not ready until heater temperature setpoint is reached
•Interlocking guard door for safe operator access
•Heater position error shuts down machine
•Conveyor jam sensing
•Operator panel must have machine performance data.
SAFETY REQUIREMENTS:
•Standards: US, EU
•Company requirements
°Fixed or interlocking guards around perimeter
°Roof on guard enclosure
°Noise level limit at operator position 80dB
°Warning label attached to thermal hazard
°Light tower 9 feet off the ground featuring green, yellow, and red at the top
SPECIAL REQUIREMENTS:
•Must allow conversion to run product 162B with 1-hour changeover time
•Operator must be able to access heater chamber from front AND back of machine.
The functional design specification is an organized list of what is known about the machine at this early stage. It should contain all the known constraints and expectations, but should be written in a way that avoids placing new constraints. For example, the product conveyance subsystem is not called an indexing conveyor here because it has not yet been decided what form the conveyance should take. An indexing conveyor is most likely, but at this point in the design process it is still open for consideration. Continuous conveyors, or pallet systems, are also possible conveyance solutions.
From this specification, the designer can begin to develop a timing diagram for the machine and determine how much time is allowed for each function. That is often the next step toward conceptual design of automatic machinery. It is important to note that the design specification details what the device must do, but not how it must be done. The question of how will be addressed in the conceptual and detail design stages. Concepts for the conveyance system and the individual devices that perform various machine operations (such as lowering and raising the heater) can then be developed and evaluated based on their ability to meet the functional design specification.
RESEARCH
The importance of research cannot be overstated. Research 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
