Infant mortality and the “mechanical failure pie"

Learn the key mechanical reasons why failures happen during system assembly and installation.

By Ian McKinnon, Reliability Solutions

Early machine failure and mediocre performance (i.e., infant mortality) is responsible for the highest probability of loss of machinery life, and can exist in any work, in any process. There are mechanical reasons why these failures happen during system assembly and installation, all of which can be approached as part of a single "failure source pie.” The three major failure pieces are misalignment, imbalance, and assembly errors, each being approximately 30 percent of the pie, with a final set of failures that just don't happen all that often.

Misalignment

The retention of alignment starts with all of the appendages that are attached to the machinery including the frame and base-work. This means that alignment starts with the “driven” machine first and then continues through the line to the driver, whatever that may be. Major alignment items that need to be in “control” (as a starting point), include the following:

  • Pipe stain
  • Angled and short foot/leg
  • Torque and torque sequence
  • Retention of bearing radial internal clearance
  • Coupling position
  • Thermal growth compensation, in all directions and planes
  • Transferred vibration from other machines in the area
  • Resonance from excited natural frequencies
  • Published final position tolerances
  • Case deflection and distortion

Imbalance

The second piece of the pie is “imbalance” or the loss of balance. Part of a precise mechanical work order is to retain the original balance as provided by the manufacturer; to potentially improve the system balance through component and system assembly; and last of all, to utilize new technologies to improve the dynamic balance as systems run over time. Some major sources of the loss of balance include:

  • Casting porosity
  • Peripheral run-out
  • Machining tolerances
  • Operation
  • Erosion
  • Corrosion

Assembly errors

This 30% slice of the pie – and at some facilities the percentage may be larger – can drive people crazy. These faults don’t show up as “independent” source frequencies, but instead “add” to or look like something else. Perhaps each of these "little things" by themselves will never cause a good machine to perform poorly, but as the “many” together, they will cause a machine that should run precisely to only perform “OK.” Here are a few, with how they might add to the other two slices:

  • Key length and positioning – unbalance
  • Bolt torque pattern & sequence – misalignment
  • Electrical termination box hard piping – misalignment
  • Set screws – unbalance and/or misalignment
  • Pulled threads – misalignment
  • Offset collars – unbalance
  • Grease quantities – unbalance
  • Position of the grease in couplings - unbalance
  • Applied lubricant torque changes – bearing life, unbalance, misalignment

Thin and final slice

The last 10% includes those failures that just don’t happen very often. Think about shorted, open, loose rotor bars. Do they happen? Yes, but do they happen often? With a dataset of 2000+ motors you might see one of these failures once every few years, however they are a part of the pie. Gearbox failures are also included in this set.

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