineering students, and provides, in an informal and entertaining style, information about engineering, its various disciplines, education options and resources, engineering societies, employment resources and representative engineering student projects. This approach to outreach takes advantage of modern computer media technology and is more efficient than traditional outreach activities.

Threaded Fastener Locking With Safety Wire and Cotter Pins

This paper presents test results and analyses of safety lock wire and cotter pins with castellated nuts used as locking features in threaded fasteners. These mechanical components are used to secure hardware in aerospace and other applications as well as to assess whether a fastener has been tampered with. Tests are performed on aerospace threaded fasteners. Locking moments up to safety lock wire and cotter pin failure are measured. Calculations of locking moment to failure are provided and compared to measured results. For fasteners with safety lock wire, the angle of turn with applied moment is measured and computed. For fasteners with cotter pins and castellated nuts, the angles of turn from clearances and applied moment are computed. Since both safety lock wire and cotter pin with castellated nut applications result in some angle of turn, analysis for loss of preload with angle of turn is developed. Calculations show modest loss in preload for typical angles of turn for long bolts, but a significant loss of preload is found for short bolts.

Effect of Fastener Preload on Structural Damping

This paper presents results from an extensive set of frequency response measurements from a simple plate structure with a single bolt lap joint. The frequency response measurements are obtained from modal impact tests. Both damping ratios and natural frequencies are found to remain fairly constant for medium to high levels of bolt preload. At low levels of preload, damping ratios increase significantly by 25–75%, whereas natural frequencies decrease by only 1–2%. These results identify the monitoring of vibration damping as a means for detecting low preload and fastener loosening. Comparative tests with monolithic plate structures show the introduction of a single bolt lap joint results in an increase in damping and a decrease in frequency for the first three modes of vibration. Similar results are found for steel and aluminum plate materials with grade 5 and 8 fasteners.