|
|
So many times we hear folks blaming their problems on Murphy's Law. That's usually when we get a call! We're on pretty good terms with Murph, planning ahead so we can comply with or avoid the jurisdiction of his laws. But, just in case you've forgotten them,
Introduction
The contributions of Edward Aloysius Murphy, specifically his general and special laws delineating the behavior of inanimate objects, have not been fully appreciated. This is, in large part, due to the inherent simplicity of the law itself where Murphy’s Law states that, “If anything can go wrong, it will."
Stated mathematically, 1+1 \/ 2 where \/ is the mathematical symbol for "seldom, if ever".
However, not often published are the additional laws and corollaries promulgated by Murphy as his electronics work continued. Below are the related elements as stated by Murphy.
General Engineering
-A patent application will be preceded by one week by a similar application made by an outsider.
-The more innocuous a design change appears, the more it will affect the circuit.
-All warranties and guarantees are void upon final payment.
-The necessity of making a major design change increase as the production process approaches completion.
-Firmness of delivery dates are inversely proportional to the tightness of the production schedule.
-Dimensions will always be expressed in the least useable term. Velocity, for example, will be expressed in furlongs per fortnight.
-Original drawings will be eaten by copiers.
-An important instruction manual or operating manual will have been discarded by the receiving department.
-Suggestions made by QA will increase costs and reduce capabilities.
Mathematics
-The validity of a calculation is inversely proportional to the number of engineers assigned to the project.
-If there are miscalculations, the fault will never be placed if more than one person is involved.
-Any error that can creep in, will. It will be in the direction that will do the most damage to the calculation.
-All constants are variables.
-In any given computation, the figure that is most obviously correct will be the source of error.
-A decimal will always be misplaced.
-In a complex calculation, one factor from the numerator will always move into the denominator.
Prototyping and Production
-Any wire cut to length will be too short.
-Tolerances will accumulate unidirectionally toward maximum difficulty of assembly.
-Identical units tested under identical conditions will not be identical in the field.
-The availability of a component is inversely proportional to the need for that component.
-If a project requires n components, there will be n-1 units in stock.
-If a particular resistance is needed, that value will not be available. Further, it cannot be created with any available series or parallel combination.
-A dropped tool will land where it can do the most damage.
-A device selected at random from-a group having 99% reliability, will be a member of the 1% group.
-When one connects a 3-phase line, the phase sequence will be wrong. When rewired, it will still be wrong.
-A motor will rotate in the wrong direction.
-The probability of a dimension being omitted from a plan or drawing is directly proportional to its importance.
-Interchangeable parts won't.
-Probability of failure of a component, assembly, subsystem or system is inversely proportional to ease of repair or replacement.
-If a prototype functions perfectly, subsequent production units will malfunction.
-Components that must not and cannot be assembled improperly will be.
-A dc meter will automatically be used on an overly sensitive range and will be wired in backwards.
-The most delicate component will drop.
-Graphic recorders will deposit more ink on humans than on paper.
-If a circuit cannot fail. it will.
-A fail-safe circuit will destroy others.
-An instantaneous power-supply crowbar will operate too late.
-A transistor protected by a fast acting fuse will protect the fuse by blowing first.
-A self-starting oscillator won't.
-A crystal oscillator will oscillate at the wrong frequency-if it oscillates.
-A crystal will resonate on an overtone unless designed to do so.
-A pnp transistor will be an npn.
-A zero-temperature coefficient capacitor used in a critical circuit will have TC of 750 ppm/C.
-A failure will not appear till a unit has passed final inspection.
-A purchased component or instrument will meet its specs long enough, and only long enough, to pass incoming inspection.
-Continuity to ground will cease to exist in wrist straps
-If an obviously defective component is replaced in an instrument with an intermittent fault, the fault will -reappear after the instrument is returned to service.
The part most critical to a repair will be on backorder. However, the grommets used in the assembly will be available.
-After the last of 16 mounting screws has been removed from an access cover, it will be discovered that the wrong access cover has been removed.
-The missing retaining ring will be noticed after all wires to the connector have been soldered
-After an access cover has been secured by 16 hold-down screws, it will be discovered that the gasket has been omitted.
-After an instrument has been fully assembled, extra components will be found on the bench.
-Hermetic seals will leak.
-Batteries only fail in the field.
Specifying
-Specified environmental conditions will always be exceeded.
-Any safety factor set as a result of practical experience will be exceeded.
-Manufacturers’ spec sheets will be incorrect by a factor of 0.5 or 2.0 depending on which multiplier gives the most optimistic value. For salesmen's claims these factors will be 0.1 or 10.0.
-An instrument or device characterized by a number of plus-or-minus errors, the total error will be the sum of all errors adding in the same direction
Contractual due dates are invalid
-Any given price estimate must be multiplied by a factor of 3.
-In specifications, Murphy's Law supersedes Ohm's.
|
|
