CHAPTER 17 TECHNICAL CASE

SITUATION

A new titanium Z Mill, a reversing cold rolling mill, is being built to create flat-rolled titanium sheets for both commercial and military aircraft. The motors that turn the rolling mill, however, run at very high RPMs and tend to become very hot due to both the ambient air temperature of the region during the summer, the speed at which the motors drive the rolls, and the pressure at which the rolls bear down on the titanium. The motors are cooled by two fans pulling hot air away from the motors and replacing it with cooler air. If one motor of the two overheats, however, the entire cooling system shuts down. This, in turn, shuts down the mill, tearing the titanium strip and causing a delay of several hours until the motor cools off and the wrecked titanium can be removed and each hour the line is down the company loses $1000 not to mention the cost to replace damaged rolls and damaged titanium.

Your assignment, as the Reliability Engineer, is to help the company determine if it is worth installing a redundant system. The current system has two fans in SERIES with individual fan reliabilities as shown:

Fan #1: R1 = .98

Fan #2: R2 = .90

Again, both fans are in a SERIES - arranged one after another - and both pull air from one giant duct that draws air off the Z Mill. If ONE FAN in the series goes down, the entire system shuts down!

This is your assignment: should you decide to accept it, it is worth 5 points. Make it good, show all your work, draw it neatly in blocks, and let me see how you calculated it. Hint: Power Point for Chapter 17 WILL HELP as will pp. 670-672 of the text.

QUESTIONS TO ANSWER:

1. DRAW the fans in SERIES; determine the overall SYSTEM RELIABILITY (RS).
2. You opt to first replace the weaker of the two fans with a more reliable fan with reliability equivalent to the more powerful fan. What is the result?
3. You decide to install a PARALLEL REDUNDANT SYSTEM with the EXACT same reliabilities as the original two fans. DRAW the PARALLEL REDUNDANT SYSTEM and determine the reliability for both systems and the entire system in total.

4. You decide, finally, as the engineer, to install a BACKUP REDUNDANT SYSTEM with one fan backing up each of the two. The back-up reliability is the same as the original two - so the back-up to Fan #1 has a reliability of .98 and the back-up to Fan #2 has a reliability of .90. DRAW the system with BACKUP REDUNDANCY.
5. MEAN TIME BETWEEN FAILURES