450E Backhoe Loader Hydraulic and Steering System Caterpillar

Illustration 1	g01281601
(1) Torque control (2) Flow compensator (3) Torque limiter (4) Solenoid for torque control (5) Pump controller (6) Pump

The piston pump (6) for the hydraulic system is controlled by the load signal from the implements. The piston pump (6) senses both pressure and flow needs. The piston pump (6) also senses hydraulic power.

If one or more circuits are being used, the single highest pressure in the system flows to flow compensator (2). The flow compensator (2) keeps the pump output at a level that is needed in order to fulfill the flow and pressure requirements of the system.

The pump outlet pressure is maintained about 2150 kPa to 2450 kPa (312 psi to 355 psi) above the needed pressure of the work port by the flow compensator (2) .

The system pressure will be greater than the requirements for the highest work port pressure unless the pump is at full stroke. Margin pressure is the difference between the pressure that is needed at the work port and the supply pressure that is higher.

Illustration 2	g01293615
(1) Torque control (2) Flow compensator (3) Torque limiter (5) Pump controller (6) Pump (7) Bias piston (8) Spring (9) Swashplate (10) Actuator piston (12) Piston (intake) (13) Piston (20) Pump output

The pump has two actuators for control, Bias piston (7) and actuator piston (10). The spring (8) in the small actuator (7) causes swashplate (9) to move. The movement of the swashplate will cause the pump to upstroke or the movement will cause the pump to destroke.

Actuator piston (10) has a larger area than the bias piston (7). Actuator piston (10) causes the swashplate to destroke the pump. Flow compensator (2) and the torque control (1) change pump output by regulating the discharge pressure of the pump that is acting on actuator piston (10) .

When none of the hydraulic circuits are being used, the pump (6) is at low pressure standby.

Low Pressure Standby

Illustration 3	g01444806
(1) Torque control (1a) Spring (2) Flow compensator (2a) Spring (3) Torque limiter (4) Solenoid for torque control (5) Pump controller (6) Pump (7) Bias piston (8) Spring (9) Swashplate (10) Actuator piston (12) Piston (intake) (13) Piston (14) Outlet manifold for the backhoe (15) Relief valve for the margin pressure (flushing valve) (16) Input for load signal (17) Drain valve for the signal pressure (18) Relief valve for the signal pressure (19) Orifice (20) Pump output (21) Stop for minimum angle

The pump is at low pressure standby, when the implement controls are in the HOLD position and the steering is inactive.

With all the implements in the HOLD position, the steering priority valve allows some supply oil to flow through a orifice. Then, the oil flows back to the tank through the metering pump. This oil flow creates a load sensing signal, which is sensed at the flow compensator (2). The low pressure standby reading will be higher than margin pressure due to an internal stop for minimum angle (21). This stop for the minimum angle is not adjustable. The stop for the minimum angle (21) prevents the swashplate (9) from moving to a true minimum angle position. This causes the margin relief (15) to open sending the pump flow to the tank through the outlet manifold for the backhoe (14). Refer to Illustration 3. The swashplate (9) is at the minimum angle. This applies the maximum setting on the spring in the torque limiter (3). The spring setting of the torque limiter (3) changes with the swashplate angle. As the swashplate angle is increased, the spring setting is reduced.

Upstroke or Destroke

Illustration 4	g01281782
(1) Torque control (1a) Spring (2) Flow compensator (2a) Spring (3) Torque limiter (4) Solenoid for torque control (5) Pump controller (6) Pump (7) Bias piston (8) Spring (9) Swashplate (10) Actuator piston (12) Piston (intake) (13) Piston (14) Outlet manifold for the backhoe (15) Relief valve for the margin pressure (flushing valve) (16) Input for the load signal (17) Drain valve for the signal pressure (18) Relief valve for the signal pressure (19) Orifice (20) Pump output (21) Stop for the minimum angle

The pump will move to the UPSTROKE position or to the DESTROKE position in order to match changes in flow requirements.

Pump supply oil flows to the flow compensator (2) and the oil acts on the compensator spool. Supply oil also flows into a cross-drilled passage and an axial drilled passage in the flow compensator spool (2). The pump supply pressure moves the flow compensator (2) upward. This opens a small passage between the spool and the body of the pump control valve. The flow compensator spool (2) meters reduced pump supply oil to the actuator piston (10) in the pump case. This moves the swashplate (9) to an angle that satisfies the needs of the system.

When pump flow matches needs of the system, the pump supply pressure equals the sum of the system pressure plus the pressure of the spring (2A). The flow compensator is in a metering position and the system is stabilized.

Constant flow is maintained until there is a change in the requirements for system flow.

Metering

Illustration 5	g01281780
(1) Torque control (1a) Spring (2) Flow compensator (2a) Spring (3) Torque limiter (4) Solenoid for torque control (5) Pump controller (6) Pump (7) Bias piston (8) Spring (9) Swashplate (10) Actuator piston (12) Piston (intake) (13) Piston (14) Outlet manifold for the backhoe (15) Relief valve for the margin pressure (flushing valve) (16) Input for load signal (17) Drain valve for the signal pressure (18) Relief valve for the signal pressure (19) Orifice (20) Pump output (21) Stop for the minimum angle

In the metering position, the signal pressure and the force of spring (2A) equals the pump supply pressure below the flow compensator spool (2) .

The pump will maintain the flow that is required in order to maintain system pressure. The pump will stay in the metering position until the flow requirements change.

High Pressure Stall

Illustration 6	g01281727
(1) Torque control (1a) Spring (2) Flow compensator (2a) Spring (3) Torque limiter (4) Solenoid for the torque control (5) Pump controller (6) Pump (7) Bias piston (8) Spring (9) Swashplate (10) Actuator piston (12) Piston (intake) (13) Piston (14) Outlet manifold for the backhoe (15) Relief valve for the margin pressure (flushing valve) (16) Input for the load signal (17) Drain valve for the signal pressure (18) Relief valve for the signal pressure (19) Orifice (20) Pump output (21) Stop for the minimum angle

When the hydraulic system stalls under load or when the cylinders reach the end of the stroke, the system pressure increases.

When the discharge pressure rises to 23800 kPa to 25800 kPa (3452 psi to 3742 psi) the relief valve (18) opens. This sends the signal oil to the tank and this limits the signal pressure. The pump sends out flow in order to maintain the margin over the signal pressure. This limits the pump pressure.

During a high pressure stall, movement of the control lever into the HOLD position causes the pump to return to the low pressure standby.

Torque Limiter

Illustration 7	g01293630
Torque limiter on the pump control valve

The torque limiter (3) maintains the displacement of the pump in inverse proportion to the discharge pressure of the pump when the torque setting of the torque limiter is reached. When the hydraulic demand reaches a set value, the torque limiter (3) will destroke the pump. If the pump does not destroke, the engine will lug. This reduces engine speed and the engine could eventually stall.

Illustration 8	g01294644
(EE) Axis for pump flow (FF) Axis for pump output pressure (A) Control of pump by torque limiter with solenoid energized. (B) Maximum pump flow (C) Maximum pump output pressure (D) Control of pump by torque limiter with solenoid de-energized.

Illustration 4 shows the effect of the torque limiter (3) on the maximum flow rate for the machine with the torque limiter open. The initial flow on the charts is low. This reflects the standby pressure from the pump. When oil flow is required, the pump increases the flow of oil. The flow of oil decreases as the pressure in the system increases.

The flow compensator spool (2) regulates pump flow when the system pressure and the flow requirements are below the curves in the upper right corner.

Line (D) indicates the position of the torque limiter's control over the pump. The torque limiter will destroke the pump as the system pressure increases. The rate of destroke will parallel the torque curve of the engine.

Line (A) indicates when the solenoid for the torque control has been energized. The pump provides more oil flow to the hydraulic system.

Parameters for the Solenoid for Torque Control with the Direction Selector (Loader)

Table 1 lists the operating parameters for the solenoid for torque control. When the solenoid is energized, the pump is in the high torque mode. When the solenoid is de-energized, the pump is in the low torque mode.

When the direction selector is in the FORWARD or REVERSE, the new solenoid for torque control (4) will not be energized until a certain engine rpm is obtained. The solenoid for torque control (4) is not energized until the engine reaches a higher engine speed. Then, all of the power from the engine can be utilized by the power train.

When you operate the machine, the power train system and the hydraulic system will utilize the engine horsepower.

When the solenoid for torque control (4) is energized or de-energized due to the engine rpm, there is a slight delay in operation. The delay occurs when you change the direction selector. The delay will help stabilize the engine and the hydraulic system.

Parameters for the Solenoid for Torque Control without the Direction Selector (Backhoe Operation)

Table 2 lists the operating parameters for the new solenoid for torque control (4) without the use of the direction selector. When the solenoid is energized, the pump is in the high torque mode. When the solenoid is de-energized, the pump is in the low torque mode.

When the direction selector is in the NEUTRAL position, the new solenoid for torque control (4) will not be energized until a certain engine rpm is obtained.

When the direction selector is in the NEUTRAL position, the solenoid for torque control (4) is de-energized in order to reduce the hydraulic demand on the engine. The solenoid for torque control (4) will also prevent the engine from a stall.

When the solenoid for torque control (4) is energized or de-energized due to the engine rpm, there is a slight delay in operation. The delay will help stabilize the engine and the hydraulic system.

Torque Limiter in the OPEN position

Illustration 9	g01293626
(1) Torque control (1a) Spring (2) Flow compensator (2a) Spring (3) Torque limiter (4) Solenoid for torque control (5) Pump controller (6) Pump (7) Bias piston (8) Spring (9) Swashplate (10) Actuator piston (12) Piston (intake) (13) Piston (14) Outlet manifold for the backhoe (15) Relief valve for the margin pressure (flushing valve) (16) Input for the load signal (17) Drain valve for the signal pressure (18) Relief valve for the signal pressure (19) Orifice (20) Pump output (21) Stop for the minimum angle (22) Relay (23) ECM

The torque limiter (3) monitors the pump swashplate angle and the pump discharge pressure. The pressure setting of the torque limiter (3) varies with the swashplate angle due to the mechanical linkage between the swashplate (9) and the torque limiter (3). As the swashplate angle decreases, the mechanical linkage increases the force on the springs of the torque limiter (3). This increases the pressure setting of the torque limiter (3) .

When a higher flow rate is required, the flow compensator (2) receives a signal from a valve. The pump moves to maximum displacement. The pressure in the line between the torque control (1) and the torque limiter (3) increases. The torque limiter opens. Oil drains to the tank.

As the oil drains to the tank, a pressure drop occurs across orifice (19). This lowers the signal pressure in the line to the torque limiter (3). The discharge oil of the pump that is below the torque control causes the torque control (1) to move up slightly. When the spool for torque control (1) moves up, the spool opens a path for the supply oil. This allows the supply oil to flow to actuator (10) .

The supply oil acts against the actuator (10). The actuator piston moves to the right. This destrokes the pump. Destroking the pump reduces the output of the pump in order to prevent the engine from stalling.

The Solenoid for Torque Control is Energized and the Torque Limiter is Open

Illustration 10	g01293628
(1) Torque control (1a) Spring (2) Flow compensator (2a) Spring (3) Torque limiter (4) Solenoid for torque control (5) Pump controller (6) Pump (7) Bias piston (8) Spring (9) Swashplate (10) Actuator piston (12) Piston (intake) (13) Piston (14) Outlet manifold for the backhoe (15) Relief valve for the margin pressure (flushing valve) (16) Input for the load signal (17) Drain valve for the signal pressure (18) Relief valve for the signal pressure (19) Orifice (20) Pump output (21) Stop for the minimum angle (22) Relay (23) ECM

When the correct engine speed for the machine operation has been obtained, the new solenoid for torque control is ENERGIZED. The flow from the pump increases.

Pump discharge oil flows through the solenoid for torque control (4) and a small orifice. This increases the pressure on the top of the spool for torque control (1). The spool remains seated. At the same time, a higher work port pressure occurs. The higher work port pressure creates a higher signal pressure that keeps the flow compensator (2) in the down position. The position of the two spools will partially upstroke the pump in order to meet the higher flow demand and the demand of the system pressure.

Due to the mechanical linkage between the swashplate (9) and the torque limiter (3), the force on the spring for the torque limiter increases. The higher force on the spring for the swashplate (9) will destroke the pump at a higher pressure.

Full pump pressure is always available. Flow from the pump is regulated in order to maintain the maximum input torque from the engine.