Mechanical Engineering Systems

Richard Gentle, Peter Edwards, William Bolton

• Publisher: Newnes
• Year: 2001
• ISBN:
  9780080496504
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

The authors of Mechanical Engineering Systems have taken a highly practical approach within this book, bringing the subject to life through a lively text supported by numerous activities and case studies. Little prior knowledge of mathematics is assumed and so key numerical and statistical techniques are introduced through unique Maths in Action features.

The IIE Textbook Series from Butterworth-Heinemann

• Student-focused textbooks with numerous examples, activities, problems and knowledge-check questions
• Designed for a wide range of undergraduate courses
• Real-world engineering examples at the heart of each book
• Contextual introduction of key mathematical methods through Maths in Action features
• Core texts suitable for students with no previous background studying engineering

"I am very proud to be able to introduce this series as the fruition of a joint publishing venture between Butterworth-Heinemann and the Institution of Incorporated Engineers. Mechanical Engineering Systems is one of the first three titles in a series of core texts designed to cover the essential modules of a broad cross-section of undergraduate programmes in engineering and technology. These books are designed with today's students firmly in mind, and real-world engineering contexts to the fore - students who are increasingly opting for the growing number of courses that provide the foundation for Incorporated Engineer registration." --Peter F Wason BSc(Eng) CEng FIEE FIIE FIMechE FIMgt. Secretary and Chief Executive, IIE

This essential text is part of the IIE accredited textbook series from Newnes - textbooks to form the strong practical, business and academic foundations for the professional development of tomorrow's incorporated engineers.

Forthcoming lecturer support materials and the IIE textbook series website will provide additional material for handouts and assessment, plus the latest web links to support, and update case studies in the book.

• Content matched to requirements of IIE and other BSc Engineering and Technology courses
• Practical text featuring worked examples, case studies, assignments and knowledge-check questions throughout.
• Maths in Action panels introduce key mathematical methods in their engineering contexts

1

Introduction: the basis of engineering

Summary

The aim of this chapter is to set the scene for the rest of the book by showing how the content of the remaining chapters will form the basis of the technical knowledge that a professional mechanical engineer needs during a career. By considering a typical engineering problem it is shown that the four main subjects that make up this text are really all parts of a continuous body of knowledge that will need to be used in an integrated manner.

The chapter concludes by looking at the units that are used in engineering and showing the importance of keeping to a strict system of units.

Objectives

By the end of this chapter the reader should be able to:

• understand the seamless nature of basic engineering subjects;

• appreciate the way in which real engineering problems are tackled;

• recognize the correct use of SI units.

1.1 Real engineering

Cast your mind forward a few years; you have graduated successfully from your course, worked for a spell as a design engineer and now you are responsibile for a team which is being given a new project. Your job is to lead that team in designing a new ride-on lawnmower to fill a gap in the market that has been identified by the sales team. The sales people think that there is scope to sell a good number of low-slung ride-on lawnmowers to places which use a lot of barriers or fences for crowd control, such as amusement parks. Their idea is that the new mower could be driven under the fences, cutting the grass as it goes, without the time wasting activity of having to drive to a gateway in order to move from one area to another. They have produced something they call a ‘concept specification’ which is really a wish list of features that they would like the new lawnmower to have.

(1) It must be very low, like a go-kart, to go under the barriers.

(2) It must be fast when not mowing so that it can be driven quickly around the park.

(3) It should dry and collect the grass cuttings as it goes so that the park customers do not get their shoes covered in wet grass.

Now comes the worst part of any engineering design problem – ‘Where do you start?’

Perhaps you should start with the framework of the mower because this is the part that would support all the other components. You have a good understanding of statics, which is the field of engineering concerned with supporting loads, and you could design a tubular steel frame without too much of a problem if you know the loads and their distribution. The trouble, however, is that you do not know the load that needs to be carried and you cannot base your design on the company’s existing products as all their current ride-on mowers are shaped more like small versions of farm tractors. You could calculate the load on the basis of an average driver weight but you do not yet know how much the engine will weigh because its power, and hence its size, has not been established. Furthermore, if the mower is to be driven fast around the park over bumpy ground then the effective dynamic loads will be much greater than the static load. It is therefore probably not a good idea to start with the frame design unless you are willing to involve a great deal of guesswork. This would run the risk of producing at one extreme a frame that would break easily because it is too flimsy and at the other extreme a frame that is unnecessarily strong and hence too expensive or heavy.

Time to think again!

Perhaps you should start the design by selecting a suitable engine so that the total static weight of the mower could be calculated. You have a good basic knowledge of thermodynamics and you understand how an internal combustion engine works. The trouble here, however, is that you cannot easily specify the power required from the engine. So far you have not determined the maximum speed required of the mower, the maximum angle of slope it must be able to climb or the speed at which it can cut grass, let alone considered the question of whether the exhaust heat can dry the grass. In fact this last feature might be a good place to start because the whole point of a mower is that it cuts grass.

First of all you could decide on the diameter of the rotating blades by specifying that they must not protrude to the side of the mower beyond the wheels. This would give you the width of the cut. A few measurements in a field would then allow you to work out the volume and mass of grass that is cut for every metre that the mower moves forward. Lastly you could find the forward speed of your company’s other ride-on mowers when cutting in order to calculate the mass of grass which is cut per second. From this you can eventually work out two more pieces of key information.

• Using your knowledge of fluid mechanics you could calculate the flow rate of air which is needed to sweep the grass cuttings into the collection bag or hopper as fast as they are being produced.

• Using your knowledge of thermodynamics you could calculate the rate at which heat must be supplied to the wet grass to evaporate most of the surface water from the cuttings by the time they reach the hopper.

At last you are starting to get somewhere because the first point will allow you to calculate the size of fan that is required and the power that is needed to drive it. The second point will allow you to calculate the rate at which waste heat from the engine must be supplied to the wet grass. Knowing the waste power and the typical efficiency of this type of engine you can then calculate the overall power that is needed if the engine is to meet this specification to dry the grass cuttings as they are produced.

Once you have the overall power of the engine and the portion of that power that it will take to drive the fan you can calculate the power that is available for the mowing process and for driving the mower’s wheels. These two facts will allow you to use your knowledge of dynamics to estimate the performance of the mower as a vehicle: the acceleration with and without the blades cutting, the maximum speed up an incline and the maximum driving speed. Of course this relies on being able to estimate the overall mass of the mower and driver, which brings us back to the starting point where we did not know either of these two things. It is time to put the thinking cap back on, and perhaps leave it on, because this apparently straightforward design problem is turning out to be a sort of closed loop that is difficult to break into.

What can we learn from this brief look into the future? There are certainly two important conclusions to be drawn.

• The engineering design process, which is what most engineering is all about, can be very convoluted. While it relies heavily on calculation, there is often a need to make educated guesses to start the calculations. To crack problems like the one above of the new mower you will need to combine technical knowledge with practical experience, a flair for creativity and the confidence to make those educated guesses. The engineering courses that this textbook supports must therefore be seen as only the start of a much longer-term learning process that will continue throughout your professional career.

• A good engineer needs to think of all the subjects that are studied on an undergraduate course in modular chunks as being part of a single body of technical knowledge that will form the foundation on which a career can be built. At the introductory level of this book it is best to keep the distinction between the various topics otherwise it can become confusing to the student; it is difficult enough coming to terms with some of the concepts and equations in each topic without trying to master them all at the same time. The lawn mower example, however, shows that you must be able to understand and integrate all the topics, even though you may not have to become an expert in all of them, if you want to be a proficient engineer.

1.2 Units

The introduction is now over and it is almost time to plunge into the detailed treatment of the individual topics. Before we do that, however, we must look at the subject of the units that are used, not only in this book but also throughout the vast majority of the world’s engineering industry.

Every engineering student is familiar with the fact that it is not good enough to calculate something like the diameter of a steel support rod and just give the answer as a number. The full answer must include the units that have been used in the calculation, such as millimetres or metres, otherwise there could be enormous confusion when somebody else used the answer in the next step of a large calculation or actually went ahead and built the support rod. However, there is much more to the question of units than simply remembering to quote them along with the numerical part of the answer. The really important thing to remember is to base ...