Dynamic Processes in a Spheroidal Seismic-Damping Mechanism

2007

Self Cite

An approach to the design of models of dynamical systems with high dry friction in the kinematic pair is developed. The members of the kinematic pair are represented by parts of rigid bodies. The system as a whole is considered to have a variable structure. According to this assumption, two modes of motion with different dissipative characteristics are possible. The states in which the modes exchange and the motion switches over into critical modes with dynamic self-locking are established. A system with a variable transfer function between members that form a nonideal constraint is described Introduction. Dry or mixed friction is to some extent manifested in movable joints (kinematic pairs) of real mechanisms. However, these forces are in most cases low in magnitude and have an insignificant effect on the motion and loading of mechanical systems. There are mechanisms in which the influence of forces of friction is significant or even governing. These are, for example, self-locking mechanisms: wedge (Fig. 1a), worm, screw, some differential, planetary, geared linkage, differential cam, link, and other mechanisms [2,6,8,10]. The fundamental feature of self-locking mechanisms is that the driving and driven members cannot exchange their functions-such mechanisms can be set in motion by applying a force to the driving member, whereas any force acting on the driven member does not activate the mechanism [1]. There are critical values of the geometrical, inertial, and energy parameters at which such mechanisms (even with a one-degree-of-freedom kinematic pair) exhibit paradoxical behavior first described in [9] in 1895 and known as Painlevé paradoxes. Later, prominent scientists of the day (Prandtl, Lecornu, de Sparre, Klein, Hamel, etc.) resolved these paradoxes by showing that in some cases it is necessary to take into account elastic strains in contacting solids [9, Discussion] or to use intermediate, rather than limiting, values of the coefficient of sliding friction [3,9], according to the essence of the dynamic process. The heightened interest to models with friction is objective [13,16] and, in particular, associated with the design of seismic isolation systems for buildings [11,12,14,15], one such system shown in Fig. 1c. The transfer function u ba of this mechanism (ratio of the velocities of its members: foundation (a) and building (b)) is a variable function of their relative position, and self-locking may occur in some range of generalized coordinates. A similar behavior is typical of many other (cam (Fig. 1b), link, etc.) mechanisms [4,6].This paper develops a mathematical model of two-or three-link mechanisms with a nonideal holonomic constraint, gives the main relations for mechanisms with a nonlinear transfer function u q ba a 1063-7095/07/4305-0554

On models of dynamic systems with dry friction

2007

Self Cite

Dynamic response of objects with shock-absorbers to seismic loads

Timokhin

2007

Self Cite

The paper outlines a mathematical model describing the vibrations of buildings and engineering structures with general-type passive shock-absorbers, rigid bodies, and ideal constraints. Two modifications of systems with passive shock absorbers are considered assuming constancy of their structure. These systems are studied numerically; the dynamic processes excited in them are compared Introduction. Considerable damage and numerous victims of earthquakes challenge researchers to look for ways of minimizing the effect of such phenomena. The design of buildings is improved to enhance their capability of resisting loads due to vibrations of the foundation (ground). Dynamic loads on various mechanical systems can significantly be reduced by using shock-absorbing devices with flexible, frictional, and other elements [7,[13][14][15]. Currently, various shock-absorbers are intensively used to reduce seismic loads on buildings and above-ground structures [2,9,11,16,17]. Relevant research efforts are important because of the destructive earthquake in Japan (Kobe, 1995) and, for example, the recent (2005)(2006) earthquakes in Indonesia, Thailand, China, Turkey, Iran, Russia (Koryakiya), Pakistan, and other regions of the globe. This problem is also of importance for Ukraine, especially for the Crimea, the Carpathians, and adjacent regions. There has been a recent resurgence of interest in new designs and structures of seismic dampers with high damping capabilities owing to rubber and plastic elements and coatings [11]. Despite the different designs of the passive shock-absorbers to be discussed below, their mathematical models in a general dynamic system have a stereotypic description for many modifications.This paper outlines a deterministic model of a system with general-type passive seismic dampers and compares two modifications of this kind of shock-absorbers to assess how they reduce the inertial effect of an earthquake on a shock-mounted body (building or engineering structure). All components of the system are considered rigid. The translational motion exciting the system is considered kinematic and is specified as time-dependent accelerations. It is adopted that the maximum accelerations range from 0.2 to 0.4 m/sec 2 , the duration of excitation from 10 to 40 sec, and the dominant periods in the excitation spectrum from 0.1 to 2 sec [4,6]. The constrains are assumed ideal, i.e., the work done by constraint forces against a possible displacement is zero. Moreover, the constraints among bodies are considered to be ideal, holonomic, and bilateral. As possible examples, we will use the systems shown in Fig. 1 [2 , 8, 9, 17], where component 1 is the body (foundation) undergoing oscillatory motions with seismic acceleration W x x = && 1 , and component 2 is the body (engineering structure) subject to the dynamic load from the foundation. The contacting surfaces of bodies 1 and 2 (Fig. 1a) or bodies 2 and 3 (Fig. 1b) are represented by circular arcs of radii R and r (in the plane of the figure); other curves can also be u...

Dynamic-model reduction for a multimass system with nonideal constraint in a seismic damper

Timokhin

2008

Self Cite

A dynamic model for a multimass system with a seismic damper as a nonlinear inertial component is reduced via balancing. The seismic damper is subject to a nonideal constraint. As an example, the order of the system of differential equations is reduced by one for a dynamic three-mass system. A comparison of the processes in the original and reduced systems confirms that it is possible to use balancing to simplify such models Introduction. Most kinematic pairs are subjected, to a greater or lesser extent, to friction, which in certain cases has a strong effect on dynamic processes in various engineering devices, machines, and mechanisms [2,8,10,14,18]. The friction in seismic dampers and structural elements of buildings and other structures contributes to the damping of seismic vibrations and to the decrease of the extreme dynamic loads [9,11,12,15]. Theoretical studies of dynamic processes play an important role in designing seismic isolation systems. This is why there is an objective need for mathematical models that would adequately describe their major properties and meet the requirements of numerical integration (see, e.g., [17]) such as that the system of differential equations must not be stiff [5,6,16,19], which frequently leads to the need to reduce the system.Due to the increasing interest to the seismoprotection of buildings and other structures [9, 20], we will outline the model of an in-line system that includes a passive seismic damper as a nonlinear inertial via balancing [16,19] for not only linear and nonlinear systems [5,13], the latter including inertialess and inertial components, but also systems that have a nonlinear inertial component with a nonideal constraint. The balancing method is detailed in [5,6,16,19].It is natural that the approach outlined here can be applied to not only buildings subject to seismic effects, but also mining, metallurgy, and farm machinery, machining tools, etc. subjected to their typical loads. Mathematical Model of a System with a Seismic Damper that Includes a Nonideal Constraint.First, we will briefly outline a model for a system with seismic dampers that include ideal constraints. With such an assumption, the reactions R i at the contact points between the bodies I and O are normal to the surface, i.e., the angles of friction r i between the normal and total reactions R are zero (Fig. 1).The body O undergoes plane translation and has only one degree of freedom because two of the three possible degrees of freedom of a free body in plane motion are excluded by the two normal constraints to the surfaces at the contact points between the bodies I and O. Each additional damper of the same shape and size (third, fourth, …, nth) would introduce a repeated (passive) constraint [4] not affecting the law of motion of the system, but making it (singly, doubly, (n -2)-tuply) statically indeterminate. The reactions in each damper cannot be found through kinetostatics. For the translating system in Fig. 1, it is convenient to use two equations of motion of the body O and an ...