When the FMEA or COFA is
finished, the maintenance tasks or actions must be chosen; to do it we will use
a logic tree.
To choose the more appropriate
task, the following factors must be considered:
1. Shall be technically feasible and worth doing, in accordance with the section
5.6.2 of SAE JA 1011. A task is considered as worth doing if it reduces (avoid, eliminate or
minimize) the failure consequences. The task must be technically feasible, applicable and effective in the
equipment work conditions.
2. Cost-effective, in accordance with section 5.6.3 of SAE JA 1011, if two or more
tasks are technically feasible the most cost-effective task shall be selected.
3. Probability of failure modes and age, the selection of tasks must
take account the fact that the probability of some failure modes shall increase
with age or that the probability not change with age, in accordance with section
5.6.1 of SAE JA 1011. To know if the failure mode ratio curve is random or
age-related is essential for the process.
4. Selection of failure management policy, a failure management policy
shall be selected if no specific task is currently being done to anticipate,
prevent, or detect the failure, in accordance with the section 5.6.4 of SAE JA
1011.
The next step is to design a logic
tree that categorizes the failure consequences and provides a selection of
failure management policies:
1. Evident failure mode with safety or environmental consequences, the task shall assess the risk
and implant tasks that reduce the probability of the failure mode to a level
that is tolerable to the owner or user of the asset, in accordance with section
5.7.1.1 of SAE JA 1011.
The standard SAE JA 1012 proposes
to use an on-condition based task, if an on-condition task is not technically
feasible and worth doing the standard proposed a time-based task, if it is not
feasible then proposes a combination of tasks and, finally, a redesign of
equipment.
2. Hidden failure mode with safety or environmental consequences, the task shall reduce the
probability of the hidden failure mode to an extent which reduces the
probability of the associated multiple failures to a tolerable level to the
owner or user, in accordance with section 5.7.1.2 of SAE JA 1011.
The standard SAE JA 1012
proposes to use an on-condition based task, if an on-condition task is not
technically feasible and worth doing the standard proposed a time-based task,
if it is not feasible then proposes a failure-finding task and, finally, a
redesign of equipment.
3. Evident failure mode with economic consequences, the direct and indirect costs
of doing the task shall be less than the costs of the failure mode, in
accordance with section 5.7.1.3 of SAE JA 1011.
The standard SAE JA 1012
proposes to use an on-condition based task, if an on-condition task is not
technically feasible and worth doing the standard proposed a time-based task,
if it is not feasible then proposes no scheduled maintenance but a redesign of
equipment is recommended.
4. Hidden failure with economic consequences, the direct and indirect costs
of doing the task shall be less than the costs of the multiple failure modes
plus the cost of repairing the hidden failure, in accordance with section 5.7.1.4
of SAE JA 1011.
The standard SAE JA 1012
proposes to use an on-condition based task, if an on-condition task is not
technically feasible and worth doing the standard proposed a time-based task,
if it is not feasible then proposes failure-finding tasks and, finally,
proposes no scheduled maintenance but a redesign of equipment is recommended.
COFA methodology takes into consideration the component classification.
For the Critical, Commitment, or Economics components an on-condition task, if this task is not
applicable or effective then proposes a time directed task, if this task also is
not applicable or effective then proposes to initiate a design change or accept
the risk.
For the Potentially Critical components
an on-condition task, if this task is not applicable or effective then proposes
a time directed task if this task also is not applicable or effective then
proposes failure finding task, if this task is not applicable or effective then
to initiate a design change or accept the risk.
Indicate that the Run-to-Failure
components not scheduled maintenance is done.
We have seen that the results of the logic trees can
be:
· On-Condition based tasks, or predictive tasks, they can be done without move
equipment, without stopping it or during a planned stop; they give information
about the failure with time enough to plan reparations.
· Time based tasks, or preventive tasks, they require to stop equipment and, usually, move
it to a workshop; this task acts overall components, even those that don’t
require maintenance.
· Failure finding tasks, their mission is to find hidden failures, but they assume that find
the failure is not immediate.
· Tasks combination, when to implant only one task is not enough to reduce risk to
reasonable values.
· Run-to-Failure, it is the best solution when the risk is under acceptable values, because
it uses all components of life.
· Redesign, they
can include equipment redesign as a process or operation redesign. Before proposing a redesign the complexity and time to implant them, the costs and the risk to no avoid the failure mode probability or consequence must be
considered.
