Dodge Sprinter 2006 Manual page 1901

VA
(9) Ver ify r epa ir.
PI ST ON S
DESCRIPTION
Th er e a r e sever a l sizes a n d t ypes of pist on s u sed in
a n a u t om a t ic t r a n sm ission . Som e pist on s a r e u sed t o
a pply clu t ch es. Th ey a ll h a ve in com m on t h e fa ct
t h a t t h ey a r e r ou n d or cir cu la r in sh a pe, loca t ed
wit h in a sm oot h wa lled cylin der, wh ich is closed a t
on e en d a n d con ver t s flu id pr essu r e in t o m ech a n ica l
m ovem en t . Th e flu id pr essu r e exer t ed on t h e pist on
is con t a in ed wit h in t h e syst em t h r ou gh t h e u se of
pist on r in gs or sea ls.
OPERA TION
Th e pr in cipa l wh ich m a kes t h is oper a t ion possible
is kn own a s P a sca l's La w. P a sca l's La w ca n be st a t ed
a s: "P r essu r e on a con fin ed flu id is t r a n sm it t ed
equ a lly in a ll dir ect ion s a n d a ct s wit h equ a l for ce on
equ a l a r ea s."
PRESSURE
P r essu r e (F ig. 213) is n ot h in g m or e t h a n for ce
(lbs.) divided by a r ea (in or ft .), or for ce per u n it
a r ea . Given a 100 lb. block a n d a n a r ea of 100 sq. in .
on t h e floor, t h e pr essu r e exer t ed by t h e block is: 100
lbs. 100 in or 1 pou n d per squ a r e in ch , or P SI a s it is
com m on ly r efer r ed t o.
Fig. 213 Force and Pressure Relationship
PRESSURE ON A CONFINED FLUID
P r essu r e is exer t ed on a con fin ed flu id (F ig. 214)
by a pplyin g a for ce t o som e given a r ea in con t a ct
wit h t h e flu id. A good exa m ple of t h is is a cylin der
AUTOMA TIC TRANSMISSION NAG1 - SERVICE INFORMA TION 21 - 163
filled wit h flu id a n d equ ipped wit h a pist on t h a t is
closely fit t ed t o t h e cylin der wa ll. If a for ce is a pplied
t o t h e pist on , pr essu r e will be developed in t h e flu id.
Of cou r se, n o pr essu r e will be cr ea t ed if t h e flu id is
n ot con fin ed. It will sim ply "lea k" pa st t h e pist on .
Th er e m u st be a r esist a n ce t o flow in or der t o cr ea t e
pr essu r e. P ist on sea lin g is ext r em ely im por t a n t in
h ydr a u lic oper a t ion . Sever a l kin ds of sea ls a r e u sed
t o a ccom plish t h is wit h in a t r a n sm ission . Th ese
in clu de bu t a r e n ot lim it ed t o O-r in gs, D-r in gs, lip
sea ls, sea lin g r in gs, or ext r em ely close t oler a n ces
bet ween t h e pist on a n d t h e cylin der wa ll. Th e for ce
exer t ed is down wa r d (gr a vit y), h owever, t h e pr in ciple
r em a in s t h e sa m e n o m a t t er wh ich dir ect ion is t a ken .
Th e pr essu r e cr ea t ed in t h e flu id is equ a l t o t h e for ce
a pplied, divided by t h e pist on a r ea . If t h e for ce is 100
lbs., a n d t h e pist on a r ea is 10 sq. in ., t h en t h e pr es-
su r e cr ea t ed equ a ls 10 P SI. An ot h er in t er pr et a t ion of
P a sca l's La w is t h a t r ega r dless of con t a in er sh a pe or
size, t h e pr essu r e will be m a in t a in ed t h r ou gh ou t , a s
lon g a s t h e flu id is con fin ed. In ot h er wor ds, t h e
pr essu r e in t h e flu id is t h e sa m e ever ywh er e wit h in
t h e con t a in er.
Fig. 214 Pressure on a Confined Fluid
FORCE MULTIPLICA TION
Usin g t h e 10 P SI exa m ple u sed in t h e illu st r a t ion
(F ig. 215), a for ce of 1000 lbs. ca n be m oved wit h a
for ce of on ly 100 lbs. Th e secr et of for ce m u lt iplica -
t ion in h ydr a u lic syst em s is t h e t ot a l flu id con t a ct
a r ea em ployed. Th e illu st r a t ion , (F ig. 215), sh ows a n
a r ea t h a t is t en t im es la r ger t h a n t h e or igin a l a r ea .
Th e pr essu r e cr ea t ed wit h t h e sm a ller 100 lb. in pu t
is 10 P SI. Th e con cept "pr essu r e is t h e sa m e ever y-
wh er e" m ea n s t h a t t h e pr essu r e u n der n ea t h t h e
la r ger pist on is a lso 10 P SI. P r essu r e is equ a l t o t h e
for ce a pplied divided by t h e con t a ct a r ea . Th er efor e,
by m ea n s of sim ple a lgebr a , t h e ou t pu t for ce m a y be
fou n d. Th is con cept is ext r em ely im por t a n t , a s it is
a lso u sed in t h e design a n d oper a t ion of a ll sh ift
va lves a n d lim it in g va lves in t h e va lve body, a s well
a s t h e pist on s, of t h e t r a n sm ission , wh ich a ct iva t e