Machine Designers Reference. J. Marrs
can be conducted in many ways, and the earlier the better. Some research methods are: reading books, searching the web, researching patents, attending seminars or tours, experimenting, and investigating the mechanical devices one is surrounded by every day.
Research of applicable theory, laws, codes, and standards is essential to any machine design project. Resources useful to this endeavor are textbooks, handbooks, trusted websites, and seminars. Research into existing devices can save the designer time and money. Commercially available solutions and items may be examined, as well as patents and mechanisms sourcebooks. Be aware that active patents must not be copied without proper licensing. Manufacturer’s websites can be a wealth of information, as are the following:
Occupational Safety and Health Administration Website: www.osha.gov
American National Standards Website: www.ansi.org
United States Patent and Trademark Office: www.uspto.gov
Machine Design Website: www.machinedesign.com
Research can also take the form of experimentation, taking measurements or simply ‘getting a feel for’ aspects of the device or its function. The following is a short list of common tools that have proven particularly useful to the author. This is not a complete list by any means.
Rule, or Scale: Scales are useful when measuring physical objects or environmental dimensions. A scale is particularly helpful when thinking about ergonomics, access, or visualizing physical part size.
Calipers: A set of calipers is useful for measuring or visualizing thicknesses, depths, or diameters. Calipers can take measurements where the traditional ruler cannot. Calipers can be purchased from any tooling supply company.
Force Gauge: A force gauge is essential for taking measurements, “getting a feel” for forces in your design, and thinking about ergonomics. Sizing springs and other light force generators is much easier when using a force gauge to physically measure the required force/load. It is also useful when simply feeling a given force to get a sense of what you are specifying. Force gauges can be expensive, but are extremely useful during the design process.
SYNTHESIS AND CONCEPTUAL DESIGN
Synthesis of elements of prior art is the ‘bread and butter’ of the machine design industry. Synthesis of existing designs is usually less costly and less risky than original design. Many machine design jobs require synthesis of existing methods, with adaptation or customization. To excel at synthesis, one needs to know about a large number of mechanical devices and methods and be capable of creative visualization. The topic of design is addressed in many great books. Consult the recommended resources for more information on design and the creative process.
It is usually beneficial to conceptualize more than one solution to the design problem and then choose between alternatives based on priorities and performance. One helpful tool for choosing between designs is a decision matrix, or Pugh matrix, like that shown in Table 1-1. In this example, the shaded fields are values that are entered. A weight value, typically between 1 and 10, is assigned each design specification or characteristic of the design. Each concept is rated on its ability to meet each specification. These values can be from 1 to 10, from 1 to 3, from zero to 3, or any meaningful assortment of numbers. For simplicity’s sake, Table 1-1 uses weights from 1 to 10 and ability values from 0 to 2. No zeros were assigned in this case because each concept that made it into the matrix met all criteria. Scores are then calculated for each entry by multiplying the weight by the ability value. The scores for each concept are then summed to give a total score for each concept. The highest score in this case is considered the best concept.
Table 1-1: Alternative Design Decision Matrix
Concepts can be broad or specific, and many choices between concepts must be made for most designs. At the very top level, a conceptual choice for an assembly machine may be the choice between a single machine and a group of modular machines that transfer in-process components between them. Another conceptual choice may be whether to use indexing motion or continuous motion for a machine. Conceptual decisions can also be as specific as deciding between an air cylinder system and a ball screw and motor system for accomplishing some movement. Some other common examples of choices made between concepts are: cam drive vs. servo positioning drive; gears vs. timing belt; photoelectric sensor vs. proximity sensor; bulk conveyor vs. nested conveyor; compliant tooling vs. compliant product holder; quality sampling vs. 100% inspection; rolling element bearing vs. plain bearing; etc. Decisions at all levels must take into account cost, risk, safety, ergonomics, reliability, maintainability, accuracy, etc.
DETAIL DESIGN AND ANALYSIS
This stage of the design process is where machine designers generally spend the majority of their time. It is during detail design that the materials and methods of manufacture are decided, the precision 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