Car lubrication

Preface

Some years ago the subject of car lubrication became one of much interest to the writer, and, when attempting to acquaint himself with the laws influencing successful practice, he was surprised to find how very little information, either of a theoretical or practical nature, could be obtained. The accompanying pages are not presented as a solution of the question, or as containing any important original research, but rather in the hope that some of the knotty problems which were then presented for solution may be made clearer.

The laws of friction, as accepted until recently, have by later experiments been limited in their application, if not confined entirely, to solids in contact. It is at least certain that they will not apply in any way as the laws governing the friction of solids separated by a lubricant. Prof. Thurston's " Friction and Lost Work," and the experiments of Mr. Woodbury with those of Mr. Tower, are quoted, and several others to a limited extent — to whom, it is hoped, proper credit has been given.

It is desired that these few words may stimulate further thought, and in that way result in a more satisfactory solution of the problem.

The Author.

Preface To Second Edition

Since the first edition of " Car Lubrication " was published the subject has received some attention by the railroads, as will be noted by the " Report of the Committee on Lubrication of Cars," contained in the Proceedings of the Master Car-Builders for 1894, and the article of Dr. Dudley and Mr. Pease in the February number of The Railroad and Engineering Journal, 1892, all of which confirms the conclusions that had been reached by the investigations of the author and set forth in the first edition. The subject, however, has not, by any means, received the consideration it deserves.

Attention is drawn to the continual drift towards the softer metals, and it now looks as though a comparatively hard white metal in shells of iron or hard brass would be finally adopted as the best bearing metal.

In the second edition a number of typographical errors that accidentally appeared in the first edition have been corrected.

The Author.

CAR LUBRICATION.

Introduction

In taking up the study of the subject of car lubrication we shall find it necessary to consider the features in the construction of the car-box and the proportion of the different parts, in addition to the care that should be given to lubrication, so that the journal will be properly supplied with oil and kept in a well-lubricated condition. While there is no reason why it should be so, yet it is a fact that there is probably no other branch of railroad engineering that has been so dependent upon empirical laws and where the practice has been at such variance with the heretofore accepted laws governing it.

The widely taught law that friction is independent of the extent of surface in contact, but varies only with the pressure, is about ready to be placed among the archives of ancient scientists. The pressure inferred in this relationship is that exerted over the whole surface, and not per square inch — that is, a surface one square foot in area under a pressure of one pound per square inch would require the same force to move it over a resisting surface as it would if made one square inch in area under a pressure of 144 pounds per square inch.

I

2 CAI? LUBRICATION.

The extent of surface in contact was supposed to have no effect upon the force or work of friction necessary to move one body upon another, and consequently required no increased effort to produce motion, provided the same total pressure was exerted although the area of the surfaces in contact might be at variance.

Recent experiments made with various grades of lubricants, to determine the coefficient of friction of lubricated surfaces under varying conditions, prove conclusively that the amount of surface in contact materially influences the work of friction. If the relationship of the/' resistance of friction as independent of the area of surfaces in contact, but dependent upon the pressure," were true, the temptation would be to reduce the work of friction and the abrasion of the materials by increasing the area of the surfaces in contact, which would allow the use of a lighter oil by reducing the pressure per square inch without increasing the abrasion. Practical demonstration, however, has proven the necessity of avoiding long journals, and, with the friction of rotation, an increase in the diameter of the journal means a corresponding increase in the work of friction.

While recent investigations have not by any means furnished data that enable the subject of car lubrication to be reduced to the desired degree of efficiency, they give information that is of much value for guidance in the design, construction, and management of lubricated surfaces, and results that will be found to accord quite closely with those obtained from practice. They indicate, and quite conclusively, that when the rubbing surfaces are kept well separated by the lubri-

IN TROD UCTION. 3

cant the friction is more dependent upon the nature and fluidity of the lubricant than upon the nature of the sohds carrying the load.

There seems to be a combined friction consisting of that inherent in the particles forming the lubricant and of the moving surface in contact with it. With constant pressure and temperature, it is dependent upon the extent of surface in contact and varies directly with it. It is also influenced by the unit pressure, and varies in some ratio with the change in the load, but not in the same ratio as had been previously supposed.

As the resistance of lubricated surfaces is made up of the resistance of the particles of the lubricant, it is evident that any influence that will change the fluidity or density of the lubricant will also affect the frictional resistance.

Increase of temperature, increasing the fluidity, causes a decrease in the coefficient of friction ; while increase in unit pressure causes an increase in the density of the fluid, and, necessarily, an increase in the friction when motion is produced.

The ideal condition of lubrication is attained when the viscosity of the lubricant at the working temperature is sufficient, and no more, to keep the surfaces of the solids apart under the maximum pressure they may have to sustain.

We should always bear in mind that frictional resistance and the abrasion of the surfaces represents an expenditure of money, and, in the aggregate, is of greater moment than is generally supposed.

The subject will be treated in the following chapters by taking up the different parts in detail and then considering the relationship that must exist to give the most economical results.

It will be considered under the two following general heads*

1st. The proportions and materials that are required to meet the demands of the service.

2d. The most economical way in which these may be applied.

THEORETICAL RELATIONS.

Chapter II. Theoretical Relations

The resistance of friction in car lubrication is that which is generally known as "sliding friction of rotation." It is similar to linear motion, but, as it is an arc of contact, it differs in the distribution of the load per unit of surface.

When the bearing is first placed upon the journal the arc of contact is small, and it is only after wear has taken place that the whole arc included within the bearing is in contact with the journal. The amount of wear which is necessary to produce this condition is comparatively small unless the radius of the bearing is made much larger than that of the journal, which must be classed as bad practice.

It will be found that the larger part of the pressure is taken at the top of the journal, and decreases in a determinable ratio from that point to the horizontal axis through the centre of the journal. The work of friction is then but a question of the space which is passed over against the frictional resistance which is offered to the rotation. The space in this case is a function of the circumference, and varies as the diameter of the journal.

The law of the distribution of the pressure is as follows ;

• CAE LUBRICATION.

Let (see Fig. i)

P z=. vertical pressure on unit surface ; P^ =. pressure in a radial direction on unit surface ; R = radius of the journal ;

GO = angle made by the radius from a point O with a vertical line through the centre of the journal ; /= length of bearing; L = total load carried.

For any point, O,

The pressure upon a surface Rdo) is PJRdao.

THEORETICAL RELATIONS. 7

The summation of the pressure should be for an equal arc on each side of the vertical, or

L= r PJRdoo ;

and by inserting the value of PJ

^— j PIR cos GO doD,

From this, the load carried by various subtended arcs can readily be obtained. The accompanying table indicates the relative loads for several values of oo.

Value of

Average pressure carried per square inch.

Square inches of surface when radius equal unity.

Percentage carried by the first 10° of the arc.

50°

1.28558 1.53209

0.1745 0,3490

0.5235 0.6980 0.8725

100.00

50.78

34-73 27.01 22.67

It is then evident that a higher percentage of the pressure is taken by a small arc of the journal, and that the lower surface of the arc of contact is the least important part of the distributing surface. It will be noticed, too, that as regards the distribution of the load there is no serious objection to giving the bearings a surface contact that is less than the width of the bearing, for instance a width of bearing surface of three (3) inches on a journal of four inches diameter. It also follows that the practice of boring out the bearing to a greater radius than the journal is open to no serious objection, but, on the other hand, must be done to prevent the bearing seizing the journal, providing

CAR LUBRICATION.

the difference in the radii is not made too great. When this difference is excessive a condition is obtained similar to that shown in Fig. 2, which will result in very poor lubrication and likely produce a heated journal, the sharp corners at A having a tendency to scrape the oil from the journal. This effect will be made more apparent further on. In practice it is found that the best results are obtained when the radius to

Fig. 2.

which the bearing is bored is about one thirty-second (sV) ^^ ^" hyoh greater than the radius of the journal. This gives a safe working margin for a new bearing and journal, and yet does not give too great a difference or opening at the sides of the bearing when a new one is placed on a journal that is worn to the minimum diameter. In cases where journals of different diameters^ are used bearings corresponding to these different sizes should be kept on hand. In any case the possibility of any heating of the journal on this score can be obviated by using the lead-lined bearings.

THEORETICAL RELATIONS. 9

Before stating the elements determining the work of friction, it is necessary to review in a general way the results of recent experiments which were made to determine the laws governing the resistance to motion of bodies when separated by a lubricant. Reference is made particularly to the experiments by Woodbury and Tower, the results from both of which, while under different conditions, are corroborative. Those by Woodbury were made with low pressures, and the curves obtained from his results give a relationship of pressure and coefficient of friction, as shown in the diagram, marked as Fig. 3. They show conclusively that frictional resistance, with an intervening lubricant, is not a direct ratio factor of the pressure, as it is with unlubricated surfaces ; but, on the contrary, the laws seem to follow those of fluid friction more closely than those of solids. The result produced by the motion of two solids underpressure is a more or less rapid abrasion of the metals in contact. With a lubricant interposed the conditions are quite changed, and follow more closely the resistance which the fluid would offer by its own friction. Whether or no this be a motion of the particles of the lubricant or of the solid upon the surface of the fluid does not concern us. The important consideration is the extent of surface in contact which should enter as an element in the calculation of the work done. The best way for our purpose is to obtain the frictional resistance of the lubricant at the pressure carried, reduced to the resistance under these conditions for a unit surface. The resistance of friction would then consist of two elements : the coefficient of friction per unit of surface for the working pressure and temperature, and the number of units of surface in contact. Representing these by/" and a respectively, would give as the relationship of the total work of friction

n = number of revolutions per unit of time ;

/f = pressure per square inch.

With given pressure and temperature the minimum value of the function/", a is determinable.

First, the pressure on the journal and the temperature attained under the maximum speed will indicate the density of the lubricant which it is necessary to use to prevent the surfaces coming in contact, and in the case of car lubrication will vary with the seasons of the year, causing a grading of the oils into those for summer, and lighter ones for winter use. Second, the value of a will depend npon the available space allowed for the journal. It will be seen further on that the results of the experiments indicate that the most economical conditions are obtained by increasing the area, within practical limits, and using a correspondingly lighter body oil for the lubricant.

The value of a for the most economical results is where any further decrease in the resistance by the use of a more fluid oil is counteracted by the increased resistance resulting from the larger surface in contact. More explicitly the experimental results indicate that, within practical limits, a lubricant of greater fluidity and correspondingly lower coefficient of friction can be used by increasing the area, and in that way reducing the unit pressure until the limits are exceeded, when

THEORETICAL RELATIONS. II

any further increase of the contact surfaces produces a reverse effect. It will be remembered we found that the additional surface obtained by increasing the arc of contact does not produce a proportionate decrease in the pressure per unit of surface, so that the oil that is selected for the purpose must depend upon the pressure that exists at the top of the bearing. Practically, an arc of contact of some magnitude is necessary for strength and stability, and to give a fairly large area to accommodate for the abrasion which also takes place. Any increase beyond this arc is economical only so long as the increased surface decreases the pressure carried at the centre of the arc to an extent that the lighter oil will, by the consequent reduction in the coefficient of friction, overbalance the increased resistance produced by a greater area. Assuming c as the constant and necessary arc of contact and w as the desired angle, it is not economical to increase go when the expression is greater than

/and/' indicate the coefficients of friction per unit of area for the lubricant which must be used to overcome the maximum pressure existing in the two cases which, in one sense, measures the fluidity of the lubricant. This is on the basis of the diameter and length of the journal remaining constant.

An increase in the length of the journal seems to be advantageous, provided it is not carried beyond the limits placed upon it by practice. The diameter of the journal is dependent upon its length and the load to be carried. Taking the usual expression for a beam supported at one end and uniformly loaded, we have

_ TTtR'

U

where T ^= safe ultimate load for the metal, and the other symbols indicate the same as in previous formulae. For the deflection we have where d represents the deflection. All are indicated in pounds and inches.

The formula for the variation of the diameter for changes in the length will be used again.

Chapter III. Coefficient Of Friction

With the exception of the method of lubrication, there is no other element in connection with the subject under consideration that has received more attention than that of the coefificient of friction, and yet there is no other that is in as crude and indeterminable a state. As investigation progresses, the subject seems surrounded with more and more variables of a complicated nature, which indicate the importance, if not necessity, of the utmost care when the best results from lubrication are desired.

The latest study of the subject has brought out some very interesting results, and has conclusively shown that it is now necessary to at least limit the old laws of friction to dry surfaces in contact, if not exclude them totally. The resistance of friction, when a medium is introduced between the so-called rubbing surfaces, follows laws quite different and more intricate than those determined by Morin, which were to the effect that " friction was independent of the surface in contact, but directly dependent upon the pressure keeping the surfaces together."

Where friction is produced it is important to distin guish between the two conditions to which the two sets of laws apply ; in one it is a solid against a solid, the particles of each interlapping and causing resistance by the efforts of the particles of one metal to tear away those of the other. Where lubrication is introduced it is intended that the two solids shall be separated by a film of the lubricant, generally a liquid. In this latter case the resistance assumes the nature of the laws of fluids, and consists of the friction of the particles of the lubricant and that of the solid against the fluid, forming a combined resistance, the percentage of each to the whole retardation depending upon the nature of the lubricant and the metal surfaces. As long as the metals are prevented by the lubricant from coming in contact, it is found that the friction is dependent upon the fluidity of the lubricant, and varies with changes of this fluid condition, decreasing with a higher temperature and increasing with a less degree of heat.

We will assume, first, that the lubricant prevents any contact of the metal surfaces. The condition then stands between the laws of solid friction on the one hand — that is, independent of the surfaces in contact, but dependent upon the total pressure, — and the laws of friction of liquids on the other, where it is independent of the pressure per unit of surface, but is directly dependent upon the area and increases as the square of the velocity. From most recent investigation this intermediate condition has been found to be, when stated in a general way, that the coefficient of friction decreases with an increase of the pressure, although the total resistance rises directly but not proportionately with the higher unit pressures and increases with the velocity, although not as rapidly as its square. It is also found to be dependent upon the extent of surface in contact. An exact relation between these varying

Coefficient of FRictioht. i^

conditions has not yet been obtained, evidently because they vary so materially with any slight variation in the method used of lubricating the surfaces. As, for instance, when the oil-bath is used the laws of lubricated surfaces, especially as regards surface and pressure, follow those of liquid friction very closely ; while with less efficient means of lubrication the results show a condition between solid and liquid friction. This matter will be brought out more prominently in the chapter on the methods of lubrication.

With the surfaces in good condition and the oil-bath method of supplying the oil, which may be considered as practically perfect lubrication, it was found that the mean resistance per square inch of surface, with pressures varying from lOO to 310 pounds per square inch, was as follows :

Lubricant. Mean resistance in pounds.

Sperm-oil 0.484

Rape-oil 0.5^2

Mineral-oil... 0.623

Lard-oil 0.652

Olive-oil 0.654

Mineral grease 1.048

[Results obtained by Tower. — See Engineering for November 16, 1883, and Feb. 6, 1885.]

The speed was 300 revolutions per minute, and journal four inches diameter and six inches long, while the temperature was maintained at 90° Fahrenheit.

A constant temperature is essential for a proper comparison, for in one case the coefficient of friction of lard oil decreased to one third (^) its value at 60° by increasing the temperature of the lubricant to 120° Fahrenheit.

l6 CAR LUBRICATION.

Probably the most accurate laboratory experiments that have been conducted for the determination of the resistance of lubricating oils were those made by Woodbury for the North-Eastern Cotton Manufacturers' Association, as published in their proceedings of April 28, 1880, and in the proceedings of the American Society of Mechanical Engineers, as contained in volume VI. They were made with the object of appropriating the results to cotton and woollen machinery where low pressures are used, and to that extent are not well adapted to the lubrication of car journals excepting as showing the action of lubricants under varying conditions of temperature and a limited range of pressure. They were presented about the same time as the results of Mr. Tower's experiments, the latter, however, under heavier pressures, but both clearly showing the different conditions under which friction must be studied when solid surfaces are lubricated by such bodies as the mineral and animal oils. It was found in the tests that uniform results could not be looked for unless constant temperature, velocity, pressure, area of surface in contact, and thickness of the film of the oil between the surfaces were maintained, the latter depending somewhat upon the method of lubrication, the results indicating clearly that the resistance of friction was dependent upon and changed with a variation in any of the above conditions. The metallic surfaces were cast iron and bronze, the latter composed of copper 32, tin 2, lead 2, and zinc I. In one case all conditions were kept constant excepting that of pressure, the diagram represented as Fig. 3 indicating a decrease in the coefficient of friction, with

COEFFICIENT OF FRICTION.

increased pressure per unit of surface, but an increase of total resistance. Similar tests were made, keeping the pressure constant and varying the temperature, which gave the eJEfect of the variation of the temperature upon the coefficient of friction.

The two diagrams, Figs. 3 and 4, indicate by the two curves the variation which takes place in the coefficient of friction under the conditions named.

In Fig. 4 it will be noticed that the variation in the

Coefficient

130 -110

poo

1 90

ig 80

.ao

Is

I r

V i c

JO . oef. of

30 .'■ Friction

If

.1 c

:i

oot-of

U .4 Frictio

Fig. 3.

Fig. 4.

coefficient of friction due to changes in temperature follows closely the laws of the straight line, indicating a proportionate decrease with the increase in temperature ; the angle of the line with the abscissae depending upon the pressure per square inch.

Combining these two diagrams gives a curve, as shown in Fig. 5, for a coefficient of friction where the two elements, pressure and temperature, vary ; and it is this relationship which most concerns the lubrication of surfaces such as car journals, as in this case the ternlo CAR LUBRICATION^

perature is subject to variations arising from changes of seasons and weather, while the pressure carried per square inch is dependent upon how long the bearing has been subjected to wear and attrition, the unit presssure decreasing with increase of service. While the results obtained by Woodbury are the most accurate that have been published and probably ever made, both as regards design of apparatus as well as its manipulation, there still lacks sufficient uniformity for the derivation of a definite law as to the variation of friction with changes in temperature and pressure. For instance, the decrease in the coefficient of friction for pressures of from one (i) to five (5) pounds per square inch is as follows:

Pressure per square

inch.

Pounds.

Coefficient of Friction.

Decrease in

Coefficient of

Friction,

Difference in

the amount of

decrease.

4 5

The decrease in the coefficient of friction from an increase of pressure of one (i) to two (2) pounds was 0.0617 more than that from two (2) to three (3) pounds, while the increase from three (3) to four (4) pounds was 0.0193, and nearly the same as took place when the pressure was increased from four (4) to five (5) pounds. The above is cited more to prevent a deduction of too wide a nature rather than to deter from the gratitude which the engineering profession must feel for the derivation of the general law of the variation of

COEFFICIENT OF FRICTION.

friction with lubricated surfaces when the temperature and pressure are varied. The care which it was necessary for Mr. Woodbury to exercise to obtain these results can be appreciated when it was found essential to run the apparatus, using gasoline or its equivalent, to clean an oil from the surfaces after testing. It required a travel of one surface over the other equivalent

12 3 4 5

Fig. 5.

Note. — The coefBcient of friction in the three cases is represented in actual pounds resistance.

to about forty (40) miles before it was advisable to commence the trial of the succeeding oil, and even then indications could be noticed in the test following of the properties of the oil previously tried. This is considered further evidence that the friction of lubricated surfaces is made up of the friction of the fluid, and tends to prove that the lubricant imbeds itself into the surface of the metal, producing a fluid resistance rather than a resistance due to the rubbing of the surface of the metal upon the surface of the lubricant. This effect will be taken up again in the chapter on Bearing Metals,

The variation in the coefficient of friction with changes of temperature can readily be carried to an extreme, as it has been found that while the resistance decreases as the temperature is raised, there is a point, depending upon the unit pressure and viscosity of the lubricant, where the coefficient starts to increase very rapidly with increase of temperature. The same holds true with a variation in the pressure ; and while the laws of changes are true as stated, in a general way, they depend and are limited by the viscosity of the lubricant used, and also the pressure which it is necessary to carry. The rapid increase, when the limits of temperature and pressure are exceeded, is due to the solids coming in contact and causing increased friction by the abrasion of the surfaces, reducing the condition from friction of fluids to that of solids.

An attempt has been made to prove a positive relationship between the viscosity of an oil and its coefficient of friction ; and while they are, no doubt, more or less dependent, there is hardly sufficient data at hand to resolve this to a definite basis. In addition to that of viscosity lubricants possess a property designated as unctuousness, which seems to influence the coefficient of friction as much, if not more, than the viscosity. It appears, and there seems sufficient information at hand to anticipate it, that a relationship of a positive and determinable nature between the three elements, coefficient of friction, viscosity, and unction, is obtainable.

The results of the experiments made by Mr. Tower, as presented before the British Institute of Mechanical Engineers, are of such a nature that they can readily be converted into practice with a resulting profitable

C0EFFICIEN7' OF FRICTION. 21

application. They go to corroborate, to a close degree, the results of Mr. Woodbury, although they have the advantage of having been made with higher pressures. It should be remembered that with high pressures, such as those obtained before seizure takes place, the film of oil separating the bearing and journal has been found to be very thin, and shows the result that maybe expected where the irregularities or projections on the surface of the bearing are greater than the thickness of the film of oil used to separate them, producing when in motion a rapid and detrimental abrasion of the metals with a marked increase in the friction. It would not be safe to allow the irregularities to project from the surface more than the thickness of the film of oil, unless, of course, the prevailing area is of this height. The other extreme, of having the surfaces too highly polished, must also be avoided, as it has been found that a moderately rough machined surface will carry something like seven (7) times more pressure before seizing than can be obtained from highly polished surfaces. The proper condition would seem to be about that produced by a boring tool, except with the soft metals, which, from their-nature, are incapable of taking a high polish.

For the purpose of comparison let us consider briefly some of the results obtained by Mr. Tower where the journal was lubricated by the oil-bath method and the surfaces were in good condition. Assuming a loaded car of total weight of 80,000 pounds would give 10.000 pounds per journal. At a pressure of 300 pounds per square inch this would require a bearing area of 33.33 square inches to carry the load. With the resistance given for mineral oil of 0.623 pounds per square inch on a journal 4 inches in diameter and a wheel 33 inches in diameter, a tractive resistance of 2.52 pounds per journal, or of 0.504 pounds per ton, would be required after motion had been produced. With the latest dynamometer readings the resistance of journal friction for loaded cars will probably reach as low a figure as 2 pounds per ton on level tangent when running at a speed of 15 miles per hour. Retracing, this figure gives a journal resistance of 82.5 pounds, and as high a figure as 2.48 pounds per square inch of bearing contact. It will be seen how low an efficiency is obtained in practice, but it should be remembered that the above laboratory tests were with the oil-bath method of lubrication, which has proven, when so tried, to be far superior to any other method that is at present known for lubricating surfaces. In most cases it has been found that the resistance of friction is a direct proportionate function of the area of surfaces in contact ; twice the bearing surface, all other conditions remaining the same, will give, approximately, twice the resistance from friction.