INDUSTRIAL A N D ENGINEERI,VG CHEMISTRY
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Vol. 18, No. 5
LUBRICATION SYMPOSIUM Papers presented before the joint meeting of the Divisions of Industrial and Engineering Chemistry and Petroleum Chemistry a t the 71st Meeting of the American Chemical Society, Tulsa, Okla., April 5 to 9, 19-06.
Introduction By Robert E. Wilson, Chairman S T A N D A R D OIL
CO.
( I N D I A N A ) , U’HITISG,
IND.
HIS, the first symposium on lubrication of the AMERI- nection with this st’ruggle of opposing forces. The smoothCAN CHEMICAL SOCIETY, reflects the increasing interest ness of the bearing surface, the clearance, the length and
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which chemists have lately been taking in this alinost universal problem. I n order to round out this program as a picture of the present state of the art we have had to call upoii a number of engineers from outside of our ranks. That they have been willing to join with us is the best possible argument for holding symposiums such as these, because it is only by planning long in advance and soliciting papers from the recognized leaders in a given field of research that we can draw together such a group of men from widely separated spheres of actkity. Thus, we have represented on our program the outstanding men on lubrication research from our largest industrial and scientific laboratories, including four of our largest oil companies, the Mellon Institute, the Bureau of Standards, our two principal manufacturers of electrical equipment, the outstanding manufacturers of thiust bearings, a large maker of internal combustion engines, and the principal manufacturer of graphite. The response from our university laboratories v a s rather disappointing so far as numbers are concerned, only one being represented here. It seems that the universities are overlooking a field of investigation to which they could contribute much, though the fault probably lies with the industries concerned in not encouraging and supporting this important line of work. The percentage response from oil company laboratories was also lower than that from the outside industries solicited. I n view of the vital interest which our industry has in this common problem, it is hoped that the efforts of our technical staffs will not be concentrated upon suitability tests and cut-and-try compounding, to the exclusion of fundamental research on lubrication which is the life blood of real progress in this field, even though it may iiot add to the sale of Compound Yo. 11,246. Theory of Bearing Lubrication
d number of papers refer to what might be termed the modern theory of bearing lubrication, without going into a detailed explanation thereof. It therefore seems worthy of a brief description, because it has not only been the means of correlating much past work which seemed conflicting or meaningless, but it has, in the past three or four years, led to several marked advances in our knowledge. While it is not yet universally accepted, i t has certainly been meeting the test of time. I n an ordinary bearing we know that the rotation of the journal, aided by the viscosity of the oil, tends to drag a fluid film of oil between the bearing surfaces. The bearing pressure, on the other hand, tends to squeeze out this film, and the conditions which prevail in any bearing depend upoii the balance between these opposing forces. Mr. Howarth’s graphical analysis brings out many interesting points in con-
diameter of the bearing, the method of oiling, and the oiliness of the lubricant are also important in det,ermining the amount of friction and the danger of abrasion. Until recent years these nine variables had all been considered independent, but Herseyl was, it is believed, the first to point out the fact that f , the coefficient of friction, being a factor “xit’hout dimensions,’’ could not be a function of these nine independent variables, but only of their combinations in such forms as would also be without dimensions. This made possible the grouping of the three principal factors controlling the operat’ion of a given bearing-namely, the viscosity % , the revolutions per minute &Y,and the bearing pressure P-in the form %N/’P. This means that the effect o n the coefficient of friction of doubling the speed, doubling the viscosity, or halving the load must be identical. The same is, of course: true of the effect on the thickness of the fluid film. The theory also indicated that the coefficient of frict’ionmas a function of the ratios of clearance to diameter and of length to diameter rather than of the three variables independently. This not only reduces the number of mriables, but’ as most ordinary bearings, even of widely different, size, have generally similar ratios of clearance to diameter and length to diameter, they should give similar relationships between the coefficient of frict.ion and X S l P . Previous Studies
These principles were first employed by Barnard and Kilson2 to correlate a great mass of data from the literature which served to verify the principles and give a clear idea of the nature of the functions. Figure 1 from that first paper shows the type of curves obtained when f is plotted against P. At, the right-hand end of this curve, where the values of ZN,’P are large-that is, a t high speeds, with viscous oils, or with low loads-the coefficient of friction is high and the fluid film is thick. As Z X j P decreases, the coefficient of friction drops off in almost direct proportion, and the fluid film gradually thins out. Eventually, however, the film thins so far that the surface irregularities begin t o touch, the friction rises sharply, and abrasion begins. This critical point of film rupture is probably the most important characteristic of a bearing. At st’illlower values of Z N / P abrasion is likely t o be serious, and the friction rises rapidly towards the static friction value, which is often fifty to one hundred times as great as that a t the critical point. This first paper2 also showed something of the effect of oil grooving and clearance on the shape and location of the curves in the fluid film region, and indicated that while the oiliness of the lubricant had no effect in the region of fluid 1
Trans. A m . SOC.Mech. Eng., 37, 167 (1915).
2
THISJ O U R N A L , 14, 682 (192’2).
INDUSTRIAL 9 N D ELVGINEERIAVG CHEA1fISTRY
May, 1926
film lubrication, it did tend to lower slightly the critical point of film rupture, and decrease friction and abrasion in the region of partial lubrication. Similar conclusions are iadicated in the papers by Parsons, Koethen, and Dover in this symposium. Since the work summarized above, Barnard3 has investigated and reported on the effect of clearance ratio on the friction curves and the Pritical point, and in his present paper
+ WHITE METAAL BEARING o BRONZE B€AU/NG
‘Y F i g u r e 1-Data O b t a i n e d by Streibeck o n Bronze a n d W h i t e M e t a l Bearings n i t h a Steel S h a f t 70 M m . (2.76 I n c h e s j i n D i a m e t e r
shows that the oil circulation through a bearing is also a function of this same modulus .ZN/P. One of I)r. Dickinson’s laboratories a t the Bureau of Standards has done some very interesting work along these lines, most of which has not yet been reported. I n particular they have obtained extremely low critical points in a large bearing with very smooth surfaces, and have bem studying the effect of dirt in oil on 3
Tms
JOURYAL,
16, 3 4 i (1924).
453
the friction and the critical point, with some very interesting results. Field for Further Study Probably the work which is now most needed is a detailed study of the effect of the smoothness and structure of practical bearing surfaces on the location of the critical point, because i t can undoubtedly be greatly lowered below that generally found today. Such a lowering of the critical point would, of course, permit the use of smaller bearings and lighter oils, giving much less friction. The present uncertainty as to the precise value of the critical point in a given type of bearing, often coupled with an insufficient appreciation of the laws of lubrication, is responsible for the frequent occurrence of factors of safety in the field of lubrication many times higher than necessary, thus increasing both the cost of the bearings and friction losses. Some bearings, on the other hand, operate too near the critical point for safety. Some intensive cooperative work along these lines should in a short time put the design of bearings and the relation of lubricants on a truly scientific basis. This does not mean that the mechanism of lubrication is even yet explained in all its phases. The very interesting observations reported in the papers of Becker and Gilson raise questions which will undoubtedly puzzle tlieoriyt, fur some time to come, but should eventually lead to the formulntion of the correct modifications of lubrication theory. Acknowledgment The chairman wishes to express his appreciation to all those who have cooperated to make this symposium a success and in particular to D. P. Barnard and E. 11. Billings, who have been of great assistance in connection with the solicitation and reviewing of papers and the mechanical work involved in arranging such a symposium.
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Characteristics of Full and Partial Journal Bearings’ By H. A. S. Howarth KISGSBCRYMACHISSWORKS, PHILADELPHIA, PA.
The types of bearings to which this analysis is applied are as follows: I-Bearings bored with running clearance: (a) full or complete bearings; (b) central partial bearings in which t h e line of action of the resultant load bisects t h e bearing angle; (c) offset partial bearings in which t h e line of action of the resultant load intersects t h e bearing surface usually more t h a n half way from t h e leading edge. 11-Bearings fitted carefully to the journal. As no running clearance is provided, these bearings m u s t be less t h a n 180 degrees long in order to function. These types have been studied graphically in order t o provide a complete and thorough set of fundamental laws which m u s t govern t h e actual lubrication of all plain journal bearings. The bearing characteristics as given i n this paper represent ideal conditions in t h a t they show the maximum film thickness which may be realized for a given load. These laws may serve as a guide in the study of actual bearings in which such factors as end leakage 1 This paper, except the parts on total friction in full bearings and the viscosity-temperature conversion chart, is a resum6 of three contributions by the author to the American Society of Mechanical Engineers in 1923, 1924, and 1925, entitled “A Graphical Study of Journal Lubrication.”
and surface roughness play a modifying part and prevent the carrying power from being as great as in the ideal case. As a n example of the use of these laws is cited the question as to the advantages of employing two or more pivoted segments instead of a single partial bearing to support a journal. I t can be readily shown t h a t the film is thinner in t h e case of the bearing of two segments, t h e total arc of each bearing and other conditions being equal. I n a similar manner a three-segmented bearing is inferior t o one of two segments. I t is quite probable t h a t many bearing problems may be solved, a t least approximately, by the graphical method once its usefulness and limitations are thoroughly understood.
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HE hydrodynainical theory of lubrication, developed
by Reynolds and amplified by Harrison2 and others, forms the basis of this analysis, and Harrison‘s equations and symbols are used so far as they apply. This graphical analysis is, therefore, a development of the twodimensional theory, originating with Reynolds, which leaves side leakage out of account, and assumes that the oil viscosity is constant throughout the film. The influence of friction 2
Trans. Phil. SOC.,22, 3, 39 (1913).
