- Email: [email protected]
Monitoring
MONITORING 3-Vibration The t h i r d a r t i c l e in this s e r i e s outlines the techniques of v i b r a t i o n m o n i t o r i n g . All m a c h i n e s v i b r a t e ; by c o m p a r i n g the v i b r a t i o n p a t t e r n s of healthy m a c h i n e s to m a c h i n e s in use incipient f a i l u r e s or faults can be detected.
E v e r y machine which has moving p a r t s v i b r a t e s and the condition of a machine can be judged by the nature of i t s v i b r a t i o n p a t t e r n . If the change in v i b r a t i o n p a t t e r n can be m o n i t o r e d and a n a l y z e d the condition of the machine can be d e t e r m i n e d The s i m p l e s t way of d e t e c t i n g v i b r a t i o n is by sounding stick. F o r g r e a t e r s e n s i t i v i t y i n d u s t r i a l s t e t h o s c o p e s a r e a v a i l a b l e . Fig 1 shows a t y p i c a l s t e t h o s c o p e i n c o r p o r a t i n g an a m p l i f i e r and a high frequency cut-off f i l t e r (above 1 kHz). However not a l l v i b r a t i o n is audible and both t h e s e methods r e l y heavily on the s k i l l of the o p e r a t o r . F o r effective m o n i t o r i n g v i b r a tion a n a l y s i s o v e r a w i d e r frequency range is r e quired. VIBRATION ANALYSIS To m e a s u r e v i b r a t i o n a p i c k - u p is p l a c e d at v a r i o u s points on a machine w h e r e m o s t useful i n f o r m a t i o n can be obtained. As v i b r a t i o n f o r c e s a r e t r a n s m i t t e d through b e a r i n g s m e a s u r e m e n t s a r e best made on
F i g 1 I n d u s t r i a l stethoscope being u s e d to check b e a r i n g s on an e l e c t r i c a l m o t o r
216
TRIBOLOGY N o v e m b e r 1970
b e a r i n g housings as n e a r to the shaft as p o s s i b l e . The points s e l e c t e d should be ones that a r e l i k e l y to show an i n c r e a s e in v i b r a t i o n when t r o u b l e does occ.ur. At l e a s t one r a d i a l and one a x i a l m e a s u r e m e n t i s r e quired for each s e p a r a t e r o t a t i n g p a r t of a machine. Axial r e a d i n g s can r e v e a l t h r e e d i f f e r e n t s o u r c e s of v i b r a t i o n ; m i s a l i g n m e n t between components of the machine, m i s a l i g n m e n t of b e a r i n g s , and bent shafts. H o r i z o n t a l r e a d i n g s a r e u s u a l l y p r e f e r r e d to v e r t i c a l v i b r a t i o n r e a d i n g s w h e r e background v i b r a t i o n s a r e likely, as they a r e not as l i k e l y to be affected by v i b r a t i o n s f r o m m a c h i n e s in the s u r r o u n d i n g a r e a . The m e t e r shown in F i g 2 could only m e a s u r e o v e r a l l d i s p l a c e m e n t and v e l o c i t y r e a d i n g s but the a n a l y z e r in F i g 3 would m e a s u r e v i b r a t i o n l e v e l s in v a r i o u s frequency bands and is used with a s t r o b e l a m p to m e a s u r e the phase of v i b r a t i o n . The f r e q u e n c y of the v i b r a t i o n is the b e s t due to its s o u r c e and hence to the defect. The r e a d i n g s a r e c o m p a r e d to c h a r t s to d e t e r m i n e the p o s s i b l e c a u s e of v i b r a t i o n and to a s c e r t a i n the a c c e p table level of v i b r a t i o n . Table 1 shows the a m p l i t u d e frequency and phase c h a r a c t e r i s t i c s of the common c a u s e s of machine v i b r a t i o n , while F i g 4 is a guide to v i b r a t i o n l e v e l s . In p r a c t i c e the a c c e p t a b l e v i b r a t i o n l e v e l s f o r p a r t i c u l a r b e a r i n g s and m a c h i n e s a r e l a r g e l y a m a t t e r of e x p e r i e n c e . F i l t e r e d v i b r a t i o n r e a d i n g s which fall in the ' r o u g h ' r e g i o n or higher indicate trouble is p r e s e n t . The t r o u b l e should be identified and c o r r e c t i o n s scheduled soon. F i l t e r e d
F i g 2 M e c h a n a l y s i s 306 v i b r a t i o n m e t e r with a f r e quency range of 300 to 240 000 c / m i n
v i b r a t i o n r e a d i n g s which fall in the ' f a i r or ' s l i g h t l y r o u g h ' r e g i o n of the F i g 4 c a l l for caution. T r o u b l e is on the way. I m m e d i a t e s t e p s to make c o r r e c t i o n s a r e not n e c e s s a r i l y r e q u i r e d but p e r i o d i c v i b r a t i o n m e a s u r e m e n t s should be taken at frequent i n t e r v a l s . A t r e n d of i n c r e a s i n g v i b r a t i o n will indicate the t r o u b l e is becoming w o r s e . If no such t r e n d is n o t e d , t h e t r o u b l e should be i d e n t i f i e d and s c h e d u l e d for c o r r e c tion during a r e g u l a r shutdown p e r i o d . F o r v i b r a t i o n a n a l y s i s to be m o s t effetive it is b e s t included in a preventative maintenance p r o g r a m m e .
t y p e s , points at which v i b r a t i o n r e a d i n g s a r e to be taken, r o t a t i n g s p e e d s , etc. When the i n i t i a l a n a l y s i s is c o m p l e t e the v i b r a t i o n l e v e l s should be checked a g a i n s t the v i b r a t i o n s e v e r i t y c h a r t (Fig 4) and the g e n e r a l v i b r a t i o n c h a r t (Table 1). If the v i b r a t i o n
PREVENTIVE MAINTENANCE PROGRAMME The f i r s t s t a g e in setting up a p r e v e n t i v e m a i n t e n a n c e p r o g r a m m e in e x i s t i n g plant is to d e c i d e which m a c h i n e s need m o n i t o r i n g , u s u a l l y ones whose f a i l u r e would affect p r o d u c t i o n and p r o f i t s . Each machine to be m o n i t o r e d has i n i t i a l v i b r a t i o n l e v e l s taken to d e t e r m i n e i t s healthy s t a t e , w h e t h e r t h e r e a r e any f a u l t s to be c o r r e c t e d i m m e d i a t e l y , and its p e r i o d i c v i b r a t i o n check points. Machines with a h i s t o r y of b r e a k d o w n s or heavy m a i n t e n a n c e should be included in the p r o g r a m m e . It is b e s t to make a c h a r t f o r each m a c h i n e to indicate b e a r i n g housings, b e a r i n g
Fig 3 M e c h a n a l y s i s 330 v i b r a t i o n a n a l y z e r with tunable f i l t e r r a n g e s c o v e r i n g 50 to 500 000 c/min
T a b l e 1 Guide to v i b r a t i o n i d e n t i f i c a t i o n Cause
Amplitude
Frequency
Remarks
Unbalance
P r o p o r t i o n a l to unbalance. Largest in radial direction.
1 x rev/min
Most c o m m o n cause of v i b r a t i o n
Misalignment couplings or b e a r i n g s and bent shaft
Large in axial direction 50°/° or more of radial vibration
1 x r e v / m i n usual 2 & 3 x rev/min sometimes
Best found by a p p e a r a n c e of l a r g e a x i a l v i b r a t i o n . Use d i a l i n d i c a t o r s o r other method f o r p o s i t i v e d i a g n o s i s . If s l e e v e b e a r i n g machine and no coupling m i s a l i g n m e n t balance the r o t o r
Bad b e a r i n g s a n t i - f r i c t i o n type
Unsteady-use velocity measurement if p o s s i b l e
V e r y high several times rev/min
B e a r i n g r e s p o n s i b l e m o s t l i k e l y the one n e a r e s t point of l a r g e s t h i g h - f r e q u e n c y vibration
Eccentric journals
Usually not l a r g e
1 x rev/min
If on g e a r s l a r g e s t v i b r a t i o n in line with g e a r c e n t e r s . If on m o t o r or g e n e r a t o r v i b r a t i o n d i s a p p e a r s when power is t u r n e d off. If on pump or b l o w e r a t t e m p t to b a l a n c e
Bad g e a r s or g e a r noise
Low--use v e l o c i t y m e a s u r e if p o s s i b l e
V e r y high g e a r teeth t i m e s rev/min
Mechanical looseness
2 x rev/min
Usually a c c o m p a n i e d by unbalance a n d / or m i s a l i g n m e n t
Bad d r i v e b e l t s
E r r a t i c o r pulsing
1 , 2 , 3 and 4 x r e v / m i n of b e l t s
Strob light best tool to f r e e z e faulty belt
Electrical
D i s a p p e a r s when power is t u r n e d off.
1 x r e v / m i n or 1 or 2 x s y n chronous f r e quency.
If v i b r a t i o n a m p l i t u d e d r o p s off i n s t a n t ly when power is t u r n e d off c a u s e is electrical
Aerodynamic hydraulic forces
1 x r e v / m i n or n u m b e r of b l a d e s on fan or i m p e l l e r x rev/min
R a r e as a cause of t r o u b l e except in c a s e s of r e s o n a n c e
Reciprocating forces
1, 2 & h i g h e r orders x rev/ min
Inherent in r e c i p r o c a t i n g m a c h i n e s can only be r e d u c e d by design changes or isolation
TRIBOLOGY N o v e m b e r 1970
217
0.1 cl
o O Q.
E
e
0<)I
"E E U
o. IA
~ 0-001 ¢. I0
0"0001
I( Vi brotion frequency [clrnin] Fig 4 Guide to level of vibration in general machinery
applications l e v e l s a r e a c c e p t a b l e t h e s e v a l u e s r e p r e s e n t the n o r m a l c o n d i t i o n of the m a c h i n e s . The i n t e r v a l f o r c h e c k i n g a m a c h i n e i s d e t e r m i n e d by the type of m a c h i n e and i t s m a i n t e n a n c e r e c o r d . When c h e c k s a r e m a d e they a r e r e c o r d e d and c o m p a r e d with the v i b r a tion h i s t o r y c h a r t . T r e n d s in v i b r a t i o n c h a n g e s can then be a s c e r t a i n e d .
SPECTRUM ANALYSIS A m o r e s o p h i s t i c a t e d m e t h o d of v i b r a t i o n a n a l y s i s i s spectrum analysis. Spectrum analyzers are far more e x p e n s i v e than o r d i n a r y v i b r a t i o n a n a l y z e r s and t h e i r u s e i s only j u s t i f i e d on v e r y e x p e n s i v e e q u i p m e n t . F o r i n s t a n c e the Navy have d e v e l o p e d a m o n i t o r ( F i g 5) f o r u s e with p r o p u l s i o n and g e n e r a t i n g e n g i n e s , p u m p s c o m p r e s s o r s etc on b o a r d ship. T h i s e q u i p m e n t can remotely monitor as many machines as required. Output f r o m an a c c e l e r o m e t e r , s c r e w e d to an a c c e l e r o m e t e r b l o c k s e c u r e d a s e l e c t e d point on the m a c h i n e u n d e r t e s t i s fed into an i m p e d a n c e c o n v e r t e r . The low i m p e d a n c e output i s then t a k e n v i a a line a m p l i f i e r to the s p e c t r u m m o n i t o r which a n a l y z e s the i n c o m i n g s i g n a l in c o n s e c u t i v e o n e - t h i r d - o c t a v e f r e q u e n c y c h a n n e l s . (In the e x p e r i m e n t a l m o d e l t h e r e a r e 27 c h a n n e l s with m i d - b a n d f r e q u e n c i e s f r o m 10 Hz to 4 kHz). A punched c a r d , c o d e d f o r the p a r t i c u l a r v i b r a t i o n t e s t point and r u n n i n g c o n d i t i o n s of the m a c h i n e u n d e r t e s t , i s fed into the m o n i t o r , a n d the s t a r t button i s p r e s s e d . S u b s e q u e n t o p e r a t i o n is e n t i r e l y a u t o m a t i c . If v i b r a t i o n l e v e l s t h r o u g h o u t the m e a s u r e d f r e q u e n c y r a n g e a r e within the t o l e r a n c e s s e t up by t h e c a r d , a g r e e n ' A c c e p t ' i n d i c a t o r w i l l be i l l u m i n a t e d . If any l e v e l f a l l s o u t s i d e the t o l e r a n c e s , a r e d ' F a i l ' i n d i c a t o r , t o g e t h e r with i n d i c a t o r s m a r k ing the o u t - o f - t o l e r a n c e c h a n n e l s w i l l be i l l u m i n a t e d . The c o m p l e t e p r o c e s s t a k e s about 40 s e c o n d s . If some abnormal vibration is indicated, further action can be t a k e n to find the c a u s e . The e q u i p m e n t w i l l 218
T R I B O L O G Y N o v e m b e r 1970
F i g 5 S p e c t r u m m o n i t o r d e v e l o p e d at the A d m i r a l t y Engineering Laboratory i n d i c a t e e i t h e r e x c e s s i v e l y high o r u n u s u a l l y low l e v e l s of v i b r a t i o n . T o l e r a n c e l e v e l s c a n be d e c i d e d e i t h e r by: m e a s u r i n g r e f e r e n c e l e v e l s of v i b r a t i o n on the m a c h i n e when it is known to be in good c o n d i t i o n , and c o m b i n i n g t h e s e l e v e l s with the a l l o w a b l e tolerances or, w o r k i n g to v i b r a t i o n l e v e l s s p e c i f i e d f o r a c l a s s of m a c h i n e , o r to a r e c o g n i s e d g r a p h of a c c e p t a b l e l e v e l s in r e s p e c t of, f o r e x a m p l e , the c o n d i t i o n of a bearing. If r e f e r e n c e l e v e l s a r e u s e d , d i f f e r e n t t o l e r a n c e s can be s e t up f o r d i f f e r e n t f r e q u e n c y b a n d s . A t o l e r a n c e of 6 dB c o u l d be s p e c i f i e d at s h a f t r o t a t i o n r a t e to c h e c k o u t - o f - b a l a n c e , w h i l e 20 dB could be p e r m i s s i b l e at s a y 4 kHz to c h e c k b e a r i n g c o n d i t i o n . ACKNOWLEDGEMENT T h a n k s a r e given to C. N i c h o l l s , M e c h a n a l y s i s L t d f o r his h e l p in p r o d u c i n g t h i s a r t i c l e . A c k n o w l e d g e m e n t i s g i v e n to IRD M e c h a n a l y s i s f o r p e r m i s s i o n to r e p r o d u c e F i g s 2, 3, 4 and T a b l e 1, and to the A d m i r a l t y f o r F i g 5.
E v e r y machine which has moving p a r t s v i b r a t e s and the condition of a machine can be judged by the nature of i t s v i b r a t i o n p a t t e r n . If the change in v i b r a t i o n p a t t e r n can be m o n i t o r e d and a n a l y z e d the condition of the machine can be d e t e r m i n e d The s i m p l e s t way of d e t e c t i n g v i b r a t i o n is by sounding stick. F o r g r e a t e r s e n s i t i v i t y i n d u s t r i a l s t e t h o s c o p e s a r e a v a i l a b l e . Fig 1 shows a t y p i c a l s t e t h o s c o p e i n c o r p o r a t i n g an a m p l i f i e r and a high frequency cut-off f i l t e r (above 1 kHz). However not a l l v i b r a t i o n is audible and both t h e s e methods r e l y heavily on the s k i l l of the o p e r a t o r . F o r effective m o n i t o r i n g v i b r a tion a n a l y s i s o v e r a w i d e r frequency range is r e quired. VIBRATION ANALYSIS To m e a s u r e v i b r a t i o n a p i c k - u p is p l a c e d at v a r i o u s points on a machine w h e r e m o s t useful i n f o r m a t i o n can be obtained. As v i b r a t i o n f o r c e s a r e t r a n s m i t t e d through b e a r i n g s m e a s u r e m e n t s a r e best made on
F i g 1 I n d u s t r i a l stethoscope being u s e d to check b e a r i n g s on an e l e c t r i c a l m o t o r
216
TRIBOLOGY N o v e m b e r 1970
b e a r i n g housings as n e a r to the shaft as p o s s i b l e . The points s e l e c t e d should be ones that a r e l i k e l y to show an i n c r e a s e in v i b r a t i o n when t r o u b l e does occ.ur. At l e a s t one r a d i a l and one a x i a l m e a s u r e m e n t i s r e quired for each s e p a r a t e r o t a t i n g p a r t of a machine. Axial r e a d i n g s can r e v e a l t h r e e d i f f e r e n t s o u r c e s of v i b r a t i o n ; m i s a l i g n m e n t between components of the machine, m i s a l i g n m e n t of b e a r i n g s , and bent shafts. H o r i z o n t a l r e a d i n g s a r e u s u a l l y p r e f e r r e d to v e r t i c a l v i b r a t i o n r e a d i n g s w h e r e background v i b r a t i o n s a r e likely, as they a r e not as l i k e l y to be affected by v i b r a t i o n s f r o m m a c h i n e s in the s u r r o u n d i n g a r e a . The m e t e r shown in F i g 2 could only m e a s u r e o v e r a l l d i s p l a c e m e n t and v e l o c i t y r e a d i n g s but the a n a l y z e r in F i g 3 would m e a s u r e v i b r a t i o n l e v e l s in v a r i o u s frequency bands and is used with a s t r o b e l a m p to m e a s u r e the phase of v i b r a t i o n . The f r e q u e n c y of the v i b r a t i o n is the b e s t due to its s o u r c e and hence to the defect. The r e a d i n g s a r e c o m p a r e d to c h a r t s to d e t e r m i n e the p o s s i b l e c a u s e of v i b r a t i o n and to a s c e r t a i n the a c c e p table level of v i b r a t i o n . Table 1 shows the a m p l i t u d e frequency and phase c h a r a c t e r i s t i c s of the common c a u s e s of machine v i b r a t i o n , while F i g 4 is a guide to v i b r a t i o n l e v e l s . In p r a c t i c e the a c c e p t a b l e v i b r a t i o n l e v e l s f o r p a r t i c u l a r b e a r i n g s and m a c h i n e s a r e l a r g e l y a m a t t e r of e x p e r i e n c e . F i l t e r e d v i b r a t i o n r e a d i n g s which fall in the ' r o u g h ' r e g i o n or higher indicate trouble is p r e s e n t . The t r o u b l e should be identified and c o r r e c t i o n s scheduled soon. F i l t e r e d
F i g 2 M e c h a n a l y s i s 306 v i b r a t i o n m e t e r with a f r e quency range of 300 to 240 000 c / m i n
v i b r a t i o n r e a d i n g s which fall in the ' f a i r or ' s l i g h t l y r o u g h ' r e g i o n of the F i g 4 c a l l for caution. T r o u b l e is on the way. I m m e d i a t e s t e p s to make c o r r e c t i o n s a r e not n e c e s s a r i l y r e q u i r e d but p e r i o d i c v i b r a t i o n m e a s u r e m e n t s should be taken at frequent i n t e r v a l s . A t r e n d of i n c r e a s i n g v i b r a t i o n will indicate the t r o u b l e is becoming w o r s e . If no such t r e n d is n o t e d , t h e t r o u b l e should be i d e n t i f i e d and s c h e d u l e d for c o r r e c tion during a r e g u l a r shutdown p e r i o d . F o r v i b r a t i o n a n a l y s i s to be m o s t effetive it is b e s t included in a preventative maintenance p r o g r a m m e .
t y p e s , points at which v i b r a t i o n r e a d i n g s a r e to be taken, r o t a t i n g s p e e d s , etc. When the i n i t i a l a n a l y s i s is c o m p l e t e the v i b r a t i o n l e v e l s should be checked a g a i n s t the v i b r a t i o n s e v e r i t y c h a r t (Fig 4) and the g e n e r a l v i b r a t i o n c h a r t (Table 1). If the v i b r a t i o n
PREVENTIVE MAINTENANCE PROGRAMME The f i r s t s t a g e in setting up a p r e v e n t i v e m a i n t e n a n c e p r o g r a m m e in e x i s t i n g plant is to d e c i d e which m a c h i n e s need m o n i t o r i n g , u s u a l l y ones whose f a i l u r e would affect p r o d u c t i o n and p r o f i t s . Each machine to be m o n i t o r e d has i n i t i a l v i b r a t i o n l e v e l s taken to d e t e r m i n e i t s healthy s t a t e , w h e t h e r t h e r e a r e any f a u l t s to be c o r r e c t e d i m m e d i a t e l y , and its p e r i o d i c v i b r a t i o n check points. Machines with a h i s t o r y of b r e a k d o w n s or heavy m a i n t e n a n c e should be included in the p r o g r a m m e . It is b e s t to make a c h a r t f o r each m a c h i n e to indicate b e a r i n g housings, b e a r i n g
Fig 3 M e c h a n a l y s i s 330 v i b r a t i o n a n a l y z e r with tunable f i l t e r r a n g e s c o v e r i n g 50 to 500 000 c/min
T a b l e 1 Guide to v i b r a t i o n i d e n t i f i c a t i o n Cause
Amplitude
Frequency
Remarks
Unbalance
P r o p o r t i o n a l to unbalance. Largest in radial direction.
1 x rev/min
Most c o m m o n cause of v i b r a t i o n
Misalignment couplings or b e a r i n g s and bent shaft
Large in axial direction 50°/° or more of radial vibration
1 x r e v / m i n usual 2 & 3 x rev/min sometimes
Best found by a p p e a r a n c e of l a r g e a x i a l v i b r a t i o n . Use d i a l i n d i c a t o r s o r other method f o r p o s i t i v e d i a g n o s i s . If s l e e v e b e a r i n g machine and no coupling m i s a l i g n m e n t balance the r o t o r
Bad b e a r i n g s a n t i - f r i c t i o n type
Unsteady-use velocity measurement if p o s s i b l e
V e r y high several times rev/min
B e a r i n g r e s p o n s i b l e m o s t l i k e l y the one n e a r e s t point of l a r g e s t h i g h - f r e q u e n c y vibration
Eccentric journals
Usually not l a r g e
1 x rev/min
If on g e a r s l a r g e s t v i b r a t i o n in line with g e a r c e n t e r s . If on m o t o r or g e n e r a t o r v i b r a t i o n d i s a p p e a r s when power is t u r n e d off. If on pump or b l o w e r a t t e m p t to b a l a n c e
Bad g e a r s or g e a r noise
Low--use v e l o c i t y m e a s u r e if p o s s i b l e
V e r y high g e a r teeth t i m e s rev/min
Mechanical looseness
2 x rev/min
Usually a c c o m p a n i e d by unbalance a n d / or m i s a l i g n m e n t
Bad d r i v e b e l t s
E r r a t i c o r pulsing
1 , 2 , 3 and 4 x r e v / m i n of b e l t s
Strob light best tool to f r e e z e faulty belt
Electrical
D i s a p p e a r s when power is t u r n e d off.
1 x r e v / m i n or 1 or 2 x s y n chronous f r e quency.
If v i b r a t i o n a m p l i t u d e d r o p s off i n s t a n t ly when power is t u r n e d off c a u s e is electrical
Aerodynamic hydraulic forces
1 x r e v / m i n or n u m b e r of b l a d e s on fan or i m p e l l e r x rev/min
R a r e as a cause of t r o u b l e except in c a s e s of r e s o n a n c e
Reciprocating forces
1, 2 & h i g h e r orders x rev/ min
Inherent in r e c i p r o c a t i n g m a c h i n e s can only be r e d u c e d by design changes or isolation
TRIBOLOGY N o v e m b e r 1970
217
0.1 cl
o O Q.
E
e
0<)I
"E E U
o. IA
~ 0-001 ¢. I0
0"0001
I( Vi brotion frequency [clrnin] Fig 4 Guide to level of vibration in general machinery
applications l e v e l s a r e a c c e p t a b l e t h e s e v a l u e s r e p r e s e n t the n o r m a l c o n d i t i o n of the m a c h i n e s . The i n t e r v a l f o r c h e c k i n g a m a c h i n e i s d e t e r m i n e d by the type of m a c h i n e and i t s m a i n t e n a n c e r e c o r d . When c h e c k s a r e m a d e they a r e r e c o r d e d and c o m p a r e d with the v i b r a tion h i s t o r y c h a r t . T r e n d s in v i b r a t i o n c h a n g e s can then be a s c e r t a i n e d .
SPECTRUM ANALYSIS A m o r e s o p h i s t i c a t e d m e t h o d of v i b r a t i o n a n a l y s i s i s spectrum analysis. Spectrum analyzers are far more e x p e n s i v e than o r d i n a r y v i b r a t i o n a n a l y z e r s and t h e i r u s e i s only j u s t i f i e d on v e r y e x p e n s i v e e q u i p m e n t . F o r i n s t a n c e the Navy have d e v e l o p e d a m o n i t o r ( F i g 5) f o r u s e with p r o p u l s i o n and g e n e r a t i n g e n g i n e s , p u m p s c o m p r e s s o r s etc on b o a r d ship. T h i s e q u i p m e n t can remotely monitor as many machines as required. Output f r o m an a c c e l e r o m e t e r , s c r e w e d to an a c c e l e r o m e t e r b l o c k s e c u r e d a s e l e c t e d point on the m a c h i n e u n d e r t e s t i s fed into an i m p e d a n c e c o n v e r t e r . The low i m p e d a n c e output i s then t a k e n v i a a line a m p l i f i e r to the s p e c t r u m m o n i t o r which a n a l y z e s the i n c o m i n g s i g n a l in c o n s e c u t i v e o n e - t h i r d - o c t a v e f r e q u e n c y c h a n n e l s . (In the e x p e r i m e n t a l m o d e l t h e r e a r e 27 c h a n n e l s with m i d - b a n d f r e q u e n c i e s f r o m 10 Hz to 4 kHz). A punched c a r d , c o d e d f o r the p a r t i c u l a r v i b r a t i o n t e s t point and r u n n i n g c o n d i t i o n s of the m a c h i n e u n d e r t e s t , i s fed into the m o n i t o r , a n d the s t a r t button i s p r e s s e d . S u b s e q u e n t o p e r a t i o n is e n t i r e l y a u t o m a t i c . If v i b r a t i o n l e v e l s t h r o u g h o u t the m e a s u r e d f r e q u e n c y r a n g e a r e within the t o l e r a n c e s s e t up by t h e c a r d , a g r e e n ' A c c e p t ' i n d i c a t o r w i l l be i l l u m i n a t e d . If any l e v e l f a l l s o u t s i d e the t o l e r a n c e s , a r e d ' F a i l ' i n d i c a t o r , t o g e t h e r with i n d i c a t o r s m a r k ing the o u t - o f - t o l e r a n c e c h a n n e l s w i l l be i l l u m i n a t e d . The c o m p l e t e p r o c e s s t a k e s about 40 s e c o n d s . If some abnormal vibration is indicated, further action can be t a k e n to find the c a u s e . The e q u i p m e n t w i l l 218
T R I B O L O G Y N o v e m b e r 1970
F i g 5 S p e c t r u m m o n i t o r d e v e l o p e d at the A d m i r a l t y Engineering Laboratory i n d i c a t e e i t h e r e x c e s s i v e l y high o r u n u s u a l l y low l e v e l s of v i b r a t i o n . T o l e r a n c e l e v e l s c a n be d e c i d e d e i t h e r by: m e a s u r i n g r e f e r e n c e l e v e l s of v i b r a t i o n on the m a c h i n e when it is known to be in good c o n d i t i o n , and c o m b i n i n g t h e s e l e v e l s with the a l l o w a b l e tolerances or, w o r k i n g to v i b r a t i o n l e v e l s s p e c i f i e d f o r a c l a s s of m a c h i n e , o r to a r e c o g n i s e d g r a p h of a c c e p t a b l e l e v e l s in r e s p e c t of, f o r e x a m p l e , the c o n d i t i o n of a bearing. If r e f e r e n c e l e v e l s a r e u s e d , d i f f e r e n t t o l e r a n c e s can be s e t up f o r d i f f e r e n t f r e q u e n c y b a n d s . A t o l e r a n c e of 6 dB c o u l d be s p e c i f i e d at s h a f t r o t a t i o n r a t e to c h e c k o u t - o f - b a l a n c e , w h i l e 20 dB could be p e r m i s s i b l e at s a y 4 kHz to c h e c k b e a r i n g c o n d i t i o n . ACKNOWLEDGEMENT T h a n k s a r e given to C. N i c h o l l s , M e c h a n a l y s i s L t d f o r his h e l p in p r o d u c i n g t h i s a r t i c l e . A c k n o w l e d g e m e n t i s g i v e n to IRD M e c h a n a l y s i s f o r p e r m i s s i o n to r e p r o d u c e F i g s 2, 3, 4 and T a b l e 1, and to the A d m i r a l t y f o r F i g 5.