Power requirements of rolling mills

POWER REQUIREMENTS O F

ROLLING MILLS

BY

WILFRED SYKES

Presented under the auspices of the

Industrial Power Committee

J O H N M . H I P P L E , CHAIRMAN, W . E . & M . C O . , EAST PITTSBURGH, P A . H. B. E M E R S O N , METHUEN, MASS. J . C . L I N C O L N , CLEVELAND, O H I O R . S . F E I C H T , PITTSBURGH, P A . R . S . M A S S O N , L O S ANGELES, CAL .

E . F R I E D L A N D E R , BRADDOCK, P A . W . H . P O W E L L , MILWAUKEE, W I S . E . H . K I F E R , MADISON, W I S . B A R T O N R . S H O V E R , YOUNGSTOWN, Ohio

C. D , K N I G H T , Schenectady, N . Y . R . H . T I L L M A N , Baltimore, M D .

A paper to be presented at the 277th Meeting of the American Institute of Electrical Engineers, New York, November 8 , 1912 .

Copyright, 1912 . B y A . I . E . E .

(Subject to final revision for the Transactions.)

P O W E R R E Q U I R E M E N T S OF R O L L I N G M I L L S

B Y W I L F R E D S Y K E S

The increasing use of electric motors for driving the main rolls in modern steel works, makes the question of the power requirements of rolling mills of considerable importance to the industrial engineer engaged in designing such installations. An error in judgment due either to inexperience or to lack of accurate information, m a y involve the loss of a large sum of money in the installation itself, but what is of still greater importance, is the loss that is incurred indirectly, due to the time lost before the error can be remedied.

The subject is one of great complexity due to the various factors controlling the power requirements and also to the variation in operating conditions in different works. The subject of rolling mills is one on which it is hardly possible to obtain reliable information from published data and the whole rolling mill practise is based upon empirical knowledge gained b y experience. During the last few years an attempt has been made in Europe to reduce the subject of rolling mill practise to some scientific basis but without very great success up to the present time.

It is not the objec t of this paper to attempt to give any set of rules for determining the correct size and characteristics of the motor required for driving any particular mill but rather to indicate the lines along which such problems must be studied and to give an idea of the factors controlling the size and equipment required. T o cover the conditions met in modern steel mills would require a great deal more space than can be allowed in a paper before this Institute and even with full knowledge of such conditions, considerable judgment is always required in working out such problems,

2017

2018 SYKES: ROLLING MILLS [Nov . 8

One of the most difficult features of this problem is to determine the set of conditions on which to design the equipment, for any particular mill, that will coincide with the actual practise. It is almost impossible to obtain accurate data from the men responsible for the operation of such installations as to operating conditions, on account of the changes that occur in practise after the mill has been installed, and for this reason any assumptions made when determining the size of machine required for driving it, may be altogether wrong in two or three years. A great m a n y superintendents are of the opinion that it is impossible to obtain, within limits, an equipment too large. This is a mistaken idea but has been based upon past experience which has shown that b y improvements, mainly in organization, it has been impossible to increase the output often as much as 100 to 200 per cent over the original estimate. Wi th our present knowledge of rolling conditions and in view of what has been done in the past it should be possible to make a reasonable estimate as to h o w much the production of a mill m a y be increased in the future, b y improvements in the auxiliary apparatus and organization, and this is a factor which must always be considered when designing an installation; and it is here that the electrical manufacturer must often take the responsibility for assumptions as to rolling conditions altogether different to those given b y the steel mill engineers. Some of our most successful manufacturers of rolling mill engines, have based their machines upon the size required to break some part of the mill so that they are certain that the engine would carry any load that could be caused b y the mill, independently of the method of operation. So long as efficiency is not considered and it is not necessary to meet competi t ion as to price of the installation, such an arrangement is an ideal one from the standpoint of the manufacturer, as there is never any doubt as to the operation of his part of the plant, but under the condit ions now existing in our steel mills, attention must be paid to the question of efficiency, and business conditions also necessitate attention being paid to the price of equipment.

In the first place it must be pointed out that the size of the mill as determined b y the size of pinions, or the width and diameter of rolls, has comparat ively little to d o with the size of motor required for driving it, as the work performed b y the same size mill may vary several hundred per cent. The fundamental basis on which the size of motor must be determined, is the product of the mill and the tonnage rolled. There are a great many

1912] SYKES: ROLLING MILLS 2019

factors entering into the proposit ion which must be considered, and dealing first with the product , the following are the principal in their usual order of importance:

1. Volume of metal displaced. 2 . M e t h o d of displacement. 3 . Temperature of metal. 4 . Class of material. 5 . Rate of displacement. 6 . Size of roll.

This order is not fixed, and the importance of any of the factors will vary with the practise at the particular mill in question.

It is of the greatest importance to have some method of comparing the actual work done on the metal in various mills and it must be admitted that such a comparison is extremely difficult. In comparing various tests, I have used as a unit of work, the

After rolling, the area has been reduced to a arid the length increased to /. T h e shaded port ion of the original section, it has been assumed, has been displaced so as to correspond with the shaded part of the metal after the pass. The displacement in practice is of course not as shown but the illustration will show what is meant b y displaced volume. From this sketch it is obvious that the vo lume, displaced is equal to (A — a) L. If the inch is taken as a unit, this formula gives the cubic inches displaced.

This unit of work takes into consideration only the volume displaced in the direction of rolling and for simple work such as rolling plates, b looms, flats, etc., practically all of the metal is displaced in this w a y as the displacement at right angles to the direction of rolling is negligible. In cases where the section of the pass is completely enclosed b y the rolls, there is very often a side displacement which this unit does not take into consideration nor is it m y opinion that any simple unit of work can provide for

V O L U M E O F M E T A L D I S P L A C E D

F I G . 1 . — D I A G R A M O F D I S P L A C E D V O L U M E

h.p.-seconds required to displace one cubic inch of metal. The displaced volume is o b tained as shown in Fig. 1. The area enclosed b y the full lines, represents the original length of material with the original area A and length L .


this condition, as it is impossible to determine exactly h o w the metal flows. Fig. 2 shows a typical pass when rolling rounds from square billets. The full line shows the section after the pass, and the dot ted line the section before the pass. These sections were obtained b y cutting pieces from the bar before and after the pass. It will be seen that the width of the material has been appreciably increased, much more than would be natural if the pressure of the rolls were only perpendicular to the bars of the metal.

At tempts have been made to introduce a factor into the c o m parisons that would take this condit ion into consideration, it being considered that the metal covered b y the area not shaded has not been displaced, but investigations have not yet reached the stage that would warrant any statement being made as to this method of comparing different passes. The instance given in Fig. 2 is a comparatively simple one, but in practise when rolling various sections such as angles, channels, rails, etc., this side displacement is often made under condit ions that make it impossible to use anything else but empirical figures. Referring to Fig. 5 showing the sections after the various passes when rolling rails from billets, it will be seen in the case of pass one of the first series, that the metal has been displaced considerably to form the basis of the flange. In this case there has been a considerable distortion of the metal in addition to the increase in the length due to displacement in the direction of rolling and it is obvious that no formula can take into consideration such distortion even if an accurate knowledge were available as to the way that the metal flows. W e have some information available as to how metal flows when rolling simple sections such as plates or blooms, but, even with this knowledge, theoretical calculations do not check up very well with practical test results. Various other units of work in addition to the power required to displace a cubic inch of metal, have been adopted b y different investigators, but they all take into consideration only the displacement in the direction of rolling, and from what has been said, it is obvious that this is the only basis on which any comparison can be made although it is admittedly open to object ion and must be used in conjunction wi th empirical constants to provide for the distortion of the metal in other directions. I have adopted the unit of h.p.-seconds per cubic inch displaced as it appears to be the most simple and direct basis of comparison. For convenience it will be referred to as " specific power consumpt ion / ' or S .P.C.


M E T H O D OF D I S P L A C E M E N T

Reference has been made to the side flow of the metal, but it is also of the greatest importance to consider how the pressure is applied to the material rolled. W h e n the pressure is vertical, or nearly so, to the surface being rolled, it may be referred to as 1 1 direct pressure " and it is obvious under such conditions that the power required, will be a minimum. When finishing material such as flanged rail or channel, where the pressure is almost parallel to the surface being rolled, it is obvious that the actual displacement for a given pressure, m a y be very small. Such a condition is illustrated in Fig. 3 which shows the condition existing when finishing a rail flange and a channel section. Under such conditions, it is obvious that the component at right angles

FIG. 2 — P A S S SECTIONS ROLLING FIG. 3—EXAMPLES OF INDIRECT ROUNDS FROM BILLETS PRESSURE

to the surface of the metal is very small, and consequently the pressure m a y be very large for a very small amount of work done. This condit ion m a y be referred to as " indirect pressure/ '

In Fig. 4 is shown a number of sections illustrating what is meant b y " direct " and " indirect pressure," and which will make this point clear.

Referring to Fig. 5 , a comparison is made of the various passes when rolling rails, and this figure illustrates the difference in practice met with in steel mill work . The second series of sections shows that the rolls are designed to have as direct pressure as possible, whereas in the first set of sections, a great deal of the work is done b y indirect pressure. The first series of sections, however , have been laid out so that the axis of the rail during

2022 SYKES: ROLLING MILLS [Nov. 8

the finishing passes, is not parallel to that of the rolls and in this way the surface of the metal is worked at a more favorable angle than in case of finishing passes of the second set of sections. It would be reasonable to expect for such condit ions that the second set of sections would require less power during the initial passes, but that the finishing passes would require somewhat greater power. This shows to some extent the local problem encountered in steel mills. In Fig. 6 is shown two methods of rolling channels, and it will be seen that in rolling the second set that the direct pressure is used as much as possible and it is only in the last pass that the actual channel section forms. In this pass the vo lume displaced is negligible, so that the mill has

E X A M P L E S O F D I R E C T P R E S S U R E

PLATES, ETC. BLOOMS, BILLETS

E X A M P L E S I N T E R M E D I A T E B E T W E E N D I R E C T A N D I N D I R E C T P R E S S U R E

E X A M P L E S O F I N D I R E C T P R E S S U R E

F I G . 4

only to straighten the sides of the channel which has already been formed b y direct pressure or pressure at favorable angles. These t w o examples show the difference in practise in various mills and illustrate to some extent the necessity of studying the particular conditions in each mill before attempting to design an equipment for driving the rolls. The question of roll turning has been based in the past, more or less, upon empirical knowledge obtained b y the roll turners from actual experience, but in Europe, some attempt has been made during the last few years to systematize the methods of reducing the metal for different sections, and when this is done the problem of comparing the results to be expected from various mills will be considerably simplified.


The pressure on the rolls due to the metal introduces additional friction but as this cannot be separated from the power actually required to displace the metal, it must be included in the specific power consumption. There is often considerable friction between the rolls and the metal due to the peripheral speeds of various parts of the section being different. On referring to Fig. 5 it is obvious that the speed of the portion of the roll in contact with the web is appreciably greater than that at the edge of the flange and therefore as the flange and the web ar;e delivered at the same rate, there must be slippage somewhere

FIRST SERIES—PASS SECTIONS ROLLING RAILS

A R E A =

AREA = 19.36 15 .8 S Q . I N . AREA ^ = AREA = A R E A = AREA = • A R E A =

SECOND SERIES—PASS SECTIONS ROLLING RAILS

FIG. 5

between the metal and the roll. In cases where a rail flange for instance is being finished, there is a tendency to m o v e the rolls laterally in relation to one another which m a y be taken up b y indirect pressure in the opposi te direction or in roll collars, in which case the friction is of course increased. A s we have no way of determining what the friction due to rolling may be, it must be included as part of the net rolling work, which is the actual input to the mill less the no-load friction. In the author's opinion, it is perfectly legitimate to consider the additional friction in the rolls, pinions and spindles as part of the net rolling


work and I d o not think we would be any better off if we had tests showing exactly how much power each i tem represented, as the problem is so complicated that I doubt if we would be able to make more accurate estimates than are now possible, although perhaps it might be possible to get along with a smaller number of tests.

T E M P E R A T U R E OF M E T A L

T h e temperature of the metal plays a very important part in the power required for any mill. Tests made indicate that the power requirements, all other things being equal, vary practically as tensile strength of the material. There is not a great deal

PASS SECTION ROLLING CHANNELS—FIRST SERIES

NO.1 NO.2 NO.3 NO.4

PASS SECTION ROLLING CHANNELS—SECOND SERIES

FIG. 6

of information available as to the tensile strength of steel at various temperatures and naturally such tests are rather difficult to make. In Fig. 7 is shown a curve of tensile strength of mild steel at various temperatures, this curve being made up from information that has been published of tests in the Wate r town Arsenal and from various European publications, as well as from tests made b y the writer. T h e curve varies somewhat from others that have been published as to the strength at high temperatures, as the tests made b y the writer indicate that previous estimates as to the tensile strength have been too low and that instead of the curve gradually tapering to zero at the melting


point, that there is a point somewhere between 1300 and 1400 deg. fahr. where the tensile strength rapidly decreases. Tests made at various temperatures when rolling plates, using only direct pressure, so that there are no other disturbing factors, indicate that this curve is approximately correct as indicating the relation between the power required to displace the metal and the temperature. It will be seen from this curve that the strength increases quite rapidly after the temperature drops below about 1400 deg. fahr. so that when rolling thin sheets,

when the metal becomes almost black the power requirements increase at a very rapid rate. This curve shows that tensile strength about 100 deg. fahr. is about 18 times greater than at 2000 deg. fahr. Tests made when rolling sheets at 2000 deg. fahr. and rolling cold, showed a variation in the power consumption per cubic inch displaced varying from 17:1 to 20:1.

The rate at which metal cools is obviously of the greatest importance and within the usual limits of rolling temperatures it m a y be said that the rate of cooling will b e practically proportional to the area exposed in relation to the volume. In

F I G . 7


Fig. 8 is shown the increase in exposed area of a particular slab as the cross sectional area was reduced; and when the rate of cooling is taken into consideration, in conjunction with the curve shown in Fig. 7, it is obvious that the power required to displace the metal will increase very .rapidly as the cross section is decreased. This will be referred to later when discussing this point.

C L A S S O F M A T E R I A L

Tests made b y the writer and b y others indicate that, providing the temperature is the same, the power required to displace a given volume of metal, is practically independent of the chemical composi t ion of the steel. This of course applies only when rolling metal hot and within the usual rolling temperatures.

G I O co 9 a _

x 61 2 3 §21 Ul

100 90 80 70 60 50 40 30 20 10 0 # O R I G I N A L O F A R E A

FIG. 8—SHOWING INCREASE IN EXPOSED A R E A OF PLATE WITH REDUCTION IN BRASS SECTION

90 80 70 * O R I G I N A L O F A R E A

FIG. 9—SHOWING INCREASE IN RELATIVE SPECIFIC POWER CONSUMPTION—COLD R O L LING STEEL PLATES

As the temperature approaches 1500 deg. fahr. the influence of the different chemical composi t ions can be noticed, but as metal is usually worked, except in the case of thin sheets or small sections, between 1800 and 2400 deg. fahr., it m a y be said that in practise, the composi t ion of the material does not directly influence the power consumption. Indirectly however, it has considerable influence, as it is necessary to roll high carbon steels and some alloy steels at comparat ively low temperatures, so that the power consumption for a given volume of displacement, may be considerably higher than would be the case when rolling mild steel.

The density of the steel also has considerable influence upon the power requirements, and when rolling ingots, the first one


or two passes made require comparat ively little power per cub ic inch displaced, as the steel is more or less porous. After the metal has had one or t w o passes through the rolls, the density when hot apparently does no t enter further into the question. W h e n rolling steel cold , there is a continual increase of the power required due to the increased density, and in Fig. 9 is shown a typical curve indicating the increase in power requirements as the cross section area is decreased.

R A T E O F D I S P L A C E M E N T

Although little information is available, there are indications that the rate of displacement somewhat affects the power requirements. Tests made b y the writer appear to show that a low rate of displacement requires less power than if metal is rolled quickly. In practise, however , metal is rolled as quickly as it can be handled, so that this feature is of comparatively little importance.

S I Z E O F R O L L S

Theoretical investigations show that when rolling plates or b looms or such sections where direct pressure only is used, the size of roll has some effect upon power requirements. Small rolls should require somewhat less power than large rolls but the writer has not been able to demonstrate the accuracy of these theoretical calculations owing to the great many other factors which influence the test results.

P R A C T I C A L D E T E R M I N A T I O N O F M O T O R S I Z E

The great majori ty of rolling mills are of the type running continuously in one direction, and to equalize the input to the motor , flywheels are used. It is of the greatest importance to determine the size of flywheel required in conjunction with the characteristics of the moto r and control apparatus, as it is only b y considering them as a unit that a satisfactory installation can be made. It is seldom that a mill is run at such a rate that it is discharging metal from the finishing pass for anything approaching 1 0 0 per cent of the running time. Depending upon the class of mill and the work performed, it is usual to find the mill actually rolling from 2 0 to 8 0 per cent of the total time. In the heavier mills, the percentage is naturally less than in the case of the mills rolling small sections, and it is therefore obvious that if the moto r size is determined upon the basis of rolling so much material per hour, it m a y be altogether too small to perform


the work while the metal is actually in the rolls, although it might be large enough to take care of the average condit ions. Wi th the ideal flywheel, a motor sufficiently large to carry the average load would be the right size to use, as all the peaks would be taken b y the flywheel, and during the intervals between passes, energy would be stored in it. In practise it is not possible to use such flywheels, as they would be excessively large, and consequently a compromise must be made between motor and flywheel. It is usual to consider that the mill will run for short periods at its maximum capacity, that is, with the minimum interval necessary to handle the material, and on this basis the load diagram must be determined. The load diagram can be determined from curves showing the power requirements per cubic inch displaced, in conjunction with the volumes displaced and the rate of rolling. From this diagram, the average load, when the mill is rolling at the maximum rate, can be determined, and also the size of the flywheel. The average production of the mill must be taken into consideration in determining the size of motor so as to have an equipment which has suitable characteristics for the normal operating conditions. T h e curve showing the power required per cubic inch displacement, shows a rapid increase as the cross section area of the material rolled decreases. It is necessary to determine this curve from test data for practically every installation, as local conditions vary so greatly that it is not possible to take any set of curves as representing universal conditions. T o illustrate the methods used in determining the size of motor , a load diagram when rolling plates is worked up in detail in Table I which it is believed will show how this problem is handled when the proper data is available. It is of course obvious that this diagram is subject to appreciable variations in practise due to the variation in the condition of the material, temperature etc., but as the curve for power consumption is based upon an average of a number of tests, the diagram is sufficiently accurate to enable the size of the motor and flywheel to be determined. The motor slip under actual operating conditions may be somewhat different from that calculated, but the flywheel will take care of these operating variations. F rom the load diagram, after allowing for friction, the average load on the motor during the period when the mill is rolling at its maximum rate, can be determined. For perfect operating conditions the flywheel should take all loads in excess of this average load. In practise this is not

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2 0 3 0 SYKES: ROLLING MILLS [Nov . 8

always feasible, but it must be remembered that the type of control also has considerable influence upon the input to the motor . If the speed of the moto r is to be regulated in such a way that the motor takes only the average load, it is necessary to automatically vary the resistance of the rotor circuit. T h e usual method of control in rolling mills, is to use a fixed rotor resistance, so that the speed falls as the load is increased. This gives a more or less satisfactory control of the flywheel, but in the majority of cases it leaves a great deal to be desired. Efficient control devices have been designed which will automatically regulate the rotor resistance quickly enough to meet rolling mill

10 20 30 40 60 60 70 80 90 100 ft O R I G I N A L O F A R E A

F I G . 1 0

conditions, and a description of one successful type has been already given b y the author.* In the table calculated, it has been assumed that automatic slip regulation is provided for, so that the flywheel will take all loads in excess of the average, and from the load diagram, the capaci ty of the flywheel can be readily determined. In Fig. 10 is shown the curve from which the S. P. C . has been obtained. This curve represents the average of a number of tests. In Fig. 11 is shown the load diagram corresponding to the table, which shows the work to be performed b y the motor and flywheel. In practice it has been found that, al-

• P R O C E E D I N G S A . I . E. E. , June 1 9 1 1 .

1012] SYKES: ROLLING MILLS 2 0 3 1

though the power required for the individual passes may vary quite appreciably from that calculated, the flywheel will have sufficient capaci ty to compensate for these individual variations, and that the general operating condit ions of the motor can be fairly accurately determined. It has been pointed out that the daily or hourly capaci ty of a mill m a y be very much less than the maximum possible capaci ty, and it is necessary to compromise between the size of machine required to handle the maximum possible output and the actual hourly and daily output. For instance, in the example that has been worked out, the average load on the mo to r is 675 h.p. when the mill is run at its maximum rate of product ion. T h e actual hourly rate of production of this mill is only 80 per cent of this maximum, so

H . P .

4000

3000

2000

1000

0 10 30 30 40 50 60 70 80 S E C O N D S

FIG.11—LOAD DIAGRAM CORRESPONDING TO TABLE I

that if the normal rating of the moto r is such that it would carry the hourly average, it would be overloaded 25 per cent when working at the max imum rate of product ion.

In practice it is advisable not to al low for an overload of more than 25 per cent when rolling at the maximum possible rate, so that there is always a certain reserve available in the motor for extraordinary condit ions that m a y arise. Rolling mill motors are usually designed so that they can carry 25 per cent overload continuously with a 50 deg. cen t . rise and 50 per cent for one hour with a 60 deg. cent. rise. Wi th motors designed on this basis, it is quite permissible to allow for them being overloaded 25 per cent when the mill is run at its maximum capacity. If the hourly capaci ty of the mill is considerably less than the

2032 SYKES: ROLLING MILLS [Nov. 8

maximum that can be rolled, it is then necessary to investigate very closely the conditions existing so as to determine on what basis the compromise must be made.

In the foregoing, attention has been drawn to some of the features controlling the power requirements of rolling mills and it is hoped that at some future date that it will be possible to discuss this problem more in detail when a fuller knowledge is available of the various constants that must be considered when dealing with this proposition. The examples given represent simple condit ions and it will be readily appreciated that the great variety of shapes rolled makes the actual determination of power requirements very difficult. Curves showing the specific power consumption are not as a rule so regular as for plates, which represent the simplest condit ion met with and the one not interferred with b y such items as indirect pressure, collar friction, etc.

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Power requirements of rolling mills