Beyond the part reliability specification, you may also add conditions or requests to your reliability specification for your supplier.
The communication with your supplier should include sufficient information that they fully understand your reliability performance expectations. When buying or contracting with a supplier, you are the customer.
Be clear about your reliability requirements including constraints and conditions.
If our product has different phases of use such as dormant and active phases, the environmental stresses will likely change.
If the product has different modes of operation or critical function elements, those may have unique reliability requirements. If the requested subsystem or component impacts system performance you may request diagnostic, repair conditions, or failure conditions.
Let’s explore a few of the additional conditions or constraints that may accompany your reliability specifications to your supplier.
Reliability during specific phases of operation
The reliability expectation may change during specific phases of product operation.
For example, you may have a very high-reliability performance target for a life support medical device in use. Whereas, when the device in calibrated or in standby mode, the consequence of product failure is not as severe. Aircraft system critical to flight may have different reliability objectives when on the ground or during flight.
It may be desirable to include a failure-free requirement. This is technically not possible as there will always be a finite chance of failure, therefore include a very high probability of success (reliability) objective for definable phases of operation.
Include definitions of the specific situation for an altered reliability and duration requirements. Include the changes to environmental and use conditions, too.
The specific reliability requirements may apply to the entire item in question or just to select functions.
Reliability during specific phases of the product lifecycle
Some products have unique reliability requirements over their lifecycle.
Transportation, storage, use, and retirement all may include unique conditions and expectations. For example, weapons systems and medicine delivery equipment may have long dormant periods followed by a very high-reliability requirement for a short duration.
Robustness and damage tolerance requirements
Do you expect the component to continue to operate even when damaged?
Be specific about the nature of the applied stresses, the nature of expected damage, and which specific functional requirements should remain active. Damage to a portable radio antenna may degrade the transmission range, for example, yet it still works.
Be specific as possible about acceptable degradation, if possible.
Product packaging should continue to protect the product within, even after drops, exposure to rain, etc. A passenger vehicle is able to move, albeit slowly, even after a tire failure.
In each case, the package and wheel assembly withstand some damage yet continue to provide some level of functionality.
Redundancy or fail-safe requirements
When the reliability requirement for the supplied parts exceeds the technical ability to meet the requirement, we often use redundancy to meet the requirements.
Providing a critical function with two or more elements in parallel permits an item to fail while the remaining elements provide the necessary functionality.
Hot swappable power supplies is an example. One supply provides power to the system while the other awaiting replacement. The ability to replace or repair an element may require unique functional specification, along with reliability requirements.
For example, the failed item may have a reporting or enhanced diagnostic set of requirements.
On failure conditions
Not all failures are pleasant when they occur.
Some may create an unacceptable condition or may damage other nearby equipment. For example, wafer fabrication equipment upon failure may not emit smoke or gasses, as it will contaminate wafers. In general, we desire failures that do not cause explosions or fires.
Beyond the safety issues, when an item fails you may desire the element to fail safe, such that is will incur no additional damage, or cause any damage to surrounding equipment. You may also desire the failed item to communicate the status change or facilitate diagnosis for faulty elements.
You may want specific service and repair activity from the supplier, include that within the requirements document.
Failures do occur. Be specific what action(s) you expect when it occurs.
Failure anticipation and maintenance inspection requirements
Since failures are going to occur, you may want to know when a failure is about to occur.
You may require alerts, warnings, or signals to permit the anticipation of impending failures. You also may want to have the element run regular self-diagnostic checks and report the results or just significant changes.
The use of prognostic health management permits the equipment to monitor and alert the need for maintenance or repair before the causing overall system failure.
The gas gauge on a passenger vehicle, with a low fuel warning light, attempts to warn the driver of the impending out of fuel failure of the system.
Degradation or graceful failure expectations
Some products fail gradually.
LED lights lose intensity over time. Paints and coating may change color with exposure to ultraviolet radiation (sunlight). The degradation may erode a performance parameter. Colors may shift to an unpleasant hue. If the technology is prone to degradation, you may specify the nature of acceptable degradation.
For mechanical systems, a failed gear or support may lead to catastrophic or sudden loss of motion.
In some systems, the ability to gracefully slow down, for example, is desired. In a printer, a paper jam should signal a stop of paper handling motion, plus permit the rapid clearance of the jam and restart. For a vehicle, the loss of engine power should not cause a sudden lock of the drive wheels, rather the ability for the driver to both continues rolling, braking, and steering.
The reliability specification you provide your suppliers is more than just a target reliability performance value.
It may also include conditions and constraints that help the supplier fully understand the reliability performance expectations.
How do you specify your reliability requirements to your supply chain? Are there other conditions or constraints we should add to this article? Add your comments and suggestions below.
How to Connect Reliability Goals to Business Objectives (article)
Establishing Part Specific Reliability Specifications (article)
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