Reliability Engineering Snapshot TM

Be careful when changing a design configuration without considering all of the aspects. Making a design better by changing one parameter, may adversely effect another, and make the equipment operate worse than before.

People in the rotary dryer and kiln business will recognize the apparatus to the left. In order to effect a tight seal between a dryer that rotates and a stationary vestibule, one method of accomplishing such a task is to use a series of flexible metal leaves. The seal leaves, as they are called, are affixed to the stationary vestibule in such a fashion as to allow the dryer to rotate while providing a tight seal against air leakage into the vestibule.

These leaves were wearing out too fast, so the immediate idea was to increase their thickness. It was felt that by simply increasing the metal thickness by a factor of two, the wear life would increase proportionally. Not true, in fact the leaves started failing within weeks of installation. The cause of failure was not due to wear, but was due to overstress. How could a thicker material fail due to overstress? One's natural train of thought would tend to arrive at the same conclusion. Stress is inversely proportional to the area of the applied stress. More area for a given force means less stress. The leaves were twice as thick, so the stress for the applied force should be half as much. That would be true for a steady state applied force. However, in this particular case, the rotary dryer was banana shaped and the end of the shell where the leaves were attached would deflect by as much as 1". This changes the whole concept. Stress is directly proportional to strain. In this case, the rotary dryer applied a strain that could not be overcome (of course) by the force of the leaves. Therefore, when the leaf thickness was increased, the stress actually increased in the leaves. The increase was enough to create a larger cyclic stress and reduce the fatigue endurance of the leaves.

The thickness was reduced below the original thickness and the life of the leaves actually increased. Doesn't sound right, but it did. Thinner leaves meant a proportionally smaller cyclic stress level for a given deflection.

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