TECHNICAL ASPECTS O F SHOT- PEENING MACHINERY AND MEDIA

H. 1. Plaster*

When Herber t experimented i n 1926 by pouring 3 mm d i a . s t e e l b a l l s down a speaking tube on t o a s t e e l specimen contained i n a metal b i s c u i t t i n , he could no t have foreseen how f a r reach ing h i s work would be. This idea of mechanical hammering was b a s i c a l l y c o r r e c t .

From t h e s e e a r l y experiments of peening has grown a whole v a s t a r e a of a n i n d u s t r y t h a t has warranted over t h e y e a r s t h e a t t e n t i o n of s c i e n t i s t s and aca- demics on t h e one hand and s p e c i a l i s t eng ineers on t h e o t h e r .

The f i r s t e s s e n t i a l f o r shot-peening t o e x a c t i n g s t a n d a r d s a r e accep tab le media of which today t h e r e i s a wide choice .

C h i l l e d I r o n Shot - Chi l led i r o n s h o t i s a c a s t m a t e r i a l . It i s manufactured by mel t ing s u i t a b l y graded i r o n i n a cupola, and a t a c r i t i c a l temperature, pouring o r p r o j e c t i n g i t i n t o a quench tank . By t h e s e means t h e meta l i s broken up i n f l i g h t and "shocked" by t h e water a s s p h e r i c a l shapes.

The l a r g e s t percen tage a r e round; many a r e pear shaped, e longa ted and worm- l i k e . As would be a n t i c i p a t e d many p a r t i c l e s c o n t a i n i n c i p i e n t s t r e s s c r a c k s . Screening and grading e l i m i n a t e a g r e a t d e a l of t h e unwanted miss-shapes and t a i l - ings . Despi te any shortcomings, c h i l l e d i r o n sho t h a s two v a l u a b l e a s s e t s , i t i s cheap and remarkably h a r d ; u s u a l l y i n t h e o rder of 700-1000 V.P.N.

Chi l led i r o n sho t f a i l e d a s a peening medium because of i t s v i r t u e , t h e very hardness , r e s u l t e d i n r a p i d f r a c t u r e and broken s h o t cannot be t o l e r a t e d i n a sho t peening o p e r a t i o n . S t e e l Shot - Made by a s i m i l a r technique a s c h i l l e d i r o n s h o t , excep t t h a t it i s melted e l e c t r i c a l l y , s t e e l s h o t was introduced commercially i n t h e l a t e 1 9 5 0 ' s . The raw m a t e r i a l i s s t e e l s c r a p , bu t t o produce a high s tandard product t h i s i s very c a r e f u l l y s e l e c t e d f o r genera l use b u t peening q u a l i t y i s u s u a l l y double and c l a s s i f i e d t o ensure a b e t t e r s i z i n g s tandard and t h e removal of s u r f a c e s c a l e . A l l s t e e l s h o t i s produced t o a range of s i z e s and s u b j e c t t o accepted grading s t a n d a r d s .

The hardness f i g u r e s of s tandard s t e e l s h o t i n Europe,and t h e United S t a t e s a r e between 350-500 V.P.N. b u t s h o t f o r peening purposes h a s been s a t i s f a c t o r i l y produced a s high a s 665 V.P.N. Cut Wire Shot - Cut wi re i s , a s i t s name i m p l i e s , wire c u t i n t o t h e form of p e l l e t s Each p e l l e t i s c u t with t h e leng th equa l t o t h e d iameter . The s i n g l e most import- a n t p o i n t i s t h a t t h e wi re from which t h e p e l l e t s a r e c u t should be of t h e recog- n i s e d s tandard .

With s p e c i f i c a l l y drawn s t e e l wi re , because of i t s manufacturing methods t h e

* Chairman B r i t i s h Impact Treatment Assoc ia t ion .

product i s f r e e of l a t e n t imper fec t ions such a s i n c i p i e n t c r a c k s , blow h o l e s e t c . In a d d i t i o n , each p e l l e t may be s a i d t o be i d e n t i c a l and uniform. Drawn v i r g i n wi re i s a c o s t l y m a t e r i a l and most wi re used f o r c u t t i n g p e l l e t s i s u s u a l l y t h a t which may have been r e j e c t e d f o r minor s u p e r f i c i a l d e f e c t s on t h e s u r f a c e o r s l i g h t incons i s tency i n t h e d iameter . The w i r e i s of h igh q u a l i t y and w h i l s t t h e q u a l i t y may f a i l on t h e s e p o i n t s a s w i r e f o r t h e o r i g i n a l purposes, i t adequa te ly meets t h e s tandard f o r shot-peening.

Obviously i n p e l l e t form, c u t w i r e i s u n s u i t a b l e a s a peening medium and r e q u i r e s t o be condi t ioned . This e n t a i l s p rocess ing t h e medium i n a s u i t a b l e sho t b l a s t machine when g r a d u a l l y t h e edges a r e rounded o f f t o form a s p h e r i c a l shape. Glass Beads - From t h e po in t of view of a man-made mass produced o b j e c t t h e g l a s s bead i s n e a r e r t o p e r f e c t i o n than any o t h e r a b r a s i v e . Unlike metal sho t which o f t e n inc ludes pear shaped and o t h e r misshapened p a r t i c l e s , g l a s s sho t keeps c l o s e -1y t o a s p h e r i c a l form. Even t h e f i n e s t s i z e main ta ins a degree of un i formi ty of shape.

The s p e c i f i c g r a v i t y of g l a s s i s one t h i r d of t h a t of c a s t i r o n . I n e f f e c t t h i s impl ies t h a t t h e medium may be picked up qu ick ly i n t o an a i r s t ream and r a p i d l y a c c e l e r a t e . The almost p e r f e c t s p h e r i c a l shape a s s i s t s i n r a p i d movement though the feed p ipe and t h e p a r t i c l e s t h e o r e t i c a l l y keep c l o s e r t o t h e a i r speed than h e a v i e r m e t a l l i c p a r t i c l e s . This i s one of t h e problems with a i r opera ted b l a s t p l a n t s i n t h a t t h e f r i c t i o n a l l o s s e s a r e h igh due l a r g e l y t o t h e p u l s a t i o n s of t h e a i r and the e r r a t i c movement through t h e supply t u b e s . The problem wi th g l a s s beads i s t h e l o s s of energy immediately they emerge from t h e nozz le and h i t the a i r b a r r i e r . This w i l l have an immediate and dramat ic e f f e c t up t h e s h o t s t ream and t h e speed i s reduced cons iderab ly i n consequence. When us ing beads i t i s necessary t o reduce t h e nozzle d i s t a n c e t o 50 t o 90 mm i n o rder t o l e s s e n t h e l o s s of k i n e t i c energy. I n s h o t peening o p e r a t i o n s a s s i s t a n c e can be gained by an i n c r e a s e i n bead s i z e and i n c r e a s i n g t h e a i r p r e s s u r e . Ceramic Beads - Of r e c e n t i n t r o d u c t i o n and produced t o a very h igh s tandard and with a good r e s i s t a n c e t o breakdown, ceramic beads may f i n d a ready accep tance i n shot-peening. Made i n s i z e s from 0.20 mm t o 0.60 mm. Niku ~ a r i z i n v e s t i ~ a t e d t h e medium and h i s conc lus ions suggest t h a t ceramic beads a r e comparable t o s t e e l sho t i n peening o p e r a t i o n s . S t a i n l e s s S t e e l Shot - Made by techniques s i m i l a r t o c a s t s t e e l s h o t . A more expensive medium and g r e a t c a r e must be e x e r c i s e d t o ensure t h a t a l l t h e f a b r i c l i n i n g of a b l a s t machine i s f u l l y p r o t e c t e d t o p r o h i b i t t h e s t a i n l e s s q u a l i t y of the s h o t be impaired by f e r r o u s contaminat ion.

ERVIN SHOT TESTING MACHINE This machine i s modelled s i m i l a r l y t o a n a i r l e s s b l a s t machine, having a

throwing wheel, a t a r g e t and a r e c i r c u l a t i n g dev ice which r o t a t e s around a common c e n t r e l i n e . Shot d u r a b i l i t y i s determined by t h e number of impacts a g a i n s t t h e a n v i l t h a t a r e r e q u i r e d t o break i t i n t o an unusable s t a t e .

PNEUMATIC SHOT PEENING SYSTEMS There a r e t h r e e systems of us ing compressed a i r f o r s h o t peening purposes, i n

which t h e s h o t i s p r o j e c t e d from a b l a s t nozz le .

1 ) . Induc t ion - syphon method. 2 ) . Induc t ion - g r a v i t y method. 3 ) . D i r e c t p r e s s u r e .

1 ) . Syphon Induc t ion method i s one i n which t h e s h o t i s a s p i r a t e d t o t h e nozz le . The method i s uncomplicated and lends i t s e l f r e a d i l y f o r use i n hand c a b i n e t s . (F ig . 1 ) .

The on ly c l a i m t h a t may be made i s t h e s i m p l i c i t y of t h e system t h a t permi t s cons tan t u n i n t e r r u p t e d working.

The one r e a l o b j e c t i o n i s t h a t most i n s t a l l a t i o n s do no t i n c o r p o r a t e sho t c lean ing and s h o t g rad ing which i n t h e c a s e of s h o t peening would no t be t o l e r a b l e . It i s t r u e t h a t many p a r t s r e q u i r i n g peening a r e ground and hardened and a r e i n the f i n i s h e d s t a t e . I n consequence they a r e c l a s s e d a s c l e a n , so t h e r e i s l i t t l e

p o s s i b i l i t y o f c o n t a m i n a t i o n of t h e s h o t , b u t a l l a b r a s i v e s b r e a k d o ~ n , ( ~ a r t i c u l a r - l y g l a s s b e a d s , which i s a p o p u l a r medium f o r u s e i n t h i s t y p e o f machine) and t h i s f a c t which h a s been emphasised c o n t i n u a l l y i s n o t t o l e r a b l e i n s h o t - p e e n i n g . Shot c l e a n i n g and s e p a r a t i o n i s j u s t a s i m p o r t a n t i n s h o t - p e e n i n g a s i t i s man- d a t o r y i n s p e c i f i c b l a s t c l e a n i n g i n t h e U.K.

2 ) . The G r a v i t y I n d u c t i o n - This sys tem o f f e r s improved e f f i c i e n c y , w i t h t h e s h o t b e i n g e l e v a t e d above t h e p o i n t o f usage , from t h e r e b e i n g p e r m i t t e d t o f r e e f a l l t o t h e n o z z l e where i t i s e n e r g i s e d by t h e compressed a i r . T h i s i s a n e a t method o f p r o v i d i n g con t inuous o p e r a t i o n by e l e v a t i n g t h e s h o t i n t h e r e c i r c u l a t i o n system. The e s s e n t i a l method o f i n c o r p o r a t i n g s c r e e n i n g , g r a d i n g and c l e a n i n g may be i n c l u d e d .

3 ) . D i r e c t P r e s s u r e - This sys tem i s t h e most u n i v e r s a l l y used . It i s based upon t h e use of a p r e s s u r e v e s s e l ( F i g . 2 ) i n which t h e s h o t i s f e d under p r e s s u r e t o t h e n o z z l e . A m e t e r i n g v a l v e i s i n c o r p o r a t e d i n t h e sys tem t o a d j u s t t h e volume of a b r a s i v e i n t o t h e a i r s t r e a m . There i s a p r e c i s e b a l a n c e between t h e volume o f s h o t and t h e b o r e s i z e o f t h e n o z z l e . The v a l u e o f t h i s sys tem i s i n t h e a d j u s t - ments t h a t may be made p o s i t i v e l y t o t h e a i r p r e s s u r e . The p r e s s u r e v e s s e l a s a u n i t may be i n c o r p o r a t e d i n t o hand c a b i n e t s and b l a s t chambers.

For f l e x i b i l i t y , t h e d i r e c t p r e s s u r e method i s s u p e r i o r t o o t h e r methods. Every t y p e o f s h o t may be used w i t h e q u a l e a s e . P r e s s u r e s a r e e a s i l y a d j u s t e d ,

c o n t r o l l e d . They s h o u l d b e l o c a t e d a t t h e c o r r e c t d i s t a n c e , wi th t h e p r e c i s e a n g l e and w i t h e x a c t speed of movement. The machine shou ld a l s o b e f i t t e d w i t h a t i m e r so t h a t t h e d u r a t i o n of each peen ing o p e r a t i o n i s c a r r i e d o u t w i t h p r e c i s i o n . Th i s does n o t e l i m i n a t e t h e n e c e s s i t y t o i n c o r p o r a t e means of k e e p i n g a c o n s t a n t s h o t c o n d i t i o n . A l l s h o t peen ing machinery shou ld b e f i t t e d wi th g r a d e r s t o e n s u r e a c o n s i s t e n t s t a n d a r d of s h o t . Broken s h o t must b e removed, s o t o o shou ld s h o t which i s u n d e r s i z e . With s m a l l a i r o p e r a t e d p l a n t s it i s d i f f i c u l t t o f i t s h o t r e p l e n i s h e r s , b u t where t h e consumption i s s u f f i c i e n t l y h i g h t h e y a r e e s s e n t i a l .

FIG. 1 FIG. 2

Syphon Type Hand Cab ine t D i r e c t P r e s s u r e Type Shot B l a s t Un i t

TARGETING

The compressed a i r method of shot-peening h a s a s i t s main a t t r i b u t e t h a t of f l e x i b i l i t y . Nozzles may be p o s i t i o n e d p r e c i s e l y , and d i r e c t e d on t o an a r e a with t h e necessary accuracy. Areas of s t r e s s c o n c e n t r a t i o n such a s f i l l e t s , grooves, shoulders e t c . a r e t y p i c a l of where t h e nozz le may be " ta rge ted1 ' with p r e c i s e accuracy. Masking by means of rubber i sed tape over t h e non-s t ressed a r e a i s o f t e n a s a t i s f a c t o r y way of l i m i t i n g t h e e f f e c t of t h e b l a s t .

Before any shot-peening o p e r a t i o n i s c a r r i e d ou t t h e workpiece should be c r i t i c a l l y examined a s t o how i t should be processed. The angle of t h e nozz le t o the a r e a r e q u i r i n g t rea tment ; t h e p r e s s u r e r e q u i r e d ; t h e s i z e and type of sho t and t h e d i s t a n c e of t h e n o z z l e . Shot emerging from a nozz le opens ou t and r a d i - a t e s from the t i p and t h e c o n c e n t r a t i o n of s h o t w i l l be t h e h i g h e s t i n t h e c e n t r e o r core of the j e t of s h o t . This i s t h e "hot-spot1 ' .

Whenever a c y l i n d r i c a l component i s r e q u i r e d t o be shot-peened, c a r e should be taken t o r e l a t e t h e "hot-spot" t o t h e p o i n t of impact and not t o overvalue t h e p e r i p h e r a l b l a s t . For example a 50.00 mm wide b l a s t when measured r e v e a l s a core c o n c e n t r a t i o n of 15.875 mm. The t r a v e r s e o r h e l i x should be based upon t h i s

CENTRIFRUGAL SHOT PEENING

PRINCIPLES OF WHEEL DESIGN

As with o t h e r forms of s h o t b l a s t i n g t h e o r i g i n a l i d e a s of p r o j e c t i n g a b r a s i v e s by means of c e n t r i f r u g a l a c t i o n were those of Tilghman*, who took o u t p a t e n t s i n 1870. The p a t e n t s covered two d i s t i n c t methods, one of which was t o feed t h e a b r a s i v e by g r a v i t y on t o t h e wheel b lades where it was energ i sed by t h e f o r c e of impact. This was r e f e r r e d t o a s t h e "Bat te r wheel". The second p a t e n t was a method of feed ing t h e a b r a s i v e t o t h e c e n t r e of t h e wheel and i t could be s a i d t h a t i t i s from t h i s t h a t t h e p r e s e n t types of " s l i d e r " wheels have been developed. Ti lghmanls wheel was designed t o p r o j e c t t h e a b r a s i v e over 360 degrees and some time was t o e l a p s e b e f o r e a wheel was evolved t h a t had d i r e c t i o n a l c o n t r o l of t h e a b r a s i v e . This c h a r a c t e r i s t i c was t o prove i n v a l u a b l e i n a p p l i - c a t i o n s of h i g h l y t e c h n i c a l m e r i t . An e a r l y form of t h i s was f i t t e d with e i g h t f l a t b lades s i t u a t e d between two s i d e p l a t e s . I t i s normal with t h i s type of wheel t o feed the s h o t by g r a v i t y through a tube t o t h e c o r e of a s t a t i o n a r y c o n t r o l cage on to t h e b lades of t h e wheel ( F i g . 4 ) . I t i s t h e c o n t r o l cage, a s i t s name impl ies t h a t determines t h e d i r e c t i o n i n which the s h o t i s thrown.

* Tilghman. B . C . F i r s t B r i t i s h P a t e n t No. 2900 - 1876

FIG. 3

B l a s t P a t t e r n I n c o r r e c t Cor rec t

As t h e sho t l eaves t h e wheel i t forms a c o n f i g u r a t i o n wherein t h e h i g h l y concen- t r a t e d a r e a i s a g a i n r e f e r r e d t o a s t h e "hot-spot" ( F i g . 5 ) It i s i n t h i s a r e a t h a t the shot-peening work i s performed, a s the c o n c e n t r a t i o n i s h i g h e s t and t h e ang le of impact a t the optimum. The c o n t r o l cage may be a d j u s t e d t o s u i t i n d i - v i d u a l components so t h a t t h e "hot-spot ' ' i s a t t h e p r e c i s e peening p o i n t . The c o n t r o l cage w i l l i n time wear and i n consequence t h e "hot-spot" w i l l move o f f t h e workpiece. Immediately t h i s occurs t h e s h o t peening o p e r a t i o n i s s u s p e c t . These a r e t h e p r i n c i p l e s which c o n t r o l a c e n t r i f r u g a l shot-peening o p e r a t i o n .

The o r i g i n a l double s ided wheel permi t s t h e g r e a t e s t c o n t r o l over the sho t s t ream. The method of blade f i x i n g however l i m i t s such a wheel t o a s i n g l e d i r e c t i o n of r o t a t i o n . This was overcome by t h e i n t r o d u c t i o n of a wheel which dispensed with b lades and used i n s t e a d a s e t of s p e c i a l l y designed t u b e s . B l a s t wheels have been g e n e r a l l y designed f o r b l a s t c l e a n i n g purposes and no t f o r s h o t - peening. They a r e designed f o r g e n e r a l purpose use , n o t with any s p e c i a l a p p l i - c a t i o n i n mind. There may w e l l be room f o r more r e s e a r c h i n t o t h e des ign of b l a s t wheels , e s p e c i a l l y with t h e growing demands f o r sho t -peen ing . Whatever t h e d i a - meter of t h e b l a s t wheel the volume of s h o t thrown i s d i r e c t l y r e l a t e d t o the energy consumed. An i n c r e a s e i n wheel speed w i l l produce an i n c r e a s e i n sho t v e l o c i t y , b u t wi th a decrease i n volume. Slowing down t h e b l a s t wheel w i l l have t h e r e v e r s e e f f e c t ( i n c r e a s i n g t h e volume of sho t a t t h e expense of v e l o c i t y ) (F ig .6 . ) . The g e n e r a l l y accep ted speed f o r a b l a s t wheel 508.00 mm i n diameter i s 2250 r.p.m. Wheels vary i n diameter and speed and t h e c r i t e r i o n i s t h e r e l a t i o n s h i p between t h e s e f a c t o r s and a proposed a p p l i c a t i o n . BLAST WHEEL DESIGN - Observat ions sugges t t h a t b l a s t wheels a r e a r b i t r a r i l y designed with t h e commercial a s p e c t a s t h e dominant f a c t o r . V a r i a t i o n s a r e i n t r o - duced t o be d i s t i n c t and t e c h n i c a l c o n s i d e r a t i o n s a r e o f t e n no t t h e prime motiv- a t i o n . The s tandard wheel, b a s i c a l l y t h e o r i g i n a l des ign , h a s been produced world wide, with s l i g h t v a r i a t i o n s i n o u t l i n e . This i s a w e l l balanced des ign and f i t t e d i n t o a s u i t a b l e housing, r o t a t e s wi th t h e minimum of tu rbu lence . The e f f i c i e n c y of t h i s wheel i s revea led by i n s p e c t i o n of the b l a d e s a f t e r a per iod of time when i t h a s been throwing c h i l l e d i r o n g r i t . Because g r i t hugs t h e s u r f a c e of t h e b l a d e , a t r a c e r y i s l e f t which r e v e a l s t h a t t h e movement down the b lade i s e x a c t l y c e n t r a l . Open s ided wheels a r e designed with a s i d e p l a t e which r e q u i r e s t h e b lades t o be h e l d i n p o s i t i o n by t h e one edge ( F i g . 7 ) . Most wheels of t h i s type a r e designed t o accommodate e i g h t b l a d e s , y e t t h e r e a r e s i m i l a r wheels which a r e f i t t e d wi th four b l a d e s . Whether e i g h t o r f o u r b lades a r e used, t h e movement of t h e wheel through t h e wheel housing must s e t up a more d i s r u p t i v e flow than a double s ided wheel. The t r a c e r y of t h e a b r a s i v e i n t h i s c a s e w i l l have a n o f f s e t f a n p a t t e r n sugges t ing some movement toward t h e unshrouded s i d e of t h e b l a d e s .

FIG. 4

Ear ly des ign of d i r e c t i o n a l l y c o n t r o l l e d B l a s t Wheel - i l l u s t r a t i n g Impel le r and Control Cage

This may be due t o t h e wheel c r e a t i n g more a i r tu rbu lence a s i t moves through f r e e a i r . This does no t sugges t t h a t t h i s type of wheel i s i n e f f e c t i v e , f o r i t h a s a proven record i n g e n e r a l a p p l i c a t i o n s of c l e a n i n g and s u r f a c e p r e p a r a t i o n , b u t i t does sugges t t h a t i n c r i t i c a l peening o p e r a t i o n s t h e r e may be room f o r more i n v e s t i g a t i o n on t h e q u e s t i o n of e f f e c t i v e n e s s .

It cannot be claimed t h a t b l a s t wheels a r e aerodynamical ly designed. One wheel which may be s o descr ibed however, i s of a more r e c e n t i n t r o d u c t i o n i n which tubes r e p l a c e convent iona l b l a d e s . I n o p e r a t i o n t h e a i r movement around t h i s wheel i s smooth and very l i t t l e d i s t u r b e d . Whether t h i s i s a n improvement upon t h e b a s i c double s i d e d wheel i s open t o i n v e s t i g a t i o n . There i s however, a funda- mental d i f f e r e n c e i n t h e flow of sho t through t h e t u b u l a r b lades e s p e c i a l l y when compared with t h e t r a d i t i o n a l f l a t b l a d e .

When t h e s h o t h a s passed from t h e impel lo r through t h e c o n t r o l cage and e n t e r s the t u b u l a r b l a d e , t h e passage i s smooth and u n r e s t r i c t e d . The s h o t flows down t h e tube i n a concent ra ted form i n a narrow pa th which r e s u l t s i n a l eng th- ened, bu t narrow b l a s t p a t t e r n . The average b l a s t p a t t e r n l e n g t h when thrown from t h e optimum from t h i s wheel i s 1600 mm whereas t h e b l a s t p a t t e r n leng th from a f l a t bladed wheel i s approximately 1066 mm. This w i l l i n d i c a t e some apprec ia - b l e d i f f e r e n c e i n t h e two forms of b l a s t wheels. Obviously i n t h e c a s e of t h e t u b u l a r wheel t h e sho t i n i t s i n i t i a l t r a v e l down t h e tube i s concent ra ted i n a f i n e s t ream i n t h e r a d i u s and opens ou t upon being r e l e a s e d i n t o f r e e a i r . With a lengthened b l a s t p a t t e r n t h e r e w i l l be shot-peening a p p l i c a t i o n s where t h e r e i s some d i s t i n c t advantage and on t h e o t h e r hand, i n s t a n c e s where t h e a p p l i c a t i o n may be s u p e r i o r with t h e use of a f l a t bladed wheel.

Some y e a r s ago Campbell* in t roduced a smal l wheel i n which t h e des ign permi t ted t h e b l a d e s t o ex tend beyond t h e per iphery of t h e s i d e p l a t e s ( ~ i g . 8 ) . This was in t roduced wi th the o b j e c t of conserving energy by e l i m i n a t i n g t h e vee b e l t d r i v e and d i r e c t coupling t h e wheel t o t h e e l e c t r i c motor. Because of t h e load on t h e b e a r i n g s of t h e motor, t h e s i z e of b l a s t wheel was l i m i t e d , espec- i a l l y as a r o t a t i o n a l speed of t h e b l a s t wheel t o be e f f e c t i v e must be over 2600 r .p .m. Therefore an e l e c t r i c motor with a speed of 2800 r.p.m. was an obvious cho ice , t o g e t h e r with heavy duty b e a r i n g s t o t ake t h e load ing . Campbell abandoned t h e des ign a f t e r some extended p e r i o d of use a s t h e method was no t considered t o be a success . It appeared t h a t t h e extended t i p s of t h e b l a d e s c r e a t e d tu rbu lence r e s u l t i n g i n a s i m i l a r e f f e c t t o b l a d e s of t h e s i n g l e s i d e d wheel, excep t t h a t t h e t r a c e r y of t h e a b r a s i v e appeared t o f a n i n both d i r e c t i o n s .

B l a s t wheels o p e r a t e i n a form s i m i l a r t o l i q u i d pumps and y e t perform with t h e f u n c t i o n of a fan . A i r i s e s s e n t i a l t o t h e f r e e flow of t h e a b r a s i v e through t h e wheel u n i t . Without a i r the a b r a s i v e w i l l choke and f a i r t o flow except i n a minute q u a n t i t y . The volume of sho t thrown is d i r e c t l y r e l a t e d t o horsepower ( F i g . 9 ) . B l a s t wheels a r e u s u a l l y d r i v e n by e l e c t r i c motors by means of m u l t i p l e

* Campbell J . A . l a t e of St.Georgels Engineers Ltd. & of C e n t r a l Gauge & Tool Co.

FIG. 5

I l l u s t r a t i n g t h e "Hot-Spot" of a B l a s t Stream.

rope d r i v e s which have t h e advantage i n t h a t t h e f i n a l wheel speed may be a d j u s t - ed when necessary by v a r i a t i o n of t h e p u l l e y s i z e s . Wheel speed d i c t a t e s sho t v e l o c i t y , a very impar tan t a s p e c t of shot-peening. This i s a n oppos i te view t o a f i x e d s i n g l e speed u n i t , with a d i r e c t coupled wheel where o f t e n when speed v a r i - a t i o n i s r e q u i r e d , i t i s impossible t o ach ieve . Sometimes t h i s i s necessary because t h e machine h a s been b u i l t down t o s i z e , and t h e a b i l i t y t o i n t r o d u c e a vee rope d r i v e i s p r o h i b i t e d . C e n t r i f r u g a l machines l ack t h e f l e x i b i l i t y of those of compressed a i r type , where f o r example t h e o p e r a t i n g p r e s s u r e may be a s low a s 0.0703 k g . / s q . cm t o a s h igh a s 7.03 KG./sq. cm. Any and every means which improve the v e r s a t i l i t y of machines i n c o r p o r a t i n g b l a s t wheels must be cons idered Wheel Blades - The range of b lade des igns i s l e g i o n , a l l may have s p e c i f i c --- f e a t u r e s which a r e purported t o be e s s e n t i a l . They a r e t h e p a r t of a b l a s t machine which i s s u b j e c t t o t h e most severe wear and because of t h i s a r e made of case hard i r o n a l l o y u s u a l l y c o n t a i n i n g 28% chrome. Each s e t of b lades i s balanced t o f i n e l i m i t s , bu t being of c a s t c o n s t r u c t i o n may c o n t a i n very s l i g h t imper fec t ions . These a r e o f t e n n o t d i s c e r n a b l e on v i s u a l i n s p e c t i o n , b u t w i l l be very ev iden t i n u s e , when the wheel u n i t may become o u t of ba lance .

The o r i g i n a l des ign of a b lade was f l a t with s l i g h t l y l ipped s i d e s t o p r o t e c t t h e s i d e p l a t e s of t h e wheel from wear.

Blades of forms o t h e r than f l a t have been in t roduced , somewith convex and o t h e r s with concave f a c e s . A form of t h e l a t t e r was patented* i n t h e pre-war y e a r s , a l though no p a r t i c u l a r c la im was made f o r them except t h a t t h e des ign permi t ted a th icken ing of t h e b l a d e t i p which he lped t o r e s i s t wear and i n c r e a s e the l i f e of t h e b lade . More r e c e n t l y a b l a s t wheel was in t roduced f o r which i t i s claimed t h a t a s i m i l a r concave b lade a s s i s t s s h o t v e l o c i t y (I?ig. lO). By coincidence a wheel in t roduced concur ren t ly c a r r i e d a s i m i l a r c la im f o r a b lade with an i n i t i a l curve i n t h e o p p o s i t d i r e c t i o n . The heavy wear e x h i b i t e d upon the wheel b l a d e s when compared with o t h e r elements of t h e wheel u n i t sugges t s t h a t t h e b l a d e s do most of t h e work and t h e i r des ign may i n consequence be most c r i t i c a l t o wheel e f f i c i e n c y . There i s l i t t l e doubt t h a t t h e l eng th of the b lades a r e a n e f f e c t i v e p a r t of wheel des ign , bu t i t i s q u e s t i o n a b l e whether any c u r v a t u r e i s s i g n i f i c a n t l y e f f e c t i v e i n s h o t v e l o c i t i e s . This i s e s p e c i a l l y so when c o n s i d e r a t i o n i s given t o t h e p e c u l i a r bounce of t h e sho t dur ing i t s passage down the b l a d e .

The b l a s t wheel in t roduced with b lades of t u b u l a r form (Fig.11) may be f i t t e d with b lades of v a r i o u s l e n g t h s according t o t h e a p p l i c a t i o n . Tubular b lades have been made of hard chrome a l l o y ; of ceramic and of p l a s t i c . A l l t h e b l a d e s , inc lud ing those of ceramic, which had bores of s u p e r f i n e f i n i s h , d i s p l a y e d the same undula t ions upon the s u r f a c e caused by t h e bounce of t h e sho t a s found on a l l o t h e r types of b l a d e s . This bounce i s very pronounced and i s o f t e n measurable , the i n i t i a l bounce be ing t h e l a r g e s t wi th p r o g r e s s i v e l y small undula t ions on t h e s u r f a c e t o t h e p o i n t where t h e sho t l eaves the b lade t i p .

* Spencer Kals tead Ltd. - P a t e n t No. 1003441

A wheel of more r e c e n t i n t r o d u c t i o n * h a s b lades of a completely i n d i v i d u a l des ign . They a r e designed i n a "V" shape and a r e a l s o wider a t t h e t i p than a t t h e roo t . This has t h e e f f e c t of throwing a wide b l a s t p a t t e r n . The normal speed of a 300 mm d i a . wheel i s 3000 r.p.m. and when used f o r shot-peening t h e speed i s i n c r e a s e d t o 4500 r .p .m.

To i l l u s t r a t e t h e e f f e c t i v e n e s s of t h e b l a s t wheel f o r shot-peening an example may be taken from an a i r c r a f t component. This was shot-peened t o Almen gauge "A" t o a n a r c h e i g h t average of 0 . 2 8 3 mrn with s t e e l sho t 5230 . The time taken was 9 minutes on a r o t a r y t a b l e type machine. The b l a s t was powered by a 15 H . P . motor. CONTROL CAGE - A s h a s been descr ibed p r e v i o u s l y , b l a s t p a t t e r n s vary with t h e types of throwing wheels. The p a t t e r n s may a l s o be a f f e c t e d by t h e shape and s i z e of t h e a p e r t u r e i n t h e c o n t r o l cage. Genera l ly openings of square and r e c t a n g u l a r form a r e favoured, though t r i a n g u l a r openings and o f f s e t openings have been used, and i n t h e case of t h e t u b u l a r bladed wheel h a l f round openings a r e e f f e c t i v e .

The opening, o r a p e r t u r e , of t h e c o n t r o l cage determines t h e leng th of t h e b l a s t p a t t e r n and t o some degree t h e s i z e of t h e "hot-spot" . (This term i s used because when t e s t i n g a b l a s t p a t t e r n wi th a s h e e t o f mild s t e e l , t h i s a r e a w i l l be found t o be s l i g h t l y warmer t o t h e touch) .

B l a s t wheels do no t have the same f l e x i b i l i t y a s compressed a i r methods and they can be w a s t e f u l i n s p e c i f i c a p p l i c a t i o n s u n l e s s every c a r e i s taken i n r e l a t i n g t h e b l a s t zone t o t h e workpiece.

The c o n t r o l cage i s a d j u s t a b l e t o enab le the s h o t s t ream t o be p r o j e c t e d a t any p o i n t i n 360 degrees . For example i f t h e c o n t r o l cage a p e r t u r e i s s e t a s on a c lock face a t 12 o ' c l o c k t h e s h o t w i l l be discharged a t a p o i n t from 3 o ' c l o c k onwards. The i l l u s t r a t i o n i n d i c a t e s t h e whole b l a s t p a t t e r n , no t t h e "hot-spot" , f o r t h i s must be checked and a s c e r t a i n e d p r e c i s e l y by means of us ing a p iece of mild s t e e l s h e e t placed i n a s t a t i o n a r y p o s i t i o n under t h e wheel. FEED IMPELLERS - This component i s o f t e n d i s regarded a s one of no r e a l s i g n i f i - - cance. Impel le r s vary s l i g h t l y i n des ign b u t a r e b a s i c a l l y the same. Two companies i n the United S t a t e s i n c o r p o r a t e coned i n l e t s i n t h e des igns and t h i s seems t o have the e f f e c t of s t i m u l a t i n g t h e movement of a g r e a t e r volume of sho t through t h e wheel u n i t .

* Sampoh Co.Ltd., Tokyo P 5 HP - - - -15 HP

1000 L

4595 9071 13608 181.44 22%8 2721 317.5 362 8

WHEEL SPEED vs. ABRASIVE FLOW.

0 100 2 0 0 3 0 0 4 0 0 5 0 0 600 7 0 0 €00 Ibr. per mln.

FIG. 6

FIG. 7 S i n g l e Sided B l a s t Wheel.

The i m p e l l e r i s c l o s e l y r e l a t e d t o t h e b l a s t wheel i n t h a t i t i s made i n a segmented form and t h e segments r e l a t e t o t h e b l a d e s . I n o p e r a t i o n t h e s h o t i s picked up by t h e i m p e l l e r and passed through t h e c o n t r o l cage a p e r t u r e on to t h e b lades . This d i s t a n c e i s smal l b u t t h e speed i s s o g r e a t and each segment i s s e t s l i g h t l y i n advance of the wheel b l a d e s i n o rder f o r t h e s h o t t o a l i g h t on to t h e face of the b l a d e .

I n time t h e i m p e l l e r w i l l wear t o a p o i n t where t h e s h o t w i l l be i n e f f e c t - i v e l y s t r i k i n g t h e i n n e r l ead ing edge of t h e b lade . It i s recommended t h a t t h i s c r i t i c a l component should be rep laced when any p a r t of i t s segments i s worn more than 3.175 mm. The wear r a t e i s h i g h ; one i m p e l l e r w i l l u s u a l l y outwear two c o n t r o l cages*. SHOT SEPARATION AND GRADING - With the advent of peening q u a l i t y s h o t which has been s e l e c t i v e l y produced t h e problems involved wi th main ta in ing a h igher q u a l i t y peening o p e r a t i o n have been cons iderab ly lessened . With a l l w e l l designed b l a s t p l a n t s t h e s e p a r a t i o n system r e c e i v e s a s much c o n s i d e r a t i o n a s t h e r e s t of t h e machine.

This s e c t i o n i s s i t u a t e d a t t h e head of t h e e l e v a t o r , which r a i s e s t h e s h o t from t h e base of t h e machine f o r r e - u s e . As t h e sho t p a s s e s through t h e s e p a r a t o r i t i s s u b j e c t t o t h e a i r drawn by t h e v e n t i l a t i o n fan which p u l l s o u t t h e d u s t , s c a l e and fragments of broken s h o t , and leaves the good s h o t f o r re -use . A w e l l designed s e p a r a t o r p rov ides a c u r t a i n of s h o t even ly d i s t r i b u t e d i n such a manner t h a t good sho t i s no t l o s t by be ing drawn o u t . This o c c a s i o n a l l y happens when t h e r e i s a break i n t h e c u r t a i n which d i s t u r b s t h e ba lance and permi t s a t t h a t po in t t h e v e n t i l a t i n g a i r t o p u l l good s p o t i n t o t h e d u s t a r r e s t e r .

It i s p r e - r e q u i s i t e of good sho t -peen ing t h a t t h e sho t should be k e p t a t a l l t imes a s n e a r t o t h e recommended s i z e s tandard s e l e c t e d f o r t h e p a r t i c u l a r opera t ion . This r e q u i r e s t h e i n c o r p o r a t i o n of a sho t g rader which i s designed t o s e l e c t o u t t h e s i n g l e s i z e r e q u i r e d . A s h o t g rader i s a s i e v i n g dev ice i n which t h e r e a r e t i e r s of s e l e c t e d wi re s c r e e n s . SHOT REPLENISHING - Shot of course , i s an expendable p roduc t , and i t s " l i f e " i n known condi t ions i s p r e d i c t a b l e . It fol lows t h e r e f o r e t h a t a s t h e s h o t l o s e s t h e optimum s i z e f o r t h e peening o p e r a t i o n , i t i s drawn from t h e machine and r e q u i r e s replacement . A r e p l e n i s h e r l o c a t e d a t t h e boot of the e l e v a t o r o r o t h e r s u i t a b l e p o i n t w i l l keep t h e volume of sho t i n t h e machine i n a balanced c o n d i t i o n . S igna ls from t h e s h o t hopper , s i t u a t e d below t h e s e p a r a t o r , i n d i c a t i n g t h e s h o t l e v e l , e n e r g i s e the s h o t r e p l e n i s h e r and s u f f i c i e n t s h o t i s added t o main ta in t h e necessary ba lance .

* Tilghman-Wheelabrator Ltd. Book of Running I n s t r u c t i o n s .

FIG. 9

FIG. 8

THE ENERGY FACTOR - The c e n t r i f u g a l method of shot-peening i s of h i g h e r e f f i c i e n c y than t h a t of the pneumatic system. It l a c k s the f l e x i b i l i t y of t h e compressed a i r method bu t h a s t h e a b i l i t y t o make much more e f f i c i e n t use of energy. The power used f o r a purpose b u i l t shot-peening machine w i l l be r e l a t i v e t o t h e demand, and u s u a l l y t h i s i s i n t h e a r e a of 20 t o 25 H.P. The r e g i s t e r of e f f i c i e n c y i s i n t h e ammeter which i n d i c a t e s the volume of s h o t thrown by t h e whee 1.

According t o s i z e , s t e e l sho t w i l l l o s e v e l o c i t y i n comparat ively s h o r t d i s t a n c e s . This should g ive some i n d i c a t i o n of t h e cub ic c a p a c i t y of a b l a s t cab ine t

With a c o r r e c t l y engineered b l a s t machine, t h e nex t e s s e n t i a l i s f o r s u f f i c i e n t a i r movement t o ensure t h e c l e a n e s t atmosphere i n s i d e t h e c a b i n e t dur ing t h e sho t -peen ing o p e r a t i o n . Many components and workpieces a r e i n t h e i r f i n a l s t a g e s and may be themselves sc rupulous ly c l e a n , and t h e s e w i l l n o t p r e s e n t a problem. I n t h e case of h e a v i l y s c a l e d workpieces, such a s l e a f s p r i n g s t h e a i r volumes must be adequate f o r t h e cont inuous removal of a l l d u s t and impedimenta.

INSTRUMENTATION : - AMMETER - This ins t rument , r e f e r r e d t o p rev ious ly i n d i c a t e s t h e volume of s h o t be ing thrown. The meter should always show a t the c o r r e c t read ing dur ing process ing and must be f r e q u e n t l y monitored.

PROCESS TIMER - A v i t a l ins t rument f o r i n shot-peening o p e r a t i o n s , t ime i s an important f a c t o r .

CONVEYOR SPEED INDICATOR - An e s s e n t i a l instrument f o r t h e p rocess per iod i s t h e speed a t which t h e workpiece passes through t h e b l a s t s t ream.

WHEEL HOUR METER - A v a l u a b l e a d d i t i o n t o a l l peening machines. The meter records t h e a c t u a l per iod of time i n which t h e wheel i s throwing s h o t .

SHOT REPLENISHER CONTROL - Methods of main ta in ing t h e amount of s h o t i n a machine may be e i t h e r e l e c t r i c a l o r pneumatic. I n each c a s e , methods may be incorpora ted t o i n d i c a t e a shor tage of s h o t i n the system and t h e r e p l e n i s h e r . These may be v i s u a l o r a u d i b l e o r a combination of bo th .

MANOMETER - Incorpora ted i n a d u s t a r r e s t e r , t h i s ins t rument i n d i c a t e s t h e p r e s s u r e drop i n the system, and i n s o doing a s s i s t s i n main ta in ing a c o n s t a n t a i r movement through t h e s e p a r a t o r .

These a r e t h e main ins t ruments . Manufacturers a r e c o n s t a n t l y improving machine des ign and i n s t r u m e n t a t i o n i s a n a r e a which i s r e c e i v i n g a t t e n t i o n . Computerised c o n t r o l s have a l r e a d y been incorpora ted i n s p e c i a l purpose machines.

FIG. 10

Curved Bladed S i n g l e Side Wheel - Courtesy Pangborn Corporat ion.

- For c i r c u l a r o b j e c t s , a simple r o t a t i o n a l method i s o f t e n . (F ig .12) . A wide v a r i e t y of machines of t h i s type a r e made Whils t s i n g l e i n d i v i d u a l t a b l e machines a r e used, many a r e m u l t i t a b l e des ign . The t a b l e s vary cons iderab ly i n s i z e and a machine may have a s many a s 20 t a b l e s

ach ine be ing purposely designed f o r a s p e c i f i c p roduc t . - Machines of t h i s d e s i g n i n which a n e n d l e s s rubber work ( ~ i g . 1 3 ) have a good d e a l t o commend them,

e s p e c i a l l y i tems such a s l e a f s p r i n g s . The b e l t s a r e heavy r e i n f o r c e d with c o t t o n p ly t o wi ths tand the arduous c o n d i t i o n s and a r e f r e q u e n t l y pegged t o l o c a t e t h e i n d i v i d u a l workpieces.

- S t e e l b a r s of s u f f i c i e n t l eng th may be conveyed upon a (F ig .14) . This presupposes t h e b a r s o r s h a f t s a r e of c l e a n and have no upse t a r e a s .

- S h a f t s which have an upset l i n e , such a s h a l f - s h a f t s which d may be s a t i s f a c t o r i l y shot-peened i n a machine of t h i s type

Due t o t h e exposure t o the b l a s t s t ream t h e s c r o l l r e q u i r e s t o be made of a s i m i l a r a b r a s i v e r e s i s t a n t meta l .

- This i s a method o f conveying gears and c r a n k s h a f t s . The s tem is t h a t t h e components may be r o t a t e d a s t h e c a r r i a g e

proceeds under t h e b l a s t . The same system has been used f o r heavy locomotive l e a f s p r i n g s (F ig .15) . S t - - Both l e a f and c o i l s p r i n g s may be s t r e s s peened upon s u i t a b l y designed machines. Machines of t h i s type a r e t h e r e s u l t of cons iderab le e x p e r t i s e and des ign exper ience .

- For t h i s purpose, t h e machines g e n e r a l l y used a r e of wide b e l t des ign wi th u s u a l l y two o r t h r e e down f i r i n g b l a s t wheels ,so t h a t t h e s p e c i a l a i r c r a f t s e c t i o n i s c a r r i e d i n t h e h o r i z o n t a l p o s i t i o n . I t would be remiss t o overlook t h e many peen-forming machines of the compressed a i r type i n which c a s e t h e workpieces a r e processed i n t h e v e r t i c a l p o s i t i o n .

FIG. 11

Tubular Bladed B l a s t Wheel Courtesy of Vacu B l a s t Co. Ltd.

FIG. 12 - FIG. 13

FIG. 14

FIG. 15

REFRENCES : - 1. Herbert E .G. "Work Hardening of Steel by Abrasion"

Journal of Iron Steel Institute 1927 pp 265 - 282 "Cloudburst Process for Hardness Testing & Hardening" Trans. A.S.S.T. 1929 pp 77 - 92

2. Niku-Lari & Sechaud R. "Grenaillage d1Eprouvettes Almen Mesures et Calcul de la Repartition des Contraintes Residuelles Minibilles S.E.P.R. pour Traitment de Surface.

The Author gratefully acknowledges the following Companies for their valuable assistance

Barton Abrasives Ltd. Cleveland Metal Abrasives Irvin Industries Inc. Lockers Ltd. Metabrasives Ltd. Pangborn Corporation Potters Industries Inc.

Sampoh Co. Societe Europeene des Produits

Refractaires Tilghman Wheelabrator Ltd. Vacu Blast Ltd. Vogel & Schemann A.G. Wheelabrator-Alevaid