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H o w t o h o o s e t h e R i gh t P o w e r f o r

U l t r a s o n i c T r a n s d u c e r s

By John B. Durkee Consultan t in Metal and Crit ical Cleaning

i~ i i D :~ ~ b~ r i :~h lt iha~: ilCie~cS~ ': :ho~ p le~oelec t r ic : :and mag netos t r i c t iv e u l t raso n ic t echno logy wor k :

: i ' : /B /Bb th : a~e Com~o~:i~o:i :many :me ~t c le ~ in g shops :. :These sy s tem s c an be m ade to comple te ma ny d i f fe r -

: ~:rJ~ J : j ~ ~ : S ~ f : i c ~ K~ g : j d b g ; : : d ~ n d ~ g i u p o n £ h e u l t r a s o n i c f r e q u e n c y c h o s en . I n t h e J a n u a ry c o l u m n , I

: ~ o ~ /e ~ h owl ~ K6 ~ i ~ s : 6 ~ i : c ~ e q u ~ a c y : : ~ f i ~ l d b~ i n : a t c he d t o t h e c h a ra c t e r o f e a c h c l e a n i n g jo b . T h i s c o l u m n

~ :~I~:i~0 ~ ~i ~ : : i~ ~5 : ~ :~ i~ t~a~0zii~ : ~6we~ ]~e l : :~ased : :on : the par t s be ing c leaned .

I t i s a h u m a n c h a ra c t e r i s t i c t o b e l i ev e " m o re i s be t -

t e r. " T h i s c h a ra c t e r i s t i c i s r e f l e c t e d i n t h e f i n a n c i a l

a d v i ce , " b e a r s m a k e m o n e y , b u l l s m a k e m o n e y , a n d

h o g s g e t s l a u g h t e r e d . " A n o t h e r e x a m p l e o f t h i s

c h a ra c t e r i s t i c i s t h e c h o ic e b y m a n y u s e r s o f e v e r -

l a r g e r p o w e r r a t i n g s f o r s on i c -p o w e r ed t r a n s d u c e r

s y s t e m s .

T h e re a r e a t l e a s t t h r e e f a c t or s t o b e c o n s i d e re d b y

a m a n a g e r wh e n c h o o s i n g t h e p o w e r le v e l f o r t h e

u l t r a s o n i c t r a n s d u c e r s i n a c l e a n i n g s y s t e m . T h e

fac to rs a re : the par t s , cyc le t ime, and ta nk s ize .

P@WEn T@ THIE PAI~TS

A g e n e ra l i z e d r e l a t i o n s h i p b e t we e n c l e a n i n g e f f e c -

t iveness fo r a p roper ly des igned sys tem i s i l lu s t ra t -

ed in F igure 1 . P lease no te tha t the re la t ionsh ip i s

"S-shaped":

• M o d e s t a p p l i c a t i on o f u l t r a s o n i c p o w e r h a s o n l y

m i n o r e f fe c ts . T h i s i s b e c a u se a n a d e q u a t e n u m b e r

o f cav i ta t io n bubb les o f su f f ic ien t size hasn ' t been

p ro d u c e d .

• At so me level of applied power, the ul trason ic clean-

ing sys te m per fo rms wel l , as des igned .

• Wh en a h igh level o f c lean ing perfo rmance has been

achieved, there is l i t t le gain by applying addit ional

ul traso nic power. In th is s i tuat ion, i f removal of the

small levels of rema inin g soi l is necessary, a second-

cry c lean ing p rocess shou ld be employed ra th er than

force th is process to pe rform be yond i ts capabili ty .

2

40

420

~ioo

; o

~ o

• 40

o

2O

o

o. 0

0 20 40 80 80 ~00

Tank Volume gal.

Figure 1: Relationship between cleaning effectiveness for a

properly designed system.

In s u m m a ry , w i t h o u t r e g a rd t o t h e c h a ra c t e r o f t h e

p a r t s , t h e r e i s a s u i t a b l e r a n g e o f p o we r l e v e l s.

T h e re i s n o p o i n t i n p a y i n g fo r m o re p o we r o r t r y i n g

to econom ize by pay ing fo r l ess .

For the f i c t i t ious s i tua t ion descr ibed in F igure 1 ,

tha t amoun t i s 1 ,000 wat t s .

EFFECT F CYCLE TIME

Cycle t ime (con tac t t ime wi th u l t rason ic ag i ta t ion )

s h o u l d b e v i e we d s i m i l a r l y . C l e a n i n g q u a l i t y w i l l

h a v e t h e s a m e g e n e ra l ( " S -s h a pe d " ) r e la t i o n s h i p v s .

t ime, as seen in F igure 1 .

• P a r t s j u s t " d i pp e d " in t o t h e u l t r a s o n i c t a n k w i l l

no t be wel l c leaned .

• Par t s cooked wel l done , as some l ike the i r s tea k to

be , wi l l no t be c leaned to a p re mi um level .

Doub l ing the cyc le t ime wi l l no t doub le the c lean -

ing qua l ity . Fo r a p roper ly des igned c lea n ing sys tem,

i f t h e p ro d u c t i o n r a t e i s r a i s e d a n d t h e a s s o c i a t e d

cycle t ime shor ten ed , c lean in g qua l i ty wi l l su f fe r.

As b e n c h m a rk s , a c y c l e t i m e o f t wo m i n u t e s c o n-

tac t wou ld be qu i te shor t , bu t perhaps sa t i s fac to ry .

A cyc le t ime o f 10 minu tes wou ld be q u i te long , bu t

p e rh a p s n e c e s s ary .

~F FI© T @F T ~K SmZIE

S i z e ( f lu i d v o l u m e ) o f t h e t a n k i n wh i c h t h e c l e a n i n g

work i s be ing done mat te rs . Less power i s u sed in

t a n k s w i t h s m a l l e r v o l u m e s .

Ul t rason ic power l eve l i s no rmal ly spec i f ied as a

d e n s i t y - -p o w e r p e r v o l u m e . S p e c if i c at i o ns fo r s t a n -

d a rd t a n k s y s t e m s p ro d u c e d b y fo u r m a j o r U . S. s u p-

p l ie rs have b een co l lec ted . The supp l ie rs a re iden t i -

fled only as "A," "B," "C," and "D."

T h e p o we r d e n s i t y p rov i d e d in s t a n d a rd s y s t e m s

i s g r a p h e d i n F i g u re 2 . P le a s e n o t e t h e s e v a l u e s a r e

s t a n d a r d - - p r o v i d e d w i t h o u t a n y d e f i n it i o n o f t h e

par t s be ing c le aned o r o f the cyc le t ime.

Please reca l l f rom Fig ure 1 tha t supp l ie r "A" i s no t

n e c e s s a r i l y p r o v i d i n g s u p e r i o r c l e a n i n g s y s t e m s

b e c a u s e t h e i r s y s t e m s h a v e a h i g h e r p o we r d e n s it y ,

no r i s supp l ie r "C" supp ly ing in fer io r sys tems .

5 2 w w w . m e t a l f i n i s h i n g . c o m

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~100 ~

80%1

. ~ 2 ~

a o

i i =

I A i

o o ~ i 90 0 ~ , oo 2.O00

Power Level Watts

Figure 2: Power density provided in standard systems

~FFE©T ©F PA~W S~ZE

Ultimately, all mechanical energy added to a clean-

ing or rinsing tank by ultrasonic transducers is con-

verted to heat. The mechanical energy is consumed

in doing frictional work, either against the mass

load of parts, against the walls of the tank, wi thin

the water, or as heat and additional frictional forces

produced by the collapse of cavitat ion bubbles.

Consequently, if the par ts are a large dense ma ss

of metal (castings, forgings, blocks, etc.), more ultra-

sonic power will be required to compensate for that

absorbed by the metal.

If parts are left too long withi n an ultrasonic-powered

cleaning tank, they and the fluid within the ta nk and the

tan k walls will become warm. And, if the p arts occupy a

large amount of he volume within a tan k, it is likely that

interna l surfaces may not be effectively cleaned.

Some suppliers recommend the weight of parts in a

ultrasonic cleaning tank be no more th an about one-

third to one-half of the weight of water in the tank.

This author's experience favors the lower value.

Such a recommendation doesn't mean tha t more large

systems be purchased. It may only mean that multiple

loads be processed in a sma ller and lower cost machine.

T~S% TE % T~ ,ST

A manager's objective, in every demonstration with a

supplier's ultrasonic (or megasonic) facilities, should

be to identify the power level and the cycle time t hat

should be used to design a commercial system.

• Excess power has negligible value. A good manag-

er should not pay for that.

• Excess cycle time is a waste of productivity. A

manager should not stand for that.

Pay attention to all the generalized relationships

and specific recommendations above matter not at

all relative to actual performance data.

C@NSUDEn M@R~ ~ J@ ~ TH~ EQUiPMmNT

Perfor mance of a sonic-powered cleaning system, for

a given set of parts, is rel ated to much more th an t he

choice of frequency and sweep rate, t ank size, and

power level. Chemicals, and their concentration,

used in the operation can affect performance.

But ther e are other factors that can be significant,

or not, which are not so obvious. Some observed by

this author are:

• Tank configuration: depth versus open area.

• Tank configuration: presence of unusual shapes

where waves aren't reflected back onto parts.

• Positioning (racking) of parts with in the open vol-

ume of a tank.

• Location of transdu cers within a ta nk (bottom,

sides, etc.).

• Operating temperature.

• Residual gas (air) content.

• Water quality.

• Smoothness of the pa rt surface.

• Fluid circulation within the tank.

• Waveform of the ultrasonic-produced pressure

pulses.

• Anyt hing present on the part surface, which would

prevent it from being wetted (and submerged).

• Accumulation of debris wi thin the tank.

It isn 't that ultrasonic cleaning in static tanks

isn't reproducible. It very often can be and is so.

Ultrasonic cleaning is reliable very often.

But specific result s (claims by vendors of superior

performance in unique applications) can often be

difficult to reproduce in ultrasonic systems provided

by other suppliers.

In oth er words, if a supplier can back up a claim

with repeatable performance data with your parts, a

manage r should give great priority to that company

in the selection process. This suggestion is consis-

tent with recommendations in the June 2004 col-

umn about selection of suppliers.

S U M M A R Y

Cleani ng of metal parts is often done using aqueous

technology (vs. solvent). Ultrasonic technology is

commonly used to provide mechanical force to aug-

ment t he aqueous detergents.

This author's experience is that ultrasonic facilities

are often trea ted by some as add-ons and by others as

insurance policies to and for cleaning equipment.

Neither viewpoint is correct. Ultrasonic technolo-

gy should be integral to and designed for a cleaning

machine. These three columns have shown how a

manage r can be certain that is done.

Jo~:~:::D:ui:k~e,i;II,::PhD,PE,::is~ :~nsui~hnt i~ met~l:~hd critical Cleaning and the auth or of the f orthc omi ng book,

andProcesses, to be publ ished in 2006 by Elsevier ISB N O-

080~i~8887).: :You can :~ontact: h~rn:.at 979~417-7707; : e~mail) durkee@precisioncleaning .com; or fax) 612-677-3170.

February 2006 53