A SEPARATE UNITS B INTEGRAL SEQUENCE

AND CHECK Directional valve Sequence valve To primary cylinder No-flow direction Free-flow direction To secondary cylinder Check valve Pump Relief valve Component enclosure

Figure 6-12. Check-valve symbol

Plugged port

To directional valve

Enclosure

Counterbalance and check valve

takes over. If the lower envelope were superimposed on the top envelope, the symbol would show that the flow path's arrow connects the pump outlet to the reservoir.

(2) Ordinary Four-Way Valve (Figure 6-17, page 6-10). If this valve is a two-position valve, the symbol will have two envelopes. If the valve has a center position, the symbol will have three envelopes. The actuating-control symbols are placed at the ends of the envelopes. The extreme envelopes show the flow conditions when their adjacent controls are actuated.

(3) Mobile Directional-Valve Section (Figure 6-18, page 6-10). The symbol for this valve section resembles a four-way-valve symbol; however, it has added connections and flow paths to represent the bypass passage. There is a separate envelope for each finite position, and connections are shown to the center or neutral position. The symbol shows a manual lever control with centering springs at

each end.

i. Accessories. The symbol for a fluid conditioner is a square (Figure 6-19, page 6-11) that is turned 45 degrees and has the port connections to the corners. A dotted line at right angles to the port connections indicates that the condi- tioner is a filter or strainer. A cooler symbol has a solid line at a right angle to the fluid line with energy triangles (indi- cating heat) pointing out. An accumula- tor (Figure 6-20, page 6-11) symbol is an oval, with added inside details to indi- cate spring load, gas charge, or other fea- tures. Reduced-pressure outlet Figure 6-14. Pressure-reducing-valve symbol Nonadjustable Adjustable

Figure 6-15. Flow-control-valve symbol

From pump

To pilot-pressure source

Two-position, controlled by external pilot pressure

Two-position, controlled by solenoids

Three-position, spring-centered, closed-center controlled by soleniod

with internal pilot pressure Solenoid-

control symbol

Solenoid control with internal pilot

pressure A B

P T

Figure 6-17. Four-way, directional-control-valve symbol

Manual control _{Check valve in} pressure line Float detent Spring centered By-pass passage View A Double-acting D-spool View C Floating C-spool View B Motor B-spool View D Single-acting T-spool

6-3. Typical Mobile Circuits. Hydraulic-lift, power-steering, and road-patrol-truck cir- cuits are considered typical mobile circuits.

a. Hydraulic-Lift Circuit. Figure 6-21 shows the lift portion of the hydraulic system. The circuit has two cylinders: a single-acting lift cylinder and a double-acting tilt cylinder. The lift cylinder moves the lifting fork up and down. The tilt cylinder tilts the mast back and forth to support or dump the load.

A two-section, multiple-unit directional valve controls the cylinder's operation. The first valve has a double-acting D-spool to operate the tilt cylinder, hydraulically, in either direc- tion. The outer envelopes show the typical four flow paths for reversing the cylinder. The second valve has a single-acting T-spool to operate the lift cylinder. This cylinder is returned by gravity;

the bypass unloads the pump.

The pump is driven by the lift truck's engine and supplies the circuit from the large vol- ume end. The enclo- sure around the two pump symbols indi- cates that both pumping units are contained in a single assembly. The same applies to the two directional valves and the relief valve that are enclosed. They are in a single assembly.

Filter or strainer

Figure 6-19. Fluid-conditioner symbols

Spring loaded Gas charged

Figure 6-20. Accumulator symbol

Lift cylinder T-spool section D-spool section To steering circuit Tilt cylinder

Figure 6-21 shows the circuit in neutral; the valves are centered. If the figure were to show the operating mode, the outer envelopes on the valve symbols would be shifted over to align with the ports at the center envelopes. The arrows in the envelopes would then show the flow paths from the pressure inlet to the cylinders and/or the return flow to tank.

b. Power-Steering Circuits. Hydraulic power steering incorporates a hydraulic boost into a basic manual-steering system. A basic manual-steering system is an arrangement of gears in a box that multiplies the input torque from the steering wheel to a much greater torque at the steering shaft (Figure 6-22). The steering shaft, through the pitman arm (or steering-shaft arm), transmits this increased torque through the steering linkage to the steering arms that turn the wheels. The basic system of manual-steering gears and steering linkage is a steering wheel, steering gear, and linkage to the steered wheel.

The hydraulic boost, which is a mechani- cally operated hydraulic servo, may be applied to the steering linkage (Figure 6-23) or within the steering gear. Steering-wheel movement actuates the steering valve, which directs the fluid under pressure to the steering-valve body that follows the valve spool. Hydraulic boost is applied only when the steering wheel is being moved.

An integral power-steering system has the hydraulic-boost subsystem built into the mechanical steering gear. The steering valve is actuated by moving the steering shaft. The valve controls the operation of the power cylin- der. Thrust from the power cylinder is trans- mitted directly to the steering shaft. Road shock transmitted back from the wheels is taken up in the steering gear.

Figure 6-24, page 6-13, shows the semi- integral power-steering system, or valve-on- gear system. The steering valve is built into the steering gear. The power cylinder is attached to the vehicle's frame and to the link- age. Road shock and thrust are absorbed in the frame.

c. Road-Patrol-Truck Circuits. Figure 6-25, page 6-14, diagrams A and B respectively, shows a road-patrol truck's hydraulic system and a hydraulic circuit's schematic, as a com- parison. The truck needs three double-acting

Wheel

Wheel pivot

(king pin or ball studs) Steering arm Linkage Pitman arm Steering shaft Steering gear Steering wheel Figure 6-22. Manual-steering-gear layout

Integral steering unit

Pitman arm C

A D

B

cylinders to operate its blades and dump body: a plow hoist cylinder for the front plow, an underblade cylinder, and a dump-body hoist cylinder. The truck also has a power-steering system operated from one-half of the double pump. (The steering system has been omitted from diagram B). The schematic shows that the three cylinders are operated through a three-spool, mobile directional valve fed from the large volume end of the double pump. 6-4. Troubleshooting. Personnel should follow a system when troubleshooting. The fol- lowing shows the STOP system:

• Study the circuit diagrams. • Test by using a reliable tester.

• Organize the knowledge gained from the circuit-test results. • Perform repairs, taking time to do the job well.

a. Causes of Improper Operations. If improper operation does occur, the cause can gen- erally be traced to one of the following:

• Use of the wrong oil viscosity or type. • Insufficient fluid in the system. • Presence of air in the system.

• Mechanical damage or structural failure. • Internal or external leakage.

• Dirt, decomposed packing, water, sludge, rust, and other foreign matter in the system.

• Improper adjustments.

• Heat exchanger that is plugged, dirty, or leaking.

b. Testing a Hydraulic Circuit. To test complete or individual parts of a hydraulic cir- cuit, use a hydraulic circuit tester (see para-

graph 2-8, page 2-18). The best tester to use is a compact portable unit that can check flow, pressure, and temperature.

c. Comparing Test Results with Specifica-

tions. Hydraulic-powered systems are power-

transmission systems. The only purpose of the components and the circuit is the controlled transfer of power from the motor shaft to the point of effective work.

where— HP = hydraulic horsepower f = flow, in GPM p = pressure, in psi HP fp 1 714, --- = Steering column

Steering valve Steering gear

Figure 6-24. Semi-integral power- steering system

By measuring those two factors at the same time, it is possible to read the effective out- put at any point. Comparing test results with specifications will give the necessary fault- finding facts.

d. Slippage. All hydraulic systems have some slippage (see paragraph 3-4, page, page 3-3) even when new. As wear increases, slippage at wear points increases, causing a decrease in GPM. However, system pressure is maintained. In time, wear can be so great that all flow is lost. Only at a complete breakdown will a pressure gauge show where the trouble is. Conducting a flow, pressure, and temperature (FPT) test would have indicated such a problem and avoided a complete breakdown.

NOTE: At low oil temperature and low pressure (or light loads) the machine will continue to operate but at less speed.

e. Flow and Pressure. Always test flow and pressure together. Connect a hydraulic tester into the hydraulic circuit at various points to isolate and check components (pumps, valves, or cylinders) for efficiency. Figure 6-26 shows a hydraulic tester, connected to the pump's output, checking the flow at various pressures that, in turn, checks the pump's per- formance against the recommended specification. When isolating and testing individual components with a hydraulic tester, direct the return fluid to the reservoir. If the fluid returns to the reservoir through the system's piping, you will not get a correct reading because of buildup of back pressure.

Test the whole circuit as described, and then isolate por- tions and test for a complete analysis of the system. If a test on a full circuit indicates a mal- function, isolate a portion and test the remain- ing portions until you find the malfunction- ing part. Gener- ally, cylinders will fail first. Packing will

wear because of friction and loading against the cylinder walls. Therefore, isolate the cylin- ders first. If test results indicate that the circuit is operating properly, the cylinders have a problem. During testing, determine the setting and condition of the relief valve. If further tests are necessary, isolate the directional-control valve to check the pump's efficiency and inlet hose.

f. Other Conditions. Other problems could occur that are not directly related to nor caused by the various parts of the hydraulic system. These problems could show the same