VELOCITY FUSE (1036Z403)
The valve spool will not shift to the blocked position
when the supply of oil is blocked, unless a leakage
path, such as a faulty counterbalance valve, exists.
One velocity fuse is mounted at the blind end of each boom
hoist cylinder. The purpose of these valves is to prevent
Oil supplied to the retract port of the cylinder is di-
rapid lowering of the boom should a hydraulic line between
rected to the rod end of the cylinder and to the spool
the counterbalance valve and the boom hoist cylinders
piston chamber. The oil in the spool piston chamber will
maintain the spool in the open position when the pressure
at the blind end of the spool is dropped to a minimal level,
by opening the extend port to tank. Retracting the cylinder
causes all the oil at the blind end of the cylinder to first
flow through the orifice in the check valve, and then
through the velocity fuse to tank.
These valves will not hold the boom in a raised posi-
tion. Therefore, the boom must be prevented from
Should the hydraulic line connected to the extend port of
lowering by some mechanical means before any
the boom hoist cylinders break while the boom is being
hydraulic line between the counterbalance valve and
raised, or when the machine is shut down, the oil pressure
velocity fuses is removed.
at the extend port would immediately drop to zero. This
would allow the pressure at the blind end of the valve spool
The operation of a velocity fuse is illustrated in Figure
to also drop to zero, allowing the spring to shift the valve
5G-1, and is described in the following paragraphs:
spool to the blocked position. With the spool in the
blocked position, all the oil at the blind end of the cylin-
ders must then pass through the orifice in the check valve.
Oil supplied to the extend port of the boom hoist cylinders
then through the orifice in the spool to the passage in the
is blocked by the velocity fuse spool. Since the oil can not
spool, and finally to atmosphere. This will cause the boom
enter the blind end of the cylinder, it is forced through the
to lower slowly.
passage in the spool to the blind end of the spool. Oil
pressure at the blind end of the spool will increase until it is
sufficient to shift the spool to the open position, allowing
oil to flow to the blind end of the cylinder through a check
At least once a year, these valves should be disassembled
and cleaned as follows to ensure free movement of the
Blocking the supply of oil to the extend port stops the
motion of the cylinders. The valve spool, however, is main-
tained in the open position by the oil pressure, caused by
1. Lower the boom onto the boom rest to prevent boom
the weight of the boom on the cylinder, trapped in the
cylinder. This pressure is transmitted through the passage in
movement while servicing the velocity fuses.
the spool to the blind end of the spool to maintain it in the
open position. The trapped pressure will also shift the
2. Remove the jam nuts and weather cap from the spool
check valve to the seated position.