Computerized Life and Reliability Modelling for Turboprop Transmissions

i i t h component n number o f components t t r a n s m i s s i o n 10 90 pe rcen t r e l i a b i l i t y S u p e r s c r i p t s : e We ibu l l s l ope p l o a d l i f e f a c t o r I n t r o d u c t i o n One i m p o r t a n t measure o f t h e per fo rmance o f t u rboprop t r a n s m i s s i o n s i s t h e s e r v i c e t ime between scheduled maintenance o v e r h a u l s . I n t h e des ign o f new p rop fan and t u r b o p r o p a i r c r a f t p ro p u l s i o n systems, s e r v i c e l i f e c a l c u l a t i o n s a r e r e q u i r e d . Too ls t h a t a s s i s t t h e t r a n s m i s s i o n des igne r i n comparing d i f f e r e n t c o n f i g u r a t i o n s on a common l i f e and r e l i a b i l i t y b a s i s for a g i v e n m i s s i o n spec t rum l o a d i n g b e f o r e t h e t r a n s m i s s i o n s a r e c o n s t r u c t e d a r e q u i t e v a l u a b l e . By de te rm in i n g t r a n s m i s s i o n system l i f e as w e l l as component l i v e s , weak l i n k s i n a d e s i g n can be i d e n t i f i e d and s t reng thened . T h i s enab les t h e des igne r t o b r i n g f o r w a r d t h e b e s t c o n f i g u r a t i o n and c o n f i g u r a t i o n embodiment p o s s i b l e t o t h e f i n a l des ign s tage of a t r a n s m i s s i o n ' s development f r o m a l i f e and r e l i a b i l i t y s t a n d p o i n t . Computer programs a r e a v a i l a b l e f o r l i f e a n a l y s i s o f v a r i o u s bear ings and b e a r i n g s h a f t arrangements based on t h e c l a s s i c a l f a t i g u e t h e o r y o f Lundberg and Pa lmgren. l .2 T h i s t h e o r y has a l s o been a p p l i e d t o t h e a n a l y s i s o f f a t i g u e l i f e for spur and h e l i c a l gear ~ e t s . 3 ~ t h e l i f e and r e l i a b i l i t y o f h e l i c o p t e r p l a n e t a r y and beve l t ransmiss ions have been developed as f u n c t i o n s o f t h e l i v e s and r e l i a b i l i t i e s o f t h e t r a n s m i s s i o n component bea r ings and gears.8-10 I n p r e v i o u s papers , The l i f e models assume t h a t t h e t r a n s m i s s i o n i s adequa te l y l u b r i c a t e d and t h a t t h e components a r e w e l l des igned. Fo r gea rs , t h i s means t h a t s u f f i c i e n t r i m th i cknesses and p r o p e r m a t e r i a l s a r e used t o p r e v e n t p remature t o o t h breakage f a i l u res . I t i s assumed a l s o t h a t t h e t o o t h form geometry and l u b r i c a n t a r e adequate t o a v o i d gear t i p s c o r i n g . Both o f these modes o f f a i l u r e a r e p r e v e n t a b l e w i t h adequate d e s i g n . l l models assume s teady power o p e r a t i o n w i t h min ima l s t a r t u p or d e c e l e r a t i o n shock l o a d i n g . The gas t u r b i n e power source o f t h e t u r b o p r o p eng ine p ro v i d e s t h i s t ype o f power i n p u t . Surface p i t t i n g i n t h e f u l l l o a d r e g i o n o f t h e gear t o o t h faces and i n t h e b e a r i n g races i s n o t p r e v e n t a b l e , however, due t o t h e l a c k o f a s u r f a c e endurance 1 i m i t f o r h i g h s t r e n g t h s t e e l s . 1-7 Both e lements w i l l f a i l e v e n t u a l l y i n s u r f a c e p i t t i n g r e g a r d l e s s o f t h e l oads . A l s o , i f s i m i l a r components o p e r a t e a t t h e same c o n d i t i o n s , t h e y w i l l e x h i b i t a s c a t t e r i n s u r f a c e p i t t i n g f a t i g u e l i f e . Thus, s t a t i s t i c a l methods, such as t h e two parameter We ibu l l d i s t r i b u t i o n a r e commonly used i n p r e d i c t i n g f a t i g u e l i f e . The l i f e