SOMETHING ABOUT PRESSURE _________
Now before bringing this somewhat lengthy lecture to a close, (for I consider it a mere lecture, a talk with the boys) I want to say something more about pressure. You notice that I have not advocated a very high pressure; I have not gone beyond 125 lbs. and yet you know and I know that very much higher pressure is being carried wherever the traction engine is used, and I want to say that a very high pressure is no gauge or guarantee of the intelligence of the engineer. The less a reckless individual knows about steam the higher pressure he will carry. A good engineer is never afraid of his engine without a good reason, and then he refuses to run it. He knows something of the enormous pressure in the boiler, while the reckless fellow never thinks of any pressure beyond the I00 or I40 pounds that his gauge shows. He says, "'O! That,' that aint much of a pressure, that boiler is good for 200 pounds." It has never dawned on his mind (if he has one) that that I40 pounds mean I40 pounds on every square inch in that boiler shell, and I40 on each square inch of tube sheets. Not only this but every square inch in the shell is subjected to two times this pressure as the boiler has two sides or in other words, each square inch has a corresponding opposite square inch, and the seam of shell must sustain this pressure, and as a single riveted boiler only affords 62 per cent of the strength of solid iron. It is something that every engineer ought to consider. He ought to be able to thoroughly appreciate this almost inconceivable pressure. How many engineers are today running 18 and 20 horse power engines that realizes that a boiler of this diameter is not capable of sustaining the pressure he had been accustomed to carry in his little 26 or 30 inch boiler? On page 114 You will get some idea of the difference in safe working pressure of boilers, of different diameters. On the other hand this is not intended to make you timid or afraid of your engine, as there is nothing to be afraid of if you realize what you are handling, and try to comprehend the fact that your steam gauge represents less than one 1-1000 part of the power you have under your management. You never had this put to you in this light before, did you?
If you thoroughly appreciate this fact and will try to comprehend this power confined in your boiler by noting the pressure, or power exerted by your cylinder through the small supply pipe, you will soon be an engineer who will only carry a safe and economical pressure, and if there comes a time when it is necessary to carry a higher pressure, you will be an engineer who will set the pop back again, when or as soon as this extra pressure is not necessary.
If I can get you to comprehend this power proposition no student of
"Rough and Tumble Engineering" will ever blow up a boiler.
When I started out to talk engine to you I stated plainly that this book would not be filled up with scientific theories, that while they were very nice they would do no good in this work. Now I am aware that I could have made a book four times as large as this and if I had, it would not be as valuable to the beginner as it is now.
From the fact that there is not a problem or a question contained in it that any one who has a common school education can not solve or answer without referring to any textbooks The very best engineer in the country need not know any more than he will find in these pages. Yet I don't advise you to stop here, go to the top if you have the time and opportunity. Should I have taken up each step theoretically and given forms, tables, rules and demonstrations, the young engineer would have become discouraged and would never have read it through. He would have become discouraged because he could not understand it. Now to illustrate what I mean, we will go a little deeper and then still deeper, and you will begin to appreciate the simple way of putting the things which you as a plain engineer are interested in.
For example on page 114 we talked about the safe working pressure of different sized boilers. It was most likely natural for you to say "How do I find the safe working pressure?" Well, to find the safe working pressure of a boiler it is first necessary to find the total pressure necessary to burst the boiler. It requires about twice as much pressure to tear the ends out of a boiler as it does to burst the shell, and as the weakest point is the basis for determining the safe pressure, we will make use of the shell only.
We will take for example a steel boiler 32 inches in diameter and 6 ft. long, 3/8 in. thick, tensile strength 60,000 lbs. The total pressure required to burst this shell would be the area exposed times the pressure. The thickness multiplied by the length then by 2 (as there are two sides) then by the tensile strength equals the bursting pressure: 3/8 x 72 X 2 x 60,000 = 3,240,000 the total bursting pressure and the pressure per square inch required to burst the shell is found by dividing the total bursting pressure 3,240,000 pounds by the diameter times the length 3,240,000 / (32 x 72) = 1406 lbs.
It would require 1406 lbs. per square inch to burst this shell if it were solid, that is if it had no seam, a single seam affords 62 per cent of the strength of shell, 1406 x .62 = 871 lbs. to burst the seam if single riveted; add 20 per cent if double riveted.
To determine the safe working pressure divide the bursting pressure of the weakest place by the factor of safety. The United States Government use a factor of 6 for single riveted and add 20 per cent for double riveted, 871 / 6 = 145 lbs. the safe working pressure of this particular boiler, if single riveted and 145 + 20 per cent=174 double riveted.
Now suppose you take a boiler the same length and of the same material, but 80 inches in diameter. The bursting pressure would be 3,240,000 / (80 x 72) = 560 lbs., and the safe working pressure would be 560 / 6 = 93 lbs.
You will see by this that the diameter has much to do with the safe working pressure, also the diameter and different lengths makes a difference in working pressure.
Now all of this is nice for you to know, and it may start you on a higher course, it will not make you handle your engine any better, but it may convince you that there is something to learn.
Suppose we give you a little touch of rules, and formula in boiler making.
For instance you want to know the percent of strength of single riveted and double riveted as compared to solid iron. Some very simple rules, or formula, are applicable.
Find the percent of strength to the solid iron in a single-riveted seam, 1/4 inch plate, 5/8 inch rivet, pitched or spaced 2 inch centers. First reduce all to decimal form, as it simplifies the calculation; 1/4=.25 and 5/8 inch rivets will require 11/16 inch hole, this hole is supposed to be filled by the rivet, after driving, consequently this diameter is used in the calculation, 11/16 inches=.6875.
First find the percent of strength of the sheet.
P-D
——-
The formula is P = percent.
P = the pitch, D = the diameter of the rivet hole, percent = percent of strength of the solid iron.
2 -.6875 ———— Substituting values, 2 = .66. Now of course you understand all about that, but it is Greek to some people.
So you see I have no apologies to make for following out my plain comprehensive talk, have not confused you, or lead you to believe that it requires a great amount of study to become an engineer. I mean a practical engineer, not a mechanical engineer. I just touch mechanical engineering to show you that that is something else. If you are made of the proper stuff you can get enough out of this little book to make you as good an engineer as ever pulled a throttle on a traction engine. But this is no novel. Go back and read it again, and ever time you read it you will find something you had not noticed before.
INDEX
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PART FIRST PAGE Tinkering Engineers . . . . . . . . . . . 5 PART SECOND Water Supply . . . . . . . . . . . . . . 31 PART THIRD What a Good Injector Ought to Do . . . 45 The Blower . . . . . . . . . . . . . . . 49 A Good Fireman . . . . . . . . . . . . 51 Wood . . . . . . . . . . . . . . . . . . 56 Why Grates Burn Out . . . . . . . . . . 57 PART FOUR Scale . . . . . . . . . . . . . . . . . 65 Clean Flues . . . . . . . . . . . . . . 67 PART FIVE Steam Gauge . . . . . . . . . . . . . . 72 How to Test a Steam Gauge . . . . . . . 74 Fusible Plug . . . . . . . . . . . . . . 76 Leaky Flues . . . . . . . . . . . . . . 79 PART SIX Knock in Engine . . . . . . . . . . . . 90 Lead . . . . . . . . . . . . . . . . . 92 Setting a Valve . . . . . . . . . . . . 94 How to Find the Dead Center . . . . . . 95 Lubricating Oil . . . . . . . . . . . . 103 A Hot Box . . . . . . . . . . . . . . . 109 PART SEVEN A Traction Engine on the Road . . . . . 111 Sand . . . . . . . . . . . . . . . . . 122 Friction Clutch . . . . . . . . . . . . 124 Something About Sight-Feed Lubricators 132 Two Ways of Reading . . . . . . . . . . 137 Some Things to Know . . . . . . . . . . 139 Things Handy for an Engineer . . . . . 159 Something About Pressure . . . . . . . . 184
End of Project Gutenberg's Rough and Tumble Engineering, by James H. Maggard