7.2.1 Establishing a Preferred
Parts List (PPL)
In the course of a design effort, equipment designers need to
select the parts and materials to be used to meet specified equipment
requirements for performance, reliability, quality, producibility and cost.
This selection task is greatly enhanced if the designer has a list of
preferred parts available to help in this selection process.
Preferred parts are those whose quality and reliability are
well-known to the industry, and are probably parts that the company is already
using in other equipments. Without a preferred parts list (PPL), designers may
tend to choose parts in haphazardly. The result is the uncontrolled
proliferation of parts throughout a manufacturer's product line, all varying
in performance and reliability. All potential candidate parts should undergo
an independent assessment before being placed on
the preferred parts list. Information sheets or data sheets from part
suppliers may paint an optimistic picture of the partís capabilities, but may
omit information regarding the partís inherent characteristics that are
critical to proper operation of the final product.
The absence of a PPL may have
wide-ranging consequences for manufacturing, purchasing, and logistics.
Manufacturing engineers may have to cope with parts that require a variety of
assembly methods and unique tooling. More inventory may be needed and, as a
result, inventory costs can mushroom out of control. Manufacturing automation
may also be adversely affected. Purchasing representatives may have to deal
with many different suppliers, making it hard for them to monitor quality and
timely delivery, and to obtain volume cost discounts. Logistics specialists
must now provide spares for many different parts, enter them into the supply
system, and find storage space for all of them.
Some consequences of designing
equipment without a PPL are:
||Proliferation of non-preferred parts and
materials with identical functions|
||Increased need for development and
preparation of engineering justification for new parts and
||Increased need for monitoring suppliers
and inspecting/screening parts and materials|
||Selection of obsolete (or potentially
obsolete) and sole-sourced parts and materials|
||Possibility of diminishing sources|
||Use of unproven or exotic technology
||Incompatibility with the manufacturing
||Inventory volume expansion and cost
||Increasing supplier base and audit
||Loss of "ship-to-stock" or "just-in-time"
||Limited ability to benefit from volume
||Increased cost and schedule delays|
||Nonavailability of reliability data|
||Additional tooling and assembly methods
may be required to account for the added variation in part
||Decreased part reliability due to the
uncertainty and lack of experience with new parts|
||Impeded automation efforts due to the
added variability of part types|
||Difficulty in monitoring vendor quality
due to the added number of suppliers|
||More difficult and expensive logistics
support due to the increased number of part types that must be
When a PPL is available at the beginning of the design process,
designers avoid using nonapproved parts and the laborious task of having to
supply engineering justification for their use.
Preferred parts databases help to flag obsolete parts and also
indicate a partís long term availability (i.e., how long a given part may
actually be available in the market).
The PMP must provide some level of standardization to minimize
the number of new parts entering the system/equipment, or the logistic support
and supply system, and yet still be flexible enough to effectively capitalize
on the advantages offered by alternative technologies. To be truly effective,
the PMP must first ensure that the parts selected will provide the necessary
level of performance and reliability over the projected life of the
system/equipment. It must also be tailored to the expected life of the
equipment to ensure, among other things, that replacement spares will continue
to be available throughout the effective life of the system/equipment. The PPL
should be updated periodically to ensure a proactive approach to minimizing
the impact of part obsolescence.