III. Reliability in Design and Development
B. Parts and systems management
2. Derating methods and principles (Analyze)
Use methods such as S-N diagram, stress-life relationship, etc., to determine the relationship between applied stress and rated value, and to improve design.
Primarily an electrical engineering concept, it applies the stress strength concept to electronic components.
Additional References
The Derating & Safety Margin Manual (article)
Derating Value (article)
Quick Quiz
1-86. Identify which technique is not used when derating parts to enhance their overall reliability.
(A) derating for voltage stress
(B) derating for temperature
(C) using a 100% rating
(D) using power derating from vendor
using a 100% rating
The derating concept is to use components at less then the vendor’s listed ratings. Thus if a capacitor is rated for 10 volts, the design should use this capacitor on a circuit that has less than 10 volts occurring across the capacitor. A 50% voltage derating would imply we should use a 10 volt rated capacitor for a situation that would experience at most 5 volts. 5 / 10 = 0.5 or 50%.
There are many stresses and depending on the specific technology used within a component, stresses such as voltage, current, temperature, power, etc. may apply. It is common to have two or more stresses derated.
1-89. Which of the following factors would be the most important to consider in designing a steel beam so as to minimize failure?
(A) that the load and strength values are fixed
(B) that the load and strength values are statistically described
(C) that a safety margin must be maintained
(D) that the means and standard deviations must be defined
(B) that the load and strength values are statistically described
The key word here is “minimize” knowing stress strength values or that they are fixed or the values meet a safety margin do not allow the designer to minimize the risk of failure. Knowing the statistical distributions of both stress and strength, as well as how the distributions change over time are critical to know.
1-93. The rated breaking strength of a nylon rope is 2,000 lb. If you use this rope in a derated condition, what maximum stress application would be appropriate?
(A) 1,000 lb
(B) 2,000 lb
(C) 3,000 lb
(D) >3,000 lb
(A) 1,000 lb
The key word here is “derated” which implies below the rated value. The only option is less then the rated breaking strength of 2,000 is the (A) response.
1-94 In general, the highest component parameter ratings feasible should be chosen so that which of the following hold?
I. The probability of secondary failure is minimized.
II. Failure rates are minimized.
III. Safety margins are offered against unforeseen transients.
(A) II only
(B) I and II only
(C) II and III only
(D) I, II, and III
(D) I, II, and III
This one requires careful reading as it uses the word “highest” meaning the most derated component for a specific application. For example, if selecting a capacitor for a 5 volt application, and the options that worked in the circuit include components rated at 10V, 15V and 20V, we should select the 20V rated component.
1-96. Why is derating parts in specific applications performed?
(A) to establish a preferred parts list
(B) to determine the stress ratios
(C) to lower failure rates
(D) to reveal which parts have inferior design
(C) to lower failure rates
Using components that have the strength to withstand applied stress with margin to handle variation operate without failure longer.
Leave a Reply