US2235928A - Apparatus for and method for controlling grinding devices - Google Patents

Description

March 25, 1941. H. HARDINGE 2,235.928

APPARATUS AND METHOD FOR CONTROLLING GRINDING DEVICES Fild Jan. 4, 1939 2 Sheets-Sheet 1 Pow-e Ava/r Z...

March 25, 1941. HARDINGE 2,235,928

APPARATUS AND METHOD FOR CONTROLLING GRINDING DEVICES Filed Jan. 4, 1939 2 Sheets-Sheet 2 Patented Mar. 25, 1941 I APPARATUS AND METHOD FOR. CONTROL- LING GRINDING DEVICES I Harlowe Hardinge, York, Pa., assignor to Hardinge Company, Inc., York, ha, a corporation of New York Application January 4, 1939, Serial No. 249,291

Claims.

by the sound produced by the balls. That is to 1 say, in most cases, during normal operation of a ball mill, the noisier the mill is, the less it is being fed with 1'resh material. There is an optimum point of noise level where best grinding occurs. It a. ball mill becomes too quiet, the balls are either coated or spaced apart and the capacity of the mill is reduced considerably.

Conversely, where there is an excessive amount of noise in a ball mill, the mill is underloaded, the balls hit themselves and the lining of the mill, and create an excess sound.

-Heretofore the operator of a mill, or other similar device, has endeavored to manually regulate the mill to secure the optimum noise level, as above indicated, where best grinding occurs. However, manual operation can only roughly approach such optimum point in view of the fact that slight deviations from the optimum sound level materially' reduce the emciency of the grinding, and the human ear is not sumciently sensitive to detect such slight variances.

Even ii such optimum noise level were secured, it would be impossible for an operator to manually maintain the mill running at a constant noise -level,-for noise meters will indicate relatively wide fluctuations in noise levels where even the most'eiilcient operator believes that he 40 is maintaining av constant level. Furthermore,

these fluctuationsare due to frequent changes in the feed size, grindability, moisture content,

etc., from time to time, and, even ii the operator were able to audibly detect such ,changes, it

4 would be impossible to regulate the feed manually to follow all such changes as soon as the occur.

An object of the invention is to provide means I for automatically controlling the rate of feed of to material to a mechanism by the sound produced by the mechanism; j

' Another object of the invention is to provide a means for controlling a feeding device of a mechanism which is actuated by varying degrees in noise level produced by. the mechanism, so

that substantially constantconditions are maintained by said mechanism.

Yet another object of my invention is to provide a feed control system for a grinding mill and the like, which includes means for regu- 5 lating the sound level set up by the mill.

A further object of my invention is to provide a method and means for accurately determining and securing the optimum grinding conditions of a mill or the like, and for thereafter causing 10 the mill to operate at such condition.

A still further object is to provide a sound sensitive device of improved construction for a Control system of the above type and to so locate Such device as to secure maximum sensitiveness. 15

With the foregoing and other objects and advantages in view, the invention consists in the construction and arrangement of the several parts which will be hereinafter more fully described and claimed. 20

In the accompanying drawings:

Figure 1 is a circuit diagram of a controlling device or system disclosing one embodiment of the invention, said device being shown in association with a well known type of ball mill. 5

- Figure 2 is a view similar to Fig. 1, of another embodiment of the invention.

Figure 3 is adiagrammatic sketch showing the preferred location of the microphone with respect to the mill. I v 30 Figure 4 is a frontal elevational view of a microphone arranged within its housing.

Figure 5 is a view taken along the line 5-5 of Figure 4 looking in the direction of the arrows and showing diagrammatically the reception of noise waves and vibrations.

Figure 6 is a graph depicting the relationship existing between the noise level of an operating mill and the emciency of its operations.

Referring to the drawings, and especially to Fig. 1, th eball mill II has a feed hopper I2 at one end," into which is delivered material to be ground in the mill, from a feeder ll of wellknown type. t

The feeder has an endless belt I! which carries the material discharged from a hopper II to the hopper l2, said belt being driven by an electric motor I. Y

The rate at which the material flows from the hopper II on to the belt I4 is controlled by a swinginggate ll whichisadaptedtob'e'adiusted tovarythcsiseotthe discharge opening oi'the hopper ll. i Dm-lng'operation,theballmi1lilrotatesata p edetermined speed and the feeder device it is adapted to supply the mill with fresh material at a rate equal substantially tothe rate at which the ground or pulverized material leaves the mill, so that the mill will operate at maximum capacity. Specifically, the rate at which the feeder delivers material to the hopper 12 may be varied in several ways, depending upon the particular type of feeder and actuating means therefor. In Figure 1, I have shown a feeder in which, when regulated by my control system, the belt operates at a constant speed, and, with the gate ll set for a given discharge opening, the quantity delivered to the hopper I2 is varied by intermittently operating the belt. Of course, the feed could be controlled by, continuously operating the belt l4 at a constant rate and adjusting the gate I! to control the amount of material deposited on the belt [4.

Since sounds are produced by a ball mill and the like, and since these sounds vary according to the manner in which the mill is operating, by this invention I propose to employ the sounds to control the operation of the mill, particularly the rate of feed to the mill. The mill or the like may also vibrate according to the manner it is operated and these vibrations may likewisebe used for the same purpose.

Located adjacent to the mill H is a microphone 29. The microphone may be sensitive to vibrations, sound waves, or both, and adjusted to pick up and handle all character of conditions prevailing at the mill H, such as vibrations produced by the mill, reacting on the foundations or other parts of the apparatus, which will cause a change in electric current, similar to that produced in a telephone or microphone pick-up ofv an amplifier or vibration sensitive device, along the same general .principles.

In a simple form, the microphone circuit utilizes a source of electric current, such as a battery 2i, the primary winding of a transformer 22, and a variable'resistance23.

The output side of the transformer 22 is in circuit with a relay 24 and an indicating device, such as an ammeter 25.

I have also shown a rectifier 26, so that direct current is supplied to the relay 24 and ammeter 25 to take advantage of better operating characteristics of the relay and ammeter where such a condition is desirable. The relay is adapted to operate at a definite energy level, and, while I have shown a conventional magnetic relay, it will be appreciated that a galvanometer relay, or any other type, may be used.

The actuating, contacts of the relay 24 are in another circuit, in which is located a time delay resistance contactor 21, and a source of power such as a battery 28, or line current from a power line.

The contact points 29, 30, of the contactor- 21 close the circuit in the control device or, as illustrated here, feeder motor l6, through a source of power, such as a battery 3|.

If desired, the contactor 21 can operate other power relays or other control devices where the electric current'is higher than the contact points 29, 30, are capableof handling properly, but for the purpose of illustration, the arrangement shown here is ,all that is necessary, as I have found by actual practice, when controlling a circuit including a small motor used to drive the feeder.

Since the noise of a ball mill or any similar device is somewhat irregular, there is quite a wide fluctuation in the instantaneous power inthe relay circuit. In order to prevent damaging ofthe feeder motor l6 and other parts of the,

time to generate enough heat in the thermal element 21 to close the contacts 29, 30.

Thus, instead of having the power of the feeder motor IS on and oif many times a minute, de-..

pending upon the current in the relay 24, the time delay resistance contactor device 21 will smooth out this fluctuation to the desired point, so that the feeder l3 will be cut in and out over a much longer period of time, which can be ad justed by adjustment of the thermal element of the contactor 21. I

I have found that satisfactory operation can be had if the thermal element of the contactor 21 is set so that it will function only after the circuit to it has been closed for a period of from a few seconds up to half a minute, although the time element in this case is not critical, so long as the power circuit is protected from too rapid make and break of contact. The time delay resistance contactor device 21 used-is of standard design and construction well-known in the art.

Where more power to actuate the relay 24 and ammeter 25 is needed than is available from the circuit shown here, or a less sensitive relay is used, it may then be advantageous to use an amplifier between the microphone and relay circuit, which amplifier can be of the vacuum tube type, well-known in the art and need not be described here.

A complete cycle of operation of the mill, feeder, and control system will now be described. Assuming that the entire system is properly installed, and that the mill and feeder are not operating, the mill will initially be started and the feedermanually regulated to' deliver the proper quantity of ore to the hopper l2, insofar as this quantity can be determined by the hu- Examination of the mill operation in the conventional manner will determine whether such rate of feed is correct or whether variations in rate of feed should be made to improve mill conditions. As heretofore indicated, the gate l! is adjustable, and therefore the regulation of the feed rate may be accomplished by running the belt 14 at a uniform speed and adjusting the ating the belt. However, the preferred form for manual operation is to operate the belt at a.

constant speed and regulate the gate [1 until the desired rate of feed is secured.

During this manual operation, the motor I6 is continuously operated by a. source of current (not indicated in the drawings) independent of the control system, or, at any rate, the control system is not actually controlling the motor l6. However, even though the control system is not actually, controlling the motor l6 as yet, the microphone and relay circuit, (and possibly time delay switch circuit) should be energized and observations made for the subsequent operation of the feeder. The relay 24 is actuated when the current in the relay circuit is of some predetermined minimum intensity-say, 35 microamperes in the instant case. With the desired rate of feed attained manually as just described, it is therefore necessary to establish in the relay circuit a current of just 35 microamperes. However, the current which is established in the microphone circuit before adjustments are made will probably be somewhat different than the amount necessary to produce 35 microamperes in the relay circuit. The current in the relay 24 depends upon the resistance 23 in the primary circuit as well as the amount of noise pickedup by the microphone 20, and therefore the resistance 23 willbe adjusted to produce a current of 35 microamperes in the relay. The current passing through the relay 24 will be indicated by the ammeter 25, which has a graduated dial.

For instance, if the ammeter 25 indicates an average current in excess of 35 microamperes, with the desired conditions in the mill still being in effect and remaining constant for the time being, the. resistance 23 in the microphone circuit must then be increased to decrease the current in the relay, and such resistance is gradually increased until an average current of 35 microamperes is established in the relay circuit. When the desired current is thus attained in the relay circuit, the control system is then cut into the motor I6, and the feeder i3 set so that, on constant operation, the amount of feed would. be increased possibly 20% or more, or sufhcient to insure a feed rate in excess of that the mill is capable of handling at any time due to variation in hardness, fineness, or other factors affecting mill operation.

This re-setting of the feeder is usually accomplished by increasing the gate opening or belt speed, and is necessary in view of the fact that,

atingconditions which had been manually established at the outset.

After the control system has been cut into the motor, the operating conditions which were initially established will be maintained within extremely narrow limits. In view of the fact that the feeder is now set to feed an excess quantity oi ore, there is a tendency for the load in the mill to increase above the desired level, but the slightest deviation from such a level is reflected in the noise vibrations, and this tendency to increase the load and quiet down the mill is reflected by a drop in the sound energy picked up by the microphone, which in turn reduces the energy in the relay circuit causing the latter to open. When the time delay relay circuit is broken, the time delay relay 2! breaks the power circuit to the motor i6, thereby stopping the motor, with consequent stoppage of feed. With this stoppage of feed, the noise level of the mill tends to increase, thereupon inducing greater energy in the microphone and relay circuit, and thus actuating the time delayand consequently no grinding takes place.

the control system, and therefore uniform conditions within narrow limits are maintained.

An important feature of my invention resides in the fact that the control system not only normally maintains conditions within the mill constant, but also this system may be used to readily vary those conditions as may be necessary or desirable. Such a change in the operating conditions may be prompted by a difference-in the type of feed, a difference in the product sought, wear of the grinding media, etc., but one of the most frequent reasons for changing the operating conditions is to secure precise optimum results. As mentioned above, the human ear is not sufficiently sensitive to determine what the precise optimum conditions are. As shown by Figure 6, which is a graph of an actual operation showing the relationship of the noise level in the mill to the useful work done in grinding, there is one best noise level for any given condition of feed, desired product, etc. This graph shows that the maximum noise level obtains when there is a full load of balls in the mill but no material, and consequently no grinding. At the other end of the curve it will be observed that there is no noise because the mill is choked and overloaded, From these two extremes, the curve ascends to a point a, which represents the optimum grinding for the conditions of feed, product, etc., which may be involved. It will be noted that this optimum noise level is a relatively sharp point, and can never be reached with certainty by the human ear when it is attempted to control the feed manually. As stated, Figure 6 represents an actual operation, in which case the point b was selected by the operator as the optimum noise level, and it was only after repeated trials and testing that the actual optimum level was plotted and found.

- While I have referred to the point a as the optimum noise level, and while this does represent the noise level at which the maximum useful work is done, under some conditions it may be desired to operate the mill at a point other than a, for instance at some point such as is designated c. This, for example, might be the case where a change in fineness is desired at the sacrifice of capacity. It will, of course, be appreciated that this desired point then becomes, in effect, the optimum operating condition under such circumstances, and can be determined and maintained in the same way as above described with references to the point a.

In order to ascertain the optimum noise level, it is necessary to vary repeatedly the noise level which is initially established by manual control, and the present control system affords ready means for effecting such variations. For instance, if it is desired to operate the mill at a lower noise level, the resistance is decreased, which thereby increases the current in the microphone circuit and consequently in the relay 24. As a result of this, the relay 24 is actuated and this in turn closes the time delay'switch 21 to actuate the motor. Due to the fact that the decreased resistance permits a greater current than before adjustment, the relay will-,consequently remain closed until the noise levelof the mill is reduced to'a point where it does not actuate the microphone sufhciently to generate a 35 microampere current in the relay circuit. As soon as this occurs, the relay 24 opens, opening the time delay relay 21 and cutting off the feed to the mill. The noiselevel of the mill then starts to rise.

As such rise continues, the microphone is sufficiently influenced to induce an excess of '35 microamperes in the relay circuit, whereupon the feed will be resumed.

The reverse of the operation just described takes place when it is desired to increase the noise level-namely, the resistance in rheostat 23 is increased, thus decreasing theenergy in the relay circuit and opening the relay which stops the feeder until such time as the noise level of the mill activates the microphone sufiiciently to induce a 35 microampere current in relay 24.

It should be pointed out that, when the resistance is increased or decreased in order to decreaseor increase the noise level, the current in the relay will be initially varied accordingly, but when the new noise level is attained, the current will remain at about 35 microamperes. However, the resistance 23 is provided with an indicator that will show that a change in noise level has taken place and can be used as a means of indicating the relative noise level of the mill by observing the position of the indicator.

While I have heretofore described a manual regulation of the feed independently of the control system, the control system itself may be used to manually regulate the feed initially. When this is desired, the mill and feeder are started, and with the control system operatively connected to the feeder, the operator then manipulates the resistance 23 over relatively wide ranges until he determines by the sound of the mill or the characterof the product what he believes to be the optimum operating conditions.

From this point on, by inching the resistance,

control of the-feeder, is that shown in Fig. 2,

in which the ball mill iii and feeder H3 are of the same general type as that shown in Fig. 1, till: feeder being operated by an electric motor As shown in Fig. 2, a microphone 529, located near the mill Ill picks up noises which are amplified by a vacuum tube amplifier l2l, having suitable output control indicated at 522. The amplifier may be of standard construction and it is connected to a relay E23 byconductors I26 and I 25. If the relay 623 requires D. C. current, a

suitable rectifier H6 is connected to the circuit between the amplifier l2! and the relay I23, in

the manner shown. The relay I23 is adapted to control the operation of a galvanometer I28 of any approved type. As the noises will have sharp variations in intensity for any given average intensity, a dash pot I29 or some dampening device is employed to smooth out the peaks, or wide fluctuations in current which occur, as previously described.

As the average intensity of noise increases, the galvanometer needle swings over until the upper contact I30 is reached. In this way the circuit of a relay I3! is closed and the relay magnet pulls an overbalanced lever I32 to the right hand contact l33, which closes the feeder motor circuit I34, and operates the feeder at the highest speed, since the maximum power is connected into the circuit, utilizing all three batteries, MD, MI, and I42, or other source of energy.

Since lever 532 is overbalanced, the contact I35 of said lever will remain over against contact I33, thereby keeping the feeder circuit I34 closed, even after the galvanometer circuit has been opened by lowering of the galvanometer needle i28 away from contact 530.

If so desired, the feeder circuit can be opened as soon, as the relay circuit is opened, if the lever I32 is the type which will swing back to normal position, as shown in Fig. 2, as soon as the current ceases to flow in the magnet of relay l3l. Such a lever is of the pendulum and not overbalanced type.

When the noise is low and galvanometer current low, the galvanometer needle will swing down and make contact with contact point 136. In this case, relay i 31 will pull the lever l32 towards theleft, so that contact I38 engages contact 39, thereby closing the feeder motor circuit, which includes battery I42 only. This causes the feeder to run at a slow speed. This circuit will remain closed until the needle of the galvanometer i28 swings upwardly and away from contact i3fi.

It will also be understood that the relay cir-' cuit may sound an alarm, or make contact with other equipment eflected by such changes in the v operation of the mill ill.

As shown in Fig. 2, the galvanometer needle is in contact with a spring finger contactor M3, which is connected to a source of power such as a battery we, said battery in turn having one terminal connected to a terminal of relay M5, The other terminal of battery MEis connected to conductor Mt, which connects the galvanometer I28 with terminalsof the relays i311 and E31, respectively. Contact it! of relay M5 is connected to one terminal of feeder motor H6 ductor M8. n

-With finger contactor M3 engaging-the galvanometer needle the contact point M! of relay M5 engages contact I49, which is connected to one terminal of battery MI by conductor i511, so that the feeder motor circuit through batteries MI and M2, and the feeder is then operated at its medium or normal rate.

In this way it will be noted that the operation of the feeder H3 can be so controlled that the feeder will operate at varying speeds in accordance with variations in'amount of noise emanating from the mill as picked up by'the microphone I34 is closed a by con- I20, and should the mill become overloaded, the- Y of any desired construction and placed at any convenient location with respect-to the mill, I have found that decidedly improved results may be obtained when the microphone is provided with a shield to prevent extraneous noises registering on the microphone, and where the microphone is positioned adjacent the mill and below the point of maximum impact for any loading.

As best shown in Figures 4 and 5, I prefer to suspend the microphone designated 20 at the focal point of a parabolic reflector 15. Suitable springs I6 may serve as suspending means, and as best shownin Figure 5, the specific location of the microphone with respect to the reflector is at the focalv point of the latter, as indicated by the arrows "representing the incoming sound or vibration waves. It will be appreciated that the provision of such parabolic reflector, and the position of the-microphone at its focal point, insures the maximum reception of sound vibrations which emanate directly in front of the micro-- phone.

To prevent the reception of any extraneous vibrations, or noises, suchas from adjoining mills, or other equipment, I provide'a dust-proof and water-proof shield for the microphone and rcflector. This shield is composed of a casing 18 and insulating material 19 intermediate such casing and the parabolic reflector 15. This insulating material 19 serves to prevent any vibrations, except those emanating directly in front of the microphone, from reaching the microphone. In addition to the protection afforded by this shield around the microphone, a covering or fabric or other material permitting the passage of sound therethrough may be placed over the front of the shield to protect the microphone from dust, moisture, or other injury.

With regard to the relative location of the microphone and mill, I have found that improved results are. obtained when the microphone is located on that side of the mill toward which the ore and balls drop, and below the point at which the mass impacts th mill when the mill is operating with the smallest load that would be encountered.

In Figure 3, I have diagrammatically shown a mill and a microphone so arranged. In this figure the mill is designated generally by H and the microphone by the numeral 20. line 8! represents the outline of the charge and grinding media when the mill is rotating in a clockwise direction and is properly loaded. The

dotted line 82 represents such outline of an overloaded mill, and the dotted line 83 represents the outline of the contents of an underloaded mill. The respective points of impact of the falling masses are at Ma, 82a and 83a.

It will be noted that the microphone 20 is positioned below the zone of impact of an underloaded mill, and is so positioned for the following reasons: The activation of the microphone is a function of the noise generated by the mill and alsothe position of the microphone with respect tosuoh noise. In turn, the noise generated by the mill is a function of the degree to which the mill is loaded. 7

-As heretofore pointed out, in general, the noise of a mill decreases as the load is increased, and, conversely, the noise increases as the load is decreased. If the microphone were positioned above the zone of impact when the mill is running with its minimum load, then, as the load is increased, the general noise level of the mill is decreased, but the diminished reaction of the microphone thus caused by the increased noise level tends to be Ofi'set by. the fact that the zone of noise, though decreased, is brought nearer to the microphone. This is due to the fact that with anincrease in the load of the mill the volume of thecontents increases, as indicated by the The solid phone or even erroneous results within certain limits, and in general a much greater change in noise level, or loadin of the mill, would be necessary in order to produce a different effect on the microphone.

However, as opposed to the above, when the microphone is located further down the mill shell so that the zone of impact is always above the microphone, as shown in Fig. 3 to be below the point 83a, the point of maximum noise on the shell moves away from the focal point of the microphone when the mill starts to load up and thereby decreases the noise level, thus insuring a dual action which increases the effective sensitivity of the microphone and consequent improvement of operation of the entire control system.

While I have described above the preferred location of the microphone, whereby it is positioned immediately below the zone of maximum impact of an underloaded mill, mention might be made of the fact that, if the microphone is positioned at any point along the arc that is defined by the contents of an unloaded mill, better results are secured than if located elsewhere.

In the previous description I have used the operation of a ball mill to show how my invention functions in one of its many forms. It will be noted that an increase in the noise in a ball mill is the indication of an underfed or underloaded machine. The reverse may be true if my invention is used in connection with some other machine, such as a jaw crusher, for example, since in this case the vibrations or sound emanating from a crusher increase as the feed increases within certain limits, then by reversing the operation of my control device the deslred results I the ball mill feeder control circuit.

There are-many other machines for a great variety of uses whose operation is accompanied by sound or vibration. It is the control of such machines, as well as the machines herein described, that I wish it understood be included in the general scope of this invention.

I claim: 1. In a control system for a mill in which vibrations set up by the mill are a function of the operating condition of the mill, a feeder for delivering material to said mill, means for actuating said feeder, a microphone positioned with respect to said mill for picking up the vibrations emanating therefrom, means responsive to the vibrations picked up by said microphone for' controlling the feeder actuating means, and separate means for varying the degree of reaction of the vibration responsive means whereby the system may be balanced with respect to a desired vibration level and to thereby maintain such level substantially constant, and which separate means may also be adjusted to establish any desired operating condition of the mill with its corresponding vibration level.

2. A control system as defined in claim 1, in which an electrical circuit is included, and in which the means for varying the degree of reaction of the vibration responsive means consists of a device for varying the amplitude of power in said circuit.

3. A control system as defined in claim 1, in which the means for'controllingthe feeder actuating means includes an electrical circuit and a relay, and in which the means for varying the degree of reaction of the vibration responsive means is a rheostat.

4. A control system as defined in claim 1, in which the means for controlling the feeder actuating means includes an electrical circuit, a relay, means for quantitatively indicating the energy in said circuit, and a time delay contactor, and in which system the means for varying the degree of reaction of the vibration responsive means is a rheostat.

5. A control system as defined in claim 1, in which the means for controlling the feeder actuating means includes an electrical circuit and a relay therein operable at a. definite electrical power level, and in which system the means for varying the degree of reaction of the vibration responsive means consists in a device for regulating the amount of electrical power which actuates the relay. I

' 6. For a .drum type mill, a control system as defined in claim 1, in which the microphone is located opposite the arc of the drum defined by the load in the mill.

7. Apparatus for controlling the operation of a machine for handling material in which vibrations set up by the machineare a function of the operation of the machine and including a motivating element, a microphone positioned witln respect to said machine for picking up vibrations emanating from the machine'to establish an electric circuit, an amplifier for amplifying the electric current responsive to said vibrations, a rectifier in said last-mentioned circuit, and a time delay relay, saidtime delay relay adapted to alternately establish three circuits depending upon the actuation of said relay, each of which circuits includes a relay, and each of which last-mentioned relays upon actuation is adapted to establish an independent circuit of a predetermined intensity, and all of which lastmentioned circuits include said motivating element. a

8. A methodof regulating the feed to a mill in which vibrations set up by the mill are a function of the operating condition 01' the mill and in which provision is made for actuating a feeding device by means of an electrical circuit, .comprising the steps of feeding material to the mill at a given rate and thereby establishing acertain operating condition in the mill having a corresponding vibration amplitude, translating the acoustic energy of the vibrations into electric energy in said circuit so that'variations in the acoustic energy efiect corresponding variations in the electric energy so as to secure a rate of feed for maintaining a definite and substantially constant operating condition in the mill, and varying the operating condition in the mill, and may be desired, by changing the ratio of translation of acoustic energy into electric energy.

9. A process as defined in claim '8, in which, in order to maintain the definite operating condition in the mill, theelectric circuit eflects an intermittent feed of the material to the mill at such a rate per increment of time and for such periods of time as to maintain a substantially HARLOW'E HARDINGE.

I CERTIFICATE OF CORRECTION. Patent No. 2,255,928. March 2 19t HARLOWE HARDINGE.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, first eolumn, line 29, for the word "or" before "fabric" read -ofpage 6, second column, line 28, claim 8, for "and" read --as; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this Zhth'day of June, A. D. l9l 1.

Henry Van Arsdale,

(S881) Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION. Patent No. 2,255,928. March 2 19M.

HARLOWE HARDINGE.

It is hereby, certified that error appears in the printed specification of the a't iove numbered patent requiring correction as. follows Page 5, first column, line 29, for the word "or" before "fabric" read -ofpage 6, sec- 0nd column, line 28, claim8,for "and" read as-; ters Patent 'should be read with this correction therein that the same may conform to the record of the casein the Patent Office.

Signed and sealed this Zhth'day of June, A. D. l9hl.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents.

and that the said Let-

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