Chevrolet 70 Series 1970, Service Manual

Page: 95 / 841

CAB AND BODY MOUNTINGS 1-64
TORQUE 40 TO 50 FT. LBS.
PUMP ASSEMBLY
T-6912
Figure 6—Hydraulic Pump Installation
LOWERING CAB (F igs. 2, 3, and 4)
1. P lace g e a rsh ift lev er(s) in N eutral and
apply parking brak e.
2. P lace valve le v e r in “ DN” position.
3. P lace pump handle in pump socket.
4. Remove safety pin from locking slo ts in
tilt le v e r m echanism . It may be n e c e ssa ry to
o p erate pump slightly to re lie v e weight from pin
befo re pulling it out.
5. Low er cab by operating pump handle up
and down.
NOTE: When cab p a s s e s o v e r-c e n te r so that
weight of cab is g re a te r to r e a r of pivot hinges
than to fro n t, the cab w ill fa ll a t a controlled ra te
due to g ravitational pull. O p erato r may fu rth e r
lim it the ra te of fre e fall by opening and closing
the pump valve. This is done by moving control
le v e r between “ DN” and “ U P ” po sitio ns until
cab has reached full-down position. The hold-
down catches o perate autom atically, and when cab
is in full-down position, hold-down catch es and
safety catch will lock. A fter cab is in full-down
position, visually in sp ect each catch to be su re
they a re fully locked.
HYDRAULIC PUMP AND TILT CYLINDER
The cab tiltin g hydraulic pump is of the
c lo s e d -re s e rv o ir type. Pump is equipped with a
p re s s u r e re lie f valve to p re v en t hydraulic p r e s ­
s u re fro m exceeding designed pump stren g th . A
standpipe is in se rte d down into re s e rv o ir to a s ­
s u re am ple a ir space is provided above hydraulic
oil. Since the sy stem is of the closed re s e rv o ir
type, a i r space above oil is n ec e ssa ry to prev en t
cavitation when oil is rem oved fro m re s e rv o ir
when tilting cab.
NOTE: F ill pump r e s e r v o ir only through
the fill plug and standpipe opening, not through
re lie f vent plug opening, when cab is in "Down"
position. T his is n e c e ssa ry to p re v en t o v er-fillin g
of hydraulic sy stem which could r e s u lt in e r ra tic
pum ping action and oil sp ra y through r e s e r v o ir
re lie f valve when cab is low ered.
PUMP OPERATION
NOTE: Key num bers r e fe r to figu re 5.
H ydraulic oil is drawn f r o m r e s e r v o ir
through suction sc re e n (21) and in let check valve
(19) due to upw ard m ovem ent of pump p isto n (4)
a s pump handle (10) and beam (9) a re ra is e d .
Oil is pulled into pump cylin der until handle
re a c h e s end of upward trav e l.
When the handle is low ered, o il p re s s u r e
clo se s suction check b all (19) and opens ou tlet
check b all (1'8). Due to downward m ovem ent of
piston, oil e n te rs p assa g e above outlet b a ll and
is d ire c te d into valve spool cavity. The valve
spool (15), shown in position fo r ra isin g cab,
d iv e rts high p re s s u r e oil through p a s s a g e s and
p o rt s c re e n leading to left hydraulic line as illu s ­
tra te d on figu re 5. H ydraulic oil re tu rn in g from
tilt cy lin der e n te rs pump through rig h t hand hose,
p a s s e s through the upper p o rtio n of valve spool
(15), into a fixed re s tr ic tio n o rifice (11), and is
re tu rn ed to the re s e rv o ir.
The fixed re s tric tio n o rifice (11) is designed
to lim it the m axim um ra te of fall when cab weight
p a s s e s “ o ver c e n te r” during n orm al cab tilting
o r low ering. This fixed o rifice is p ro tec ted by
a sc re e n to p re v en t d irt fro m clogging p assag e.
Also, ex tern ally ad justable stops a r e provided
which contact ro ll pin (16) on valve spool (15).
As re q u ired , these stops m ay be ex tern ally ad­
justed to fu rth e r lim it the ra te of cab fall.
If o p e ra to r continues to actuate handle when
cab is fully tilted o r if prop pin was not rem oved
fro m lock position and con tro l le v e r is in “ UP”
position, extrem e hydraulic p r e s s u r e s can be
g en erated which will open a p r e - s e t re lie f valve
(2Q). The re lie f valve is c a lib rated to allow su f­
ficient p re s s u r e to tilt cab under a ll n o rm al
conditions, y et provide a m oderate safety fa cto r.
When oil p re s s u r e beneath pump p isto n exceeds
re lie f valve setting , valve will bypass oil d ire c tly
to pump re s e rv o ir.
Quad rin g s and se a ls a re u sed throughout
pump to elim inate leaks due to oil p re s s u r e .
S creens a re u sed in oil p a ssa g e s to p ro te c t pump
from ex tern al d irt th at could e n te r through hy­
d ra u lic lin e s o r r e s e rv o ir f ille r opening.
PUMP REPLACEMENT (Fig. 6)
1. Remove hydraulic h o ses from co ntro l
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCKSHOP MANUAL