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Becht Engineering Blog

In this section of the site contributing authors submit interesting articles relating to the various services, industries and research & development efforts of Becht Engineering.

Reducing Coupling weights in Large Motor Driven Pumps

coupling Pump Coupling Assembly
One of the major issues in designing large motor driven pump applications is minimizing the overhung weight on the pump rotor.  Motor or gearbox drives are designed and sized independently of the driven unit.  They are based on motor frame size and/or gearbox frame sizes and the drive component manufacturer has no information on the actual driven unit.  In motors, the design of engineered motors is based on years of experience and rules of thumb.  The motor rotor is significantly heavier than the pump rotor, thus requiring it to be much larger in diameter than the driven unit size to support the rotor weight and give an acceptable bearing surface area.  It is not unusual for the motor output shaft size to be twice the input shaft size of the driven unit shaft. Size Matters The shaft size differences are basically a product of economics.   It cost money for a motor...
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Your pump's Mean Time Between Repairs (MTBR) is what?...Or what should it be...

Your pump's Mean Time Between Repairs (MTBR) is what?...Or what should it be...
Many of the tourist spots around the world have panhandlers. Some of the more creative ones stand on street corners advertising that they will tell you where you got your shoes, for a dollar. Of course it turns out that “you got your shoes” at the corner of Bourbon and St. Peters - or wherever you happen to be standing at the moment. It’s a play on words. However, being able to predict your pump Mean Time Between Repairs (MTBR) is not a play on words. Indeed it can be done quite accurately. I am often asked to do a facility rotating equipment reliability audit. After all, the plant’s management has heard that top tier plants have pump MTBR’s in excess of 100 months. They would be happy to have half that number. Hence the need to understand the major MTBR influencing factors. The main broad reliability categories that determine what...
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Shaft Fatigue Failures – Part I

Shaft Fatigue Failures – Part I
Many years ago when I first started my career in a rotating equipment maintenance organization, I was spending the day with a senior metallurgist, which was part of my orientation process as a new technical employee.  He was well known to not have the best “bedside manner” and it was clear he was not thrilled to have a young mechanical engineer by his side for the day.  As I walked into the materials lab, looking for something to start a conversation, I picked up a small broken ANSI pump shaft and asked if he knew why this shaft had failed?  His response I vividly remember to this day – he said, “Son, I have not analyzed that failure yet, but if it is a shaft out of a piece of rotating equipment and is broken into two pieces, there is about a 90% chance that it is a fatigue failure”. While...
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Shaft Fatigue Failures – Part II

Shaft Fatigue Failures – Part II
Note:  This blog is a continuation from the previous article from Becht Engineering, “Shaft Fatigue Failures – Part I”.  It is suggested that the reader review “Shaft Fatigue Failures - Part I” before proceeding with this article.   Click Here to view Part I. Before getting into the calculation methods, first a quick refresher on high cycle fatigue.  High cycle fatigue failures are typically acknowledged to be fatigue failures resulting from alternating loading cycles in excess of 10 6 cycles.    While that may sound like a large number, in high speed rotating machinery, one million cycles will occur in hours.  For high cycle fatigue, the fatigue test data is often reported in the form of alternating stress vs number of cycles (S-N, or Stress-Life method).  Fortunately, many of the common shaft and rotor alloys exhibit a fatigue strength “endurance limit” at about 10 6 – 10 7 cycles, beyond which the...
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