Chevrolet 70 Series 1970, Service Manual

Page: 715 / 841

STEERING SYSTEM 9-36
2 3 4 5 6 7 8
1 Pump C over A ssem b ly
2 C ontrol V alve Spring
3 Control V alve A ssem b ly
4 Control V alve O -rin g
5 Control V alve Plug
6
V alve R etain er P in
7 O -rin g
8 Pump Body
9 O il Seal
10 Key
11 Pump Shaft
12 Shaft B earing
13 Snap Ring
14 O -ring
15 Pump Rotor
16 R otor Vane (10 Required)
17 Pump Ring
18 P r e s s u r e P late
19 P r e s s u r e P la te Spring
20 Pump C over Cap S crew s
T-4505
Figure 70—Cross Section of Vane Type Pump (Model VTM-271 (Typical)
FLOW CONTROL AND RELIEF VALVE
An in tegral flow control and r e lie f valve w ith­
in the pump op erates on a p r e ssu r e d ifferen tial to
lim it m axim um p r e s s u r e within the hydraulic c ir ­
cuit.
Maximum pump d elivery and maximum sy ste m
p r e ssu r e a re determ ined by the in tegral flow con­
tr o l and r e lie f valve in the outlet co v er (fig.
1 2
).
An o r ific e in the cover lim its m axim um flow . A
p ilo t-o p era ted type r e lie f valve sh ifts to divert
e x c e s s fluid d elivery to r e se r v o ir , thus lim itin g
sy ste m p r e ssu r e to a p r e sc r ib e d maxim um .
When sy ste m p r e ssu r e is equalized a s shown in
figu re 12, View A, total pump output can be p a sse d
through the o r ific e to the pow er cylin d er. T his con­
dition usually o ccu rs only at low drive sp e e d s. The
la r g e spring cham ber is connected to the p r essu r e
port through an o r ific e . P r e s s u r e in th is chamber
eq u alizes p r e ssu r e at the other end of the r e lie f
valve sp ool and the light sp rin g holds the spool
clo se d . Pump d eliv ery is blocked from the r e s e r ­
voir port by the spool land.
When pump d eliv ery ex ceed s the flow rate,
determ ined by the o r ific e plug, a p r e ssu r e build­
up fo r c e s the sp ool open against the light spring.
The pump ring is shaped so that two opposing
pumping cham bers a r e form ed, thus can cellin g any
hydraulic loads on the b earings. Radial movem ent
of the vanes, and rptation of the rotor, c a u ses the
cham ber area betw een vanes to in c r e a se in s iz e
at the inlet (large diam eter) se ctio n of the ring.
T his r e su lts in a low p r e ssu r e , or vacuum in the
cham ber. T his p r e ssu r e differential c a u se s o il to
flow into the in let, w here it i s trapped betw een the
rotating vanes and is forced , through porting in the
p r e ssu r e p late to d isch arge into the sy ste m as the
cham ber s iz e d ec r e a se s at the p r e ssu r e quadrant
(sm a ll diam eter) of the ring.
OUTLET
Figure 11 —Pump Operation and Fluid Flow
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL