Mechanical Design

The word ‘ design ’ derived from the latin ‘ designare ’ ‚ which means ‘ to designate or mark out ’. Webster’s gives several definitions ‚ the most applicable being ‘ to outline ‚ plot or plan ‚ as action or work … to conceive ‚ invent-contrive ’. Engineering design has been defind as “ … the process of applying the various techniques and scientific principles for the purpose of defining a device a process ‚ or a system in sufficient details to permit its realization … . Design may be simple or enormously complex ‚ easy or difficult ‚ mathematical or non-mathematical ; it may involve a trivial problem or one of great importance ”. Mechanical design means the design of things and systems of a mechanical nature ‚ machines ‚ products ‚ structures ‚ devices and instruments. For the most part ‚ mechanical design utilizes mathematics ‚ the materials sciences ‚ and the engineering-mechanics sciences.

In ancient times ‚ design was combined with simple manufacturing based upon trial – and – error methods . Later ‚ projected ‚ two dimensional scale drawings were used to represent products . This made it possible to plan the product before it was manufactured ‚ and marked the birth of modern design . Computer graphics ‚ numeric analysis and artificial intelligence have been introduced into daily design activities ‚ stimulating research in design theory and methodology . Today ‚ most theorists and practitioners agree that design can be roughly devided into four major stages :

1-      Specification – In this stage ‚ the main task is to determine the design specifications or clarify the design requirements . In the end a set of specifications is generated to meet the requirements stated by the customer or generated from market analysis or other resouerces .

2-      Conceptual or Preliminary Design – In this stage ‚ the task is to conceive solution concepts ‚ typically represented by drafts ‚ symbols or abstract diagrams ‚ that will functionally meet the above specifications .

3-      Layout Design or Embodiment Design – In this stage ‚ one optimal alternative is chosen based upon rough analysis ‚ experience ‚ and other heuristic rules . This solution concept is then fleshed out to generate a layout ( sometimes called a general arrangement ) . If problems are detected at this stage ‚ the selected concept must be modified ‚ or replaced by other alternatives .

4-      Detail Design – In this stage ‚ each component is precisely specified . This includes engineering analysis ( such as stress analysis ) ‚ dimensioning ‚ etc . During this analysis ‚ problems that were not obvious in the earlier stages may be uncovered . Then ‚ the design iteration begins ; a new cycle starts again from the layout design either by selecting other solution concepts or even one step back to the design specifications and make some compromises in order to manufacture the product economically . Finally ‚ a set of documents is generated which include engineering drawings and other relevant descriptions . All information needed for manufacturing the product must be completed at this stage .

It can be seen that the conceptual design is the beginning of the actual design process ‚ therefor it has the largest leverage on the quality final product . If a bad design concept is chosen ‚ no matter how much effort is put into later design stages and manufacturing ‚ the final product cannot be satisfactory .

This point can be best illustrated by the following example . The example is about the cutting of involute gears . The old solution concept was to use a cutter shaped like the tooth space . Because the shape of the tooth space varies with the diametral pitch ‚ whereas there are a limited number of cutters ‚ the cutter is only approximate to the true shape of the tooth space in most cases . Consequently ‚ it is difficult to use this method to product high-precision gears . To solve this problem ‚ a new solution concept is generated from observing the motion of a pair of engaged gears . If one gear is used as the cutter ‚ then theoretically an ideal mating gear can be producted . This new concept gave birth to shaping and hobbing machines ‚ and brought about a breakthrough in gear manufacturing .

There are many valuable rules of thumb for better conceptual design of machine parts . But how can an engineer generate better design concepts ? This is a question many researches are trying to answer . Of the four design stages ‚ conceptual design requires the most creativity and therefore is the least understood . Research in conceptual design is still in its infant stage . Researchers from engineering ‚ computer and cognitive sciences are working together ‚ trying to gain better and deeper understanding of this design phase . Generally speaking ‚ engineering design can be classified into two categories : modification of existing design and creation of new design . The first case includes correcting short comings ‚ adapting the existing design to meet new needs ‚ improving performance ‚ etc. This kind of design accounts for more than 70% daily design activities . The second category covers creation of new products that never existed before . Things that were originally conceived ‚ such as the first automobile ‚ the first space shuttle ‚ the first compact disc player belong to this category . Different methods have been proposed for the two categories .

Ref………………….................Mechanical Engineering Design of Solids .

Author…………………………….……………….Jamaloddin Jalalapour .