expression of cylinder pressure when between maximum service and maxispeaking of braking ratio clears the mum emergency braking ratio. question of all confusion; while fixing

If the foregoing is accepted as coron some specific figure gives this ratio a rect reasoning, we have then reached a definite value and affords a working starting point in the endeavor to accom. basis, or starting point, for general plish the purpose as expressed above, brake installations, but even this only namely, that 90 per cent braking ratio affords a means for discussing in a gen- be declared as the standard maximum eral way the question of braking ratio, braking ratio for service applications. its chief merit in this connection being If this be accepted, then the writer that it is free from the term which mis- would suggest that, with a pneumatic leads, misleading because it must give brake, this 90 per cent be obtained by the impression either that cylinder pres- the reduction of 24 pounds pressure in sure is indispensable to the expression the auxiliary reservoir. This reduction of braking ratio, or, that by its state- to be caused by a flow of air at an apment the expression is made full and proximately uniform rate from the complete, or both these, whereas, as a auxiliary reservoir to the brake cylinder, matter of fact, in neither case is it so. this rate to be such that the time of obtaining it is 7 seconds.

Proposed Standard Recommendation. These three requirements cover all With regard to the suggestion that with which we are concerned in the de90 per cent braking ratio, obtained by a sign and installation of the air valves. 24 pound reduction in the auxiliary From these we may go on to the leverreservoir pressure, be recommended as age ratio that should be considered the most logical I may say that it is not standard and the cylinder pressure upon the writer's intention to inquire fully which this percentage of braking ratio into the question of whether or not the should be based, but these are derived faclong used and generally accepted 90 per tors and not fundamental factors, since cent braking ratio for service applica- the leverage ratio permissible is detertions is either the scientific percentage mined by the car and truck construction or the best practicable percentage. and by the kind of foundation brake rigPersonally, he believes this to be the case and can furnish very good evidence that this is so, but whether so or not, it would be a very hard task in the first place to change it, and, in the second, the supplanting percentage would have to demonstrate marked superiority before it would be considered worth while to make the attempt to change, and, certainly, before it would be accepted. This is supposing that the change would be great. If it would not be so, it certainly does not deserve consideration.

Briefly, my reasons for suggesting that 90 per cent braking ratio be recommended as the standard are:

ging employed; while the cylinder pressure upon which the 90 per cent braking ratio must be based is determined by various other considerations, such as, type of brake equipment, class of service, etc. Please observe that the writer is not concerned with any specified set of figures, for he is willing to accept any change in these that shall be shown to have greater warrant than those submitted. What he is concerned with is the adoption of some criterion or standdard that will prevent many of the present vagaries and conflicting propensities. In other words, he desires that we may be able to say "that after due deliberation and consideration of all the interests, conditions, and other considerations, such and such a standard has been adopted and before you, whoever it may be, can hope for the acceptance of SECOND: This percentage braking what you proposed you must attack the ratio happens to be the critical point for standard and demonstrate that it can the installation of new apparatus and be obsoleted and conditions improved by any recommendation which calls for the acceptance of your proposition." higher than 90 per cent braking ratio Assuming now that 90 per cent brakfor a full service brake application ing ratio is accepted as the standard for would be open to the charge of requiring service brake operations, the solution increased expense and troubles which of the braking ratio problem for emerappear prohibitive. gency applications is simple and must THIRD: By keeping the braking proceed along certain definite, and not power down to 90 per cent for a full to be varied from, lines, for only two service brake application, it is possible avenues are open for its accomplishto obtain a safe and satisfactory margin ment, namely, (1st) by an increase in

FIRST: 90 per cent has been a common standard for years and, therefore, requires that sufficient reasons be advaned to justify a change and the writer knows none.

crease of piston area, is followed, again an unlimited value in braking ratio is possible, since the area of the emergency brake piston as compared with the service brake piston, may be of any ratio, requiring only that the necessary pressure or volume be carried to give the desired cylinder pressure.

cylinder pressure and (2nd) by an in- quirements, such as ability to release crease in brake piston area. The the brake, practical volume permissible, method by an increase in brake cylinder etc., will permit. volume is susceptible of accomplishment If the second method, namely, inin two different ways-(1) by increasing the auxiliary reservoir pressure until at equalization with the brake cylinder the desired emergency braking ratio be obtained, and (2) by an increase in stored volume to such a degree that at its equalization into the brake cylinder the pressure required to give the emergency braking ratio will be obtained. As a corollary from these two, it is evident that it may be advisable in some cases and necessary in others that both of them be employed.

With regard to the second, namely, increase of brake piston area, it is plain that to increase the piston area by an increase in the diameter of a single piston would change the service standards of ratio and time, and therefore, this is manifestly not a permissible method. We are reduced to the necessity of using an additional brake cylinder of such area, pressure considered, that the desired emergency braking ratio will be obtained.

Steps in a Brake "Lay Out."

At the outset we must decide the basis upon which the "lay out" must rest; that is, whether it is to be from some arbitrarily chosen braking ratio, in which case the retardation may be what it will, or from some specified retardation, in which case the braking ratio can only be determined after many other factors are known.

Lay Out with Arbitrarily Chosen Braking

Ratio.

Dividing then the subject into these two problems, and considering first the one starting with an arbitrarily chosen The conclusion from these statements braking ratio, we would particularly call is that 90 per cent braking ratio ob- attention here to the fact that when some tained from 24 pounds reduction of aux- values are decided upon or chosen, others iliary reservoir pressure once being ac- with which many people seem to think cepted, the means and method of ob- they can play shuttlecock and battledore, taining the emergency braking ratio is must necessarily follow if we are to absolutely fixed. Reflection will also have a consistent whole. Commencing show that the quantity of emergency with (a) a car of known weight, (b) braking ratio may be unlimited, if it be the braking ratio to be employed, it is obtained, by an increase of auxiliary next necessary to fix upon values, for reservoir pressure. The practical limit (c) the range of flexibility, that is, the is determined by what brake pipe pres- decrease in auxiliary reservoir pressure sure may be carried. For instance, if to be permitted before the braking ratio the 90 per cent braking ratio was based is realized, (d) shoe clearance, (e) upon 60 pounds cylinder pressure and brake piston travel, (f) the cylinder the 60 pounds cylinder pressure was pressure from which the braking ratio is obtained by a 24-pound reduction, it is to be obtained, (g) the fiber stresses to plain that the pressure carried was 84 be permitted, and (h) if the "emergency" pounds, assuming that the 24 pounds re- braking ratio is to be greater than that duction produced equalization of auxil- for "service," this must be stated in iary and brake cylinder pressures. quantity. These are thought to be all Therefore, if it was desired to obtain of the necessary factors or absolute 180 per cent braking ratio in emer- values involved in a brake "lay out" gency, it would be necessary to carry an where the braking ratio is specified, but auxiliary reservoir pressure of 168 there are certain other values which we pounds, equalization again being ob- must know; for instance, (i) the prestained. If the increase of emergency sure that must be carried, (j) the auxilbraking ratio is to be obtained by an in- iary reservoir volume required, (k) the crease of reservoir volume, (called sup- leverage ratio, (1) the size of the brake plementary reservoir) it is likewise cylinder, (m) the size of the brake levplain that 150 per cent ratio is the limit ers, etc. But these are all contingent that could be obtained in emergency, for factors, or derived values, and must be the reason that from 110 pounds reservoir obtained from the others. In other pressure carried, 100 pounds brake cyl- words, they cannot be arbitrarily deinder pressure is as high an equaliza- termined, or fixed upon as can all of the tion in the brake cylinder as other re- preceding fundamental elements. Το

specify them without deriving them, Choosing Arbitrary Values. would be an unwise risk, as the result All the inconsistencies that exist in would probably be the expression of con- brake design and installation today are tradictory or inconsistent values, and due to the non-observance of these rewe submit the following (which makes lationships, that is, one or more has evident how the derived values are se- been changed without changing the cured from the fundamental values) in whole, and this cannot be done if the proof of this. design is to contain but one set of The dimensions. Remember the physical assertion is made that it cannot be done if a practical brake is to be had-the answer will be, that it is done. This, we admit, but with chaos and controas the inevitable result. versy Then

car weight times the braking ratio specified gives the total shoe pres

sure.

The piston travel divided by the shoe clearance gives the leverage ratio.

The total shoe pressure divided by the leverage ratio will give the total cylinder pressure required.

The total cylinder pressure divided by the unit cylinder pressure will give the area of the piston, which fixes the size of the cylinder.

The volume of the cylinder multiplied by the unit cylinder pressure and divided by the range of flexibility will give the volume of the auxiliary reservoir and this fixes the size of the auxiliary reservoir.

follows the next statement of, I don't know, things are not so bad, and certainly better than if we had no brake at all. Admitted again, but with the question why is it as bad as it is, when it is far easier and much more profitable to all concerned to have the design as it should be.

It seems to the writer that the chief reason why the idea prevails among so many that the derived factors or values can be juggled arbitrarily, as can the fundamental or absolute values, is that, previously a distinction has not been emphatically drawn between them, showing that they will no more mix than will

The unit cylinder pressure plus the range of flexibility will give the necessary auxiliary reservoir pressure which fixes the brake-pipe pressure to be car- oil and water.

ried.

some other

Another reason is that a number of The total cylinder pressure with the the factors have at sometime been fixed total shoe pressure and fiber stresses to by the manufacturers, or be permitted gives all the necessary in- authority, and the reason for this formation that is required to design the largely forgotten, or an analysis is not foundation brake gear, the particular made to see why they were so fixed. In form which this shall take depending other words, it is not understood that upon car and truck design and the de- practical considerations fix the absolute signer's preference. (It is my personal values, and therefore, the others are a judgment however, that only a "clasp mathematical result. brake gear" should be employed for modern heavy cars. I have had one or two experiences where the very best air brake mechanism was made very ineffective by an inefficient brake gear.)

If the brake is a duplicate brake, that is, having a service braking ratio and an emergency braking ratio, the size or strength of the foundation brake gear must be based upon the maximum total cylinder pressure.

Still another reason is that some not knowing that the factors involved in a brake "lay out" are composed of arbitrary and derived values, mix them indiscriminately and give an arbitrary value to one that must have a derived value. For example, they think that the auxiliary reservoir volume can be arbitrarily fixed, when, as a matter of fact, this cannot be done.

So much then for the considerations where the problem commenced with an arbitrarily chosen braking ratio.

Stops in a Brake Lay Out When Braking

It will be seen that not one of these eight values can be included among what we have called the fundamental elements, and further, and even more important, not one of them can be arbitrarily chosen, but must follow in the same manner that four follows the adding of two and two if we are to have a harmonious and consistent brake, and to avoid this being dismissed with the assertion that the ideal is not attainable, I will state that by harmonious and consistent, I mean a practical brake. preceding

Ratio is Not Arbitrarily Chosen. We will reserve for the present our conclusions from the discussion of this first problem and will now consider the second, namely, that one in which either the length of stop or the rate of retardation in miles per hour per second is specified. In this, all the factors of the problem necessarily enter.

However, this problem must be ap- will fix the braking ratio required as proached in an entirely different man- shown by the following. ner. It should be obvious that other data must be either known or assumed, after which the braking ratio that must be employed is fixed unalterably by a strictly mathematical relationship.

First, we must ascertain the retard ing force necessary to produce the specified stop or retardation; then whether the length of stop desired or the retardation in miles per hour per second specified is a physical possibility and to do this, we must either determine or sume the capacity of the rail for adhesion.

as

We have a coefficient of friction of 10 per cent and an efficiency of foundation brake gear of 90 per cent, a maximum cylinder pressure of 100 pounds and a retarding force required of 12,000 pounds. For the first case, the braking ratio will then be 12,000 (.10 × .90 × 100,000) 133 per cent. This however, is without taking into consideration the time required to get the brake fully applied which in the equipment we are considering is in a mean time of 1 second. This is equivalent to the vehicle running 1 second or 88 feet without Therefore, the stop the brakes applied. must be actually made in 912 feet which will require an average retarding force of (1 × (100,000 ÷ 32,2) × 88 × 13,140 pounds. There88) ÷ 912 fore, the actual braking ratio (.10 X .90 sary is 13,140 100,000)

=

=

neces

X

146 per cent. The next step is to satisfy ourselves that this braking ratio is possible of employment without the likelihood of wheel sliding, and to do this, it is necessary to know whether or not the brake shoe pull required will exceed the rail pull on the wheel.

X .10

Since we know that

To arrive at this, we must work with the figures given in the specification which we will assume to be a stop in 1000 feet from a speed of 60 M. P. H.. and in a second case, a retardation of 3.0 M. P. H. per sec. from the same speed. In furnishing the specification for the retardation, it is obviously both necessary and possible to include the weight of the vehicle therein, which in our case we will take to be 100,000 pounds. We now have all the factors from which to ascertain what our average retarding force must be, namely, (a) a speed of 60 M. P. H. (b) weight of vehicle in pounds, (c) distance in from 20 to 25 per cent of the weight of which the stop has to be made, that is, the vehicle on the rail can be realized the retarding force in pounds will equal in rail pull, or adhesion, it is clear that one-half the weight of the vehicle di- we are entirely within the limit, since vided by 32.2 times the square of the the brake shoe pull will be 1.46 x .90 speed in feet per second divided by the 13 per cent of the weight of length of the stop in feet. For the first the vehicle, while the potential of the case taken, the retarding force will be, rail is 20 per cent of the weight of the vehicle. We will not work out the sectherefore, ( × (100,000 ÷ 32.2) × 88 X 88) 1000 ond case as it would be merely a repeti12,000 lbs. For the tion. second case, the retarding force is secured by changing the M. P. H. to feet per sec., dividing by 32.2 and multiply- The foregoing analysis is necessarily ing by the weight of the vehicle, that is, quite lengthy, still it is not complete, (3 × 1.466÷32.2) × 100,000 13,630 at least as far as the reasons for some pounds, the retarding force required to requirements are concerned. It is a long meet these specifications. With the av- and obtuse subject, particularly when erage retarding force now known, the all that is involved is considered. So next step is to convert this to the brak- far as the writer knows this is the first ing ratio required, and to do this, we time that the braking ratio problem or must know the characteristics of the science has been treated comprehenphysical equipment to be employed; sively, the usual discussion being only that is, the coefficient of friction which of parts or phases or sections, which has will be realized (and in the case we are only served as a rule to make conconsidering, we will assume that the fusion worse confounded, both in practice maximum and minimum coefficient real- and when real problems have presented ized varies but little from the mean themselves. Doubtless some opine that which we may well do under the con- what has been here presented has not ditions); the efficiency of the founda- made the matter simple by any means. tion brake rigging; the maximum cylin- No, but neither does a text book on any der pressure attainable, and the time in science, Calculus, for instance. Its purwhich the maximum cylinder pressure is pose and usefulness consists in developobtained. The combination of all these ing the science completely, which, of

=

Conclusion.

leaves simplicity, or other- Fig. 6 is a detail perspective view of one wise, as a quality of the subject-this of the clamping jaws. being greater or less to the individual according to his aptness and previous knowledge. It must, of necessity, leave comprehension and understanding to the patience and diligence of the student, whose competency, both to judge of its value and to use it in practice, will depend upon how completely he gets to know it and his ability to apply it in practice.