Design for Serviceability / Maintainability

Service Drivers

With more complex and expensive products, customers expect that their products can be used over a long period of time. As a result service may be required for the following reasons.

Planning

Design for Serviceability / Maintainability begins with understanding the customer needs related to availability, reliability and service expectations. From this requirements will be defined in terms of such factors as availability, mean time between failures (MTBF), mean time to repair (MTTR), lifecycle cost (LCC), warranty period, etc. This should then result in a general service strategy and plan for a product that would address such things as:

• Who will perform the service? Where will they be based?
• Where will the service be performed – onsite at the customer location, a service center or the factory?
• What is the level of repair strategy – dispose of a use a new product, repair to the field replace unit (FRU) level, repair to the shop replaceable unit (SRU) level?
• Where will spare parts be stocked – central location, distributed locations, customer sites?

Reliability and Durability

To the degree that the product is more reliable and durable, it will require less service. Techniques such as failure modes and effects analysis (FMEA) and failure reporting and corrective action systems (FRACAS) can be used to enhance reliability and durability. Design for reliability principles can be followed such as:

• Design based on expected range of operating environments
• Design to minimize / balance stresses & thermal loads
• Provide cooling to hot components
• Avoid stresses on boards & component leads due to flexing & bending
• De-rate components for added margin
• Provide critical subsystem redundancy
• Use more reliable / robust parts & materials
• Reduce part count & interconnections
• Provide lubrication to minimize wear
• Provide filtering to air, lubrication, fuel & hydraulic lines and subsystems

Diagnosibility

The first step in any service is to diagnose what the problem is. This is done by testing where a stimulus is applied to the system or to a module and the output is observed and checked to see if it conforms to test expectations or specifications. It the results are not to specification, diagnosis needs to isolate where the fault is. This requires visibility dorn to the replaceable item level (field replaceable unit or shop replaceable unit). To accomplish this it is important to measure or monitor the input and output of each module as illustrated in the diagram below.

The next issue to address is whether this diagnostic capability if provided externally (external test equipment) or internally (built into the system). The cost of electronics is reducing the cost to provide built-in self test (BIST) capabilities. Further, with more expensive products and where products have built in computing and communication capability, it make more economic sense to develop an internal diagnostic capability. This internal diagnostic capability relies on inputs from BIST, electronic signals and sensors to determine where a operating parameter is out of specification. This information is then considered through fault tree analysis to suggest what the most like fault and related maintenance action is. Further integration with operating history (hours of operations, cycles, miles, etc.) can improve the fault tree analysis.

Design for Serviceability Guidelines

The starting point is to consider the functional and physical architecture of the system. Based on knowledge of similar existing systems, what items are going to most likely fail or require service (inspection, adjustment, cleaning, consumable replacement, etc.). These items should be placed closer to the exterior of the system in the physical architecture of layout to minimize access issues.

The key design for serviceability (DFS) guidelines or principles are summarized below.

Simplification. Simplification is one of the most basic principles.  The benefits of simplification are:

• Fewer items to fail / wear out
• Fewer items to diagnose
• Less disassembly & reassembly effort
• Lower service parts inventory

Standardization. Standardization not only refers to parts used in the design but also to design approaches, service procedures and methods, and service tools. The benefits of standardization with parts and modules are:

• Standardized parts and modules can be bought or produced at lower cost
• Parts and modules produced in larger quantities generally have better consistency & quality
• More failure & reliability data for better service planning
• Better accessibility of replacement components; less inventory required to obtain the same spare parts stock-out protection
• Easier for customers and field service personnel to maintain inventory of common standard parts

Access. Provide access panels and hatches to gain access to items that need to be serviced and make it easy to open or remove access panels or hatches. Minimize unfastening and re-fastening effort with access panels by using hinged panels and hatches, quick release latches and levers, or integral attachment unfastening and fastening. For items on interior of the system, provide slide-out drawers or rails or put assemblies on hinges to swing out to improve access. Provide physical and visual access to items that need to be serviced with a minimal amount of parts and interconnections that need to be disassembled and removed.

Ergonomics. Consider ergonomics in gaining access for a maintenance task. Avoid the need to lean over, reach into, crawl on top of, crawl under, climb up, repeatedly reposition oneself, or work over your head. Avoid the need for many repetitive motions such as unscrewing many screws or bolts that can lead to strains.

Safety. Shield high voltage terminals or prevent high voltage access when the system is powered. Provide mechanism such as interlocks to insure power disconnected when product opened for service. If interlocks are not possible, rely on lock-out, tag-out procedures. Provide an easy way to bleed stored energy from any system before beginning a maintenance procedure. Avoid sharp edges or parts and protect service personnel from burns due to contact from hot parts by incorporating shields.

Disconnecting/Reconnecting. Minimize the number of connections between modules to facilitate disassembly & assembly. Provide access to disconnect and reconnect. Label and mark interconnections and connectors to insure that only the applicable interconnections are being removed and facilitate correct reconnection. Specify connectors that are easy to remove & reinsert, i.e., large grip surface, pull tabs, release levers, quick disconnect and connect features, etc. Use keyed connectors with external features to define their orientation or maximize connector asymmetry to facilitate proper orientation when re-connecting. Simplify the routing of interconnections; avoid routing that interferes with disassembly and re-assembly.

Unfastening/Refastening. Minimize the number of fasteners; use integral attachment features or features that can provide EMI shielding without a large number of fasteners. Use captive attachment hardware that require no tools for unfastening and fastening ease and avoids loose fasteners getting dropped or lost in the system. Specify fasteners that will be easy to remove as product ages, oxidizes or corrodes (e.g., stainless steel). Use common fasteners with readily available replacements. Use mechanisms that allow removal with a minimum of manipulation and time such as quick release latches and levers and quarter-turn fasteners. Provide access to place and manipulate tools.

Part Handling. Considering what parts or modules may need to be removed or re-assembled as part of service procedures, minimize negative handling characteristics such as very large size, very small size, weight, fragility (breakable, ESD sensitive, etc.), nesting or tangling, toxic materials, etc. Provide gripping features on parts that must be disassembled and re-assembled.

Location and Insertion. Minimize axes of re-assembly and need to reposition service technician; design for top-down assembly. Avoid blind or restricted vision when locating or inserting parts during re-assembly. Provide features to guide parts into proper position, e.g., self-locating features and features to facilitate insertion (chamfers, tapers, lead-in’s, guides, etc.). Provide features to align parts to one another during re-assembly. Minimize the number of surfaces or points that need to be simultaneously located. Maximize insertion clearance and minimize insertion or hold-down force.

Mistake-Proofing. Provide features on parts and modules that will only allow the parts or modules to be assembled one way – the correct way. Label, color-code or mark parts to facilitate correct disassembly and re-assembly. Make the design intuitive to facilitate correct dis-assembly, adjustment, and re-assembly. Use a service process FMEA to help identify potential mistakes or failure modes in a service procedure.

DFS Development Process

Service engineers need to be involved early in the development process to understand customer needs, participate in requirements definition as it pertains to service, availability of the system and reliability. They need to identify typical service issues for similar existing systems, provide reliability experience, and consider the implications of the physical architecture. Service engineers need to collaborate during detailed design to identify service issues and collaborate to develop solutions. As the design matures, they need to conduct virtual service procedures with CAD assembly models and later the physical service procedure with prototype or qualification hardware to identify specific service issues and suggest changes. As necessary, service may need to develop a business case to justify the added cost of hardware features that will facilitate service or maintenance.