CHAPTER 6

Circuit Diagrams and Troubleshooting

Hydraulic-circuit diagrams are complete drawings of a hydraulic circuit. Included in the diagrams is a description, a sequence of operations, notes, and a components list. Accurate diagrams are essential to the designer, the people who build the machine, and the person who repairs it. Hydraulic mechanisms are precision units, and their continued smooth operation depends on frequent inspection and servicing. Personnel must maintain the equipment and system by performing frequent inspections and servicing. The systems must be kept clean, with the oil and filters changed at established intervals.

6-1. Hydraulic-Circuit Diagrams. The four types of hydraulic-circuit diagrams are block, cutaway, pictorial, and graphical. These diagrams show the-

• • • Components and how they will interact.
    • Manufacturing engineer and assembler how to connect the components.
    • Field technician how the system works, what each component should be doing, and where the oil should be going so that the technician can diagnose and repair the system.

  a. Block Diagram. A block diagram shows the components with lines between the clocks, which indicate connections and/or interactions.

  b. Cutaway Diagram. A cutaway diagram shows the internal construction of the components as well as the flow paths. Because the diagram uses colors, shades, or various patterns in the lines and passages, it can show the many different flow and pressure conditions.

  c. Pictorial Diagram. A pictorial diagram shows a circuit's piping arrangement. The components are seen externally and are usually in a close reproduction of their actual shapes and sizes.

  d. Graphical Diagram. A graphical diagram (Figure 6-1), the short-hand system of the industry, is usually preferred for design and troubleshooting. Simple geometric symbols represent the components and their controls and connections.

  

6-2. United States of American Standards Institute (USASI) Graphical Symbols. The USASI, the old American Standards Association (ASA), and the Joint Industry Conference (JIC) are three systems of symbols used in circuit diagrams. This manual uses the USASI symbols shown in Figure 6-2.

a. Reservoir. The symbol for a reservoir is a rectangle; the horizontal side is the longest side (see Figure 6-3). If a reservoir is vented to the atmosphere, the top of the symbol is open. If a reservoir is pressurized, the top is closed. Lines that connect to a reservoir usually are drawn from the top, regardless of where they connect. If the line terminates below the fluid level, it is drawn to the bottom of the symbol. A line connected to the bottom of a reservoir may be drawn from the bottom of the symbol, if the bottom connection is essential to the system's operation. For example, when the pump's inlet must be charged or flooded by a positive head of oil above the inlet's port, they would be positioned above the pump symbol and the suction line drawn out the bottom of the symbol. Every reservoir has at least two hydraulic lines connected to it; some have more. The reservoir is usually the only component pictured more than once so that components and return or drain lines to and from the reservoir are represented correctly.

b. Lines. Figure 6-4 shows the symbols for hydraulic lines, which are as follows:

• Working line: A solid line that represents a hydraulic pipe, tube, hose, or other conductor that carries the liquid between components.
• Pilot line: Long dashes that represent control lines.
• Drain line: Short dashes that represent the drain lines for leaking oil.
• Flexible line: A solid, arced line that is drawn between two dots which represents a flexible line in the system.

Figure 6-5, diagram A, shows crossed lines that are not connected. Systems 1 and 2 represent two ways to indicate an intersection, one with a loop, one without a loop. Diagram B shows lines that are connected. The lines in system 1 use a dot at the crossing, indicating that loops are used to designate the crossing. The lines in system 2 do not use a dot at the crossing, indicating that loops are not used at the crossing.

c. Pump. The basic symbol of a pump is a circle with a black triangle in the circle pointing outward (see Figure 6-6). The pressure line from the pump is drawn from the tip of the triangle; the suction line is drawn opposite it. The triangle indicates the flow direction. If a pump is reversible, it will have two triangles, one pointing out of each port. Port connections to the pump (or any other component except the reservoir) are at the points where the lines touch the symbols. A variable (or adjustable) component is designated by an arrow drawn through the components at a 45-degree angle.

d. Motor. Motor symbols are circles with black triangles pointing inward, indicating that the motor receives pressure energy (see Figure 6-7). One triangle indicates a nonreversible motor; two triangles indicate a reversible motor. Flow direction in a single triangle is the way the triangle points. In the reversible motor, studying the pump and valve symbols is the way to trace the flow direction. The arrows that are outside the lines show the flow direction, which is always away from the pump's pressure port and into the motor port that is connected to the pressure line. The opposite port then discharges back to the tank.

e. Cylinder. The basic cylinder symbol is a simple rectangle (a barrel) and a T-shaped figure (a piston and a rod). The symbol can be drawn in any position. The following describes four different cylinder symbols (see Figure 6-8):

• Single-acting cylinder: One hydraulic line drawn to the basic cylinder symbol; the end opposite the port is open.
• Double-acting cylinder: Both ends of the symbol are closed; two lines meet the basic cylinder symbol at the port connections.
• Double-end rod cylinder: A rod line extends from each end of the basic cylinder symbol.
• Cushioned cylinder: Small rectangles are placed against the piston line. If the cushion has an adjustable orifice, a slanted arrow is drawn across the symbol. There is no symbol for flow direction, so lines must be watched to see where they are connected, which should help determine flow.

f. Pressure-Control Valves. The basic symbol is a square with external port connections and an arrow inside to show the flow direction (see Figure 6-9). This valve operates by balancing the pump outlet to the reservoir.

(1) Relief Valve (Figure 6-10). The relief valve's symbol goes between the pressure line and the tank. The flow-direction arrow points away from the pressure-line port and toward the tank port. When pressure in the system overcomes the valve spring, flow is from the pressure port to the tank port.

(2) Sequence Valve (Figure 6-11). A sequence valve uses the relief valve. However, the inlet port is connected to a primary cylinder line; the outlet port is connected to the secondary cylinder line. Pilot pressure from the primary cylinder line sequences the flow to the outlet port when it reaches the valve's setting. Since the sequence valve is externally drained, a drain connection is added to the symbol at the drain's location in the valve.

(3) Check Valve (Figure 6-12, page 6-8). A check valve uses a sequence valve for free return flow when the cylinders are reversed. In Figure 6-12, diagram A shows the valves as separate units. Diagram B shows the check valve built into the sequence valve. The box around the valves is an enclosure, which shows the limits of a component or an assembly that contains more than one component. The enclosure is an alternate long and short dashed line. External ports are assumed to be on the enclosure line and indicate connections to the components.

(4) Counterbalance Valve (Figure 6-13). A counterbalance valve is a normally closed pressure-control with an integral check valve. A directly controlled valve uses the same symbol as in Figure 6-13, with the primary port connected to the bottom port of the cylinder and the secondary port to the directional valve. The valve is drained internally, so the symbol shows no drain connection. If the valve body has two primary ports, the symbol should show one of them plugged.

(5) Pressure-Reducing Valve. Figure 6-14 shows the normally opened pressure-reducing valve. The symbol shows the outlet pressure opposite the spring to modulate or shut off the flow when the valve setting is reached.

g. Flow-Control Valves. Figure 6-15 shows the symbols for the basic flow-control, adjustable and nonadjustable valves. The figure also shows the symbol for a completely adjustable, pressure-compensated, flow-control valve with a built-in bypass.

h. Directional-Control Valves. A directional-control-valve symbol uses a multiple envelope system that has a separate rectangle for each position. All the port connections are made to the envelope, which shows the neutral condition of the valve. Arrows in each envelope show the flow paths when the valve shifts to that position.

(1) Unloading Valve (Figure 6-16). The symbol for this valve has two envelopes. In the normally closed position, flow is shown blocked inside the valve. The spring control is placed adjacent to this envelope, indicating that the spring controls this position. The external pilot pressure is placed against the bottom envelope, indicating the flow condition when the pilot pressure 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). 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). 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) 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 conditioner is a filter or strainer. A cooler symbol has a solid line at a right angle to the fluid line with energy triangles (indicating heat) pointing out. An accumulator (Figure 6-20) symbol is an oval, with added inside details to indicate spring load, gas charge, or other features.

6-3. Typical Mobile Circuits. Hydraulic-lift, power-steering, and road-patrol-truck circuits 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 direction. 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 volume end. The enclosure around the two pump symbols indicates 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.

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 mechanically 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 cylinder. Thrust from the power cylinder is transmitted directly to the steering shaft. Road shock transmitted back from the wheels is taken up in the steering gear.

Figure 6-24 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 linkage. Road shock and thrust are absorbed in the frame.

c. Road-Patrol-Truck Circuits. Figure 6-25 diagrams A and B respectively, shows a road-patrol truck's hydraulic system and a hydraulic circuit's schematic, as a comparison. The truck needs three double-acting 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 following 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 generally 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 circuit, use a hydraulic circuit tester (see paragraph 2-8). The best tester to use is a compact portable unit that can check flow, pressure, and temperature.

  c. Comparing Test Results with Specifications. 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

By measuring those two factors at the same time, it is possible to read the effective output 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, Chapter 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 performance 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 portions and test for a complete analysis of the system. If a test on a full circuit indicates a malfunction, isolate a portion and test the remaining portions until you find the malfunctioning part. Generally, cylinders will fail first. Packing will wear because of friction and loading against the cylinder walls. Therefore, isolate the cylinders 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 general malfunctions of an improperly operating system. Examples are leaking hose, packing glands, and seals, which would be visually evident; a bind in the directional-control valve or the cylinder's piston rod; a dented or deformed hydraulic cylinder; or a crimped or restricted pressure line, which would be harder to detect.

g. Specific Troubles, Causes, and Solutions. Tables 6-1 through 6-5, pages 6-17 through 6-21 list some possible problems and solutions in a hydraulic system.

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