Product Features / Requirements

Safety and Product Liability

The strict liability concept of product liability generally prevails in the United States. This concept states that the manufacturer of an article is liable for any damage or harm that results because of a defect. And it doesn’t matter whether the manufacturer knew about the defect, or even could have known about it. For example, suppose an article was manufactured, say, 10 years ago. And suppose at that time the article could not have been considered defective on the basis of all technological knowledge then available. Ten years later, according to the concept of strict liability, the manufacturer is still liable. Thus, under this concept, the plaintiff needs only to prove that the article was defective and that the defect caused some damage or harm. Negligence of the manufacturer need not be proved.

The best approaches to the prevention of product liability are good engineering in analysis and design, quality control, and comprehensive testing procedures. Advertising managers often make glowing promises in the warranties and sales literature for a product. These statements should be reviewed carefully by the engineering staff to eliminate excessive promises and to insert adequate warnings and instructions for use.

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 critical locations. 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.

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.

Uncertainty

Uncertainties in machinery design abound. Examples of uncertainties concerning stress and strength include

• Composition of material and the effect of variation on properties.
• Variations in properties from place to place within a bar of stock.
• Effect of processing locally, or nearby, on properties.
• Effect of nearby assemblies such as weldments and shrink fits on stress conditions.
• Effect of thermomechanical treatment on properties.
• Intensity and distribution of loading.
• Validity of mathematical models used to represent reality.
• Intensity of stress concentrations.
• Influence of time on strength and geometry.
• Effect of corrosion.
• Effect of wear.
• Uncertainty as to the length of any list of uncertainties.

Engineers must accommodate uncertainty. Uncertainty always accompanies change. Material properties, load variability, fabrication fidelity, and validity of mathematical models are among concerns to designers.

There are mathematical methods to address uncertainties. The primary techniques are the deterministic and stochastic methods. The deterministic method establishes a design factor based on the absolute uncertainties of a loss-of-function parameter and a maximum allowable parameter. Here the parameter can be load, stress, deflection, etc. Thus, the design factor n_d is defined as

n_d = loss-of-function parameter/maximum allowable parameter

Design Factor and Factor of Safety

A general approach to the allowable load versus loss-of-function load problem is the deterministic design factor method, and sometimes called the classical method of design. The fundamental equation is above Equation where n_d is called the design factor. All loss-of-function modes must be analyzed, and the mode leading to the smallest design factor governs. After the design is completed, the actual design factor may change as a result of changes such as rounding up to a standard size for a cross section or using off-the-shelf components with higher ratings instead of employing what is calculated by using the design factor. The factor is then referred to as the factor of safety, n. The factor of safety has the same definition as the design factor, but it generally differs numerically.

Since stress may not vary linearly with load, using load as the loss-of function parameter may not be acceptable. It is more common then to express the design factor in terms of a stress and a relevant strength. Thus above Equation can be rewritten as

n_d = loss-of-function strength/allowable stress

Dimensions and Tolerances

The following terms are used generally in dimensioning:

• Nominal size. The size we use in speaking of an element. For example, we may specify a 1½-in pipe or a ½-in bolt. Either the theoretical size or the actual measured size may be quite different. The theoretical size of a 1½-in pipe is 1.900 in for the outside diameter. And the diameter of the ½-in bolt, say, may actually measure 0.492 in. 
• Limits. The stated maximum and minimum dimensions 
• Tolerance. The difference between the two limits. 
• Bilateral tolerance. The variation in both directions from the basic dimension. That is, the basic size is between the two limits, for example, 1.005±0.002 in. The two parts of the tolerance need not be equal. 
• Unilateral tolerance. The basic dimension is taken as one of the limits, and variation is permitted in only one direction, for example, 

• Clearance. A general term that refers to the mating of cylindrical parts such as a bolt and a hole. The word clearance is used only when the internal member is smaller than the external member. The diametral clearance is the measured difference in the two diameters. The radial clearance is the difference in the two radii. 
• Interference. The opposite of clearance, for mating cylindrical parts in which the internal member is larger than the external member (e.g., press-fits). Allowance. The minimum stated clearance or the maximum stated interference for mating parts.