US1646373A - of connebsville - Google Patents

Description

\ l646,3 Oct. 18,1927. WSILKIN 73 HIGH SPEED LOBED IMPELLER BLOWER Original Filed April 14, 1926 3 Sheets-Sheet 1 l INVEN'TOR.

' John T. Wzlkm;

' ATTORNEYS v 7 1,646,373 J. T. WlLKlN HIGH SPEED LOBED IMPELLER BLDWER Oct 18, 1927.

v Original Filed April 14, 1926 3 Sheets-Sheet 2 I INVENTOR c/b/m T. Wz'lkz'n,

ATTORNEYS Patented a. 18, 1927.

UNITED STATES PATENT OFFICE.

JOHN T. WILKIN, OF CONNERSVILLE, INDIANA, ASSIGNOR TO THE CONNERSVILLE BLOWER COMPANY, OF CONNERSVIIJLE, INDIANA, A CORPORATION OF INDIANA.

HIGH-SPEED LOBED-IMPELLER BLOWER.

Substitute for application Serial No. 101,903, filed Apri114, 1926.

- Serial No. 171,913.

In that type of rotary gas pumps and blowers primarily designed for increasing or decreasing gaseous pressures, and wherein the fluid flow is caused by the action of two interdigitated rotary lobed impellers ar ranged within a casing, the two impeller elements have mating surfaces of such a character as to form gas forwarding pockets between the impellers and the casing and also to maintain minimum and uniform clearance between the impellers themselves and between each impellerand the casing. Such impellers do not coact for relative rotation but are rotatively connected by intermeshing timing gears which also serve to transmit power from one impeller shaft to the other. 1

For convenience of description I shall designate such a structure as a blower,

whether it be designed or used as a blower or gas pump, because the coaction of the several elementsis the same in either case.

In the early period of development of blowers of this type their use was largely smelting furnaces, forge fires and theoperation of cash carrying tubes for store service work; also for gas exhausters in gas works, all of which uses required pressure differences rarely exceeding one or two pounds per square inch. The demand for struc tures of this type has gradually increased until now they are used for many purposes which require pressure differences as high as fifteen pounds. For convenience of description I shall. use the term low pres sure for pressure differences below two and a half pounds, and the term high pressure for pressure differences between two and a. half and fifteen pounds.

Heretofore in the manufacture of blowers of this type it has been the practice to make the lobes of the impellers hollow and as light as is consistent with necessary rigidity.

I have found by long experience with blowers of this type having light impellers, each with two hollow lobes, that where the driving power is applied to one shaft, and the other impeller is driven through the gears, the wear on. the teeth of the gears in limited to the blowing of foundry cupolas,

This application filed March 2, 1927.

not uniform in all parts of the revolution, but is concentrated on a few teeth at two' opposite portions of the revolution.

It has long been known, in the operation of blowers of this type, where each impeller has two. lobes, that there is, twice in. each cylic angular distortion or twist of eac shaft between the connecting gears and the impellers, and that this distortion or twist permits a change in the angular relation of the impellers to each other. The amount of this angular distortion increases directly with the increase of the unbalanced area of each impeller and the pressure actin there- Theoretically the mating sur aces of the impellers should just avoid contact, and the actual clearances between such mating surfaces should, of course, be made as small as possible in order to decrease slippage or leakage of the gas from the high pressureside to the low. pressure side between the mating lines of the two impellers.

In blowers of this type the highly fluctuating torque and unavoided inaccuracies of gearshave necessitated excessive clearances betweenthe impellers, andhave limited the practicable speed to a pitch line velocity of approximately 700-feet per minute in small machines and approximately 1400 feet per minute in large machines, more or less depending on workmanship, materials, etc., because when higher speeds were attempted there would be a noisy and destructive action of the gears and impellers unless further and more excessive clearances were maintain continuous contact of the working faces of the gear teeth, and that this failure due 'to inaccuracies of commercial produc tion, which give fluctuating impulses to the driven gear, such impulses, as is well-known, increasing in intensity as the square of the increasedspeed, p 7

I have alsoff'oun'd that, in such blowers this 'destructive an'd noisy action increases as the gearsbecome worn, because, as previou-sly stated, the wearof the gear teeth, when'light impellers are used, is not uni- 7 form throughout -the circumference but is concentrated on'a few teeth'at two opPO gen-ions of the gear. It is, therefore, evient itlrat thisfuneven wear of the gears woiild tend to transmitto the driven impeller additional cyclic impulses. 7 one object of my invention is to so construct a blower or gas pump ofthe type described that *the impellers maybe commerciallysuccessfully driven at very much higher speeds than has heretofore been found -'practicable and'to commercially successfully drive such impellers at such in til creased speeds -against higher pressures than'have heretofore been found practicable at-such highspeeds, the construction beings'u'ch "that necessary clearances between the impellers may be substantially decreased.

"Terms end my invention contemplates the provision of impeller elements of a and sufficient flywheel effect (most conveniently obtained by substantially increa'sing the, weight of the impellers which 'in'the higher pressure'machincs having relatively short impellers would be solid or nearly solid) coordinated with and'proportioiiailtothe variable torque exerted on'the impellers by the gas forces, thus decreasing objectionable torsional angular displacements" of the impellers with relationto each other" and avoiding torsional reactions of the shafts'thereby preventing breaking of ccntact of the driving faces of the gear teeth by such reactions; and further by providing gears with teetlrwhich willnot impart to the driven gear suchfiuctuating impaissae-wnrcause the breakingof contacts of the driving faces of the teeth.

Bythes'e'means I eliminate, or practically eliminate the noise and destructive impacts of the gear teeth and impellers at speeds much higher than has heretofore heenfound possible, namely at speeds from 2000 to 5000 (and above) feet per minute pitch line velocity, thus making possible the utilization of smaller, and more efficient units at lower cost,

In other words, I have discovered that if the impellers themselves be made sufficiently heavy, so as to give them a very much greater inertia value than they have heretofore possessed, the cyclic distortion of the shafts between the impellers and the gears, particularly in that impeller which is driven through the gears, will be decreased and that consequently a blower in which the impellers themselves have a sufficientinertia valuemay be driven at speeds very much in excess of present practices, while at the same time the clearances between such impellers may be substantially reduced, and more uniform wear of the gears will be attained.

In orderto make the matter clear:

impeller and'the impeller which is rotated through the medium of the gears may be called the driven "impeller. 'When the driven impeller has thetip of one of its lobes in immediate association with thewaist'of the driving impellenthe pressure on the gear teeth and the resistance to rotation offered by the driven gear, due tothe pressure in the pressure side of the system,

one impeller, to the shaft of which the driving .power is applied, may be called the driving are at their minimum. As this drive'nim v peller advances the gear pressure and e'ffective resistance to forward movement of the driven impeller increase and, as this (:lrivenimpeller is being forwardedby the gear connection, the shaft of the driven im peller, between the impeller and the a gear is twisted. lVhen the driven impeller 'reachesa point where its waist is in association with the tip of the driving impeller, the gear pressure andresistance'to forward movement of the driven impeller are at their maximum.

Immediately thereafterthe advanced tip of the driven impeller leaves association with the casing and the pressure in the pressure side of'the system enters the on-coming delivery pocket between the impeller and the casing thus causing a sudden pressure equalization, and the resistance to forward movement of'the driven impeller decreases until the advancing tip-of the driven inipeller is in association with the'waist of the driving impeller, at which time the gear pressure and resistance to forward move. ment of the driven impeller have returned to minimum. During this period of decreasing impeller torque, the twist of the shaft of the driven impeller between that impeller and its gear has been correspondingly decreased, such decrease taking effect quite promptly upon the-entry of the pres. A

sure into the on-coming delivery pocket justmentioned, and the lighter the impeller, the more promptly its shaft twist .will be released. The release of this shaft twist causes, or has a tendency to cause, the driven impeller to accelerate because of the energy delivered by the untwisting shaft, and this acceleration causes, or tends to cause, a separation of the driving connection between the gear teeth.

During the above described period. ofaction of the driven impeller and its shaft, a similar action is occurring in the driving impeller and its shaft, but, because of the cycles of the two being spaced ninety degrees apart, the combined effect is to produce twice per revolution an increase of pressure on the few teeth trasmitting maximum power to the driven impeller, and a subsequent increased tendency to cause a separation of the driving connection between the gear teeth at the time of minimum transmission of power to the driven impeller.

It will now be recognized that the smaller the inertia value of the impellers, the greater will be the acceleration of the driven impeller as the crest of resistance to forward movement is passed, and that consequently an increase in the inertia value of the impellers will decrease the amplitude of their torsional fluctuations. \Vhen this is realized it becomes apparent that, because inertia varies directly as the mass and as the square of the velocity, the necessary inertia value to substantially eliminate objectionable torsional fluctuations of the impellers and produce more uniform gear wear, maybe obtained by'sufficient increase in mass and speed and that such increase of speed becomes possible because the inertia value of the impeller has been made sufliclent to mam- ,tain. practically uniform angular velocity of the impellers and their tion toeach other.

According to recognized practice prior to my present discovery, blowers of this type with impellers having two hollow lobes have proper angular relagenerally been designed with standard shafts and bearings for each gear diameter so that the standard elements, i. e.,impellers, shafts, bearings, gear teeth, etc., were capable of transmitting a given normal H. P. per revolution. Also the lengths of the impellers have been made to vary inversely as the pressure against which the blower operates. That is to say, low pressure operation permitted use of axially long impellers and, as desired pressures were greater, these impellers were axially shortened in a mannor to maintain approximately the same H. P. per revolution, the same total pressures on impellers, bearings, gears, etc.

Under that practice the shortening of the impellers for highpressure work reduced their mass and thus reduced thier inertia value, whereas, according to my invention, the mass of the shorter impellers, relative to their length, is increased and their inertia value is maintained or increased. Since, according. to my present discovery, the shorter impellers for higher pressures are very substantially heavier than heretofore,

their inertia value will be sufficient to practically eliminate the objectionable cyclic acceleration of the driven impeller and prevent the breaking of driving contact of the gears, and this in itself permits additional speed which, in turn increases the inertia value of the impellers.

When a sufficient mass is provided, proportionate to the pressure against which the blower is to work, the impellers should, of course, be speeded up, in order to gain the advantage of my invention.

The design should be such therefore, that desired volume and pressure of delivery being known, impeller'proportions should be determined on the basis of a large inertia value of the impeller, such inertia value to be a function of speed and mass, the mass to be relatively large and sufficient at the adopted speed to prevent objectionable relative cyclic angular fluctuations of the impellers.

Methods of calculatingthe variation of angular velocity of masses subjected to fluctuating torque, can befound in engineers hand books and text books dealing with the v design of flywheels for reciprocating steam engines.

There is another distinctly beneficial result attained by the use of high inertia impellers which deserves consideration. Just as each gas forwarding pocket is opened to the delivery, or high pressure, side of the system, there is a sudden increase of pressure in that pocket and this increase of pressure, acting upon the entire side area of the impeller, delivers to the impeller, transversely of its axis and towards the mating impeller, a heavy blow which, if the impeller be light, as has heretofore been the practice, actually displaces the impeller and its shaft sidewise, the extent of this lateral displacement being determined by the looseness of the bearings, the flexibility of the bearing supports and the flexibility of the impellers and shafts. It now, the shorter impellers for higher pressures be made solid or nearly solid, and the longer impellers be made adequately heavy, their increased inertia value is not only effective as a resistance to fluctuations in angular velocity but also has beneficial. effect as resistance to the above mentioned, lateral displacements caused by the sudden blow of pressure entering the oncoming pocket. This lateral displacement of the impeller and its shaft is ,my invention.

tionof a blower or gas pump intended to 7 very objectionable, because it makes it almost impossible'to maintain tight packing around the shaft .where sucha structure is used as a gas .pump and where leakage around the shaft is objectionable. This lateral deflection of the shaft also produces uneven side wear of the, journal bearings thus changing the distance between the gear centers and causing irregular and'noisy action-of thegears.

The accompanying drawings illustrate Fig. 1 isa transverse seccomputation diagram for detern'iining a de-V .sirable gear tooth form, and Fig. 6 an adiarbatlc compression diagram.

In the drawings 10 indicatesthe casing, 11 and 112 the impeller shafts, 13 and 14: the lobed impellers interdigitated with each other, 15 the inlet, 16 the outlet and 20, 21

the connecting gears- With the exception of the above described inertia value of the impellers and reduced clearance between the impellers, theseele- :ments are of the general form commonly known in the art and the particular contours-of the impellers may be considerably varied in accordance with well-known practice. The relatively short impellers, however,are shown as solid because, in the practice of my invention, these impellers must'haveoa. very high inertia value and the most convenient way to obtain such high inertia value is to make their lobes solid or nearlysolid.

I am :awarethat liquid pumps of the type under consideration have been made with. impellers having solid lobes, but the speed of suchpumps was too low to develop the inertia values of the impellers as a function ofihi'gher speed of operation.

'My-invenilion does not apply to pumps of this type designed for pumping liquids, because-operation of such pumps at high speeds becomes destructive to the pump. due

to the weight and incompressibility of the liquids.

The increase in compression efiiciency by high speed operation of a blower or gas pump of the'type under consideration .becomes apparent from a study of Fig. 6, which shows'the relations of pressures and volumes and the percentage of the discharge strokeire uiredto. compress the gases to specified disc arge pressures. It will be noted that the higher the pressure againstwhi'ch the bloweris-operating, the greater the percentage of" the stroke required to produce the necessary compression. In the operation the gas which is being transferredfrom a lower livery pocket at an early point in the com; pression period and the impellers must,

therefore, make their advance against the full discharge pressure.- y

It'is evident from a study of these relations that the higher the pressure against which'the blower is operating,the higher should be the speed in order to take advantage of the inertia of the gas and accomplish the compression of each dischargevolume before the gas in the discharge linehas time to overcome its inertia and expand backward into the oncoming discharge pocket. By such action the compression efliciency Ofgthfi blower t higher pressures is very muchincreased. Taking for example a blower operating against 10- lbs. pressure, it will be noted that the mean effective adiabatic pressure when compressing to 10 lbs. is 8.27

lbs. If a slow running blower is working against 10 lbs. pressure,the .mean effective pressure against the impellers will at all times closely approximatelO lbs, butiftthe blower is so designed and speeded as to accomplish approximately I adiabatic compression, the mean effective pressure'would only be 8.27 lbs, and a consequent savingin horsepower of 17.3 per cent.

shorter leakage lines and smaller clearances.

if that smaller blower maybe run at a higher speed than has heretofore beenfound practicable.

I have found that, inzconnection with providingthe impellers of high inertia =value, some further advantages may be obtained by providing connecting "gears having teeth which are fiexible'to an extent equahxor substantially equal to the unavoidedidepartures from theoretical accuracy'of'the'gears existing in tooth form, toothspacing and mounting.

Another object of my invention tetherefore, to combine connecting gears; having suitably flexible teeth, with the high-inertia hi h-speed impellers.

Where the teeth have been produced by commercially practicable methods there are unavoidable inaccuracies from theoretical perfection and these inaccuracies vary in individual teeth. At the point Where two teeth first come into action, the power-deliverytooth is contacted at apoint slightly below the pitch line, while the power-receiving tooth 1s contacted at, or approximately at, the top of the tooth which is, of course, much further beyond the pitch line than is the point of contact below the pitch line on the power-delivery tooth. Bearing in'mind the fact that the shapes of the teeth should be such that, when in contact at the pitch line there should be a true rolling contact, it becomes apparent for several reasons that, in order to avoid hammering at themoment of initial contacting oftwo teeth, thepower receiving tooth is the one which at the point of initial contact, should have greater yield than thepower-delivery tooth.

For instance, if the powerreceiving tooth is incapable of suflicient yield, then one of two things must take place. Either the power-receiving element must, as a whole, be suddenly accelerated, or the power=delivery element must, as a whole, be suddenly retarded. It is apparent, therefore, that the preferable construction, by which individ ual yield of a tooth may be obtained, should be such that the greatest yield shall take place in the tooth to which the driving force is being applied, and this greatest yield should take place atv the timeof initial con tacting i. e., when the tooth is coming into action.

It is also desirable that whatever of yield there be in each tooth will take place under such conditions as to avoid as much as possible tendencies'towards crystallization at any particular section. i

The epicycloidal type of tooth, having a radial flank below the-pitch line is, of course that type of tooth which in itself is capable of greater yield, but the weakest point of this tooth is 'at'its narrowest cross-section which is at the root, and this type of tooth is therefore incapable, without modification, of satisfactorily providing the yield which is necessary for the practice of my invention, without danger of crystallization and breaka e atthe root.

Bearing in mind that the tooth may be considered as receiving its load on lines nor mal to its surface, and that at the time when a load-receiving tooth comes into action the load may be considered asbeing applied along the line LL, (Fig. 5) thetooth may be analyzed as beam of uniform flexure (one having a parabolic section) fixed at one end and having its load applied at its outer end T. Applying, then the .;usual analysis for such a beam loaded at the point T and having, at the root line, a thickness AB equal to the normal root thickness of a n0rmal epicycloidal tooth, it is apparent that, that tooth may be given an additional resilience theoretically capable of uniform stress throughout its length by forming a sub-root with sides approximating the parabola secof the tooth defined by the points ABCD may of course be varied,idepending upon the metal used, the amount of velocity considered to be necessary, the inertia value of the impellers and various other factors whichwill readily suggest themselves to a competent designer, and dependent upon circumferential velocities desired, etc, being in all cases sufficient to insure a quiet openation of the gears.

.At first glance it might be supposed that the tooth at the points AB would have a weak cross-section, but. upon further consideration it becomes apparent that that would not be the case because the parabola MAC or MBD is the uniform stress line and therefore the additional metal in the tooth which lies between the exterior of the tooth and the-parabola section MA or MB is simply additional strength or wearing material and there are no lines of weakness at the points A and B. i

In order that the method of computation may be understood I give herewith. the

mathematical analysis for a specific example:

a 1 Gear specifications. Pitch diameter 26". Face ll. Speed=400 R. P. No. teeth=37. Three teeth to be always in contact. H. P. to be transmitted=346fi H. P.

Velocity pitch line= 400 X 26 X 3.1416 2722.7 ft. per min.

t 12 a U r .547 X 35000 7 LQfldPGI'llOOth'Q 2W =1,4 ()0# (app.)-

If the load is borne by one tooth g 1 ==382i$per 1 face. Teeth to be epicycloidal radial flankform,

51.- ini' oluteofsulficient tug tandem? to "Now iii order a) find the thickness (21) I permit deflection as desired.

I thepoint of initial'conta'ct at T (Fig:- 5); the tooth pressure is exerted along the hneof' action, cutting'the center line of tooth atM'. The distance 0.7 5'T isselected' as'a inimum safe. flank-length which will" allow proper tooth" action Without interfer ence, and such a flank-length according tofl root accepted rules of design, requires a thickness, at AB, 0151.040.-

Gircular pitch'=2. 2076:

i Thickness of tooth 1.1038;

It parabolic curve'isnow generated with the point M as the vertex and-the distaneeAb as adouble ordinate, it' willireprecent-i'leverbeam of uniform strength havingtheloa concentrated at: M. Allof the tooth outside of the parabolic curves wil1;,be excess-material, If theparabo'lie curves; and- MYB are. continued: as, shown} on; the drawing to G7 andv D, then the portion,.A iB C-D-Willbea portion of uniform strength. fI'n other ords, the fiberv it apprioaches a straight'line so that for all practical purposes the sides of: the tooth along: AG-and; 13D may be made'straight.

The mathematical calculations used to findi thethickness (d) of the sub-root A, whiohis a double ordinate of the parabola,

' at: any: place-within-the portion of uniformstnength is follows.

First calculate the maximum; fiber stress i SQQL BL); of the section AB.

M=bending moment.

C= thickness of tooth at-section under consideration. i v I=moment of inertia of tooth at section a under consideration. M=382 1.84L=7O2.88

It should be noted that have selected the Worst condition possible by considering a the entire'load of per'l of itace to be borne by the' oneimperfect toot-hat its point of initial. contact. Thisgives- S(AB) a maximum value.

at any other distance (L) from the Vertex: We must keep the maximum fiber stress s constant at 3900# and solvefor in: the equation+ and thusgenerate the arabolic curves,

The values of d and" however shouldbej such as to give'the desired amount of, de-

flection. I v I v Now consider that We desire a' deflection of 0.00108 in the; tooth taking the tooth as:

a straight cantilever beam with the load concentrated the free end t 3 then Itis thus seen" that W6 5 can find Values: of

By giVingL'difierent valueswe may solve any number of correspondingvalues for d E uaat (im- 0 beam of uniform strength and load acting on the pitch line.

It should be noted that in solving for and L any deflection formula may be used. Also the figures and methods which are given are to act as an illustration only, and do not limit the design or construction of the tooth which is essentially a gear tooth having a portion of uniform flexure.

This application is filed as a substitute for application No. 101,903, filed by me April 14th, 1926.

I claim as my invention:

1. A blower comprising a casing, two interdigitated lobed impellers mounted with in the casing, and a pair of meshing gears rotatively connecting said impellers, the said gears having teeth capable of circumferential yield under the applied load of approximately the unavoided departure of the contacting surfaces of said teeth from theoretical accuracy.

2. A high speed blower comprising a casing, two interdigitated lobed impellers mounted within the casing and having an inertia value which is high relative to their gas load torque variations, and a pair of meshing metal gears rotatively connecting said impellers, the said gears having teeth capable of circumferential yield under applied load of approximately the unavoided departure of the contacting surfaces of said teeth from theoretical accuracy, whereby the tendency of tooth wear is toward uniformity throughout the circumferences of the gears.

3. A blower comprising a casing, two interdigitated lobed impellers mounted within the casing and a pair of meshing gears rotatively connecting said impellers, the said gears having teeth capable of circumferential yield under applied load of approximately the unavoided departure of the wearing surfaces of said teeth from theoretical accuracy, each of said teeth having a subroot of approximately uniform flexure,

4. A high speed blower comprising a casing, two interdigitated lobed impellers mounted within the casing and having an inertia value which is high relative to their gas load torque variations, and a pair of meshing metal gears rotatively connecting said impellers, the said gears having teeth having a sub-root ofapproximately uniform flexure and capable of circumferential yield under applied load of approximately the unavoided departure of the wearing surfaces of said teeth from theoretical accuracy, whereby the tendency of tooth wear is toward uniformity throughout the circumferences of the gears.

75. A high speed blower comprising a casing, two interdigitated lobed impellers mounted within the casing, and a pair of meshing metal gears rotatively connecting the said impellers and having teeth of a form providing at least two pairs of teeth in mesh at all times and capable of circumferential yield at the contacts, the inertia value of said impellers being relat-ivelyhigh, whereby the tendency of tooth wear is toward uniformity throughout the circumference of the gears.

6. A high speedblower characterized by a casing, two interdigitated lobed impellers rotatably mounted within and coacting with the casing, and a pair of meshing gears rotatively connecting said impellers, the said impellers having an inertia value which is suflicient to maintain continuous contact of the working faces of the gear teeth at high speeds and which is high relative to their gas load torque variation, whereby they may be rotated at high speed with small clearance.

7 A high speed blower characterized by a casing, two interdigitated lobed impellers rotatably mounted within and co-acting with the casing, a pair of meshing gears rotatively connecting said impellers, said impellers having such great mass as is adequate, at the intended high speed of rotation, to develop inertia value which is sufficiently high relative to the cyclic gas load torque fluctuation to maintain continuous contact of the working faces of the gear teeth at such speeds, whereby small clearance between the impellers may be employed and eflicient, normal, high-pressure, highspeed operation assured.

8. A high speed blower comprising a casing, two interdigitated lobed impellers mounted within the casing, and a pair of gears rotatively connecting said impellers and having teeth of a form providing at least two pairs of teeth in mesh at all times, the inertia value of said impellers being high relative to their gas load torque variations, whereby the tendency of tooth wear is towarduniformity throughout the circumference of the gears.

In testimony whereof, I have hereunto set my hand at Connersville, Indiana, this 24th day of February, A. D. one thousand nine hundred and twenty-seven.

JOHN T. WILKIN.

Images