The following procedures must be used when inspecting or working near the mast. Additional precautions and procedures can be required when repairing or removing the mast. See the correct Service Manual section for the specific mast being repaired. 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 Hyster Company lift trucks. The operation and repair procedures for the different lift cylinders are similar. 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.
All lift cylinders are single-acting cylinders. See Figure 4. The hydraulic force is applied only in one direction. 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 replaced.
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 pressures. Some lift trucks have a lift system design using 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 variation 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 Figure 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 completely 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. Hyster C176 Service Manual,Hyster C176,Hyster C176 User Manual.
Two-Speed Lift Cylinder
During lowering, oil from the lift cylinder moves the orifice sleeve. The orifice sleeve moves away from the larger inner diameter area of the bore in the valve body. This movement makes a restriction to the oil flow. As the pressure increases, the plunger begins to move against the spring. The movement begins to close the openings of the large holes in the main sleeve. Additional pressure will push the plunger against the main sleeve to close the large holes completely. All the oil must then go through the small holes to the center of the main sleeve. This restriction permits the piston rod to lower only at a maximum controlled speed.
CYLINDERS (GENERAL)
Standard masts use a single-stage lift cylinder. Most three-stage and four-stage masts use two single-stage lift cylinders. A single-stage lift cylinder has one piston rod. See Figure 6. Most full free-lift masts and some three-stage masts use a two-stage lift cylinder. A two-stage lift cylinder has two telescopic piston rods. See Figure 7. The two-stage lift cylinder operates similar to a one-stage lift cylinder. Hydraulic pressure pushes the primary (larger) piston rod from the cylinder first because it has a larger diameter. The secondary piston rod moves with the primary piston rod. When the primary piston rod reaches the limit of its stroke, the secondary piston rod extends from the primary piston rod.
CYLINDERS (H520-620B, H700-800A)
The rod side of the lift cylinder is used for the storage of hydraulic oil on the H520-620B and H700-800A lift trucks. This storage decreases the required size of the hydraulic tank. When the piston raises in the cylinder, the oil on the rod side of the piston transfers to the hydraulic tank. When the piston lowers in the cylinder, some of the oil transfers to the rod side of the cylinder. Approximately one half of the oil in the bottom of the cylinder goes through the main control valve to the rod side of the cylinder. See the Main Control Valve of Service Manual for more information. See Figure 9.