05 Steering Hydraulic LG958L

LG958L Training Material

Chapter VII Steering Hydraulic System Tuesday, May 19, 2015

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CONTENTS Overview of Steering System Load-Sensing Fully Hydraulic Steering System Working Principle for Hydraulic Units of Steering System Cause Analysis and Troubleshooting for Common Malfunctions of Steering System

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Section I Overview of Steering Hydraulic System I. Classification of steering system

The steering system of the wheel loader is functioned to control the traveling direction of the loader. It can stably maintain the linear traveling of the loader and can flexibly change the traveling direction as required. By steering mode, the wheel loader can be classified into deflection wheel steering mode, skid steering mode, and articulated steering mode.

For the loader with articulated steering mode, the working device is installed on the front frame. When the frame is swinging, the direction of the working device is always consistent with the direction of the front frame, which can enable the working device to rapidly aim at the working face during the operations, in order to reduce the travel and time of working cycles and improve the working efficiency of the loader. Therefore, the articulated steering mode has become the mostly applied steering mode in modern loaders.

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II. Requirements of steering system

The good steering performance is one important factor to guarantee the traveling safety of the loader, relieve the labor strength of driver, and improve the working efficiency. The basic requirements on the steering system include light and flexible operations, stable and reliable working, and good durability. The steering oil line requires relatively stable supply of oil flow. However, the steering system commonly adopts fixed displacement pump. As the flow of fixed displacement pump varies along with the speed, the flow of the steering oil line is reduced when the engine runs at low speed, which will cause delayed response of steering speed and easily lead to accidents. If the high flow pump is adopted, when the engine runs at high speed, the excessive oil flow will be relieved through the relief valve, which is not economical due to high power loss and easy heating of oil.

A relatively appropriate method is to choose auxiliary pump and flow changeover valve. The pressure oil of the auxiliary pump varies along with the engine speed under the control of the flow change-over valve to fully or partially flow into the steering oil line, in order to guarantee the flow of steering oil line. The remaining oil flows into the working oil line.

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III. Types of steering system

There are many types of steering system and the different types of steering system represent the development levels of different hydraulic technologies respectively. At present, the types of the steering system applied in the wheel loader are as below: ① Fully hydraulic steering system composed of single stabilizer valve and open center no-reaction steering gear.

② Load-sensing fully hydraulic steering system composed of priority valve and loading sensing steering gear.

③ Load-sensing fully hydraulic steering system composed of priority valve and coaxial flow amplifying steering gear. ④ Flow amplifying steering system.

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Section II Load-Sensing Fully Hydraulic Steering System The steering system of LG958L loader is a load-sensing fully hydraulic steering system composed of priority valve and coaxial flow amplifying steering gear. This system is mainly composed of steering pump, coaxial flow amplifying fully hydraulic steering gear, steering cylinder, hydraulic oil tank, and pipelines and accessories, as shown in Figure 6-1. This steering system features the following characteristics: 1. Compact structure and small size for constituent units.

2. Automatic lubrication and long service life for all units. 3. Reliable steering and light and flexible operations.

4. It can realize the converging with the hydraulic system of working device to reduce the power loss and improve the system efficiency.

At the time of steering, the system supplies oil preferentially to the steering hydraulic system and the remaining oil converges with the return oil of steering system to return to oil tank through radiator. The safety valve is set on the priority valve, with the regulated system pressure at 16MPa.

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7-1 Schematic diagram of LG958L steering system

1. Priority valve 2. Steering pump 3. Oil suction filter element 4. Steering cylinder 5. Coaxial flow amplifying steering gear 6. Radiator 7. Oil return filter element 8. Hydraulic oil tank

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Section III Structure and Working Principle of Steering System Hydraulic Units I. Steering gear 1. Meaning of steering gear model

Steering Gear BZZ6-800 Model of steering gear BZZ1 BZZ2

BZZ3 BZZ4

BZZ5 BZZ6

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Type of steering gear

Open center no-reaction fully hydraulic steering gear

Open center reaction fully hydraulic steering gear Closed center no-reaction fully hydraulic steering gear Multi-functional no-reaction fully hydraulic steering gear

Displacemen t: 800ml/r

Load-sensing fully hydraulic steering gear

Load-sensing flow amplifying fully hydraulic steering gear

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2. Types of fully hydraulic steering gear: 1) The BZZ fully hydraulic steering gear is mainly classified into: Open center no-reaction type (BZZ1); Open center reaction type (BZZ2);

Closed center no-reaction type (BZZ3); Load-sensing type (BZZ5);

Coaxial flow amplifying type (BZZ6); and other types.

The functionality symbols are shown in the left figure:

Type symbols of BZZ

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2) Corresponding types of steering gears for specific loader models

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NO.

Applied model

1

952/952H/952L

2

968/969

3

916/918

4

936/938

5

946L

6

933L

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953/956/958

Name of steering gear

Type of steering gear

Steering gear BZZ1-800

Open center no-reaction hydraulic steering gear

Steering gear BZZ5-320

Dynamic signal load-sensing hydraulic steering gear

Steering gear BZZ3-100

Closed center no-reaction hydraulic steering gear

fully fully fully

Steering gear BZZ5-500

Static signal load-sensing hydraulic steering gear

Steering gear BZZ6-630

Static load-sensing flow amplifying fully hydraulic steering gear

Steering gear BZZ6-500

Steering gear BZZ6-800

fully

Static load-sensing flow amplifying fully hydraulic steering gear

Static signal flow amplifying hydraulic steering gear

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fully

3. Structural composition of steering gear

As shown in Figure 5-2 (a) and (b), the structure of the BZZ6 fully hydraulic steering gear is mainly composed of valve body, valve core, valve sleeve, linkage shaft, locating spring, shifting pin, rotor, stator, and rear cap.

There are total 5 oil ports on the steering gear, namely oil inlet port P, oil return port T, left steering oil outlet port L, right steering oil outlet port R, and feedback oil port LS, which are connected with the oil outlet port CF of priority vale, oil return port of hydraulic oil tank, rodless chamber of right steering cylinder, rodless chamber of left steering cylinder, and port Ls of priority valve respectively.

T: Oil return P: Oil inlet

R: Right steering L: Left steering

LS

T L P R

Location of steering gear oil ports 11

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Figure 7-2 Structure of fully hydraulic steering gear (a)

1 – Connecting block 2 – Front cap 3 – Valve body 4 – Spring plate 5 – Shifting pin 6 – Valve sleeve 7 – Valve core 8 – Linkage shaft 9 – Rotor 10 – Rear cap 11 – Limit post 12 – Spacer disc 13 – Stator 14 – O-ring 15 – Steel ball 16 – O-ring 17- X-shaped ring 18 – O-ring

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Valve body

Left Right steering steering Oil inlet port Spacer disc

Oil return port

Linkage shaft

Limit block

Valve core

Bearing

Valve sleeve

Return spring

Check valve

Shifting pin

Rear cap Stator-rotor pair

Figure 7-2 Structure of fully hydraulic steering gear (b)

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The valve body, valve core, and valve sleeve of the steering gear constitute the follow-up swing valve of steering gear to control the oil flow direction. The valve core is within the inner chamber of valve sleeve. It is connected directly with steering column via connecting block and can be driven for rotation by the steering wheel via steering column. The valve sleeve is within the inner chamber of valve body and is driven by the rotor via linkage shaft and shifting pin for movement within valve body.

The rotor and stator constitute the metering motor at the lower portion of the steering gear. The stator has 7 teeth and the rotor has 6 teeth. The stator is fixed and the rotor rotates around the center of stator in radius of eccentric distance. The tooth profile for metering motor is equilong arc epicycloidal tooth, which can ensure that each point on the rotor curve can become the engagement point. During the rotation, seven oil orifices are connected with the oil orifices on the valve sleeve at all times so that the valve sleeve and valve core distribute the flow to the metering motor, namely enable the pressure to enter half of tooth chamber and drain oil from the other half of tooth chamber, in order to convey the pressure oil to the steering cylinder.

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The metering motor composed of rotor and stator is also referred to as cycloid gear engagement pair. During the power steering, it functions as the metering motor to ensure that the flow into the steering cylinder is proportional to the rotation angle of the steering wheel. During the manual steering, it’s equivalent to a manual oil pump. The linkage shaft and shifting pin connect the rotor with valve sleeve. During the power steering, they ensure the synchronization between valve sleeve and rotor (for feedback function). During manual steering, they are functioned for conveyance of torque.

The spring plate is functioned to ensure the neutral position of follow-up valve for centering purpose. Therefore, the spring plate is referred to as centering spring. The check valve is installed between oil inlet port and oil outlet port. During the manual steering, the check valve opens so that the oil in one chamber of the steering cylinder is sucked into the oil inlet port through oil return port and then is compressed into the other chamber of steering cylinder by cycloid gear engagement pair, namely it’s functioned for oil suction during the manual steering. 15

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4. Working principle of steering gear

The schematic diagram for the working principle of BZZ6 fully hydraulic steering gear is shown in Figure 5-3 (a), (b), and (c). The ports A and B are connected with two chambers of steering cylinder respectively, the port P is connected with oil outlet port of steering pump, and the port T is connected with hydraulic oil tank.

Figure 7-3 Structure of fully hydraulic steering gear (a) 1 – Connecting block 2 – Front cap 3 – Valve body 4 – Spring plate 5 – Shifting pin 6 – Valve sleeve 7 – Valve core 8 – Linkage shaft 9 – Rotor 10 – Rear cap 11 – Limit post 12 – Spacer disc 13 – Stator 14 – O-ring 15 – Steel ball 16 – O-ring 17 – X-shaped ring 18 – O-ring

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Figure 7-3 (b) Schematic diagram of fully hydraulic steering gear (at neutral position)

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Figure 7-3 (c) Schematic diagram of fully hydraulic steering gear (at steering position)

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1. No rotation of steering wheel (namely neutral position)

The valve core 1 and valve sleeve 3 are at neutral positions under the action of the centering spring plate. The hydraulic oil from the steering pump 7 enters into the valve core through the valve core and the orifice at the end of valve sleeve and then returns to oil tank 8 via port T. When the steering wheel is not rotated, the centering spring is functioning. As the oil ports A and B are blocked by the valve core 1, the oil in the steering cylinder is restrained against input and output and the piston can’t move so that the loader is traveling towards the preset direction. 2. Leftward rotation of steering wheel

The steering wheel drives the valve core 1 for counterclockwise rotation so that the centering spring is unilaterally compressed. As there is an up to 10º30’ rotation amount between the valve core 1 and valve sleeve 3, the valve core can rotate with respect to the valve sleeve. In such case, the oil groove of the valve core is connected with oil inlet port P of valve sleeve so that the oil from the oil pump flows, through oil grooves of valve sleeve 3 and valve core 1 and the valve sleeve, to the rotor 5 and stator 4 to drive the rotation of rotor 5 with respect to the starter 4. At the same time, the outlet oil from the rotor and stator enters into the rodless chamber of right steering cylinder through the valve sleeve and port A so that the cylinder piston rod is extended to drive the wheels via frame for leftward steering. The oil in the rod chamber of the cylinder flows into the valve sleeve 3 via oil port B and flows back to the hydraulic oil tank through oil return groove of valve core 1, oil return port of valve sleeve, and the port T. 19

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During the leftward steering, the rod chamber of left steering cylinder is connected with the rodless chamber of right steering cylinder so that the rod chamber is supplied with oil to retract the piston rod into the cylinder and drive the leftward steering of the frame. The rodless chamber of left steering cylinder is connected with rod chamber of right steering cylinder so that the oil from the rod chamber of right steering cylinder enters into port B via rodless chamber of left steering cylinder and returns to the oil tank through oil return port and oil inlet port of valve sleeve. When the relative rotation angle between valve core and valve sleeve is approximate 1.5º, the oil line is connected so that the rotation of rotor drives the oil from steering pump to the cylinder, of which the oil supply amount is proportional to the rotation angle of the steering wheel.

When the steering wheel is rotated for a certain angle and held, as the above-mentioned oil line is connected, the oil from the steering pump drives the rightward rotation of rotor 5. When the rotation angle of rotor 5 is same with the rotation angle of the steering wheel, as the valve sleeve 3 and rotor 5 are mechanically connected by linkage shaft, the rotor drives the leftward “follow-up” rotation of valve sleeve 3, till the stator spring drives the valve sleeve and valve core to neutral position and the angle rotated is same with the rotation angle of steering wheel. In such case, the relative rotation angle between the valve sleeve 3 and valve core 1 is eliminated and the oil line to the rotor and steering cylinder is closed so that the outlet oil from the steering pump 7 enters into the valve sleeve via port P and returns to oil tank through oil return groove of valve core 1, oil return port of valve sleeve 3, and oil port T. In such case, the loader stops the steering. When the steering wheel is rotated further, the rotor and valve sleeve will follow up, till the left limit position is reached. This is the hydraulic “feedback” follow-up function.

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3. Rightward steering

When rotating the steering wheel rightward, the steering wheel drives the valve core for clockwise rotation, of which the working principle is same with the leftward steering.

For the steering at low speed (the rotation speed of the steering wheel is less than 10r/min), the effective displacement of the steering gear is same with the metered displacement. When the input speed of the steering wheel is increased (the rotation speed of the steering wheel is 10~40r/min), the effective displacement is proportional to the rotation speed of the steering wheel. In such case, only a part of oil from the oil inlet port P enters into the rotor-stator pair for metering and the remaining oil enters into the cylinder directly via ports A and B. Therefore, the flow amplifying function is available only at this stage. When the input speed of the steering wheel exceeds 40r/min, the effective displacement of steering gear is basically constant at its rated equivalent displacement. 4. Manual steering

In event of sudden flameout of the engine or the malfunction of steering pump, rotate the steering wheel with hand for static steering. While rotating the steering wheel rightward, the valve core rotates for a 10º30’ angle to drive the rotation of valve sleeve, linkage shaft, and rotor via shifting pin. In such case, the rotor and stator are functioned as oil pump. The rotation of rotor 5 sucks out the oil from the oil port T and inputs the oil into the oil inlet chamber of rotor pump via check valve, valve sleeve, and valve core. The oil pumping action during the manual steering compresses the hydraulic oil sucked into the steering oil so that the compressed oil enters into the rodless chamber of steering cylinder to extend the piston rod and steer rightward the loader. The oil in the rod chamber flows to the oil inlet chamber of rotor pump from oil port A through valve sleeve, valve core, valve sleeve, and check valve and continually refills into the rodless chamber, in order to maintain the steering action.

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5. Combination valve block The valve block is a combined hydraulic unit that is mainly composed of check valve, two-way damping valve (overload valve), and oil refilling valve. It’s connected between the steering pump and steering gear and is assorted to the fully hydraulic steering gear (Generally, it’s directly installed on the valve body flange of the steering gear and works along with steering gear to form an integral unit). It’s functioned to guarantee the normal and stable working of steering gear and entire steering system under rated pressure on one hand and guard the steering cylinder and connecting pipelines against damage in event of sudden overload and protect the steering pump on the other hand.

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1) Check Valve

The check valve is composed of the valve seat 1, valve core 2, and spring 3 and is installed within the oil inlet port of the valve block body so that the high pressure oil from the oil pump flows into the oil inlet port of steering gear via the check valve. It's functioned to prevent the backflow of oil from automatically deflecting the steering wheel and leading to steering failure. 2) Two-way damping valve

The two-way damping valve incorporates two constant pressure direct-acting safety valves composed of the spring, ball valve seat, and steel ball. It’s installed within the valve port for connecting the valve body with the orifices of left and right chambers of steering cylinder and is connected with the oil return port, in order to protect the hydraulic steering system against impact of over-high pressure and ensure the safety of oil lines. 3) Oil refilling valve

The oil refilling valve incorporates two check valves composed of steel ball and is installed within the valve port for connecting the valve body with the orifices of left and right chambers of steering cylinder and is connected with the two-way damping valve. When the pressure within one chamber of cylinder is higher than the pressure setting of damping valve, the damping valve relieves the load and the oil refilling valve on the other chamber of the cylinder refills the oil to prevent the formation of cavitation in the system. 23

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II. Priority valve

The priority valve is mainly composed of the valve body, valve core, spring, safety valve assembly, and screw plug, as shown in Figure 7-4.

Figure 7-4 Structural diagram of priority valve 1 – Safety valve assembly 2 – Control spring 3 – Valve core 4 – Valve body 5 – Screw plug

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II. Priority valve

There are 5 oil ports on the priority valve, namely oil inlet port P, oil outlet ports EF and CF, feedback oil port LS, and oil return port T, which are connected with oil outlet port of steering pump, oil inlet port of hydraulic oil radiator, oil inlet port of steering gear, port LS of steering gear, and oil return port of hydraulic tank respectively.

This valve is matched with the BZZ6 steering gear to form the load-sensing steering system. At the speed variation of the steering wheel, it can preferentially guarantee the flow required by the steering gear and the remaining oil flows into the working device hydraulic system.

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Port EF Port P

Port CF

●

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When the steering wheel is stationary, the pressure oil from the steering pump flows to port EF from port P through valve core and enters into the working device hydraulic system or returns to oil tank directly. RELIABILITY IN ACTION

Port EF Port P

T

Port CF

Ls

●

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When the steering wheel is rotating, under the joint action of the spring force and LS pressure, the valve core moves rightward to connect the port P with port CF so that the pressure oil enters into the steering gear and drives the cylinder for steering of loader and the excessive oil diverges into the working device hydraulic system or oil tank through port EF.

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Section IV Cause Judgment and Troubleshooting for Common Malfunctions of Steering Hydraulic System

1) Heavy steering (analysis and judgment procedure)

Cause s y m p to m

J u d g e a s p e r m a lf u n c ti o n

Yes

Cylinder creepage, air bubbles in oil, and regular sound

Check steering column for flexible rotation No

Is the steering heavy at high speed and light at low speed?

air from system Air content in system Bleed and check oil inlet port of oil pump for air leakage

Yes

Heavy steering and no action of steering cylinder

Measure

Yes No Does system pressure meet requirement?

No

Yes

Check feedback oil pipe for blockage

Check steel ball for presence and blockage
Replace FK combination valve
Check cylinder for internal leakage




Failure of manual steering check valve
Leakage of FK overload valve
Internal leakage of cylinder




Damage of steering column

Repair or replace

Blockage of pipeline

Clean or replace

No

Yes

Adjust the system pressure. Is there any pressure change?

Yes

Low system pressure Repair or replace

No Breakage of priority Adjustment system pressure valve spring or blockage of valve core

Yes

Yes

Is hydraulic oil level too low?

Shortage of oil

Add hydraulic oil

Blockage of pipeline

Clean or replace

Wear or internal leakage of steering pump

Repair or replace

No Yes

Is oil suction pipe blocked?

No

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2) No steering end or failure for steering to limit position (analysis and judgment procedure) No end After the steering cylinder rotates to limit position, when the steering wheel is rotated with high force, the steering wheel can rotate lightly, namely there is no feeling of end.

Malfunction cause Low pressure of overload valve

Troubleshooting Appropriately increase pressure of overload valve

failure for steering to limit position

The steering cylinder can’t be rotated to the limit position, with heavy steering

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Malfunction cause

Troubleshooting

Low pressure of safety valve

Appropriately increase pressure of safety valve

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3) Cause for off-tracking of machine

① Due to leakage towards the cylinder port when the closed core steering gear is at neutral position, the slight offtracking of the closed core steering gear is normal. ② Check the cylinder connecting rod for presence of looseness. ③ Internal leakage of cylinder. ④ High pressure difference between two tires. ⑤ Unilateral leakage of two-way overload valve or two-way oil refilling valve. ⑥ Air content in oil.

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4) Incorrect steering ① Air content in system. ② Loose cylinder pin.

③ Stagnation of priority valve or shunt valve core. ④ Internal leakage of cylinder.

⑤ Low pump efficiency, leading to unstable pressure.

L

R

T

P

p Fixed Pump

Engine

Filter Reservoir

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5) No rotation or slow rotation of steering wheels, though the steering wheel can be rotated flexibly.

① Serious internal leakage of two-way overload valve

② Serious leakage of cylinder piston.

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6) Idle travel of steering wheel

② Loose nuts of steering wheel.

③ Air content in oil. ④ Internal leakage of two-way overload valve ⑤ Internal leakage of steering cylinder

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① Worn or damaged connection between steering column and steering gear.

L

R

T

P

p Fixed Pump

Engine

Filter Reservoir

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7) Vibration or autogiration of steering wheel

① Incorrect assembly relationship. At the

time of reassembly after the disassembly for repair, it’s required to align the

spline teeth of the linkage shaft shifting pin slot with the corresponding inner spline teeth of rotor.

② When the oil pumped is connected to

the port R or L, the steering gear will rotate on its axis like a motor.

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8) Bounce of steering wheel Cause: The check valve at oil inlet port of steering gear is damaged. Function of check valve: It prevents the backflow of oil in steering cylinder under the action of external force when the pressure is higher than the pressure at oil inlet port. If the check valve is damaged, the backflow of oil will lead to bounce symptom of steering wheel.

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THE END

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