Akella S.R. Murty

Availability and maintenance cost optimization of a production plant

Discusses the relationship between plant availability and maintenance expenditure and their limiting features. Achieving higher plant availability always necessitates a higher maintenance budget and may not be economically feasible in many cases. Through a mathematical modelling the variation of net income with respect to plant availability has been studied and the limiting availability values have been established. Expressions for optimum availability and maintenance cost have also been obtained. An illustrative example has been worked out.

Condition monitoring strategy : a risk based interval selection

Abstract Condition monitoring (CM) has been widely applied in most of the modern process as well as manufacturing industries as a predictive maintenance tool for incipient fault detection and trouble shooting but the interval at which the measurements are to be taken to achieve the best results has not yet been rationally discussed. This paper suggests a method for designing condition monitoring measurement intervals for all types of machines, from commissioning to replacement, by considering various factors such as risk, interval availability, debugging, useful life and wear-out regions of the failure rate curve, as well as economic factors governing the replacement prospects of the machine

Reliability strength design through inverse distributions—exponential and Weibull cases

This paper illustrates the inverse reliability design techniques for exponential and Weibull stress-strength models. Expressions for factor of safety, mean strength and its variability have been derived for a given range of target reliability and external load conditions. In the case of exponential models a global solution has been found out whereas in the case of the Weibull model only some specific cases have been considered. The difficulty in obtaining a global solution in the Weibull model has been discussed with possible methods to attain it.

Inverse normal and lognormal distributions for reliability design

In a reliability design problem it is often the case that the design parameters are found after the design reliability, for which cost and availability of resources may be a deciding factor, has been fixed. For an accurate design it is necessary to calculate the design values as precisely as possible. It is often the case that tables do not give the desired accuracy in the inverse problem of parameter search. For the conventional search of parameters, readily available tables suffer from the disadvantage of their inability to provide suitable and accurate values where the inversion is in a very narrow limit. A direct inversion, by using an expression which can facilitate any desired accuracy, is a promise of this paper. The authors have developed inverse normal and lognormal distributions which find an application in a wide variety of problems particularly in designs involving steel and structural alloys.

Influence of entrained air on frequency response of hydraulic actuator controlled by open center three way spool valve

Abstract A theoretical investigation on the effect of entrained air frequency response of hydraulic actuator controlled by open center three way spool valve is made. It is shown that entrainment of air gives rise to additional non-linearity into the system equation. Both nonlinear and linearized system equations, derived in dimensionless form, are solved numerically and analytically respectively. Increase of air content or decrease of area ratio in presence of air increases the resonant peak amplitude and decreases the resonant frequency leading to a reduction in bandwidth. Similar effect is observed also when the process of change of state of entrained air is adiabatic rather than isothermal. Entrainment of air also reduces the allowable open loop gain making the stability in the closed loop inferior.

Machinery selection ‐ process capability and product reliability dependence

Reliability of a product is highly dependent on the process capability index of the manufacturing process. Discusses a mathematical modelling technique for deriving the relationship between the product reliability strength and the process capability requirement to meet the product reliability strength for different types of external stress/load distributions which the product undergoes in the actual working environment. Considers four cases of external stress distributions: normal, log‐normal, expotential and Weibull. These techniques can be applied effectively in industrial production plants while selecting machines with required process capability to meet the product reliability strength demand.

Mechanical/structural reliability evaluation through Esscher's method of approximation

Abstract A simple and effective computational procedure is presented for computing probabilities of failure of mechanical and structural systems. The method provides very close approximate solutions for well behaved linear limit state functions with independent normal or nonnormal, continuous or discrete random variables. But the random variables should have moment generating functions. This method is also applicable when the limit state function is a hyper sphere. Numerical examples are given including one for hyper sphere limit state.

Inverse distributions in reliability design—a revisit

Abstract This is a revision paper on inversion of normal and lognormal failure distributions. Higher order terms of the inverse series have been derived and the convergence possibility has been studied in detail. Limitations of the method are also provided. The design procedure of an I-section beam for the target reliability strength has been illustrated.

Special cases of generalized functions

The generalized function is an indispensable tool in the study of complex physical processes. Owing to its universal property it encompasses a wide range of mathematical functions and can be used for modelling complex physical processes. In this paper the authors demonstrate some special cases of generalized functions. The application of such special cases forms a subject of further study.

Study of Drift in an Asymmetric Linear Hydraulic Motor

It is well known that in a hydraulic servomechanism the ram exhibits in the open-loop a linear motion in addition to its usual oscillatory motion for a sinusoidal input. This phenomenon, called drift, gives rise to an apparent instability of the system and the study of the dynamic behaviour becomes difficult. A theoretical investigation into the causes and remedy of this phenomenon is made on a loaded system with an asymmetric linear hydraulic motor controlled by an open-centre three-way spool valve. The original non-linear system equation in dimensionless form is derived taking into account the effects of leakage, compressibility (both fixed and varying) and area ratio, and solved numerically by the fourth-order Runge–Kutta method. It is shown that the determination of the reverse flow boundary is essential prior to the study of drift, since the rate of drift will be maximum near the boundary frequency. It is also shown that while the effect of compressibility due to the change in initial volume is mainly responsible for drift for an area ratio of 1 : 2, the effect of area ratio itself is predominant for an area ratio not equal to 1 : 2. It is observed that in most of the cases the drift can be removed by giving a proper input shift to the valve. A method of calculating the input shift required is suggested and the effects of different system parameters on its magnitude are studied.