Page 604 - Mechatronics with Experiments

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FIGURE 7.117: Hydraulic system circuit diagram of the Caterpillar wheel loader model 950G.

actuated pilot valves instead of EH pilot valves. Proportional flow control valves for mobile
equipment are typically fabricated in stackable blocks, that is a valve stack may have up to
ten valve sections for ten different functions (Figure 7.118).
In some cases, the pilot and main flow sections of the valves are designed as separate
blocks. The advantage of this is that the main section of the valve accepts the pilot control
pressure. The pilot control pressure may be controlled either by a mechanically actuated
pilot valve or solenoid actuated EH valve. Hence, the main flow section of the valve can be
used with either mechanically actuated pilot valves or electrically (solenoid) actuated pilot
valves. It is instructive to note that the main flow control valve for multi function hydraulic
circuits, such as the the lift and tilt circuit in Figure 7.52 (component V on the figure), is
manufactured as a single valve block with all the individual valve functions incorporated
into one block design that has the external port connections (pump, tank, service ports A
and B for lift and tilt, and pilot pressure control ports, Figure 7.119).
Notice that the difference between mechanical lever controlled and electrical solenoid
controlled pressure reducing pilot control valves is the source of actuation power to shift
the spool of the pilot valve (see Figure 7.66). In a mechanical lever controlled system,
as the operator moves the lever, the lever motion moves a spring in the pilot valve which
then moves the pilot valve spool and changes the output pilot control pressure. In solenoid
controlled pilot valves, the lever displacement is measured by a sensor, the electronic control