A kriging model for dynamics of mechanical systems with revolute joint clearances

Over the past two decades, extensive work has been conducted on the dynamic effect of joint clearances in multibody mechanical systems. In contrast, little work has been devoted to optimizing the performance of these systems. In this study, the analysis of revolute joint clearance is formulated in terms of a Hertzian-based contact force model. For illustration, the classical slider-crank mechanism with a revolute clearance joint at the piston pin is presented and a simulation model is developed using the analysis/design software MSC.ADAMS. The clearance is modeled as a pin-in-a-hole surface-to-surface dry contact, with an appropriate contact force model between the joint and bearing surfaces. Different simulations are performed to demonstrate the influence of the joint clearance size and the input crank speed on the dynamic behavior of the system with the joint clearance. In the modeling and simulation of the experimental setup and in the followed parametric study with a slightly revised system, both the Hertzian normal contact force model and a Coulomb-type friction force model were utilized. The kinetic coefficient of friction was chosen as constant throughout the study. An innovative design-of-experiment (DOE)-based method for optimizing the performance of a mechanical system with the revolute joint clearance for different ranges of design parameters is then proposed. Based on the simulation model results from sample points, which are selected by a Latin hypercube sampling (LHS) method, a polynomial function Kriging meta-model is established instead of the actual simulation model. The reason for the development and use of the meta-model is to bypass computationally intensive simulations of a computer model for different design parameter values in place of a more efficient and cost-effective mathematical model. Finally, numerical results obtained from two application examples with different design parameters, including the joint clearance size, crank speed, and contact stiffness, are presented for the further analysis of the dynamics of the revolute clearance joint in a mechanical system. This allows for predicting the influence of design parameter changes, in order to minimize contact forces, accelerations, and power requirements due to the existence of joint clearance.Fundação para a Ciência e a Tecnologia (FCT

Fear of Neighborhood Violence During Adolescence Predicts Development of Obesity a Decade Later: Gender Differences Among African Americans

Background: African American youth are more likely than other racial and ethnic groups to be obese. African American youth are also more likely to live in disadvantaged neighborhoods which increase their victimization, observation, and fear of violence. Objectives: This study tested if victimization, observation, and fear of violence in the neighborhood during adolescence predict trajectory of body mass index (BMI) in the 3rd decade of life in African Americans. Patients and Methods: Data came from an 18-year community-based cohort. We used multi-group latent growth curve modeling for data analysis, considering neighborhood violence at age 15 (i.e. victimization, observation, and fear) as predictors, and the linear slope for the average change in BMI from age 21 to 32 as the outcome, with age and socioeconomic status (i.e. intact family and parental employment) as covariates. Results: Fear of neighborhood violence at age 15 was predictive of an increase in BMI from age 21 to 32 among female but not male African Americans. Victimization and observation of violence at age 15 did not predict BMI change from age 21 to 32 among female or male African Americans. Conclusions: Fear of neighborhood violence is a contributing factor to increased risk of obesity for female African American youth who live in disadvantaged areas. This finding has implications for prevention of obesity among African American women who are at highest risk for obesity in the United States. Initiatives that enhance neighborhood safety are critical strategies for obesity prevention among African American women

Compliant contact force models in multibody dynamics : evolution of the Hertz contact theory

Over the last decades, several compliant contact force models have been proposed. However, no complete and systematic comparison has been done on these models, which provides information on their range of application and accuracy for use in different contact scenarios. Thus, the selection of an appropriate model for a given contact problem is still an important and challenging issue to be addressed. The Hertzian contact theory remains the foundation for almost all of the available force models, but by itself, it is not appropriate for most impacts in practice, due to the amount of energy dissipated during the impact. A good number of contact force models have been offered that augment the Hertzian law with a damping term to accommodate the energy loss during the impact process for small or moderate impact velocities. In this work, the main issues associated with the most common compliant contact force models of this type are analyzed. Results in terms of the dynamic simulations of multibody systems are presented, which allow for the comparison of the similarities and differences among the models considered.Fundação para a Ciência e a Tecnologia (FCT) - DACHOR - Multibody Dynamics and Control of Hybrid Active Orthoses (MIT-Pt/BSHHMS/0042/2008), BIOJOINTS - Development of advanced biological joint models for human locomotion biomechanics (PTDC/EMEPME/ 099764/2008), SFRH/BD/40164/2007, SFRH/BD/64477/200

Spatial revolute joints with clearances for dynamic analysis of multi-body systems

This paper presents a new technique for assessing the influence that clearance of spatial joints has on the kinematics and dynamics of multi-body systems. The technique is developed for the revolute joint in which the basic elements are the journal and bearing. Under certain working conditions, these two mechanical elements collide with each otherbeing the contact treated as coupled forces. The forces between each pair of contacting bodies are equal and opposite, and the magnitude and direction are calculated based on the positions and velocities of the bodies, as well as on the geometry of the joint. A continuous contact force model is used to evaluate the contact–impact force. The proposed methodology is developed in the framework of the multi-body systems formulation; therefore, the forces developed during the contact between the joint elements are introduced into the systems’ equations of motion as external forces. A double pendulum is used as a numerical example to show the application of the proposed methodology, being the efficiency of the developed methodologies discussed in the process of the results obtained.FEDER - Project POCTI/2001/EME/ 38281Fundação para a Ciência e a Tecnologia (FCT

Dynamic behaviour of planar rigid multi-body systems including revolute joints with clearance

This paper deals with a general methodology to assess the influence of the clearance size and the friction coefficient on the dynamic response of planar rigid multi-body systems including revolute joints with clearance. When there is a clearance in a revolute joint, impacts between the journal and the bearing can occur, and consequently, local deformations take place. The impact is internal and the response of the system is performed using a continuous contact force model. The friction effect because of the contact between joint elements is also included. The dynamic response of the systems is obtained numerically by solving the constraint equations and the contact-impact forces produced in the clearance joint, simultaneously with the differential equations of motion and a set of initial conditions. Numerical results for two simple mechanisms with revolute clearance joints are presented and discussed. In the present work, the clearance size and friction effects are analysed separately. Through the use of Poincaré maps, both periodic and chaotic responses of the systems are observed. The results predict the existence of the periodic or regular motion at certain clearance sizes and friction coefficients and chaotic or non-linear in other cases.FEDER - Project POCTI/2001/EME/38281Fundação para a Ciência e a Tecnologia (FCT

A study on dynamics of mechanical systems including joints with clearance and lubrication

In this work a methodology for the dynamical analysis of mechanical systems considering realistic joint characteristics, namely, joints with clearance and lubrication is presented. For the case of the joints with clearance modelled as a contact pair with dry contact, a technique using a continuous approach for the evaluation of the contact force is applied, in which the energy dissipation in form of hysteresis damping is considered. The friction forces are calculated using a modified Coulomb’s friction law. For the lubricated case, the hydrodynamic theory for dynamically loaded journal–bearings is used to compute the forces generated by lubrication action. The numerical results point out that the existence of dry joint clearances causes high peaks on the kinematic and dynamic system’s characteristics due to contact–impact forces when compared to those obtained with lubricated model. The performance of the lubricated joint is closer to that of an ideal joint.FEDER - Project POCTI/2001/EME/38281Fundação para a Ciência e a Tecnologia (FCT

Financing of physical rehabilitation services in Iran: A stakeholder and social network analysis

Background: Inadequate financing is one of the major barriers in securing equitable access to high-quality physical rehabilitation services, without imposing financial hardship. Despite this, no sufficient attention has been paid to physical rehabilitation services and no specific financial resources have been allocated to such services in many countries including Iran. Owing to the fact that effective decision- and policy-making requires identifying possible stakeholders and actors and their characteristics, in the current study a stakeholder analysis and also a social network analysis (SNA) was conducted to identify the potential stakeholders and also their characteristics involved in physical rehabilitation financing (PRF)-related policies in Iran. Methods: The present study was performed in two phases. Firstly, semi-structured interviews and relevant document review were conducted to identify the stakeholders. Then, the position, power, interest, and influence of each stakeholder were determined using a web-based questionnaire. Secondly, SNA approach was utilized to map and visualize the interactions among stakeholders. Results: The findings showed that there are different stakeholders in PRF-related decision- and policy-making processes in Iran. In addition, the position, power, interest, and influence level of the identified stakeholders were varied. Moreover, although some stakeholders, like the Ministry of Health and the parliament have the highest level of power and position, they lack sufficient interest to participate in PRF-policies. Furthermore, SNA demonstrated that social network density was low, which indicates the lack of proper collaboration and interaction among the stakeholders. Conclusion: As many powerful and influential stakeholders had low interest levels to warrant participate in the FPR-related decision- and policy-making processes in Iran, employing careful and effective strategies, that is ongoing negotiations, receiving advocacy, and making senior managers and policy-makers aware can be helpful. © 2020 The Author(s)

Physical rehabilitation financing in Iran: a policy analysis using Kingdon�s multiple streams

Background: Adequate financing is a crucial function, securing that physical rehabilitation services (i.e., physiotherapy, occupational therapy, prosthetics and orthotics) are available with no financial hardship. Like many other countries, despite the adoption of various policies and strategies in recent decades, Iran enjoys no desirable physical rehabilitation financing (PRF). Accordingly, this qualitative study aimed to explore the PRF-related strategies and issues as well as their impacts on relevant policies in Iran. Methods: An analysis of PRF-related policies was conducted in Iran using semi-structured interviews and policy documents review. Purposive and snowball sampling techniques were employed to select key informants, including health-policy makers, civil society, rehabilitation-policy makers, university professors, and practitioners. Thematic analysis was used to analyze the collected data. The analysis was framed within Kingdon�s multiple streams. Results: The hindering factors for desirable financing were weak insurance coverage, lack of sustainable financial resources, fragmented financing, lack of split between provider and financer, high-cost of physical rehabilitation services, low engagement of relevant experts in policy-making processes, and corrupt activities. In the policy stream, the following factors were highlighted: involvement of sustainable financial resources, the use of external revenue sources, allocated resources� earmarking, the integration of the current funds to have better pooling, the use of incentive and timely payment mechanisms, the implementation of strategic purchasing principals, and the employment of effective rationing strategies. Moreover, parliament support, changes in administrations, international effects, pressures from interest campaigns and NGOs, and international sanctions were found as factors affecting the politics stream. Conclusion: The study findings revealed that a variety of national and international factors affect PRF-related issues in Iran. The recently enacted laws indicate that the PRF policies have already been on the national health political agenda. The study reflected the multifaceted nature of barriers to optimal PRF in Iran. © 2021, The Author(s)