MAINTENANCE POLICY: cirteria & strategies we use in making maintenance.

-> There are two types of maintenance:

1. CORRECTIVE MAINTENANCE (CM): maintenance intervention is performed after failure occurrence;
2. PREVENTIVE MAINTENANCE (PM): maintenance intervention is performed before failure occurence.

-> TYPE:

• CYCLE MAINTENANCE, TBM: PM actions are carried out at:
  • Set-times (clock based)
  • Fixed age/usage of the component (age-based);
• CONDITION BASED MAINTENANCE, CBM: PM actions are based on the condition of the component being maintened.

-> Involves monitoring of one (or more) parameters characterizing the wear process (e.g. crack growth in a mechanical component).

• PREDICTIVE MAINTENANCE: PM actions are based on the evaluation of the trend of one or more parameters, that are clearly linked to the wear process (normally thorough a mathematical model).

Accompanying Maintenance Measures:

• OPPORTUNISTIC MAINTENANCE: making maintenance actions on secondary components (e.g. replacements of secondary components), in correspondence of an asset/ machine stand-by, due to a corrective or preventive maintenance operation.
• PRODUCTIVE MAINTENANCE: set of continuous improvement actions, inspired by the JIT production “phylosophy” that can improve equipement reliability and maintainability.

Preventive Maintenance:

Cyclic Maintenance:

-> DEF: PM actions are carried out at set-times (clock-based) or at a fixed age/usage of the component (age-based)

-> requires statistical knowledge of the failure mode;

-> Well suited if:

• Failure rate has a well defined and growing shape (refer to the bathub shape);
• Cost of the TBM policy lower than the cost of the CM policy.

Condition Based Maintenance (CBM):

-> DEF: PM actions are based on the condition of the component being maintened.

-> Well Suited if:

• Cost of the TBM policy is lower than the cost of the CM policy;
• Cost of the CBM policy is lower than the cost of the TBM policy.

Analysis of Failure Times:

TTF or TBF (Time To Failure, Time Between Failures): it is the calendar time intercurring between 2 sequential failures (for a repairable/ not repairable entity);

OTTF or OTBF (Operating Time To/ Between Failures): time between 2 subsequential failure measured on the cumulated operating time (for a repairable/ not repairable entity).

• MTTF (Mean Time To Failure): mean time to failure of non repairable entities (can be evaluated considering both the calendar or the operating time, i.g. OTTF);
• MTBF (Mean Time Between Failures): mean time between failures of repariable entites (can be evaluated considering both the calendar or the operating time, i.e. OTBF)

System Availability:

Problem Setting:

-> Failures (or breakdown) leads to performance losses at each machine and, subsequently, at system level;

• The impact of failures can be different depending on the configuration of the system.
• DOWNTIME STATE: performance of the machine are lost (0 productivity);

-> The system availability can be evaluated by analyzing the configuratno of the system, based on the availability of each single machine.

-> We have to study the availability of the system starting from the availabilities of each component  (machine/ station).

Study:

• Analyze the function carried out by the machiens for the production capacity of the whole system.
• Evaluate the impac of unabailability of the machines on the availability/ unavailability of the manufacturing system, thus on the whole production capacity (in order to meet the demand).

Availability Analysis:

-> Devise the configuration fo the system (a Functional Block Diagram of the manufacturing system), in order to analyze how much the production flow is affected by failures:

• Helps planning a reference scenario where in the expected technological routings of the production mix are considered.

-> EXAMPLE:

-> Now the system is working at ist theoretical production capacity.

-> In this configuration we have a reduction of the production capacity.

-> The production capacity is equal to zero.

Performance Evaluation: State Space Method

-> HP:

• State of the system depends on the state reached by its machines.
• Each machine is working/ degrading independently of the others.
• States of the system are mutually exclusive.

-> The method can be used to provide a rough estimate of the expected systema availability & production capacity.

• Firstly introduced as a method for logical analysis of the functions played by a system of interest.
• Grounded on a logical analysis of the functions required by the “system of interest”.
• It prepare the ground for a quantitative evaluation.

-> IDEA: an event may happen/a state may be subsequently reached by each machine during a given time interval; then the state of the manufacturing system is defined by considering the functions relevant for the system under concern, based on the states reached by the machines => is possible to calculate the total production capacity actually/ effectively available due to the still functioning machines after a given time interval.

-> OBJ: provide a rough estimate of the expected availability/ production capacity of the system.

Steps:

1.The system is modelled through a state space table where:

• Each machine is assigned its own state indicator;
• System is assigned its own state indicator:

2.The expected system availability & production capacity is calculated based on the states of the system defined in the state space table.

Procedure:

1. );
2. );
3. Evaluate the probability of each state of the system:

4.Evaluate the production capacity due to each state:

5. Calculate the expected production capacity due to all states:

6. Calculate the expected system availability due to all states:

-> The probability of a state of the system is calculated based on a rough estimate of the probability of the state reached by each machine (alternative 1 – “operating times”):

📌Bernoulli distribution => We can modellate the use of operating times as probababiliti Sj=1 / 0

The availability depends on:

• Technological characteristics of product types (as materials to be manufactured on given machine types);
• Number of machines and their state (dependent on their characteristics as reliability and maintainability);
• Ability to schedule preventive maintenance;

-> It is difficult to evaluate the impact of unavailability of the machines on the availability/unavailability, thus the production capacity (i.e. to meet the demand), of the manufacturing system.