4000 SRM 135
Description
Legend for Figure 3
A. TWO-STAGE MAST
B. FREE-LIFT MAST
C. THREE-STAGE MAST
D. FOUR-STAGE MAST
1.
OUTER WELDMENT
2.
INNER WELDMENT
3.
INTERMEDIATE WELDMENT
4.
FIRST INTERMEDIATE WELDMENT
5.
HOOK
6.
FREE-LIFT CYLINDER
7.
CROSSMEMBER
8.
CROSSMEMBER
General
This section has the description for lift cylinders used
in masts and the instructions for their repair. There
are many different sizes of lift cylinders used in Hys-
ter Company lift trucks. The operation and repair
procedures for the different lift cylinders are simi-
lar. The illustrations in this section are for typical
lift cylinders. All of the variations in lift cylinders
used in lift trucks are not shown. A section for Lift
Cylinders for VISTA
®
Masts is included later in this
section.
Description
All lift cylinders are single-acting cylinders. See Fig-
ure 4. The hydraulic force is applied only in one di-
rection. When hydraulic oil enters one end of the lift
cylinder, the hydraulic force extends the piston rod.
When the force is removed, the weight of the carriage
and inner mast retracts the piston rod.
A common maintenance problem is the repair of oil
leaks from the seals and wiper around the piston rod.
If the bore of the shell of the lift cylinder is damaged
and cannot be repaired, the lift cylinder must be re-
placed.
LOWERING CONTROL VALVE
A lowering control valve is installed in the inlet port.
See Figure 5. It permits easy entry of hydraulic oil
into the cylinder, but gives a restriction when the rod
is retracted. This restriction gives limits to the speed
that a load on the forks can be lowered.
There are two types of lowering control valves. Both
types have the same function and similar operation.
The most common type of valve has a design that
has good operation using large flows and lower pres-
sures. Some lift trucks have a lift system design us-
ing higher pressures and a lower flow. These systems
need a different type of lowering control valve.
The most common type of lowering control valve uses
these parts: (1) a special fitting for the valve body, (2)
a spring, and (3) a special washer. There is a varia-
tion in the shapes of the special washer. Different
lift cylinders use different shapes of special washers.
The volume of the hydraulic oil flowing through the
inlet port controls the shape of the special washer.
(A typical shape of a special washer is shown in Fig-
ure 5.) When the piston rod is lowered, the oil flow
pushes against the special washer and spring. When
the oil flow reaches the limit, the special washer is
moved against the special fitting. The oil then flows
only through the hole in the center of the special
washer. This restriction permits the piston rod to
lower only at a maximum controlled speed.
The other type of lowering control valve uses these
basic parts: (1) valve body, (2) spring, (3) washer, (4)
cylinder, (5) orifice sleeve, (6) plunger, and (7) main
sleeve. The orifice sleeve position is controlled com-
pletely by oil flow. The plunger position is controlled
by oil pressure and spring tension. During lifting,
oil entering the lift cylinder goes through the center
of the main sleeve to the large holes. The oil flow
through the plunger and bore moves the orifice sleeve
to the end of the plunger. The flange of the orifice
sleeve is then aligned with the large part of the bore
in the body. This alignment lets the oil flow past the
orifice sleeve to the cylinder.
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