PRODUCT DESIGN

Stress and Strength

The survival of many products depends on how the designer adjusts the maximum

stresses in a component to be less than the component’s strength at specific locations of interest. The designer must allow the maximum stress to be less than the strength by a sufficient margin so that despite the uncertainties, failure is rare.

In focusing on the stress-strength comparison at a critical (controlling) location, we often look for “strength in the geometry and condition of use.” Strengths are the

magnitudes of stresses at which something of interest occurs, such as the proportional

limit, 0.2 percent-offset yielding, or fracture. In many cases, such events represent the stress level at which loss of function occurs.

Strength is a property of a material or of a mechanical element. The strength of an element depends on the choice, the treatment, and the processing of the material.

Consider, for example, a shipment of springs. We can associate a strength with a specific spring. When this spring is incorporated into a machine, external forces are applied that result in load-induced stresses in the spring, the magnitudes of which depend on its geometry and are independent of the material and its processing. If the spring is removed from the machine unharmed, the stress due to the external forces will return to zero. But the strength remains as one of the properties of the spring. Remember, then, that strength is an inherent property of a part, a property built into the part because of the use of a particular material and process.

Various metalworking and heat-treating processes, such as forging, rolling, and cold forming, cause variations in the strength from point to point throughout a part. The spring cited above is quite likely to have a strength on the outside of the coils different from its strength on the inside because the spring has been formed by a cold winding process, and the two sides may not have been deformed by the same amount.

Remember, too, therefore, that a strength value given for a part may apply to only a particular point or set of points on the part. In this book we shall use the capital letter S to denote strength, with appropriate subscripts to denote the type of strength. Thus, Ss is a shear strength, Sy a yield strength, and Su an ultimate strength.

In accordance with accepted engineering practice, we shall employ the Greek letters ó (sigma) and ô (tau) to designate normal and shear stresses, respectively. Again, various subscripts will indicate some special characteristic. For example, ó1 is a principal stress, óy a stress component in the y direction, and ór a stress component in the radial direction.

Stress is a state property at a specific point within a body, which is a function of load, geometry, temperature, and manufacturing processing. In an elementary course in mechanics of materials, stress related to load and geometry is emphasized with some discussion of thermal stresses. However, stresses due to heat treatments, molding, assembly, etc. are also important and are sometimes neglected.

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

Budynas−Nisbett

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Economic and Market Analysis

The net result or purpose of most engineering designs is to produce a product that generates a profit for the company. Obviously, each alternative design has to be evaluated against criteria such as sales features, potential market, cost of manufacturing, advertising, and so on. Large companies often conduct marketing surveys to obtain a measure of what the public will buy. These surveys may be conducted by telephone interviews with randomly selected people, or they may be personal interviews conducted with potential users of a product. Our society is based on economics and competition. Many good ideas never get into production because the manufacturing costs exceed what people will pay for the product. Market analysis involves applying principles of probability and statistics to determine if the response of a selected group of people represents the opinion of society as a whole. Even with a good marketing survey, manufacturers never know for certain if a new product will sell.

ENGINEERING DESIGN PROCESS

Education Transfer Plan

Prepared by

Seyyed Khandani, Ph.D.

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Product Safety and Liability

The primary consideration for safety in product design is to assure that the use of the design does not cause injury to humans. Safety and product liability issues, however, can also extend beyond human injury to include property damage and environmental damage from the use of your design. Engineers must also consider the issues of safety in design because of liability arising from the use of an unsafe product. Liability refers to the manufacturer of a machine or product being liable, or financially responsible, for any injury or damage resulting from the use of an unsafe product.

The only way to assure that your design will not cause injury or loss is to design safety into the product. You can design a safe product in three ways. The first method is to design safety directly into the product. Ask yourself, "Is there any probability of injury during the normal use and during failure of your design?" For example, modern downhill ski bindings use a spring-loaded brake that brakes the ski automatically when the ski disengages from the skier's boot. Older ski bindings used an elastic cable attached to the skier's ankle, but this had a tendency to disconnect during a severe fall.

Inherent safety is impossible to design into some products, such as rotating machinery and vehicles. In such cases you use the second method of designing for safety: You include adequate protection for users of the product. Protection devices include safety shields placed around moving and rotating parts, crash protective structures used in vehicles, and "kill" switches that automatically turn a machine off (or on) if there is potential for human injury. For example, new lawnmowers generally include a protective shield covering the grass outlet and include a kill switch that turns the motor off when the operator releases the handle.

The third method used in considering safety is the use of warning labels describing inherent dangers in the product. Although this method does not implement safety in design, it is primarily used as a way to shift the responsibility to the consumer for having ignored the safety guidelines in using the product. In most cases, however, a warning label will not protect you from liability. Protective shields or other devices must be

included in the design.

A product liability suit may be the result of a personal injury due to the operation of a particular product. The manufacturer and designer of a device can be found liable to compensate a worker for losses incurred during the operation or use of their product.

During a product liability trial, the plaintiff attempts to show that the designer and manufacturer of a product are negligent in allowing the product to be put on the market. The plaintiff's attorney may bring charges of negligence against the designer.

To protect themselves in a product liability trial, engineers must use state-of-the-art design procedures during the design process. They must keep records of all calculations and methods used during the design process. Safety considerations must be included in the criteria for all design solutions. The designer must also foresee other ways people could use the product. If a person uses a shop vacuum to remove a gasoline spill, is the

designer responsible when the vacuum catches fire? The courts can decide that a design is poor if the engineer did not foresee improper use of the product. It is imperative that you evaluate all of your alternative solutions against safety considerations. Reject or modify any unsafe elements of your design at this stage in the design process.

ENGINEERING DESIGN PROCESS

Education Transfer Plan

Prepared by

Seyyed Khandani, Ph.D.

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Ergonomics

Ergonomics is the human factor in engineering. It is the study of how people interact with machines. Most products have to work with people in some manner. People occupy a space in or around the design, and they may provide a source of power or control or act as a sensor for the design. For example, people sense if an automobile air-conditioning system is maintaining a comfortable temperature inside the car. These factors form the basis for human factors, or ergonomics, of a design.

A design solution can be considered successful if the design fits the people using it. The handle of a power tool must fit the hand of everybody using it. The tool must not be too heavy or cumbersome to be manipulated by all sizes of people using the tool. The geometric properties of people-their weight, height, reach, circumference, and so on-are called anthropometric data. The difficulty in designing for ergonomics is the abundance of anthropometric data. The military has collected and evaluated the distribution of human beings and published this information in military standard tables. A successful design needs to be evaluated and analyzed against the distribution of geometry of the people using it. The following Figure shows the geometry of typical adult males and females for the general population in millimeters. Since people come in different sizes and shapes, such data are used by design engineers to assure that their design fits the user.

A good design will be adjustable enough to fit 95 percent of the people who will use it.

ENGINEERING DESIGN PROCESS

Education Transfer Plan

Prepared by

Seyyed Khandani, Ph.D.

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