Graseby 3000/500, Manual

Page: 85 / 231

G r a s e b y M e d i c a l L t d .
5 — 6
5
I s s u e A ( A p r i l 2 0 0 2 )
3 0 0 0 / 3 1 0 0 a n d 5 0 0 / 5 0 5 S e r v i c e M a n u a l
V o l u m e t r i c I n f u s i o n P u m p s
Cams, pistons and valves of the pumping mechanism
The motor’s mounting shaft is connected to a vertically mounted cam
shaft (see Fig. 5.3, item 13) by the motor coupler (item 4), interface
coupler (item 5), and hub coupler (item 6). Four cams (items 8, 9, 11
and 12) and an encoder wheel (item 10) are moulded onto the cam
shaft. The cams actuate four horizontal shafts, which in turn drive
two valves and two pistons which produce the pumping action.
Pumping cycle description (Figs 5.3, 5.4 and 5.5)
1. The cam shaft revolution causes the upper valve (item 17) to open,
allowing flow to the upper cassette chamber and upper piston (see
item 18). The lower valve (item 20) closes, thus stopping flow
between the upper and lower cassette chamber (lower piston).The
smaller, lowest chamber on the cassette is in contact with the
overpressure piston (item 22) and is the point where internal
pumping pressure is continuously regulated.
2. The upper piston (see item 18) begins to draw back as fluid flows
from the upper tubing into the upper cassette chamber. At the
same time, the lower piston (item 21) begins to slowly push
forward, forcing the fluid in the lower cassette chamber into the
lower tubing, towards the patient.
3. Once the upper piston (item 18) has moved fully back and the
lower piston (item 21) fully forward, the upper valve (item 17)
closes and the lower valve (item 20) opens. The upper piston (item
18) then begins to push forward to force the fluid in the upper
cassette chamber downward into the lower cassette chamber,
where the lower piston (item 21) is beginning to draw back to
receive the fluid. The upper piston moves forward delivering fluid
more rapidly when the lower piston draws back, causing a
continuous stream of fluid to be forced out of the lower tubing to
the patient.
4. The lower valve (item 20) closes and the upper valve (item 17)
opens as the upper piston (item 18) reaches its furthest forward
point of travel and the lower piston (item 21) its furthest back
travel. The cycle then repeats.
The result of the above cam, piston and valve action is a smooth flow
of fluid through the lower tubing to the patient. Each full revolution
of the cam shaft delivers approximately 1.7 mL of fluid as governed by
the precise and unvarying travel of the pistons. The rate of fluid
delivered is determined by the rate at which the motor is stepped.