OP-755 Part 2 Pages 197-401 - Personal Page of GENE SLOVER

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striction. The limits of adjustment permit hoist acceleration
and deceleration greater than the rates as determined by the
B-end cams but not in excess of one-half gravity. Dashpot
reservoir 70 supplies oil to take care of the differential
area of the piston and to replace seepage loss.
56. Power failure - Servo Electrical. - The diagram of
plate 10 shows the circuit during power failure whenoperating
with servo electrical control. If power should fail during
servo operation, both solenoids 1 and 28 wil,l be de-energized,
which vents the brake cylinder and applies the brake, and also
positions valve 2 to by-pass the main circuit lines. However,
with the pump in hoi.st Lng position, line 24 is open to line
25 only through restriction 49 until the pumpis near its centered
position. Restriction 49 is small enough to prevent a
pressure drop in line 23 which would permit the car to drop
due to a slack cable (power failure when.hoisting) ,but is
sufficiently large to prevent hoisting when the car is at the
lower unloading station with door open. At occurrence of power
failure the pumpcontinues to drive the B-end carried on by
electric. motor inertia. The brake under these conditions is
engaged, but the free wheeling device 11 allows over-r-un.of the
drum and .the B-end. Thus the car gradually decelerates, without
cable whl.p , and as it comes to rest the ·load is taken by
the brake. Simultaneously pressure falls off in line 23 and
centering plunger 22 restores the pumpyoke to its center posi
tion. The electric motor cannot be started until the pump
yoke has been returned to near the center position. At this
position neutral switch 13 (in the motor starting circuit) is
closed.
57. If power failure should occur during a lowering
cycle (servo electrical control), solenoids 1 and 28 will be
de-energized to by-pass and vent the main circuit, and to vent
the control circuit. Venting the latter applies the brake to
decelerate and stop the hoist. Centering plunger 21 is vented
and moves to return the yoke to centered position.
58. St op pos it ion - Servo Meehan i c a l. - The diagram of
pLate 11 showsthe circuit in the stop position with the controls
set for servo mechanical operation. The arrangement shown
provides for hoist control and operation in event that the electrical
control circuit is inoperative. The controls are set
:for servo mechanical operation by shifting the selector control
lever 12 to "Servo Mechanical" position. This movement
shifts the brake release valve 14, the venting valve 2 and releases
latch 52. The latter drops into the notch on cam 53
(provided lever 31 is at neutral). This allows latch valve 54
to shift to vent the main circuit and the control circuit to
the tank. Main circuit lines 23 and 41 are vented to the tank
through check valves 62. and 64 and thence around the power