Machine Designers Reference. J. Marrs
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
| Safety Factor | Application |
| 1.3 - 1.5 | For use with highly reliable components or materials where loading and environmental conditions are not severe, and where weight is an important consideration. |
| 1.5 - 2 | For applications using reliable components or materials where loading and environmental conditions are not severe. |
| 2 - 2.5 | For use with ordinary components or materials where loading and environmental conditions are not severe. |
| 2.5 - 3 | For less tried and for brittle materials where loading and environmental conditions are not severe. |
| 3 - 4 | For applications in which component or material properties are not reliable and where loading and environmental conditions are not severe, or where reliable components or materials are to be used under difficult loading and environmental conditions. |
| 4+ | For applications with a high degree of uncertainty, high risks, or where unreliable components or materials are to be used where loading and environmental conditions are severe. |
•Safety is of paramount importance in all designs. Understand and obey all applicable laws, codes, and standards.
•Apply appropriate safety factors when conducting design and analysis.
•The design specification should account for all design requirements and foreseeable needs.
•Research and test. This will improve your results and help prevent mistakes.
•Avoid overly descriptive terms early in the design specification phase. They can limit creativity.
•State and question assumptions made during the specification and design process.
•Build your list of known mechanical devices to maximize your synthesis capabilities. Work to improve your understanding and ‘feel for’ forces, distances, and machining methods.
•Allow enough time for research and conceptualization. Ensure that the work environment allows long periods of concentration. This is often a challenge in the modern office.
•Come up with multiple solutions for every problem and select the best one. Use a decision matrix to aid the selection process.
•When visualization stalls, some designers find it helpful to begin drawing or modeling the known surfaces, parts, and points in an assembly. A second round of visualization after accurately modeling what is known is often more fruitful.
Proper units are critical when performing measurement and analysis. Most engineering books contain some discussion of units