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Paperboard machine wire wear
Paperboard machine wire wear D. Eynon*
The t y p i c a l p a p e r or b o a r d m a c h i n e c o n s i s t s of a wet end, p r e s s s e c t i o n a n d d r i e r s e c t i o n . At the wet end a s u s p e n s i o n of f i b r e s i n w a t e r i s d i s c h a r g e d onto a belt-like continuous wire mesh. Water is drained t h r o u g h the w i r e l e a v i n g a wet m a t r i x of f i b r e s on the s u r f a c e . The wet m a t r i x i s t r a n s f e r r e d to a p r e s s s e c t i o n w h e r e m o r e w a t e r is e x p e l l e d . F i n a l l y the s h e e t i s d r i e d on a s e r i e s of s t e a m h e a t e d c y l i n d e r s . The b o a r d m a c h i n e c o n s i d e r e d h e r e , w h i c h i s an I n v e r f o r m m a c h i n e , u s e s l a r g e q u a n t i t i e s of w a s t e p a p e r i n the m a n u f a c t u r i n g p r o c e s s . The w a s t e p a p e r u s e d c o n t a i n s a s u b s t a n t i a l a m o u n t of fine a b r a s i v e m a t e r i a l , m u c h of which i s r e m o v e d i n the e x t e n s i v e c l e a n i n g s y s t e m . H o w e v e r , s m a l l q u a n t i t i e s of v e r y fine a b r a s i v e m a t t e r , s u c h as c l a y , t i t a n i u m dioxide and fine g r i t and m e t a l , r e a c h the m a c h i n e . The q u a n t i t i e s i n v o l v e d a r e so s m a l l as to have no a d v e r s e effects on the p r o d u c t q u a l i t y , b u t give one of the m o s t s e v e r e w e a r p r o b l e m s i n the p a p e r and board industry. The m a j o r w i r e w e a r o c c u r s at the s t a t i o n a r y d e w a t e r i n g u n i t s l o c a t e d on the opposite s i d e of the w i r e m e s h to the f i b r e m a t r i x . As w a t e r d r a i n s t h r o u g h the w i r e m e s h it t a k e s with it fine f i b r o u s m a t t e r a n d o t h e r fine a b r a s i v e s u b s t a n c e s which a r e u s e d i n the m a n u f a c t u r e of p a p e r a n d b o a r d to i m p r o v e s u c h t h i n g s as b r i g h t n e s s and o p a c i t y .
F i g 2 One s t a t i o n of I n v e r f o r m m a c h i n e wet end
and i s then s c r a p e d off by the a u t o - s l i c e . A v a c u u m box u n d e r the bottom w i r e r e m o v e s w a t e r d o w n w a r d s u n d e r a v a c u u m of up to 130 m m m e r c u r y . The wet m a t r i x i s held f i r m l y on the b o t t o m w i r e at the r o t a r y v a c u u m box w h e r e the top w i r e l e a v e s the m a ~ t r i x . The r o t a r y v a c u u m box i s a p r e s s with the b o t t o m r o l l e n c l o s e d i n a v a c u u m box.
THE I N V E R F O R M MACHINE W E T END The I n v e r f o r m m a c h i n e wet end c o n s i s t s of five ply f o r m i n g u n i t s shown d i a g r a m m a t i c a l l y i n F i g 1. At the f i r s t s t a t i o n a s u s p e n s i o n of wood f i b r e s i n w a t e r i s d i s c h a r g e d f r o m the flow box onto the b o t t o m c o n t i n u o u s w i r e m e s h . D r a i n a g e at f i r s t t a k e s p l a c e d o w n w a r d s t h r o u g h the b o t t o m w i r e and the w a t e r i s r e m o v e d by a s e r i e s of f o i l s . At the f o r m i n g r o l l w a t e r is e x p e l l e d u p w a r d s t h r o u g h the top w i r e m e s h
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F i g 1 I n v e r f o r m m a c h i n e wet end
TRIBOLOGY N o v e m b e r 1970
229
m a t r i x is held f i r m l y on the bottom w i r e , a t the r o t a r y v a c u u m b o x , w h i l e the top w i r e s e p a r a t e s f r o m the m a t r i x . The o p e r a t i o n of the t h i r d , f o u r t h and fifth s t a t i o n s i s s i m i l a r to that of the s e c o n d . WIRES AND WIRE WEAR In c h o o s i n g a w i r e m e s h for a p a r t i c u l a r a p p l i c a t i o n the following c o n s i d e r a t i o n s a r e i m p o r t a n t : 1
The pore size m u s t be s m a l l enough to r e t a i n the f i b r e s
2
The open a r e a m u s t be a s high as r e q u i r e d for good d r a i n a g e
3
The o v e r a l l m e s h c a l i p e r ( i . e . t h e d i s t a n c e b e t w e e n flat s u r f a c e s p l a c e d on e i t h e r side of the m e s h ) m u s t be low so that as little w a t e r as p o s s i b l e is r e t a i n e d in the w i r e a f t e r a d r a i n a g e element
4
The s t r a n d c a l i p e r m u s t be a s high as p o s s i b l e to allow for w e a r
T y p i c a l l y the m e s h s i z e for this I n v e r f o r m m a c h i n e i s b e t w e e n 11/2 and 2 s t r a n d s / m m and the o v e r a l l m e s h c a l i p e r b e t w e e n 635 and 760 yrn. Bottom w i r e a n d w i r e w e a r The bottom w i r e m e s h shown in Fig 3 i s m a d e in p h o s p h o r - b r o n z e and is a t w i l l weave. T w i l l weave m e a n s that the w a r p s t r a n d s c r o s s two weft s t r a n d s on the w e a r i n g side. T h i s r e d u c e s the s h a r p n e s s of the w e a r i n g k n u c k l e s that a r e p r e s e n t in a p l a i n weave w i r e and i n c r e a s e s the w e a r i n g s u r f a c e a r e a . Fig 4 shows the w e a r that has t a k e n place on a p h o s p h o r - b r o n z e t w i l l weave bottom w i r e a f t e r an o p e r a t ing p e r i o d of 13 days on the m a c h i n e . The w i r e a f t e r this t i m e w i l l have t r a v e l l e d a p p r o x i m a t e l y 4200 k m . T h i s is a t y p i c a l o p e r a t i n g r e s u l t . At t h i s stage the s t r e n g t h of the w i r e h a s b e e n r e d u c e d to a n u n a c c e p t able l e v e l and the open a r e a has b e e n r e d u c e d by the w e a r i n g and f l a t t e n i n g of the k n u c k l e s shown in Fig 4. The o v e r a l l c a l i p e r of the m e s h will have b e e n r e duced by b e t w e e n 100 a n d 130 pan. The m a j o r p a r t of the bottom w i r e t a k e s p l a c e at the foils on the f i r s t s t a t i o n and at the r o t a r y v a c u u m boxes on a l l s t a t i o n s . The s u r f a c e s of t h e s e u n i t s , which a r e m a d e of high d e n s i t y p o l y e t h y l e n e , a r e set
F i g 4 W o r n p h o s p h o r b r o n z e bottom w i r e (twill we ave)
i n the plane of the bottom w i r e and do not s i g n i f i c a n t ly d e f l e c t the w i r e . To a m i n o r extent w e a r is c a u s e d by s l i p p a g e at v a r i o u s s u p p o r t r o l l s , w h i c h a r e d r i ven by the w i r e , a n d at the d r i v e n r o l l s which p r o p e l the w i r e , s e e F i g 1. The bottom w i r e is 63 m long and t r a v e l s at a n a v e r a g e s p e e d of 230 m / m i n . Each p a r t of the w i r e s u r f a c e c o n t a c t s a p p r o x i m a t e l y 2000 s t a t i o n a r y and 4000 r o t a t i n g w e a r p o i n t s i n o n e hour. The f r i c t i o n a l f o r c e s a c t i n g on the w i r e at the v a r i ous w e a r points depend on the w i r e t e n s i o n , t h e weight of w i r e plus the wet f i b r e s , t h e v a c u u m a p p l i e d , and the c o n t a c t a r e a . The w a t e r p r e s e n t a c t s a s a l u b r i cant and the fine m a t e r i a l c a r r i e d t h r o u g h the w i r e with the w a t e r a c t s as an a b r a s i v e . The effects of t h e s e v a r i a b l e s on w e a r r a t e s has not b e e n q u a n t i f i e d and it is v e r y difficult, if not i m p o s s i b l e , to do so. W i r e w e a r t a k e s place u n e v e n l y a c r o s s the w i r e m e s h due to a n u m b e r of m a c h i n e o p e r a t i n g d e f e c t s which can i n c r e a s e w e a r r a t e s . F o r i n s t a n c e , f i b r e s can be t r a p p e d , p a r t i c u l a r l y on the edges of foils and r o t a r y v a c u u m box t o p s , f o r m i n g h a r d l u m p s . T h e s e f i b r e s c a n c a u s e s t r e a k s of w e a r in the d i r e c t i o n of w i r e t r a v e l . E v e n if the f i b r e s a r e c l e a r e d away they m a y have c a u s e d a p e r m a n e n t r i d g e to be f o r m e d a l o n g the w i r e length. T h i s r i d g e will c o n t i n u e to w e a r at a h i g h e r r a t e t h a n the r e s t of w i r e . S i m i l a r u n e v e n w i r e w e a r can be c a u s e d by u n d u l a t i o n s , a t r i g h t a n g l e s to the d i r e c t i o n of w i r e m o v e m e n t , i n the s u r f a c e s of the foils and r o t a r y v a c u u m box t o p s . T h e s e u n d u l a t i o n s in t u r n a r e c a u s e d by the u n e v e n w e a r i n g a c t i o n of the w i r e on the s t a t i o n a r y s u r f a c e s . The u n e v e n w e a r that t a k e s place a c r o s s a w i r e m e a n s that~ at the end of a 13 day o p e r a t i n g p e r i o d , the m a j o r p a r t of the w i r e m a y be as l i t t l e as 60% of its a c c e p t a b l e w e a r l e v e l . The w i r e is r e m o v e d a f t e r this p e r i o d of t i m e b e c a u s e of the e x i s t e n c e of the high w e a r a r e a s .
F i g 3 U n w o r n p h o s p h o r b r o n z e bottom w i r e (twill weave)
230
TRIBOLOGY N o v e m b e r 1970
The m a c h i n e d o w n t i m e to change a bottom w i r e is b e t w e e n 3 and 31/2 h o u r s , b u t m a i n t e n a n c e r e q u i r e m e n t s on the m a c h i n e a r e c o n s i d e r a b l e and this t i m e and m o r e would n o r m a l l y be i n d e p e n d e n t l y r e q u i r e d . Only in the e v e n t of an u n s c h e d u l e d w i r e r e n e w a l , due p e r h a p s to the w i r e s p l i t t i n g in a high w e a r a r e a , would 31/2 h o u r s be r e q u i r e d s o l e l y for a w i r e change. The e x t e n s i o n of s c h e d u l e d d o w n t i m e due only to w i r e r e n e w a l s t e m s p r i m a r i l y f r o m the s e a m i n g t i m e of a p p r o x i m a t e l y 11/2 h o u r s . The w i r e i s p u r c h a s e d in a 63 m length to allow it to be t h r e a d e d t h r o u g h the
m a c h i n e . A f t e r t h r e a d i n g it i s s e a m e d by s o l d e r i n g the w a r p s t r a n d s t o g e t h e r . T h e new p h o s p h o r - b r o n z e w i r e c o s t s b e t w e e n £1500 and £2000
Top w i r e w e a r T h e t o p w i r e m e s h shown in F i g 5 i s a p l a i n w e a v e m a d e f r o m a p o l y e s t e r . P l a s t i c top w i r e s h a v e b e e n c h o s e n b e c a u s e p h o s p h o r - b r o n z e w e a r out in a few h o u r s. The all plastic wires used here are normally plain weave with the weft knuckles contacting the stationary water removal units. The plain weave gives a m o r e stable m e s h than the twill weave shown in Fig 3 and is normally used for plastic wires because of the relatively low rigidity of the plastic strands. The wear rates of the polyester are sufficiently low to allow the use of the plain weave. Fig 6 shows the wear that has taken place on a plastic top wire after an operating period of approximately 40 days. At this stage, as in the case of the bottom wire,the strength of the wire is too low, but the open area has been reduced m u c h m o r e than that exhibited by the bottom wire. The top wires are subjected to higher compression forces and the plastic is a softer material than phosphor-bronze, thus the strands of the top wires flatten m o r e than those of the bottom. The drainage rate allowed by a worn top wire m a y curtail its operating life before its strength characteristics fall too low. W e a r of the top wires takes place primarily at the auto-slice and inverted v a c u u m box, see Fig 2. The auto-slice is used to control water removal rates and can be depressed into the wire to increase drainage. Depressing the auto-slice into the wire increases the pressure on the matrix of wet fibres through the action of the tension in the bottom wire. The point contact of the auto-slice on the top wire therefore gives a relatively high pressure between the autoslice wire. It is not possible to determine this pressure due to the indeterminate nature of the wrap of the wire round the auto-slice blade tip. The autoslice blades are constructed in high density polyethylene.
T h e i n v e r t e d v a c u u m b o x e s c o n s i s t of a s e r i e s of s t a i n l e s s s t e e l f o i l s with 7 c o n t i n u o u s s l o t s at r i g h t a n g l e s to the d i r e c t i o n of the w i r e r u n . T h e v a c u u m in t h e s e s l o t s p u l l s the top w i r e and f i b r e m a t r i x f i r m l y onto the f o i l s . T h e c o n t a c t p r e s s u r e i s d i f f i c u l t to d e t e r m i n e s i n c e t h i s i s d e p e n d e n t on the t o p
F i g 6 W o r n p l a s t i c t o p w i r e (plain w e a v e )
a n d b o t t o m w i r e t e n s i o n s , the v a c u u m , the p e r m e a b i l i t y of t h e w i r e / f i b r e / w i r e s a n d w i c h a n d t h e c o n t a c t c o n f i g u r a t i o n b e t w e e n the t o p w i r e and t h e f o i l s . At both the a u t o - s l i c e and the i n v e r t e d v a c u u m box t h e r e i s a c o n s i d e r a b l e q u a n t i t y of w a t e r p r e s e n t c a r r y i n g with i t fine a b r a s i v e f i b r e s and the v a r i o u s other abrasive materials previously mentioned. This c a u s e s w e a r on both the w i r e and the s t a t i o n a r y d e w a t e r i n g u n i t s . The a u t o - s l i c e b l a d e w e a r s p a r t i c u l a r l y q u i c k l y and u n e v e n l y w h i c h r e f l e c t s in t h e u n e v e n w e a r in the top w i r e s . A t o p w i r e i s 1 1 . 6 m long and t r a v e l s at the s a m e l i n e a r v e l o c i t y a s t h e b o t t o m w i r e i . e . 230 m / m i n a v e r a g e . T h i s m e a n s t h a t a g i v e n point on the w i r e c o n t a c t s the s t a t i o n a r y u n i t s , w h e r e s e v e r e w e a r t a k e s place,2500 times per hour. P l a s t i c top w i r e s a r e p u r c h a s e d in c o n t i n u o u s f o r m a n d c a n be f i t t e d r a t h e r l i k e a s l e e v e due to the f l e x i b l e n a t u r e of the w i r e . The r e p l a c e m e n t of a top w i r e t a k e s a p p r o x i m a t e l y 1/2 h o u r . T h i s i s n o r m a l l y c a r r i e d out d u r i n g a s c h e d u l e d s h u t and the c o n t r i b u t i o n to d o w n t i m e can be c o n s i d e r e d to be n e g l i g i b l e . T h e c o s t of a new p l a s t i c t o p w i r e i s a p p r o x i m a t e l y £600. IMPROVEMENT OF WIRE LIFE I m p r o v e m e n t s in w i r e life can be a c h i e v e d by e i t h e r i m p r o v i n g the s t a t i o n a r y w e a r i n g s u r f a c e s o r by i m p r o v i n g the w e a r c h a r a c t e r i s t i c s of the w i r e s o r p e r h a p s by the a d d i t i o n of c h e m i c a l l u b r i c a n t s . The u n e v e n e s s of w e a r on the s t a t i o n a r y s u r f a c e s can be d r a m a t i c a l l y i m p r o v e d by the u s e of l o n g - l i f e m a t e r i a l s l i k e c e r a m i c s . If e v e n w e a r , p a r t i c u l a r l y on the b o t t o m w i r e , can be a c h i e v e d t h i s m a y i m p r o v e w i r e life by up to 2 5 0
F i g 5 Unworn p l a s t i c t o p w i r e (plain w e a v e )
Plastic (polyester or other suitable polymers) bottom w i r e s w i l l p r o b a b l y r e p l a c e the p h o s p h o r - b r o n z e when d i f f i c u l t i e s in s e a m i n g p l a s t i c w i r e s on m a c h i n e a r e o v e r c o m e . The low r i g i d i t y of the p l a s t i c m a t e r i a l m a y a l s o g i v e s o m e p r o b l e m s with the long b o t t o m w i r e r u n s . W i t h i n c r e a s e d r i g i d i t y of the p l a s t i c s t r a n d s a m u c h w i d e r c h o i c e of w e a v e t y p e w i l l be a v a i l a b l e , f o r both top and b o t t o m w i r e s , which c o u l d i m p r o v e life a n d d r a i n a g e c h a r a c t e r i s t i c s . T h r o u g h d e v e l o p m e n t s in p l a s t i c s a n d c o m b i n a t i o n s of p l a s t i c s and m e t a l s f o r w i r e c o n s t r u c t i o n t h e o p e r a t i o n a l life of w i r e s s h o u l d be e x t e n d e d s i g n i f i c a n t l y , p o s s i b l y tenfold.
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The t y p i c a l p a p e r or b o a r d m a c h i n e c o n s i s t s of a wet end, p r e s s s e c t i o n a n d d r i e r s e c t i o n . At the wet end a s u s p e n s i o n of f i b r e s i n w a t e r i s d i s c h a r g e d onto a belt-like continuous wire mesh. Water is drained t h r o u g h the w i r e l e a v i n g a wet m a t r i x of f i b r e s on the s u r f a c e . The wet m a t r i x i s t r a n s f e r r e d to a p r e s s s e c t i o n w h e r e m o r e w a t e r is e x p e l l e d . F i n a l l y the s h e e t i s d r i e d on a s e r i e s of s t e a m h e a t e d c y l i n d e r s . The b o a r d m a c h i n e c o n s i d e r e d h e r e , w h i c h i s an I n v e r f o r m m a c h i n e , u s e s l a r g e q u a n t i t i e s of w a s t e p a p e r i n the m a n u f a c t u r i n g p r o c e s s . The w a s t e p a p e r u s e d c o n t a i n s a s u b s t a n t i a l a m o u n t of fine a b r a s i v e m a t e r i a l , m u c h of which i s r e m o v e d i n the e x t e n s i v e c l e a n i n g s y s t e m . H o w e v e r , s m a l l q u a n t i t i e s of v e r y fine a b r a s i v e m a t t e r , s u c h as c l a y , t i t a n i u m dioxide and fine g r i t and m e t a l , r e a c h the m a c h i n e . The q u a n t i t i e s i n v o l v e d a r e so s m a l l as to have no a d v e r s e effects on the p r o d u c t q u a l i t y , b u t give one of the m o s t s e v e r e w e a r p r o b l e m s i n the p a p e r and board industry. The m a j o r w i r e w e a r o c c u r s at the s t a t i o n a r y d e w a t e r i n g u n i t s l o c a t e d on the opposite s i d e of the w i r e m e s h to the f i b r e m a t r i x . As w a t e r d r a i n s t h r o u g h the w i r e m e s h it t a k e s with it fine f i b r o u s m a t t e r a n d o t h e r fine a b r a s i v e s u b s t a n c e s which a r e u s e d i n the m a n u f a c t u r e of p a p e r a n d b o a r d to i m p r o v e s u c h t h i n g s as b r i g h t n e s s and o p a c i t y .
F i g 2 One s t a t i o n of I n v e r f o r m m a c h i n e wet end
and i s then s c r a p e d off by the a u t o - s l i c e . A v a c u u m box u n d e r the bottom w i r e r e m o v e s w a t e r d o w n w a r d s u n d e r a v a c u u m of up to 130 m m m e r c u r y . The wet m a t r i x i s held f i r m l y on the b o t t o m w i r e at the r o t a r y v a c u u m box w h e r e the top w i r e l e a v e s the m a ~ t r i x . The r o t a r y v a c u u m box i s a p r e s s with the b o t t o m r o l l e n c l o s e d i n a v a c u u m box.
THE I N V E R F O R M MACHINE W E T END The I n v e r f o r m m a c h i n e wet end c o n s i s t s of five ply f o r m i n g u n i t s shown d i a g r a m m a t i c a l l y i n F i g 1. At the f i r s t s t a t i o n a s u s p e n s i o n of wood f i b r e s i n w a t e r i s d i s c h a r g e d f r o m the flow box onto the b o t t o m c o n t i n u o u s w i r e m e s h . D r a i n a g e at f i r s t t a k e s p l a c e d o w n w a r d s t h r o u g h the b o t t o m w i r e and the w a t e r i s r e m o v e d by a s e r i e s of f o i l s . At the f o r m i n g r o l l w a t e r is e x p e l l e d u p w a r d s t h r o u g h the top w i r e m e s h
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F i g 1 I n v e r f o r m m a c h i n e wet end
TRIBOLOGY N o v e m b e r 1970
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m a t r i x is held f i r m l y on the bottom w i r e , a t the r o t a r y v a c u u m b o x , w h i l e the top w i r e s e p a r a t e s f r o m the m a t r i x . The o p e r a t i o n of the t h i r d , f o u r t h and fifth s t a t i o n s i s s i m i l a r to that of the s e c o n d . WIRES AND WIRE WEAR In c h o o s i n g a w i r e m e s h for a p a r t i c u l a r a p p l i c a t i o n the following c o n s i d e r a t i o n s a r e i m p o r t a n t : 1
The pore size m u s t be s m a l l enough to r e t a i n the f i b r e s
2
The open a r e a m u s t be a s high as r e q u i r e d for good d r a i n a g e
3
The o v e r a l l m e s h c a l i p e r ( i . e . t h e d i s t a n c e b e t w e e n flat s u r f a c e s p l a c e d on e i t h e r side of the m e s h ) m u s t be low so that as little w a t e r as p o s s i b l e is r e t a i n e d in the w i r e a f t e r a d r a i n a g e element
4
The s t r a n d c a l i p e r m u s t be a s high as p o s s i b l e to allow for w e a r
T y p i c a l l y the m e s h s i z e for this I n v e r f o r m m a c h i n e i s b e t w e e n 11/2 and 2 s t r a n d s / m m and the o v e r a l l m e s h c a l i p e r b e t w e e n 635 and 760 yrn. Bottom w i r e a n d w i r e w e a r The bottom w i r e m e s h shown in Fig 3 i s m a d e in p h o s p h o r - b r o n z e and is a t w i l l weave. T w i l l weave m e a n s that the w a r p s t r a n d s c r o s s two weft s t r a n d s on the w e a r i n g side. T h i s r e d u c e s the s h a r p n e s s of the w e a r i n g k n u c k l e s that a r e p r e s e n t in a p l a i n weave w i r e and i n c r e a s e s the w e a r i n g s u r f a c e a r e a . Fig 4 shows the w e a r that has t a k e n place on a p h o s p h o r - b r o n z e t w i l l weave bottom w i r e a f t e r an o p e r a t ing p e r i o d of 13 days on the m a c h i n e . The w i r e a f t e r this t i m e w i l l have t r a v e l l e d a p p r o x i m a t e l y 4200 k m . T h i s is a t y p i c a l o p e r a t i n g r e s u l t . At t h i s stage the s t r e n g t h of the w i r e h a s b e e n r e d u c e d to a n u n a c c e p t able l e v e l and the open a r e a has b e e n r e d u c e d by the w e a r i n g and f l a t t e n i n g of the k n u c k l e s shown in Fig 4. The o v e r a l l c a l i p e r of the m e s h will have b e e n r e duced by b e t w e e n 100 a n d 130 pan. The m a j o r p a r t of the bottom w i r e t a k e s p l a c e at the foils on the f i r s t s t a t i o n and at the r o t a r y v a c u u m boxes on a l l s t a t i o n s . The s u r f a c e s of t h e s e u n i t s , which a r e m a d e of high d e n s i t y p o l y e t h y l e n e , a r e set
F i g 4 W o r n p h o s p h o r b r o n z e bottom w i r e (twill we ave)
i n the plane of the bottom w i r e and do not s i g n i f i c a n t ly d e f l e c t the w i r e . To a m i n o r extent w e a r is c a u s e d by s l i p p a g e at v a r i o u s s u p p o r t r o l l s , w h i c h a r e d r i ven by the w i r e , a n d at the d r i v e n r o l l s which p r o p e l the w i r e , s e e F i g 1. The bottom w i r e is 63 m long and t r a v e l s at a n a v e r a g e s p e e d of 230 m / m i n . Each p a r t of the w i r e s u r f a c e c o n t a c t s a p p r o x i m a t e l y 2000 s t a t i o n a r y and 4000 r o t a t i n g w e a r p o i n t s i n o n e hour. The f r i c t i o n a l f o r c e s a c t i n g on the w i r e at the v a r i ous w e a r points depend on the w i r e t e n s i o n , t h e weight of w i r e plus the wet f i b r e s , t h e v a c u u m a p p l i e d , and the c o n t a c t a r e a . The w a t e r p r e s e n t a c t s a s a l u b r i cant and the fine m a t e r i a l c a r r i e d t h r o u g h the w i r e with the w a t e r a c t s as an a b r a s i v e . The effects of t h e s e v a r i a b l e s on w e a r r a t e s has not b e e n q u a n t i f i e d and it is v e r y difficult, if not i m p o s s i b l e , to do so. W i r e w e a r t a k e s place u n e v e n l y a c r o s s the w i r e m e s h due to a n u m b e r of m a c h i n e o p e r a t i n g d e f e c t s which can i n c r e a s e w e a r r a t e s . F o r i n s t a n c e , f i b r e s can be t r a p p e d , p a r t i c u l a r l y on the edges of foils and r o t a r y v a c u u m box t o p s , f o r m i n g h a r d l u m p s . T h e s e f i b r e s c a n c a u s e s t r e a k s of w e a r in the d i r e c t i o n of w i r e t r a v e l . E v e n if the f i b r e s a r e c l e a r e d away they m a y have c a u s e d a p e r m a n e n t r i d g e to be f o r m e d a l o n g the w i r e length. T h i s r i d g e will c o n t i n u e to w e a r at a h i g h e r r a t e t h a n the r e s t of w i r e . S i m i l a r u n e v e n w i r e w e a r can be c a u s e d by u n d u l a t i o n s , a t r i g h t a n g l e s to the d i r e c t i o n of w i r e m o v e m e n t , i n the s u r f a c e s of the foils and r o t a r y v a c u u m box t o p s . T h e s e u n d u l a t i o n s in t u r n a r e c a u s e d by the u n e v e n w e a r i n g a c t i o n of the w i r e on the s t a t i o n a r y s u r f a c e s . The u n e v e n w e a r that t a k e s place a c r o s s a w i r e m e a n s that~ at the end of a 13 day o p e r a t i n g p e r i o d , the m a j o r p a r t of the w i r e m a y be as l i t t l e as 60% of its a c c e p t a b l e w e a r l e v e l . The w i r e is r e m o v e d a f t e r this p e r i o d of t i m e b e c a u s e of the e x i s t e n c e of the high w e a r a r e a s .
F i g 3 U n w o r n p h o s p h o r b r o n z e bottom w i r e (twill weave)
230
TRIBOLOGY N o v e m b e r 1970
The m a c h i n e d o w n t i m e to change a bottom w i r e is b e t w e e n 3 and 31/2 h o u r s , b u t m a i n t e n a n c e r e q u i r e m e n t s on the m a c h i n e a r e c o n s i d e r a b l e and this t i m e and m o r e would n o r m a l l y be i n d e p e n d e n t l y r e q u i r e d . Only in the e v e n t of an u n s c h e d u l e d w i r e r e n e w a l , due p e r h a p s to the w i r e s p l i t t i n g in a high w e a r a r e a , would 31/2 h o u r s be r e q u i r e d s o l e l y for a w i r e change. The e x t e n s i o n of s c h e d u l e d d o w n t i m e due only to w i r e r e n e w a l s t e m s p r i m a r i l y f r o m the s e a m i n g t i m e of a p p r o x i m a t e l y 11/2 h o u r s . The w i r e i s p u r c h a s e d in a 63 m length to allow it to be t h r e a d e d t h r o u g h the
m a c h i n e . A f t e r t h r e a d i n g it i s s e a m e d by s o l d e r i n g the w a r p s t r a n d s t o g e t h e r . T h e new p h o s p h o r - b r o n z e w i r e c o s t s b e t w e e n £1500 and £2000
Top w i r e w e a r T h e t o p w i r e m e s h shown in F i g 5 i s a p l a i n w e a v e m a d e f r o m a p o l y e s t e r . P l a s t i c top w i r e s h a v e b e e n c h o s e n b e c a u s e p h o s p h o r - b r o n z e w e a r out in a few h o u r s. The all plastic wires used here are normally plain weave with the weft knuckles contacting the stationary water removal units. The plain weave gives a m o r e stable m e s h than the twill weave shown in Fig 3 and is normally used for plastic wires because of the relatively low rigidity of the plastic strands. The wear rates of the polyester are sufficiently low to allow the use of the plain weave. Fig 6 shows the wear that has taken place on a plastic top wire after an operating period of approximately 40 days. At this stage, as in the case of the bottom wire,the strength of the wire is too low, but the open area has been reduced m u c h m o r e than that exhibited by the bottom wire. The top wires are subjected to higher compression forces and the plastic is a softer material than phosphor-bronze, thus the strands of the top wires flatten m o r e than those of the bottom. The drainage rate allowed by a worn top wire m a y curtail its operating life before its strength characteristics fall too low. W e a r of the top wires takes place primarily at the auto-slice and inverted v a c u u m box, see Fig 2. The auto-slice is used to control water removal rates and can be depressed into the wire to increase drainage. Depressing the auto-slice into the wire increases the pressure on the matrix of wet fibres through the action of the tension in the bottom wire. The point contact of the auto-slice on the top wire therefore gives a relatively high pressure between the autoslice wire. It is not possible to determine this pressure due to the indeterminate nature of the wrap of the wire round the auto-slice blade tip. The autoslice blades are constructed in high density polyethylene.
T h e i n v e r t e d v a c u u m b o x e s c o n s i s t of a s e r i e s of s t a i n l e s s s t e e l f o i l s with 7 c o n t i n u o u s s l o t s at r i g h t a n g l e s to the d i r e c t i o n of the w i r e r u n . T h e v a c u u m in t h e s e s l o t s p u l l s the top w i r e and f i b r e m a t r i x f i r m l y onto the f o i l s . T h e c o n t a c t p r e s s u r e i s d i f f i c u l t to d e t e r m i n e s i n c e t h i s i s d e p e n d e n t on the t o p
F i g 6 W o r n p l a s t i c t o p w i r e (plain w e a v e )
a n d b o t t o m w i r e t e n s i o n s , the v a c u u m , the p e r m e a b i l i t y of t h e w i r e / f i b r e / w i r e s a n d w i c h a n d t h e c o n t a c t c o n f i g u r a t i o n b e t w e e n the t o p w i r e and t h e f o i l s . At both the a u t o - s l i c e and the i n v e r t e d v a c u u m box t h e r e i s a c o n s i d e r a b l e q u a n t i t y of w a t e r p r e s e n t c a r r y i n g with i t fine a b r a s i v e f i b r e s and the v a r i o u s other abrasive materials previously mentioned. This c a u s e s w e a r on both the w i r e and the s t a t i o n a r y d e w a t e r i n g u n i t s . The a u t o - s l i c e b l a d e w e a r s p a r t i c u l a r l y q u i c k l y and u n e v e n l y w h i c h r e f l e c t s in t h e u n e v e n w e a r in the top w i r e s . A t o p w i r e i s 1 1 . 6 m long and t r a v e l s at the s a m e l i n e a r v e l o c i t y a s t h e b o t t o m w i r e i . e . 230 m / m i n a v e r a g e . T h i s m e a n s t h a t a g i v e n point on the w i r e c o n t a c t s the s t a t i o n a r y u n i t s , w h e r e s e v e r e w e a r t a k e s place,2500 times per hour. P l a s t i c top w i r e s a r e p u r c h a s e d in c o n t i n u o u s f o r m a n d c a n be f i t t e d r a t h e r l i k e a s l e e v e due to the f l e x i b l e n a t u r e of the w i r e . The r e p l a c e m e n t of a top w i r e t a k e s a p p r o x i m a t e l y 1/2 h o u r . T h i s i s n o r m a l l y c a r r i e d out d u r i n g a s c h e d u l e d s h u t and the c o n t r i b u t i o n to d o w n t i m e can be c o n s i d e r e d to be n e g l i g i b l e . T h e c o s t of a new p l a s t i c t o p w i r e i s a p p r o x i m a t e l y £600. IMPROVEMENT OF WIRE LIFE I m p r o v e m e n t s in w i r e life can be a c h i e v e d by e i t h e r i m p r o v i n g the s t a t i o n a r y w e a r i n g s u r f a c e s o r by i m p r o v i n g the w e a r c h a r a c t e r i s t i c s of the w i r e s o r p e r h a p s by the a d d i t i o n of c h e m i c a l l u b r i c a n t s . The u n e v e n e s s of w e a r on the s t a t i o n a r y s u r f a c e s can be d r a m a t i c a l l y i m p r o v e d by the u s e of l o n g - l i f e m a t e r i a l s l i k e c e r a m i c s . If e v e n w e a r , p a r t i c u l a r l y on the b o t t o m w i r e , can be a c h i e v e d t h i s m a y i m p r o v e w i r e life by up to 2 5 0
F i g 5 Unworn p l a s t i c t o p w i r e (plain w e a v e )
Plastic (polyester or other suitable polymers) bottom w i r e s w i l l p r o b a b l y r e p l a c e the p h o s p h o r - b r o n z e when d i f f i c u l t i e s in s e a m i n g p l a s t i c w i r e s on m a c h i n e a r e o v e r c o m e . The low r i g i d i t y of the p l a s t i c m a t e r i a l m a y a l s o g i v e s o m e p r o b l e m s with the long b o t t o m w i r e r u n s . W i t h i n c r e a s e d r i g i d i t y of the p l a s t i c s t r a n d s a m u c h w i d e r c h o i c e of w e a v e t y p e w i l l be a v a i l a b l e , f o r both top and b o t t o m w i r e s , which c o u l d i m p r o v e life a n d d r a i n a g e c h a r a c t e r i s t i c s . T h r o u g h d e v e l o p m e n t s in p l a s t i c s a n d c o m b i n a t i o n s of p l a s t i c s and m e t a l s f o r w i r e c o n s t r u c t i o n t h e o p e r a t i o n a l life of w i r e s s h o u l d be e x t e n d e d s i g n i f i c a n t l y , p o s s i b l y tenfold.
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