0b Agitator Drive Design

8/2/2019 0b Agitator Drive Design

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A G I T A T O R D R I V E D E S I G N

D o n a ld L . K im eC h e m in e e r , I n c . , D a y t o n , O h io

R e p r in te d f r o m :P O W E R T R A N S M IS S IO N D E S IG N


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,.

Fig l=-Common process agitator.

PowerTransmissionDesig"

top view

side view

Fig 2-Agitation forms flow patterns, thus aiding mixing.

Agitator drive designers blendright m ix of gears and bearingsBy Donald L. Kime. Fluid agitation poses tougb problems to the designerof power-transmission systems. Cbemineer surmounts them with taperedroller bearings, spiral bevel gears, and helical gears in their speed reducers.

Commonl y, a process agitatorhas a prime mover (an air motor,steam turbine, hydraulic mawr, orelectric motor) and a gearboxmounted on top of a long verticalshaft that extends into a mixingvessel and terminates in an impeller(Fig 1). The impeller may be a flat,pitched, or disc turbine, propeller.paddle, or helix.

Through rotation, an axial flowimpeller imparts energy to the fluidin a vessel. Flow patterns form intwo directions:

axially, to the bottom of thevessel and back up the side walls rotary, around the vessel

To prevent swirling, baffles di-rect [he fluid from the wall inca thecenter of ehe vessel (Fig 2). Thisrandom flow exerts bending forceson the impeller, perpendicular tothe agitator shaft. If the agitator isnot designed properly, these fluidforces are large enough to causemechanical failure.


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fsRl ~F (xIs)R2 .. F [1+ (xIs) ]

F

F .. bending load due to fluid flowRl = proportion of load reacted at top bearingR2 = opposing reaction at bottom bearings .. bearing spanx '" overhung length

shaft support bearing

Fig 3-Bending force affects bearing reactions.

IIi l. . . shaft deflectionK .. constantE .. modulus of elasticity1.. moment of inertiaF .. bending load due to fluid flows .. bearing spanx .. overhung length

Fig 4-Sbaft deflection increases with large bearingspan.

'.

quiUsupport bearing

reduction gears

quill support bearing

Complications-Early agitators hadregular cornmerical gear drives thatdid not take into account the bear-ing overhung-load capacity. Bear-ings were subjected to high loads,especially with small supportedspans, large overhung lengths, andlarge fluid forces (Fig 3). Conse-quently, bearing failures were com-mon.An initial answer was an in-tank

bearing, at or near the mixing im-peller. on the agitator shaft. Thisprovided acceptable drive protec-tion, but it also provided an unde-sirable and unacceptable mainten-ance item.

Increasing bearing span to re-d uce loading was an answer.Greater bearing spans increaseddrive bearing life. However, shaftdeflection also increased (Fig 4). Asa result, gear failures from toothmisalignment were common.

Another solution included aquill, a hollow shaft supported onbearings (Fig 5). The: driveshafr hada flexible coupling and was sup-ported at a large: span by separateFig 5-A quill can support tot! ag1t:Jtorsbaft;


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Fig 6-Chemineer Model HT agita-tor drive with double reduction.

bearings. The quill shaft reducedbearing and gear difficulty, but itintroduced new complications. Thequill, flexible low-speed coupling,and two extra bearings were expen-sive and added parts to maintain.It's feasible-Two developments al-lowed the solution that is mostprevalent today (Fig 6): tapered roller bearing improve-ments fluid technology advancementBearing spans were reduced andoutput shafts were enlarged to pre-clude destructive shaft deflections.The ta per ed roller bearings handledthe. higher bearing loads comfort-ably.

ItBearing selecriorr-Lsing a computer,Che mine er optimizes bearing span,shaft diameter, and beari ng selec-tion. They rate a. I tapered rollerbearings In the reducer for a30,OOO-hour minimum B10 life.

B10 life is a rating where 10% of alarge random bearing sample can beexpected to fail. Average bearinglife, or 50% failure, is five times theB10 life.Helical and spiral bevel gears-TheAGMA classifies gears according toestablished tolerances. The degreeof precision in making the gearteeth is related to a quality number.A higher AGMA quality numberclassification specifies smaller 3 . 1 -Iowa b l e tolerances. Cnemineerlimits effective tooth error due IOshaft deflection to 1/20 of rheallowable AG,\1A tolerance. AJlgearing IS inspected to meet theAG.'vL';'Quality 10 classification.

Ch emirie er uses helical and spiralbevel gears throughout their ModelHT agitator line. Helical and spir albevel gears have a high load CJ.P.l-city. produce low noise. and areefficient. Spira: bevel gears are g~n-erur ed, rnacche d, \3 . p ped, and :n-spect ed. Helical geJ.rs are through

hardened, hobbed, shaved, and in-spected. Although the more ductilethrough hardened helical gears aremore expensive, Chernineer chosethem rather than case hardenedgears to withstand the high shockloads that agitators experience.Quiet-Low noise level operationwas an important parameter in se-lecting gears and bearings. Chemi-neer found that tapered roller bear-ings distribute loads over a largecontact line, thus generating lownoise. Helical gears are used forhigh- speed reduction, where somemanufacturers use spiral bevelgears.Two para meters affect helical gearnoise; the nu rnber of helical overlaps too th a ccu r acvThe nurr.o e r of helical overlaps is

a measure 0; the total number ofteeth In cor.r act at one time. Pitch,helix angie, and face width govern


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+50

g:) - s_ ,~.-10I'! -IS0co . -20'"~ -25.~ -30;" E -35

-40

SO 100 200 500 1000 2000 sooo lO,OOOfrequency, cps

Fig 7-Tbe /l..Weigbting Scale represents bearing response.

Fig 8-Noise profile for Cbemineer's Model HT agitator drives.contact ratio. Helical gears are de-signed commonly for 1.5 to 1.6helical overlaps. Chemineer uses aminimum contact ratio of 2.0. Asdrive size and noise potential in-crease, the contact ratio is increasedto 2 . 5 .

Meets OSHA standards-Sound is avibratory air disturbance that isconverted, by the ear, into a sensa-tion called hearing. The ear re-sponds to sound pressure level.Thus, higher sound pressure levelsare sensed as degrees of intensity.Sou nd pressure level is measuredin decibles (dB), a logarithmic ratio

of sound pressures. Human hearingresponse varies according to soundpressure level and frequency. Soundis measured for pressure level atdiffcrenr frequencies and thenweighted to represent average hu-man hearing response.

There are several weightingscales, but the A-Weighting Scale ismost commonly used (Fig 7). Whensound is destructive or undesirable,it is called noise.Acceptable noise levels dependon the installation and are governedby local ordinance, plant codes, or

the Walsh-Healy Act.Th e Walsh-Healy Act, laterstrengthened by the 1970 Occupa-tional Safety and Health Act(OSHA), limits the amount of timeexposure an employee can have tonoise if the employer sells to thefederal government or is involved ininterstate commerce. The OSHAstandard limits noise exposure to90 dBA (decibels on the A-Weighting Scale) maximum for 8hours each day.Approximately 95% of Chemi-rieers Model HT agitators are sizedat less than 100 hp at 100 rpm. A

noise profile indicates that the op-erating noise level is below 80 dBA,well within present OSHA stan-dards (Fig 8) .Wide application-Chemineer's agi-tators span an application range of1 to 3000 hp at 1 to 500 rpm. T()minimize shaft vibration, the agita-tors ordinarily operate at or below65% of the first shaft critical speed.Fabricated steel housing, motorbrackets, and pedestals are used.The housings have heavy walls andare heat treated and pressure tested.The Model HT line includes 16case sizes. Each mo del has a case

designed for a specified torque ca-pacity. The power and speed marvary for a model, but the torquecapacity must not be exceeded. Thespline or taper hub-mounted gearsmay be removed through a coverplate. You can vary gear ratio aslong as you don't exceed torquecapacity. Sizes 1 through 9 havedouble reduction gear boxes; sizes10 through 16, triple reduction. Adry-well output-shaft seal mini-mizes oil leakage possibility. Oillubricates high-speed shaft bearings;grease, low-speed bearings.Open-tank agitators have mount-ing rails for bolting to supportbeams. dosed tank agitators, forapplications to 1000 psi, have stuf-fing box housings integral with themounting flange. For pressures to5000 psi, agitators have removablecartridge single or double mechani-cal seal assemblies.The agitators can be suppliedwith regular or multispeed electricmotors, variable-speed belt drives,hydraulic drives, SCR-controlled

drives, and steam turbines. Variousimpellers are available, includingturbines, anchors, or helixes. Atwo-component, catalyzed, andair-cur ed polyurethane coating pro-tects the drives.More than 1000 agitators havebeen installed in the first 18months after introducing the ModelHT line.Donald L. Kime is .Hanager of Salesand Engineering. Cbemine er Agitil'tors, Dayton, Ohio.

Reprinted from the November 1973 issue of POWER TRANSMISSION DESIGN, the magazine of motors, drives, bearings and coCopyright 1973, Industrial Publishing Company. Division of Pittway Corporation, all rights reserved.


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P.O. Box 11231939 East F:rst S::S'':::Dayton, Oh io 454:

0b Agitator Drive Design

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