Infant Mortality Theory

While it may not be refered to as infant mortality, the theory and its effects are seen quite frequently throughout daily life. This node shall try to explain one aspect of this theory, specifically to do with engineering systems, not the life cycles of living beings.

Infant mortality can be described at the high rate of failure early in a sample of product's intended life span.

Of course this explanation does not say why this is so, thus the rest of the node!

There are many explanations for why this occurs, but there a few points that seem to stick out the most when looking at this common occurrence. The most common reason is the product was simply manufactured under the desired level quality. This could occur for any number of reasons. We will trace the life of a small bronze gear, lets say a distributor gear, in an automotive ignition system as it is replaced in an already functioning system.

The first reason that the gear is defective could be a defective base material. The bronze used for the gear could be a bad mix of raw materials making a weaker alloy than intended. Or perhaps an impurity was in the metals, again making a weaker alloy.

Next reason could be maybe the cast that the gear is formed in is defective. Somewhere in the life of the casting, it was either made defective, or became defective due to wear and tear, mishandling, or a bad alloy that caused a defect. This defect in the mold would then pass on to each and every gear afterwards, making a batch of gears that are sub par.

Continuing along the chain, perhaps during the inspection of the gear the inspector picked it up to look at it and in doing so introduced some damage by dropping it back on the conveyor belt. Or when packaged it was damaged, thus it is prone to fail sooner rather than later. Once shipped out, damage could occur during shipping.

A quite common reason for failure is improper installation. In the case of our distributor gear, if when attached to the rod it is installed upside down, the gear is installed along the wrong part of the shaft, or the mechanism to hold the gear in place is not tighten correctly thus allowing the gear to shift, failure will soon result. Again, this is a quite common reason for infant mortality to occur, as human error has been introduced.

The last reason for failure is that the other parts in the system that a replacement is operating with have a defect. In the case of a distributor gear, the gear is in direct contact with two very important pieces; the distributor rod that holds the gear in place which turns the distributor contacts above, and a second gear that is somehow connected to the crankshaft so that the timing can be sensed and transmitted along the ignition system. Should either of these two components have a defect (such as the rod is bent and/or out of alignment or the second gear was damaged during the failure of the distributor gear) then the replacement part will soon fail, thus infant mortality will effect the system.

Infant Mortality in Electrical Systems

Many of the same reasons that a mechanical system may fail apply to electrical systems. I am not an electrical engineer, so I do not have the necessary background to present all the issues. But here are some additional reason that an electrical system may fail that may not a apply to mechanical systems.

  • The voltage may be incorrect for the part being installed. This mistake may occur from the part not being designed to handle such a level, the total system the part is being installed into has a second defect causing incorrect levels, or an incorrect installation (Human error again!).
  • The circuit is in AC current but the device is designed for DC, or vice versa. Similar voltage in all other respects.
  • The amperage is incorrect. Similar to voltage in all other respects.

    Human Error.

    Human error. Yes, I know I have stated this before, but I cannot state it enough. Most systems that have been working for a period of time do not just randomly break down without an outside force to initiate the failure. In mechanical systems this can come from exceeding the design specifications; in an engine going above redline or hauling trailers with a sports car. In electrical systems this could be unplugging a computer before shutting it down, or leaving it plug into a wall outlet with no surge protector. In either case a person may intentionally cause the system to fail.


    This theory is very important to understand when performing preventive maintenance (PM), as the person performing the PM must take into account all of the above factors and make the necessary inspections to prevent the part from failing. Auto mechanics, power plant operators, any manufacturing plant owner, and even computer repair personnel are quite concerned with this theory as it plays a factor into determining reliability and warrantee that a certain product or repair will have.

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