ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Conclusionr
Fundamental engineering concepts of fluid flow have been applied to roll milling and correlating equations have been developed for a single nip with a nonpigmented oil. The variables correlated were viscosity, clearance, power input, roll speed, roll speed ratio, roll diameter, and roll length. The density of the fluid was constant throughout the tests. The term representing fluid density is included in the equation to suggest its possible relationship based on other fluid flow examples. Apron and feed nip loadings have been investigated. The correlating equations for various conditions are 1. For the apron nip or feed nip, a t constant roll speed ratio, the general correlating equation is
The values for K appear in Table 11. 2. For the apron nip, at any roll speed ratio, the correlating equation is
3. For the feed nip, a t any roll speed ratio, the correlating equation is
literature cited
Badger, W. L., and RIcCabe, W. L., “Elements of Chemical Engineering,” 2nd ed., Chap. 2, RlcGraw-Hill Book Co., New Y- o. r k~. 1936. ~ ~ , Ibid., Chap. 4. Langhanr, H. L., “Dimensional Analysis and Theory of Models,” Cham 2, I). 21, Wiles. New York, 1951. Murpcy, N. F., Bu1l:Virginia Polytech. Znst.,42, No. 6 (1949). Rushton, J. H., Costich, E. W., and Everett, H. J., Chem. Eng. Progr., 46, KO.8, 395 (1950). Worthington, A. G . , and Geffner, J., “Treatment of Experimental Data,” Chap. 1, p. 20, Wiley, New York, 1943. RECEIVED for review December 16, 1953. ACCEPTED October 6, 1954. Presented a t the 122nd Meeting of the ACS, Atlantic City, N . J , September 1952. From a dissertation submitted b y Louis Maus, ,Jr,, t o the Graduate School of Lehigh University in partial fulfillment of the requirements for the degree of doctor of philosophy, J u n e 1951.
(DISPERSION STUDIES)
Floating Roll Mill LOUIS MAUS, JR.~,ALBERT C. ZETTLEMOYER, AND ERNEST GAMBLE National Printing Ink Research Institute, Lehigh University, Befhlehem, Pa.
I
ix
N T H E preceding paper the operation of the conventional three roll paint or ink mill was described. This age-old design is capable of excellent dispersion performance but has basic difficulties of adjustment and control. The operation of a three roll mill has been an art, and a t least several months have been necessary for the training of a good mill hand. With the usual screw jacks there is no indication of how the mill is set, and the mill hand must judge the correctness of his adjustment by the appearance of the ink films on the rolls and apron and by the fineness-of-grind achieved. There are 80 different ways or combinations of ways in which a conventional four-point adjustment mill can be out of adjustment. This situation has led to the development of several schools of thought as to how a three roll mill should be set. The conflicts among these schools of thought have remained unresolved for many years because of the lack of basic information on roll mill behavior and lack of means for measuring mill conditions. The great needs in three roll mills today, then, are for a simple system for bringing a mill to proper adjustment and for a method of numerically evaluating the mill setting. In recent years some progress. has been made toward satisfying these needs, but adoption by the industry has been very slow in most cases. Twenty years ago Vase1 (3)developed a mill with hydraulic elements in place of the conventional screw jacks so that a pressure gage indicated the forces exerted on the roll bearings. Thus, the operator could reproduce force conditions on the mill and could intelligently study optimum conditions. The Vasel mill could also be arranged so that the four hydraulic elements were interconnected and thus maintained a force balance 1 Present address, Department of Chemical Engineering, Lehigh University, Bethlehem, Pa.
April 1955
around the mill. This arrangement also permitted adjustment of the mill from a single point of control. The unified hydraulic system did not work as well as anticipated, however, because of unbalanced thrusts from the gears connecting the rolls. Gears are normally mounted on only one end of each roll so that their spreading forces tend to open one end of the mill and produce an unbalance. This tendency can be corrected with the four-point arrangement but not with the four elements tied in a single system. The problem was finally solved by mounting gears on both ends of the rolls. The Vasel mill was not accepted by the industry a t the time, but mills with four-point mechanical adjustment with hydraulic gages are now coming into use. A mechanical system of simplifying roll mill adjustment was designed by Brasington (1). This system brings both end rolls to the fixed center roll simultaneously by turning a single control wheel. The mechanism is usually designed to permit a clearance ratio of 2:1 between the feed and apron nips. The four corners of the mill have individual adjusting screws to bring the two outer rolls into proper relationship to one another and to the fixed center roll. Once this adjustment has been made, the mill can be opened and closed reproducibly by the single control wheel. In practice, however, each screw needs periodic readjustment. Since one oE the factors affecting machine adjustment is maintenance of the initial roll relationships, and since the same degree of skill is required to readjust as to set a conventional four-adjustment machine, frequent readjustment may be necessary. Not only does this one-point control simplify mill adjustment but also the mill is equipped with an indicator for setting tightness on an arbitrary scale. Although this indicator is helpful in reproducing settings, i t must be connected t o one of the adjusted points and if the operator does not maintain this reference adjustment, loss of the zero point occurs.
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
where
Pg/w = power per unit mill width ut = total run velocity L = roll diameter D = clearance in nip g = gravitational constant K = constant dependent on roll speed ratio p = fluid density p = fluid viscosity
If all conditions except power and clearance are held constant, the following relationship is valid
n
Figure 1.
At the high rates of shear encountered in the nips, the viscosity may be assumed to be constant for the practical limits of clearance settings. From this proportionality, any decrease in clearance is accompanied by an increase in the required power. Thus, if equal forces are applied to the bearing blocks of the free end roll of a floating roll mill, any deviation from parallelism will be corrected automatically. Thus, from correlating the variables the prediction was made that parallelism would automatically be maintained on the floating roll mill.
Roll arrangement in conventional three roll mill
The Brasington design is popular because of its simplicity, but it has not been completely accepted by the industries. A third method of control based on gravitational loading was proposed by Shurts and Rosa ( 2 ) . In this design the two end rolls are fixed parallel and in a horizontal plane, with the space between them smaller than the diameter of the center roll. The center roll is then placed between and above the two fixed end rolls. Tightness is adjusted by loading the center roll to force it, between the two end rolls. Relatively light weights can be used to create large forces in the nips. To counteract the unequal spreading force of the driving gears, the point for applying force cannot be a t the center of the roll but slightly toward one end. Unfortunately, exact parallelism of the two fixed rolls is essential for proper operation of this type mill. The work reported by these authors was done only on a laboratory size mill.
n
Floating roll mill i s a simplified three roll mill
The problems of mill adjustment have been attacked in a somewhat different manner in this work which has resulted in a new type three roll mill design. Instead of having a fixed center roll, the new floating roll mill has one of the end rolls fixed as shown in Figure 2. The center roll is free to move, and the other end roll is positioned with a screw jack on each end of the roll. Thus there are only two points of adjustment at one end of the mill. When a three roll mill is in operation, the center roll acts as half the feed nip and half the apron nip. This roll never actually touches either of the other two rolls but only the ink in the nips. In the floating roll mill the ink of the two nips is free to align the center roll between the two end rolls. When the two end rolls are parallel, the center roll adjusts its position until it, too, is parallel. This roll actually floats on the ink in the two nips around it. The fact that proper alignment could be attained with the floating center roll became apparent from the engineering studies previously reported. The preceding paper shows that the behavior in a mill nip a t a given speed ratio obeys the equation
702
Figure 2.
Roll arrangement in floating roll mill
Furthermore, the floating center roll will-position itself so that the spreading forces in the nips will be equal. Experienced mill operators disagree as to whether the front or the rear nip should be run tighter than the other. No published scientific data support either view. However, the condition of equal forces maintained on the floating roll mill is supported by the performance data obtained here. Ease of Adjustment. Adjusting the floating roll mill is much easier than adjusting the conventional four-point adjustment mills as a comparison of ways in which the mill can be out of adjustment in the two cases indicates. In the case of fourpoint adjustment the misadjustments encountered are
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING. DESIGN, AND PROCESS DEVELOPMENT Rear roll tighter on the right Rear roll tighter on the left Front roll tighter on the right Front roll tighter on the left Apron nip tob tight for feed nip setting 6. Apron nip too loose for feed nip setting 7. Both nips too tight 8. Both nips too loose 1. 2. 3. 4. 5.
These 8 types of misadjustment and 72 combinations of them are possible. Thus there is a total of 80 ways in which a fourpoint adjustment mill can be out of adjustment. In contrast, the floating roll mill can have only four types of misadjustment 1. Mill tighter on the right 2. Mill tighter on the left 3. Mill too tight 4. Mill too loose And four combinations of them are possible so there is a total of 8 ways in which the floating roll mill can be out of adjustment. This is only a tenth as many possibilities for misadjustment as there is with conventional mills; consequently, the floating roll mill is much easier to put and to keep in proper adjustment. This marked relative ease of adjustment has been amply borne out by experience. Floating roll mill has been laboratory and production tested
Laboratory Scale Tests. Three small roll mills have been converted to the floating roll arrangement a t the institute. These mills are a Kent 4 X 8 inches, a Brasington one point 6 X 14 inches, and a Fritsch 2.5 X 5 inches. Each mill has a different gearing arrangement and is a different size. The most marked change created in these mills by the conversion was the ease of adjustment. Laboratory mills must be particularly easy to set since very small batches are normally run, and little time is available before the entire batch has passed through the mill. When a four-point adjustment mill was used, as many as two or three passes were required before the mill was properly set. The amount of milling which the batch received was poorly defined. A floating roll mill, however, can be set within the first pass, and the batch can be milled almost entirely a t the final setting. Furthermore, the training of new laboratory personnel in the proper operation of a mill is no longer a major problem. Floating roll mills have been used in this laboratory for over 2 years, and a four-point mill would no longer be considered. The single point mechanism used in the Brasington mill is excellent when it is properly aligned and adjusted, but this adjustment requires skill and is not permanent. The mechanism tends
to drift slowly from proper adjustment so that eventually a good setting can no longer be obtained with the single point adjustment. After this mill was converted to a floating roll mill by merely releasing the center roll bearing blocks, much of the usual difficulty apparently disappeared, and this mill is now much more reliable. A comparison was made in this laboratory of the dispersion effected by the floating roll mill and a standard roll mill. Although it was difficult to make a rigorous comparison, the results indicate that the floating roll mill behaves essentially like a fourpoint adjustment mill a t optimum setting. To date a four-point adjustment mill properly set has not out performed an otherwise similar floating roll mill. Additional information has been obtained on this point by subsequent work on production mills. Use in Plant Production. Since the initial announcement of the floating roll mill to the-members of the Printing Ink Institute, a considerable number of production mills have been converted to operate on the floating roll principle. The returns from a recent questionnaire provide considerable information on the adoption of the floating roll mill by the industry. The 21 replies indicate that 15 companies have converted 80 mills in sizes to 16 X 40. The reports show that these mills are easier to adjust, and production has increased an average of 15% with a maximum of 30%. The training of mill hands was considerably simplified. Actually in some cases a better dispersion was obtained as well as increased production. These reports from the industry confirm the simplicity of adjusting the floating roll mill. They show clearly that the floating roll mill is practical for production use, and that it has important advantages over the conventional four-point adjustment mill as used. The generally increased production with the floating roll mill is ample evidence that the imposed condition of equal spreading forces in the two nips is nearly optimum for a three roll mill. This conclusion was also reached on the basis of laboratory work where no setting of a four-point adjustment mill was found that gave results superior to those obtained on a floating roll mill. It should be emphasized that the conversion of most types of present three roll mills to floatin8 roll mills takes only a short time and can usually be handled readily by ink plant personnel. Literature cited (1) Brasington, C. P., U. S. Patent 2,254,512(September 1941). (2) Shurts, R. B., and Rosa, Prisco. Natl. Paint, Varnish Lacquer Assoc., Sci. Sect. Circ. 759, October 1952. (3) Vasel, G. A., U. S.Patent 1,788,964(January 1931). ACCEPTED October 6. 1954. RECEIVED for review December 16, 1963. Presented a t the 124th Meeting, ACS, Chicago, Ill., September 1953.
(DIs PERSION ST uDIES)
Correlation of Floating Roll Mill Variables ALBERT
C. ZETTLEMOYER,, JAMES H. TAYLOR,
AND
LOUIS MAUS, Jr.'
National Prinfing Ink Research Insfifote, Lehigh University, Bethlehem, Pa.
IIV
T H E first paper ( 2 ) published here a method of correlation for roll mills was presented, and in the second (3) an improved and simplified three roll mill-the floating roll mill-was described. The initial correlation of roll mill variables was derived from 1 Present address, Depar6ment of Cheniical Engineering, Lehigh Univeraity, Bethlehem, P a .
April 1955
data taken on an experimental two roll mill operated under conditions simulating individually both a feed nip and an apron nip as found on a conventional three roll mill. The present study was designed to extend the correlation work t o apply to a floating roll mill as described in the second paper. On the experimental used in this work the two types of nips operated simultaneously with the apron roll fixed and a take-off knife in use. The
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