247B and 257B Multi Terrain Loaders Machine Systems Caterpillar

Illustration 1	g01057768
247 Multi Terrain Loader hydraulic schematic (1) Control Valve (work tool) (2) Accumulator (3) Tilt Cylinder Group (4) Gear Motor (fan drive) (5) Gear Motor (air conditioner) (6) Hydraulic Tank Group (7) Lift Cylinder (8) Hydraulic oil filter (9) Return Manifold Group (10) Gear Pump (11) Ball Valve Group (12) Piston Pump (hydrostatic system) (13) Relief Valve (Charge oil) (14) Solenoid Valve and Mounting Group (parking brake) (15) Speed Sensing Valve Group (16) Pilot Valve (work tool) (17) Solenoid Valve Group (hydrostatic system) (18) Solenoid Valve Group (work tool) (19) Pressure Switch (20) Pilot Valve (hydrostatic system)

Illustration 2	g01057772
257 Multi Terrain Loader hydraulic schematic (1) Control Valve (work tool) (2) Accumulator (3) Tilt Cylinder Group (4) Gear Motor (fan drive) (5) Gear Motor (air conditioner) (6) Hydraulic Tank Group (7) Lift Cylinder (8) Hydraulic oil filter (9) Return Manifold Group (10) Gear Pump (charge oil, implement, air conditioner and fan drive) (11) Ball Valve Group (12) Piston Pump (hydrostatic system) (13) Relief Valve (Charge oil) (14) Solenoid Valve and Mounting Group (parking brake) (15) Speed Sensing Valve Group (16) Pilot Valve (work tool) (17) Solenoid Valve Group (hydrostatic system) (18) Solenoid Valve Group (work tool) (19) Pressure Switch (20) Pilot Valve (hydrostatic system)

The pilot hydraulic system is used to move the spools in the Control Valve (work tool) (1). The pilot hydraulic system is also used to control the operation of the hydrostatic system. The pilot system uses lower pressure oil to control higher pressure oil.

In order for the pilot system to function, the interlocks must be disengaged. The interlock system consists of the following components: the Solenoid Valve Group (hydrostatic system) (17), the Solenoid Valve Group (work tool) (18) and the Solenoid Valve and Mounting Group (parking brake) (14). In order for the interlock system to be disengaged, the following conditions must be met:

• The seat must be occupied.

• The armrest must be in the DOWN position.

• The parking brake must be disengaged.

The second section of the Gear Pump (10) provides oil for the pilot oil circuits. The Relief Valve (Charge oil) (13) regulates the pressure of the charge system. The charge oil pressure is 3300 ± 200 kPa (480 ± 30 psi) at engine HIGH idle. The engine HIGH idle speed is 3240 ± 50 rpm.

When the interlock system is disengaged, oil flows from the hydraulic tank (6) to the second section of the Gear Pump (10). From the gear pump, the oil flows to the Gear Motor (fan drive) (4). After the gear motor, oil flows through the hydraulic oil filter (8). Then, the oil flows to the Piston Pump (hydrostatic system) (12) .

At the Piston Pump (hydrostatic system), the pilot system oil for the hydrostatic system is regulated by the Speed Sensing Valve Group (15). The pilot system oil that flows to the Pilot Valve (hydrostatic system) (20) is regulated at a pressure of 2550 ± 100 kPa (370 ± 15 psi) with the engine at HIGH idle. The pilot system oil that flows to the Pilot Valve (work tool) (16) is regulated by the charge relief valve (13) .

The rest of the charge oil is makeup oil for the hydrostatic system. Any leakage in the hydrostatic system is replaced by the charge oil.

Pilot Valve (Hydrostatic System)

Illustration 3	g01055937
Hydraulic schematic for the pilot valve (hydrostatic system) (12) Piston Pump (hydrostatic system) (13) Relief Valve (Charge oil) (14) Solenoid Valve and Mounting Group (parking brake) (15) Speed Sensing Valve Group (17) Solenoid Valve Group (Hydrostatic System) (20) Pilot Valve (Hydrostatic System) (21) Resolver (22) Piston Motor Group (Hydrostatic Left Side) (23) Piston Motor Group (hydrostatic right side) (24) Actuator for the Piston Motor Group (Hydrostatic Left Side) (25) Actuator for the Piston Motor Group (Hydrostatic Right Side) (B) Ports to both piston motor groups (X3) Port for actuator (24) actuator (25) (X4) Port for actuator (24) and actuator (25)

Oil flows from the speed sensing valve group (15) to the pilot valve (hydrostatic system) (20). The load of the hydrostatic system determines the pressure of the pilot oil that is maintained by speed sensing valve (15). The rpm of the engine determines the pressure of the pilot oil that is maintained by the Speed Sensing Valve Group.

Illustration 4	g01055928
Schematic for the Pilot Valve (hydrostatic system) (17) Solenoid Valve Group (Hydrostatic System) (20) Pilot Valve (Hydrostatic System) (21) Resolver network (26) Plunger for the Forward travel (A) Return to tank port (B) Pump supply port (C) Line to "X3" Pilot pressure port (hydrostatic right side) (7) (D) Line to "X4" Pilot pressure port (hydrostatic left side) (18) (E) Line to "X4" Pilot pressure port (hydrostatic right side) (8) (F) Line to "X3" Pilot pressure port (hydrostatic left side) (22)

When the interlock system is disengaged, an electrical signal is sent to the solenoid valve group (17). The energized solenoid causes the valve spool to shift upward. With the valve spool in this position, oil from speed sensing valve (15) that is in line (B) is now able to flow through the solenoid valve (17). The oil then flows to the plungers for pilot valve (20) .

The pilot oil will be blocked at the plungers if the control lever for the hydrostatic system is in the NEUTRAL position. Because the pilot oil is blocked at the plungers the oil will flow through pilot valve (20) back to the hydraulic tank through line (A). Centering springs cause the travel control lever to return to the HOLD position whenever the travel control lever is released. Table 1 shows the direction of machine travel when the pilot ports are pressurized by the Pilot Valve (hydrostatic system).

When the travel control lever is moved to the FORWARD travel position, pilot oil flows from speed sensing valve (15) through the energized solenoid valve (17). The oil then flows to the plungers for the pilot valve (20). Because the travel control lever is in the FORWARD travel position, the oil now flows through the plunger for the FORWARD travel (26). The oil then flows into the resolver network (21) .

The oil pressure causes the two resolver balls on the right side of the resolver network to move to the left. When the resolver balls move to the left, two paths open that allows the pilot oil to flow into the line (D) and the line (E) .

Pilot oil that is in line (D) will flow into port (X4) in order to shift actuator (24). The pilot oil that flows into port (X4) causes the piston in actuator (24) to shift left. At the same time, pilot oil that is in the line (E) will flow into port (X4) in order to shift actuator (25). The pilot oil that flows into port (X4) causes the piston in actuator (25) to shift right. The movement of the actuators causes the swashplates to move. This will cause the pump to upstroke. When the pump upstrokes, the flow of oil from port (B) in illustration 3 will increase.

Oil from both ports (B) will flow to piston motor (22) and to piston motor (23) in illustration 3. The flow of oil from the pump will be restricted when the oil reaches the piston motors. The restriction to the flow of oil will cause the pressure of the oil to rise. The rise in the pressure of the oil will cause the piston motors to start to rotate. This will propel the machine in the forward direction.

When the operator releases the travel control lever, the travel control lever returns to the NEUTRAL position. With the travel control lever in the NEUTRAL position, the plunger for the FORWARD travel (26) will move to the left. When the plunger for the FORWARD travel moves to the left the plunger blocks the flow of pilot oil. The plunger for the FORWARD travel (26) will now open a path for the oil in the line (D) and the line (E) to flow back to the hydraulic tank.

Since the pilot oil that is acting against the actuator piston (24) and the actuator piston (25) is now routed back to the hydraulic tank, the centering springs for the actuator pistons will force the actuator pistons back to the NEUTRAL position. When the actuator pistons move to the NEUTRAL position the swashplates move back to the minimum displacement position. When the swashplates move to the minimum displacement position the pumps will be at low pressure standby. Since the flow of oil has stopped to the Piston Motor Groups, the Piston Motor Groups will cease to rotate. The machine will come to a stop.

Refer to the Systems Operation, "Piston Pump (Hydrostatic)" for more information on the piston pump. Refer to the Systems Operation, "Piston Motor (Hydrostatic)" for more information on the piston motor.

Pilot Valve (Work Tool)

Illustration 5	g01057778
247 Multi Terrain Loader hydraulic schematic (1) Quick disconnects for the auxiliary circuit (2) Control Valve (work tool) (3) Accumulator (4) Tilt Cylinder Group (5) Hydraulic Tank Group (6) Lift Cylinder (7) Return Manifold Group (8) Gear Pump (charge oil, implement, air conditioner and fan drive) (9) Ball Valve Group (11) Solenoid Valve and Mounting Group (parking brake) (12) Check Valve (10) Piston Pump (hydrostatic system) (13) Relief Valve (Charge oil) (14) Pilot Valve (work tool) (15) Solenoid Valve Group (work tool)

Illustration 6	g01057839
257 Multi Terrain Loader hydraulic schematic (1) Quick disconnects for the auxiliary circuit (2) Control Valve (work tool) (3) Accumulator (4) Tilt Cylinder Group (5) Hydraulic Tank Group (6) Lift Cylinder (7) Return Manifold Group (8) Gear Pump (9) Ball Valve Group (11) Solenoid Valve and Mounting Group (parking brake) (12) Check Valve (10) Piston Pump (hydrostatic system) (13) Relief Valve (Charge oil) (14) Pilot Valve (work tool) (15) Solenoid Valve Group (work tool) (16) Quick disconnects for the high flow auxiliary circuit

The pilot oil for the work tool is supplied by the second section of gear pump (8). The pilot oil for the work tool is controlled by relief valve (13). Relief valve (13) is part of piston pump (12). Relief valve (13) maintains the pressure for the pilot oil at 3300 ± 200 kPa (480 ± 30 psi) at engine HIGH idle. The engine HIGH idle speed is 3240 ± 50 rpm.

Illustration 7	g01056082
Locations of the Accumulator and Check Valve (2) Control Valve (work tool) (3) Accumulator (11) Solenoid Valve and Mounting Group (parking brake) (12) Check Valve (14) Pilot Valve (work tool)

The oil that flows through the check valve (12) is used as pilot oil for the work tool circuits. The check valve is installed in a fitting that is installed to the bottom of piston pump (10). The pilot oil overcomes the force of the spring in the check valve. The pilot oil then forces the check valve to open. A pressure of 35 kPa (5 psi) is required to open the check valve. When the check valve opens the pilot oil flows through the check valve. The oil then flows to the control valve (1). The oil also flows to the pilot valve (14) .

The pilot oil that flows to control valve (2) is also used to activate the auxiliary circuit spool. The flow of the pilot oil is controlled by two solenoid valves. The pilot oil is also used to charge the accumulator (3). The pilot oil that flows to pilot valve (14) is used to activate the lift spool and the tilt spool.

The interlock system must be disengaged in order for the pilot oil to flow to the work tool circuit. The auxiliary circuit can function with the armrest in the UP position, if the machine is equipped with an interlock override. Refer to the Operation and Maintenance Manual, SEBU7732, "Hydraulic Shutoff and Interlock Override" for more information.

When the interlock system is disengaged, an electrical signal is sent to the Solenoid Valve Group (work tool) (15). The energized solenoid causes the valve spool to shift upward. The pilot oil is able to flow through the valve spool and on to the plungers for pilot valve (14) when the valve spool is in the upward position.

The pilot oil will be blocked at the plungers so long as the control lever for the work tool is in the HOLD position. When the control lever is in the HOLD position, the oil will not flow through pilot valve (14). Centering springs cause the control lever for the work tool to return to the HOLD position whenever the control lever for the work tool is released from any of the positions except for the FLOAT position. The FLOAT position has a detent that keeps the control lever for the work tool in this position until the operator moves the control lever for the work tool into a different position.

Illustration 8	g01056083
Schematic for the Pilot Valve (work tool) (14) Pilot Valve (work tool) (15) Solenoid Valve Group (work tool) (17) Plunger for work tool TILT BACKWARD (A) Line to work tool RAISE (B) Line to work tool LOWER (C) Line to work tool TILT BACKWARD (D) Return to tank port (E) Pump supply port (F) Line to work tool TILT FORWARD

For the following description, refer to illustration 5, illustration 6, and illustration 8.

When the control lever for the work tool is moved to the TILT BACKWARD position, pilot oil flows from the check valve (12) through the energized solenoid valve group (15). The oil then flows to the plungers for the pilot valve (14) .

Because the control lever for the work tool is in the TILT BACKWARD position, the oil now flows through the plunger for the TILT BACKWARD position (17). Then, the pilot oil flows to the right side of control valve (2) in order to shift the tilt spool to the TILT BACKWARD position. The pilot oil overcomes the force of the centering springs for the tilt spool. Then, the pilot oil forces the tilt spool to move to the left.

When the tilt spool moves to the left a path is opened for the implement oil to flow to the tilt cylinders. The implement oil for the work tool is supplied by the second section of gear pump (8). The implement oil flows to the tilt spool. Oil flows from the tilt spool to the rod ends of tilt cylinders (4). The oil pressure causes the tilt cylinders to retract in order to tilt the work tool backward.

When the tilt spool moves to the left a path is also opened for the implement oil to flow from the tilt cylinders back to the hydraulic tank. As the tilt cylinders retract the oil that is in the head end of the tilt cylinders is forced out of the cylinders. The implement oil that is in the head ends of the tilt cylinders flows into the port for the head ends of the tilt cylinders that is in control valve (2). The oil then returns to the hydraulic tank.

When the operator releases the control lever for the work tool, the control lever for the work tool will move back to the HOLD position. The plunger (17) now moves to the right. This shuts off the flow of pilot oil. Since the flow of pilot oil is blocked to the tilt spool, the centering springs for the tilt spool will move the tilt spool back to the HOLD position. Since the tilt spool is in the HOLD the tilt cylinders will stop retracting.

Refer to the Systems Operation, "Control Valve (Work Tool)" for more information on the control valve.

Accumulator

The accumulator (3) is mounted on control valve (2). The accumulator provides a limited amount of pilot oil after the engine is shut OFF. The oil capacity of the accumulator is 0.28 L (0.30 qt). The accumulator is charged to 900 ± 100 kPa (130 ± 15 psi) with 40 cc (2.44 in³) of dry nitrogen.