locating configuration is designed, fasteners are selected, the forms of the parts take shape, surface finishes and treatments are specified, and the design is analyzed in detail regarding strength, function, safety factors, and compliance with design specifications. Consider materials and methods of manufacture early in detail design, because both influence things like joint configurations and part shapes. Sizing and analysis should always be conducted using factors of safety. Machine performance and reliability should be planned for, and analytical techniques like Failure Modes, Effects, and Criticality Analysis (FMECA) should be undertaken in the detail design phase if not earlier. FMECA is covered in Chapter 12. The following are some best practices for detail design:
ASSEMBLIES AND SYSTEMS
•Consider accuracy, stresses, failure modes, wear, environmental exposure, operating temperature, life expectancy, spare parts, assembly procedure, alignment, and maintenance early in the design process.
•Design one or more forms of overload protection into machinery to reduce the consequences of a jam or crash.
•Use the smallest number of fasteners required for the application. The use of three screws instead of four, for example, reduces cost and assembly time. Use the same size screws wherever possible in an assembly to save cost and ease the assembly process.
•Always allow room for assembly, service, and adjustment. Tools, hands, and line of sight must be accommodated. If possible, design such that systems can be assembled and serviced from one side. Provide inspection points with easy access for all critical or at-risk components.
•Plan for insulation against galvanic corrosion when fastening dissimilar metals. Many coatings are capable of insulating materials against galvanic action. Helical coil threaded inserts with special coatings can act as galvanic insulators when a screw and part are made from dissimilar metals.
•When the orientation of a part at assembly is critical, design the parts so that they cannot be assembled incorrectly. The Japanese term for this is “Poka-yoke,” which means “mistake-proofing.” This is often accomplished by making the mounting or locating holes asymmetric.
•Use kinematic principles to exactly constrain parts. Never overconstrain parts.
•For easier setup, design an assembly such that each alignment direction is isolated from the others. This is an extension of the exact constraint principle in which a single-degree constraint prevents movement of a part in only one direction. Separate constraints, independently adjustable (if adjustment is needed), should be provided for each degree of freedom.
•Design setup gauges for your assemblies if alignment is critical. Make setup gauges open-sided so that parts can be pushed into the gauge along each adjustment direction. Avoid setup gauges that require that a pin or feature drop into a hole because these tend to be much harder to work with.
•Keep the ratio of length divided by width (bearing ratio) of all sliding elements above 1.5. This will maximize precision, reduce wear, and reduce the tendency to jam.
•Design assemblies to use stock items like commercial parts, or use parts common to other assemblies. This will usually shorten design time and save fabrication cost.
PARTS
•Consider the method of manufacture, stresses, failure modes, wear, environmental exposure, and life expectancy early in the design process.
•Place material in line with the path of forces through a part or assembly. Remove material that does not see any stresses, if weight is more critical than cost.
•Design symmetrical parts when possible. Avoid asymmetric parts that look symmetric, because this can cause assembly mistakes.
•Design parts for multiple uses when possible. For example, instead of a left and right gib in an assembly, it will be more cost effective to use two identical gibs.
•Use purchased or standardized components whenever possible to save cost.
•Specify through holes instead of blind holes where possible unless the part thickness is extremely large. Through holes are often more cost effective than blind holes, especially if threaded.
•For cost and weight reduction, apply liberal tolerances and rough surface finishes where possible.
•Minimizing setups during machining will save cost. If possible, design parts to be machined from one side only.
•Design parts to match existing stock material sizes when possible to save cost. Reduce machining when possible.
•When a part is to be mass produced, consider near-net-shape casting as a cost effective alternative to machining.
•To prevent interference, chamfers should be applied to outside edges that fit snugly into machined pockets in other parts. Use small chamfers with loose tolerances to reduce cost.
These tools, in addition to those mentioned earlier, have been useful to the author during detail design:
“Feeler” or Thickness Gauges: A set of feeler gauges is useful for measuring and setting gaps during set up and maintenance of tools and mechanical devices. They are also useful in the office for visualizing sheet metal or other thin parts. The stiffness of stainless steel sheet metal of various thicknesses can be understood by playing with a stainless steel “feeler” gauge of the selected thickness. “Feeler” or thickness gauges can be purchased at most tooling or automotive supply companies.
Surface Finish Comparator: Several manufacturers offer a set of surface finish comparators with samples of common surface finishes arranged on a card. These are useful for understanding the look and feel of the different surface finishes.
Screw Selector Slide Chart: These slide charts are available from a variety of manufacturers. They are a convenient and fast way of looking up fastener dimensions and related information.
FACTORS OF SAFETY
Factors of safety in machinery design are used to represent the risk of failure of a component, part, or system. Factor of safety of a part, device, or system is its theoretical capacity divided by the maximum of what is expected. In machinery design, factor of safety is often defined as the maximum safe load (or stress) for a component divided by the expected maximum load (or stress) on the component. It can also be expressed as a maximum safe speed divided by the maximum expected service speed, maximum overturning moment divided by expected moment, or some other measure of failure or risk.
Sometimes factor of safety is dictated by laws or codes. When the designer is free to set a safety factor, some common values are provided in Table 1-2. It is customary to assign higher safety factors in situations where risk or uncertainty is higher. It is also customary to assign factors of safety to brittle materials that are double that for ductile materials. Higher safety factors generally result in designs that are heavier, larger, more costly, and more powerful. In cases where this must be avoided, safety factors must be kept relatively low and steps taken to reduce uncertainty to a level where the lower safety factor is acceptable. For reference, light industrial machinery is often designed with a factor of safety around 2. Critical components like bearings are often designed with a larger factor of safety, commonly between 3 and 5.
Table 1-2: Common General Factors of Safety
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Safety Factor
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