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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 17, Assembly Errors Due to Sharp Corners

The title seems strange as a part's sharp edges (referred to here as "corners"), do not themselves create a vibration problem. However, for example, a pump impeller's bore may have been re-machined with the machinist relieving its sharp corners while the impeller was still in the lathe. Often the corners are rounded only slightly with a quick touch of a file or emery cloth to the rotating part. The same can be true for the sharp corners on the pump shaft. Although the corners are not sharp enough to cut one's finger, they may still be rounded only slightly. For such situations, the slightest impact of a shaft's sharp edge on the sharp edge of the bore creates a very large concentrated load as the contact surface areas between two sharp edges are so small.

The resulting very concentrated pressure is large enough to move the metal by at least a few mils. If the metal is displaced radially, it can offset the rotor relative to the shaft by a few mils, primarily resulting in static or quasi-static unbalance. If the metal is displaced axially, it can cause the rotor to be cocked a few mils, resulting in couple unbalance.

All of this may seem like a gross exaggeration to craftspeople who have never before had to be so careful. With previous standards for machinery balance and performance, such small errors of rotor location relative to its shaft would usually create only a small percentage increase in vibration. For example, assembly of two rotating parts without these defects could result in a vibration of 0.1 in/sec, but with these defects, a vibration of 0.15 in/sec, could result and would usually be accepted. A precision balanced rotor without these errors could result in a vibration amplitude of 0.04 in/sec, but with the errors, 0.09 in/sec. By obsolete reasoning, the vibration levels for the higher amplitude are still "good enough." But notice that the centerline orbit diameter more than doubled! New standards for precision balancing require what are, so far, relatively unfamiliar precision tolerances. Obtaining these tolerances requires more careful attention to assembly errors. (See section, "Unbalance Standards for New and Rebuilt Machinery.")

To prevent these problems, all sharp corners should be relieved far more than previously specified. Also, the craftsperson should, before assembling any two parts, automatically and routinely feel all corners and mating edges with a fingernail to determine if there is displaced metal. If so, the dents and burrs should be removed by a file or abrasive stone until dents are all below the surface.

 

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