Reliability engineering refers to the term for the reliability given to work equipment or machinery before failure or breakdown occurs. For this, Reliability Engineering seeks to measure or quantify by means of numerical and probabilistic methods the risks in terms of costs of a stoppage or defect occurring, as well as the probabilistic estimate that any of the equipment or elements of a production system fails..
Basic concepts
Engineering
“Engineering is called the name of that discipline that deals with the study and application of the knowledge that results from this and the experience, so that through designs, techniques and problems, the different problems that affect to humanity." (Ucha, 2009)
Engineers are responsible for bringing the knowledge of mathematics and exact sciences to solve problems in society, in the same way they use this knowledge to create innovative inventions that seek to improve the quality of life of people and solve problems more quickly and efficient.
Reliability
"It is the probability that a team fulfills a specific mission under certain conditions of use in a certain period." (BS Group)
The reliability defined from a point of view to the maintenance area seeks to determine the duration or life time that the equipment has before a failure or defect occurs. The reliability of a piece of equipment can be measured from the failure frequencies, which will allow us to make failure predictions by calculating probability distributions.
Types of maintenance
Corrective maintenance
It is that maintenance that is dedicated to repairing all the failures or defects of the equipment that are presented on a day-to-day basis. This type of maintenance is one of the most common in small and medium industries, where there are no preventive controls or indicators or even lack of failure or breakdown history.
Preventive Maintenance
This type of maintenance seeks to have all the equipment at an optimal level, that is, it seeks to carry out maintenance schedules, which are previously planned, and maintenance of some equipment or machinery is carried out despite the fact that it does not present any fault symptoms. Mainly, this type of maintenance is followed by the indications of the suppliers, where the suppliers establish the duration or maximum estimated time in which a unit can operate with a guarantee.
Predictive Maintenance
This type of maintenance is more advanced, it seeks to anticipate maintenance by means of physical or mathematical models, in such a way that it is necessary to know the operability and current state of the equipment, where variables such as (vibration, temperature, consumption of power, revolutions RPM).
Zero Hour Maintenance (Overhaul)
This type of maintenance is preventive, the main objective is to leave the equipment with "zero operating hours", that is, all those elements of the equipment that are subject to wear and tear are replaced. This is done with the aim of ensuring the proper functioning of the equipment for a longer time and is executed through programmed time intervals.
Maintenance in Use
It refers to the maintenance of basic use that the equipment has, mainly carried out by the equipment operators. This type of maintenance tends to be very basic and instinctive such as visual inspections, cleaning, lubrication. It is possible that to carry out this type of maintenance, the operators will have to carry out a previous training on the use and maintenance of the equipment.
Maintenance Models
Unlike types of maintenance, "maintenance models" refer to the different strategies that are implemented for the implementation of maintenance to the different teams in an organization. It is very possible that the different equipment in the production area require an exclusive maintenance model or a combination of several of these models. It is important to identify which model or models apply to each of the company's teams.
Corrective Model
It is the easiest model to run and the most basic. It refers to all maintenance that is not a technical and economic problem for the company, since it can be carried out by the equipment operators. This type of maintenance can be visual inspections, cleaning and lubrication of the equipment.
Conditional Model
This type of maintenance is intended to carry out tests or trials in search of possible failures or indications that the equipment is not working correctly. In case of finding any indication of anomaly in the equipment, preventive maintenance or zero hours can be carried out in order to ensure that the equipment remains in correct operation. This type of model can be applied to equipment with little use, up to critical equipment in the organization.
Systematic Model
This type of model is applied mainly to medium-duty equipment, in which it is sought to have an algorithm of steps to follow and tasks to be carried out in the event of a failure symptom. It is possible that tests can be carried out to determine the degree of maintenance to be done, add more maintenance tasks or not according to the need or severity of the failure. This type of maintenance can be seen in the maintenance of vehicle engines, which depending on the mileage, one or more steps are carried out.
High Availability Model
This type of maintenance model is applied to all equipment that is critical, that is to say to equipment that has a high level of utilization (90% or more). Therefore, this type of equipment cannot suffer any type of breakdowns or failures, since that would cause serious economic losses for the organization. The maintenance of this equipment can be carried out in progress or in advance schedules, where a total and exhaustive review of the equipment is carried out, the ideas of "zero hours" are applied where all those elements that suffer wear are replaced 100%. It is worth mentioning that this type of maintenance does not necessarily have to be identical year after year.
Cost-Risk Analysis
Risk as an Indicator
The risk estimates are made through a probabilistic analysis, for this, samples of events or incidents are required with which the calculations on the probability of equipment failure or breakdowns can be made. The basic mathematical model of reliability is the following: (Probability of Reliability = 1 - Probability of failures).
Risk analysis allows organizations to make investment decisions. For this, two important aspects are taken into consideration: the risk curve (Risk = Probability of failure X Cost of failure) and maintenance costs. It is intended that maintenance costs are kept in balance with the costs of risk of failures or unscheduled stoppages, in such a way that the negative economic impacts on the organization due to equipment failures or breakdowns are reduced.
Maintenance Costs Chart
Reliability Indicators
- Average life: It refers to the time expected for a failure to occur in one of the non-repairable components (Example 5000hrs).Frequency of failures per year: Number of expected failures that a unit has in the year (Example 1 failures / year). Unavailability: Number of hours that the equipment is expected to be unavailable (Example 10hrs / year). Load Loss: Expected value not attended per year (Example 135.6W) Average Repair Time: Time expected for each repair (Example 7.5hrs /reparation) LOLE: Refers to the expected number of hours per year in which the demand will not be able to be met (Example 0.1hrs / year) Endurance: Number of operations that a contact, switch or disconnector can perform before entering its obsolescence period (Example 2500 operations).
Types of Analysis or Reliability Studies
Qualitative vs Quantitative
Qualitative: It is an assessment that is considered subjective, in this process indexes or numbers are not established. Examples (It will not fail, It is very reliable, This equipment is better than another). This type of analysis does not serve to make alternatives or make economic analyzes.
Quantitative: This assessment is objective, in this type of analysis indices are established that can be deterministic or probabilistic. Examples (Probability of failure of 80%, Reliability 65%, Margin of 30%).
Deterministic Vs Probabilistic
Deterministic: It refers to all those variables that can be calculated and are considered fixed. In order to determine their values, it is necessary to determine all the factors that model the system. This method eliminates the uncertainty of its values thanks to the use of equations that give certainty to the value of the parameters. Example (Pressure = Force / Area).
Probabilistic: All those values that are not precise and are expressed in the form of probability and the values that they express are considered random. These types of variables can determine the risk of an event occurring or not. (Example P. of failure where we have Z = 0.10%).
Analytical Vs Simulation
Analytical: A system is represented by a mathematical model (equation or set of equations) where reliability is evaluated and direct mathematical solutions are offered. Some of these tools use Block Diagrams or Markov Processes.
Simulation: It is evaluated numerically and the behavior of the system is simulated, evaluating the reliability indices. The results have to be run through specialized simulation software and the results obtained must be analyzed. Examples (Use of simulation programs such as Promodel or Simio).
Historical Vs Predictive
Historical: It is based on the performance data or historical indices. They are mainly established through descriptive statistics. Examples (Tables of frequencies, paretos, fault frequencies).
Predictive: Reliability values are determined by means of probability functions, either for an instant of time or future period. Examples (LOLE = 0.1 days / year, Expected time per outage = 4 days, Expected failure frequency = 5 shutdowns / year).
Probability of Failure Calculations
Data Analysis Process
Each of the equipment in the organization has its own risk of failure, this risk of failure can be determined thanks to the analysis of the failure history. We can consider a good failure history statistically reliable if the number of data is equal to or greater than thirty data (N> 30 data). In case we have a smaller amount of data, we increase the probability of bias in our results.
Counting on a good data history, we can continue to analyze the data. The data analysis process is as follows.
Data Analysis Process in Maintenance
The main objective of the data analysis process is to find the probability distribution that represents the behavior of the failures of the analyzed component. The most common distributions to find are the following:
- NormalExponentialUniformLognormalGammaWeibull
Normal Distribution (Example)
| Engine Failure Time | |||||
| 1.30 | 2.47 | 4.37 | 4.07 | 2.33 | 2.74 |
| 3.37 | 2.20 | 1.46 | 3.34 | 0.80 | 1.25 |
| 2.60 | 1.67 | 3.11 | 3.39 | 0.27 | 2.25 |
| 1.17 | 1.29 | 3.85 | 2.34 | 1.91 | 0.94 |
| 3.70 | 1.67 | 1.15 | 0.05 | 3.36 | 1.81 |
Histogram - Diagram and Distribution in Maintenance
The data are distributed under a Normal distribution with a Mean = 2,207 (years) and with a standard deviation = 1,144 (years).
- What is the probability that the equipment will fail in 4 years of use?
A: The probability that the equipment will fail within the first 4 years of use is 94.14%
- What is the probability that the equipment will fail in 3-4 years of use?
A: The probability that the equipment fails between 3 and 4 years of use is 18.55%
Maintenance Outsourcing
When an organization does not have all the knowledge or the means or tools necessary to be able to carry out maintenance on its own, it is required to hire a contractor external to the organization.
These external contractors can be other individuals or organizations, which have the characteristic of being specialized in some equipment and / or service. This maintenance can also be carried out by the same supplier or manufacturer which is trained or specialized in their products.
It is important that before using any maintenance service, you must be sure that the company or contractor that is going to enter has the sufficient technical knowledge and tools necessary to carry out the work. It is very possible that, to reduce the risk of a contractor failure, different percentages of bonds have to be agreed, this in order to assure the organization that the maintenance investment is insured.
One of the characteristics of this type of maintenance is that they tend to be an alternative that is generally expensive, mainly these types of maintenance should be avoided since it can generate a kind of dependence on a third party. The most advisable thing is that the company has resilience and little by little obtain the basic knowledge and tools to be able to carry out this maintenance the organization by its own means.
References
BS Group. (sf). Propellant Reliability Indicators in Maintenance Management. Retrieved on 12/16/2018, from bsgrupo.com:
García, LG (12 of 11 of 2014). Reliability engineering. Retrieved on December 16, 2018, from Gestiopolis:
renovetec. (sf). TYPES OF MAINTENANCE. Retrieved on 12/16/2018, from renovetec.com:
Ucha, F. (13 of 05 of 2009). Definition of Engineering. Retrieved on December 16, 2018, from DefinitionABC:
Zapata, CJ (2011). Reliability in Engineering. Retrieved 12/16/2018, from
