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Student Case Study Guidelines - Introduction1
Two Speed Coiler Gear Box (A) Peter Sawchuck had just completed the design of a two-speed coiler gear box. He was pleased to see it fully assembled on the shop floor. He felt good about it and was certain that nothing could go wrong. This was important because for a unit this size there were no facilities to run the unit under design loading. Therefore, the unit had to work the first time.
Peter was in his early career. He had graduated from an eastern engineering school and was now applying for registration as a professional engineer. He had been working for Northern Gear Works for 5 years, in which time had established himself as their specialty gear designer.
Note how the writer set up the protagonist at the beginning of this case. There is no question about the subject of this case, Mr. Sawchuck, his name and position and what he had to accomplish. The paragraph below sets the context of the case. It describes the company and the significance of the coiler box in the companies business.
Northern Gear Works was a division of a multi-national company. The parent company divided up the work on a regional basis rather than on a product line basis. They felt that any cost advantage that would be gained by quantity production would soon be offset by warehousing and shipping costs. Besides, regional responsibility meant that the divisions could respond quicker to customers' needs, and in a highly competitive market customer satisfaction often made the difference between profit and loss. Consequently Northern gear produced a mixed line of standard and special power transmission equipment.
The following two paragraphs set the time frame and the specific application this case is addressing. Note that every sentence adds relevant information. It doesn't take long to set the stage and introduce a case study.
Three months earlier Peter received the work order for the coiler drive. Although coiler drives were not a stock item, Peter was familiar with the application.
For steel cold rolling mills, coilers are used to reel and unreel the steel being processed. Such coilers are capable of providing the high forward tensions that may be required in rolling, as the strip is stretch leveled and wound tightly and uniformly on the drum.
The next four paragraphs describe the specific application in more detail. They don't describe the coiler itself, so the information may be considered as introductory context material. Note how the reading slows down when you reach this part of the introduction.
Matching the rolling and coiling speeds and maintaining constant tension is the strip are complicated by the changing diameter of the coil as it is being built up. In modern mills, the coilers are driven by various speed motors which ensures matched speeds at the desired tension level. Moreover, in reversing mills, when the strip is being paid off a coiler, the motor acts as a generator returning energy from the back tension to the power supply.
The diameters of drums (when expanded) are usually about 16 inches. To minimize the possibility of coils collapsing or telescoping, the drum diameter should be as small as possible. However, the diameter should not be so small that the strip is plastically deformed as it is bent around it. Moreover, it must have sufficient strength to bear the weight of the strip, the effect of the strip tension and the radial pressures exerted on it by the strip. Unfortunately, the internal components of the drum required to reduce its diameter for coil removal also diminish the strength of the drum and necessitate its being of the order of 16 inches.
Mandril design for cold mills varies considerably ranging from simple drums with no mechanisms for fastening the strip to the mandril to more elaborate types that clamp the head end of the strip. With the former, a common practice is to fasten the end of the strip onto the drum with a piece of adhesive tape and then rotating the drum to accumulate several wraps on it before applying high tension.
The power required to drive a coiler is stated to be:
Where: is the bending moment of the strip when uncoiling and in practical calculations is often taken as equal to the moment of plastic bending (pound-feet) where
is the yield stress and S is the moment of resistance of the strip to plastic bending.
T is the tension in the strip (lbs). R R is the minimum radius of the coil (practically the radius of drum) (feet). is the total moment of friction in the bearings of the drum shaft (allowing for the weight of the drum and the coil) (pound-feet). are the speed of coiling (feet per minute) and the efficiency factor of the transmission. While the author of this case study included the formula in the introduction, I don't recommend that you do. It is better in the analysis part of the case study. The next three paragraphs get back on track.
The following paragraph gets to the heart of Peter's problem and the two paragraphs after that provide a good transition to the body of the case. One should always ask " Why is this material here?" Will this cause the reader to get sleepy or will it move the narrative along? Think of each paragraph as one you are telling to either a peer or to an interested, intelligent, but non specialist, supervisory group. They are expecting you to organize the flow in a way such that the key information is presented at the right time. The keys to good writing are transitions (from paragraph to paragraph and section to section ) and organization.
For his registration as a professional engineer Peter was required to submit a thesis. He chose to write about the design of the coiler gear box. In it he describes the problems of designing such a gear box.
The coiler reducer had to be designed and manufactured to satisfy a number of special requirements delineated by the customer as the "Design Specifications".
Although specifications list only the bare essentials, it is the designer's responsibility to consider many other factors whether they appear in the basic specification or not. In addition to considerations of strength, durability and efficiency he must ensure that his design meets temperature rise limitations for the anticipated range of ambient temperatures; does not exceed acceptable noise levels, meets weight limitation, and that the design is economical.
1. Smith, C. and G. Kardos, D. E. E. D. Case Workshop for Engineering Reality in the Classroom. ASEE Annual Conference. Toledo, Ohio. 1992. ECL 255.
Last update 9-3-98
