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Item type:Publication, Access status: without files , Friction induced dynamics of ball joints : instability and post bifurcation behaviorIt is well known that ball joints may be subject to friction induced oscillations. In the present analysis the stability behavior of a ball joint model with Coulomb and Stribeck-type friction is studied and the post bifurcation behavior of the resulting limit cycles is determined. The bifurcating limit cycles can be followed for both types of friction, and velocity dependent friction is not necessary to yield vibrations, as was proposed in the literature before. The linear instability is found to be of the whirl or flutter type. The results suggest that, depending on the ball joint and friction parameters, a complicated variety of nonlinear periodic solutions exists. It is shown that the previously often used approach of computing parameter values where stable sliding turns unstable can not even serve as a first rough indicator for the existence of friction induced oscillations. Instead, existence areas for stable/unstable sliding and co-existing stable limit cycles give a far better impression of the system characteristics, and can even give hints on measures that can help to eliminate friction whirl vibrations from ball joints. The analysis yields that a low friction coefficient, low loads and high system or interface damping seem promising to calm squeaking joints, as is physically plausible. Furthermore, the combination of the momentary speed of rotation and the relative component kinematics has a huge impact on the existence of limit cycles - hence the human gait is likely to influence the occurrence of squeaking in total hip arthroplasty.Publicationtype: Journal Article5 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Vibration transfer in the ball-stem contact interface of artificial hips(Elsevier, 2013-05-23); ; ; ; Audible squeaking has put the reputation of ceramic bearings for total hip replacements into question. Inter-articular friction induces vibrations in the ceramic head which are transferred through the head-taper interface to the femoral stem. If excited to sufficient amplitudes, squeaking can be emitted by the stem. Hence, the damping and amplification properties of this interface have a crucial influence on stem vibrations.The aim of this study was to determine the vibration transfer behavior between the head and the taper of a femoral stem and its dependence on the assembly force, in order to assess its influence on the development of audible squeaking. A ceramic head was assembled on a titanium femoral stem taper with high and low forces. Frequency response functions characterizing the head-stem interface were determined experimentally.The interface demonstrated negligible influence on vibration transfer in the squeaking frequency range (1-5. kHz). However its damping effect in lower and higher frequency ranges was significant and some areas of amplification were also found.Although squeaking vibration was not influenced by the particular interface studied, the method established can be utilized to trace taper designs with dynamic properties that decrease the susceptibility to squeaking. © 2013 IPEM.Publicationtype: Journal Article172 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Friction-induced whirl vibration : root cause of squeaking in total hip arthroplasty(Elsevier, 2012-01-10); ; ; ; Squeaking is reported for ceramic-on-ceramic hip arthroplasty, and risk factors leading to this phenomenon have been investigated empirically in the past, this way giving hints to when this phenomenon occurs. The aim of this study is to present an experimentally validated explanation for the dynamical mechanism underlying the squeak, i.e. a description of what happens when noise is generated.First the kinematics of the ceramic bearing couple in relative motion are reconsidered. The relative motion at the contact zone can be understood as superposition of relative rotation and translation. The relative weight of both components depends substantially on the instantaneous load vector, which primarily determines the position of the contact area, and the instantaneous relative rotation vector. For the investigated gait scenarios, both load vector and rotation axis vary strongly during the gait cycle.Second, experimental vibration analysis during squeak is performed. A pronounced micrometer scale elliptical motion of the ball inside the liner is found. It is shown that the rotational component of the relative kinematics during gait indeed leads to friction induced vibrations. We show that a generic whirl type friction induced flutter instability, also known from similar (non bio-) mechanical systems, is the root cause of the emitted squeaking noise.Based on the identified mechanism, the role of THA system parameters (materials, design), patient risk factors, as well as the role of the gait cycle, will have to be reconsidered and linked in the future to develop effective measures against squeaking.Publicationtype: Journal Article10 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , The influence of component design, bearing clearance and axial load on the squeaking characteristics of ceramic hip articulations(Elsevier, 2011-02-04); ; ; ; Squeaking of hip replacements with ceramic-on-ceramic bearings has put the use of this material into question despite its superior wear behavior. Squeaking has been related to implant design. The purpose of this study was to determine the influence of particular acetabular cup and femoral stem designs on the incidence of squeaking and its characteristics. The dynamic behavior of the stem, head and stem assembled with head was investigated by determining their eigenfrequencies using experimental and numerical modal analysis. Four different stem and three different cup designs were investigated. Operational system vibrations resulting in audible squeaking were reproduced in a hip simulator and related to the respective component eigenfrequencies. The applied joint load and bearing clearance were varied in the clinically relevant range.Stems with lower eigenfrequencies were related to lower squeaking frequencies and increased acoustic pressure (loudness), and therefore to a higher susceptibility to squeaking. Higher load increased the squeaking frequency, while the acoustic pressure remained unchanged. No influence of the clearance or the cup design was found.Stem design was found to have an important influence on squeaking characteristics and its incidence, confirming and explaining similar clinical observations. Cup design itself was found to have no major influence on the dynamic behavior of the system but plays an important indirect role in influencing the magnitude of friction: Squeaking only occurs if the friction in the joint articulation is sufficient to excite vibrations to audible magnitudes. If friction is low, no squeaking occurs with any of the designs investigated.Publicationtype: Journal Article7 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Noises originating from artificial hip replacements : where do they come from and how can they be prevented?Publicationtype: Journal Article3 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A numerical study on stick-slip motion of a brake pad in steady slidingA numerical model for an elastic brake pad sliding under constant load and with constant velocity over a rigid surface is investigated by finite element analysis. The geometry is taken to be two-dimensional, the contact is assumed to follow the laws of continuum mechanics and temporal and spatial resolution are such that dynamical effects localized at the interface are resolved. It turns out that at the contact interface localized slip events occur either in the form of long-lasting slip pulses, or in the form of brief local relaxations. Macroscopically steady sliding, macroscopic stickslip motion or slipseparation dynamics occurs, depending on the macroscopic relative velocity. While structural oscillations of the brake pad do not seem to play a significant role during steady sliding at least one structural oscillation mode becomes synchronized with the interfacial dynamics during stickslip or slipseparation motion. Assuming a given friction law for the interface, the macroscopically observed friction coefficient depends considerably on the underlying dynamics on the interface.Publicationtype: Journal Article5 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Deformation characteristics and eigenfrequencies of press-fit acetabular cupsBackground: Elastic deformation of press-fitted acetabular cups during implantation provides primary stability. Excessive deformation can lead to chipping or improper seating of ceramic inlays and is dictated by cup stiffness, which also affects its vibrational characteristics. Purpose was to investigate the influence of cup design on deformation during press-fitting and on vibration properties. Methods: Deformation of ten acetabular cups (with and without ceramic inlay) was tested for radial loads clinically occurring during press-fitting (0-2000 N). Eigenfrequencies were measured using experimental modal analysis and related to mass and stiffness. Findings: The first eigenfrequency of the shells varied greatly (4-9 kHz); insertion of inlays caused an increase (16-33 kHz). The range of shell stiffness was high (2.7-48.4 kN/mm), increasing due to inlay insertion (124.7-376.2 kN/mm). Stiffness and mass were sufficient predictors for eigenfrequencies (p < 0.001,R2 = 0.94). Interpretation: The cups investigated represent a large stiffness range. Lower cup stiffness can increase primary stability but jeopardize inlay seating, and a suitable balance must be achieved by the designer. Eigenfrequencies also decrease with decreasing stiffness but were all found to lie considerably above clinically observed squeaking frequencies, indicating that these cup designs play no predominant role in the squeaking phenomenon. The observed relation between eigenfrequencies and the quotient of stiffness and mass might be used in the development of new thin walled cup designs so that their contribution to system vibrations is prevented. Presently, surgeons should be aware of the deformation characteristics of cups in order to select a suitable press-fit magnitude.Publicationtype: Journal Article9 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Squeak in hip endoprosthesis systems : an experimental study and a numerical technique to analyze design variants(Elsevier, 2010-07-01); ; ; ; Hip endoprosthesis systems are analyzed with respect to their susceptibility to self-excited vibrations and sound or noise generation. Experimental studies reveal that certain configurations can become unstable causing exponentially growing regular high-frequency oscillations that asymptotically approach a limit-cycle with considerable amplitude. Ultimately the vibrations do also lead to the emission of sound that is perceived as squeaking or squeal. To identify dominant influence factors and critical parameters, stability analyses were conducted on the basis of finite-element modeling. The resulting numerical approach, based on the determination of complex eigenvalues and eigenvectors, is shown to be an effective tool to analyze and show differences between endoprosthesis designs with respect to their susceptibility to develop squeaking phenomenons. © 2010 IPEM.Publicationtype: Journal Article5 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , 6 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Friction-induced vibration of artificial hip jointsA hip replacement system is analysed with respect to its susceptibility to friction induced vibrations under laboratory conditions and in a pseudo-in vivo setup. The method of complex eigenvalue extraction within the framework of finite-element modelling and analysis is used to identify the geometric properties and system parameters that might lead to unstable sliding states. The approach presented reveals negatively damped eigenmodes for both the pseudo-in vivo setup and the not-implanted system under laboratory conditions without boundary conditions. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Publicationtype: Journal Article7