* parlv working model electing - w * * " 9*
Fig. 50
The same model demonstrating the Trendelenberg position by altering the distance between the p arallel legs.
te s t t h i s m o b i l i t y a p i n - j o i n t e d car dboar d model was made e n a b l i n g t he s t r u c t u r e and j o i n t i n g t o be as s e s s e d. As t h i s pr oved s a t i s f a c t o r y t h e nex t st age de v e l o p e d t he mechanism f u r t h e r by use of s c a l e d wor ki ng model s. F i g s . 4 8 , 49 and 50 show a model of t he b a s i c s t r u c t u r e of t he mechanism t h a t was used t o d e v e l o p t he s l i d i n g mechani sm, l e n g t h o f t he l egs t o g i v e t he r e q u i r e d h e i g h t and t he d i s t a n c e t he c e n t r e s l i d e would need t o move to a c h i e v e t h i s . I t a l s o enabl ed t he T r e n d e l e n berg c o mb i n a t i o n t o be e x p l o r e d as shown i n f i g . 50. Two models were t hen b u i l t t o show t he shape and form o f t he t r o l l e y s t r u c t u r e u t i l i z i n g t h e two d i f f e r e n t m a t e r i a l s e c t i o n s i n or de r t o e v a l u a t e t he appear ance of t h e t r o l l e y . Fi g s . 5 1 , 52 and 53 show t he use of t u b u l a r s e c t i o n m a t e r i a l and f i g s . 54 and 55 show t he appear ance o f a
r e c t a n g u l a r s e c t i o n . However , i n e x a mi ni ng t h e s e s e c t i o n s i t was n e c e s s a r y t o con si d er t he b e n e f i t s and d i s a d v a n t a g e s of each as w e l l as t h e i r a p p e a r ance. T u b u l a r m a t e r i a l had t he advant age t h a t i t i s c u r r e n t l y i n use f o r t h e e x i s t i n g s t r e t c h e r t r o l l e y s t h e r e f o r e t h e r e would be m a t e r i a l , c a s t i ngs and components a v a i l a b l e , however, u n l e s s t h e new d e s i gn u t i l i z e d t he e x i s t i n g c a s t i n g s whi ch would be r e s t r i c t i v e t hen new f a s t e n i n g components would need t o be made. The r e c t a n g u l a r s e c t i o n m a t e r i a l i n t he model r e p r e s e n t e d a f l a t - o v a l s e c t i o n which has t he advant age t h a t t h e f l a t si des a f f o r d a s u r f a c e t o which o t h e r components can be more e a s i l y f a s t e n e d . However, t h i s shape r e q u i r e s s p e c i a l c a s t i n g s and t o o l i n g where
components f i t i n t o or around t he s e c t i o n . I t was f e l t , however , t h a t t he advant ages o u t we i g h e d t he d i s a d v a n t a g e s p a r t i c u l a r l y as c a l c u l a t i o n s shown bel ow demon s t r a t e d t h a t t he f l a t - o v a l
s e c t i o n has a g r e a t e r s t r e n g t h t o wei ght r a t i o i n t he r e q u i r e d d i r e c t i o n t han t he t u b u l a r s e c t i o n and as i t s appear ance was more s u i t e d t o t h e chosen
1 1 7 - -
mechanism i t was dec i de d to use t h i s s e c t i o n f o r t he t r o l l e y d e s i g n .
Tube used f o r t he e x i s t i n g t r o l l e y f r a m e : -
D i a me t e r 27.78mm
Wal l t h i c k n e s s 2.34mm
Usi ng I = second moment of ar ea D = e x t e r n a l d i a me t e r d = i n t e r n a l di a me t e r Then I = tt ( D4 - d4 ) 64 = k ( 2 7 . 7 8 4 - 2 3 . 1 4 ) 64 1 5 . 2 5 8 x 1 0 3mm4 Wei ght = . 506Kg/ m
For new de s i gn usi ng r e c t a n g u l a r s e c t i o n
41.9mm x 19.05mm wi t h .91mm w a l l t h i c k n e s s Where I = second moment of ar ea
b = br e a d t h d = depth I = bd3 72 1 9 . 0 5 x 41 .91 3 _ ^ /c. „ n 3 --- = 2 4 . 3 4 5 x 10 mm 12 Wei ght = -281Kg/m
The r e c t a n g u l a r s e c t i o n has a h i g h e r ’ I 1 v a l u e and weighs l ess per met re l engt h t han t he t u b e .