SERV7107-05 Vol. 5, No. 1 May 2007
GLOBAL GLO BAL SER SERVIC VICE E LEARNI LEARNING NG TECHNICAL PRESENT PRESENTA ATION
300D SERIES HYDRAULIC EXCAVATORS - TIER III ENGINES MAIN HYDRAULIC PUMPS AND CONTROLS (INCLUDES 320D, 321D, 323D, 324D, 325D, 328D AND 330D)
New Product Introduction (NPI)
300D SERIES HYDRAULIC EXCAV EXCA VATORS - TIER III ENGINES MAIN HYDRAULIC PUMPS AND CONTROLS AUDIENCE Level II - Service personnel who understand the principles of machine systems operation, diagnostic equipment, and procedures for testing and adjusting.
CONTENT This presentation provides an introduction and describes the components and systems operation of the 300D Series main hydraulic pumps and controls. controls. Additional presentations will cover the machine walkaround, engines, pilot system, main control valve group, implements swing system, travel system, and tool control systems in more detail. detail. This presentation may be used for self-paced and self-directed training.
OBJECTIVES After learning the information in this presentation, the technician will be able to: 1. identify the components and explain the operation of the 300D Series hydraulic excavators main hydraulic pumps and controls, and 2. diagnose problems in the main hydraulic pumps and controls.
REFERENCES 320D Hydraulic Excavator Specalog 324D Hydraulic Excavator Specalog 325D Hydraulic Excavator Specalog 328D Hydraulic Excavator Specalog 330D Hydraulic Excavator Specalog NPI "325D Hydraulic Excavator - Introduction" NPI "330D Hydraulic Excavator - Introduction" Machine Monitoring System - Systems Operation Self-study "300D Series Hydraulic Excavators, 345C Hydraulic Excavator, and 365C & 385C Large Hydraulic Excavators iTIM iT IM " '30 '300C 0C'' Se Seri ries es Hy Hydr drau aullic Ex Exca cava vattor orss-E -Ellec ecttro roni nicc Con Conttro roll Sys Systtem ems" s" iTIM "325C Hydraulic Excavators-Hydraulic Systems" 325D Hydraulic Schematic
Estimated Time: 1 hour Illustrations: 22 Form: SERV7107-05 Date: May 2007: Vol. 5, No. 1 © 2007 Caterpilla Caterpillarr Inc.
AEHQ5856 AEHQ5663 AEHQ5665 AEHQ5706 AEHQ5667 SERV7105-12 SER V7105-12 SERV7106-02 SER V7106-02 RENR8068 SERV7032 SER SE RV2 V269 693 3 SERV2701 KENR6157
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TABLE OF CONTE CONTENTS NTS INTRODUCTION .................................................. ........................................................ ......................................................................5 ..............5 320D - 328D Main Hydraulic Pump Group.........................................................................11 330D Main Hydraulic Pump Group Group ...................................................... .....................................................................................21 ...............................21 CONCLUSION...........................................................................................................................35
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PREREQUISITES "Fundamentals of Mobile Hydraulics Self Study Course" "Fundamentals of Power Train Self Study Course" "Fundamentals of Electrical Systems Self Study Course" "Fundamentals of Engines Self Study Course"
NOTES
TEMV3002 TEMV3003 TEMV3004 TEMV3001
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INTRODUCTION
This section of the presentation will cover the main hydraulic pumps and pump controls for the 300D Hydraulic Excavators. The main pump group consists of a variable displacement piston drive pump and an idler pump. The drive pump and the idler pump are contained in an integral housing. The drive pump and the idler pump are identical in construction and operation. The pumps are sometimes referred to as S.B.S. (side by side) pumps The main difference between all of the pumps is the maximum pump flow for each model. Both the drive pump and the idler pump have individual pump control valves to control the pump flow. The 320D through the 328D use the same type of pump control valve. The 330D pump control valve is the same as the pump control used on the 345C pump.
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Power shift pressure is controlled by the Machine ECM, and assists in pump regulation. Power shift pressure is one of three pressures to control the pump. The pilot pump supplies the power shift PRV solenoid with pilot oil. The Machine ECM monitors the selected engine speed (from the engine speed dial), the actual engine speed (from the engine speed sensor and Engine ECM), and the pump output pressures (from the output pressure sensors). The power shift PRV solenoid regulates the pressure of the power shift oil depending upon the signal from the Machine ECM. When the engine speed dial is in position 10, the Machine ECM varies the power shift pressure in relation to the actual speed of the engine. The power shift pressure is set to specific fixed values dependent upon the position of the engine speed dial. The fixed power shift pressures assist cross sensing pressure with constant horsepower control.
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When the engine speed dial is on position 10 and a hydraulic load is placed on the engine, this condition causes the engine speed to decrease below the engine's target rpm. When this decrease occurs, the Machine ECM signals the power shift solenoid to send increased power shift pressure to the pump control valves. The increased power shift signal causes the pumps to destroke, and reduce the horsepower demand placed on the engine. With a decreased load from the hydraulic pumps the engine speed increases. This function is referred to as engine underspeed control. Engine underspeed control prevents the engine from going into a "stall" condition where engine horsepower cannot meet the demands of the hydraulic pumps. The power shift signal to the pump control valves enables the machine to maintain a desired or target engine speed for maximum productivity. Power shift pressure has the following effect on the main hydraulic pumps: - As power shift pressure decreases, pump output increases. - As power shift pressure increases, pump output decreases. Power shift pressure ensures that the pumps can use all of the available engine horsepower at all times without exceeding the output of the engine. NOTE:
The target rpm is the full load speed for a specific engine no load rpm. Engine target rpm is determined by the opening of one of the implement, swing, and/or travel pressure switches at the end of an operation. The Machine ECM then waits 2.5 seconds and records the engine speed. This specific rpm is the "new" no load rpm. The Machine ECM then controls the power shift pressure to regulate pump flow to maintain the full load (target) rpm for the recorded no load rpm. Target rpm can change each time the pressure switches open for more than 2.5 seconds.
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The solenoid operated proportional reducing valve for the power shift pressure is located on the right/drive pump control valve. The proportional reducing valve receives supply oil from the pilot pump. The solenoid receives a pulse width modulated signal (PWM signal) from the Machine ECM. The PWM signal sent from the Machine ECM causes the proportional reducing valve to regulate the pilot pressure to the pump control valves to a reduced pressure. This reduced pressure is called power shift pressure (PS). The output flow of the right/drive pump and the left/idler pump is controlled in accordance with the power shift pressure. The power shift pressure is used to control the maximum hydraulic pump output in relation to the engine rpm. A decrease in engine speed causes an increase in power shift pressure and a decrease in pump flow.
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When the speed dial is at dial position 10, if the Machine ECM senses a decrease in engine speed below target rpm, the Machine ECM increases the PWM signal sent to the solenoid. The magnetic force of the solenoid increases. As the magnetic force of the solenoid becomes greater than the force of the spring, the spool moves down against the force of the spring. The downward movement of the spool blocks the flow of oil to the tank. More power shift pressure oil is now directed to the pump control valve. The increased power shift pressure acts on the drive pump control valve and the idler pump control valve. If both pumps are upstroked, then both pumps will destroke as a result of the increase in power shift pressure. If only one pump is upstroked, only the upstroked pump will destroke.
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If engine speed is above the target rpm, the Machine ECM decreases the power shift pressure to increase the pump flow. When the Machine ECM senses an increase in engine speed above the target speed the Machine ECM decreases the PWM signal sent to the solenoid. As the magnetic force of the solenoid becomes less than the force of the spring, the spool moves up. The upward movement of the spool restricts the pilot oil flow to the power shift passage and opens the power shift passage to the drain. The power shift pressure is reduced. The reduced power shift pressure acts on the drive pump control valve and the idler pump control valve. Depending on which circuits are activated, the drive pump and/or the idler pump will upstroke as a result of an decrease in power shift pressure.
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320D - 328D Main Hydraulic Pump Group
This illustration shows the main hydraulic pumps groups. The drive pump (right pump) (1) is driven by the engine and the idler pump (left pump) (2) is driven by the drive pump. The pilot pump (3) is mounted on the drive pump. The medium pressure pump (4) is driven by the idler pump. The drive pump supplies oil to the right half of the main control valve group and the following valves: - stick 2 control valve - boom 1 control valve - bucket control valve - attachment control valve - right travel control valve
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The idler pump supplies oil to the left half of the main control valve group and the following valves: - left travel control valve - swing control valve - stick 1 control valve - boom 2 control valve - auxiliary valve for tool control (if equipped) The output of the variable-displacement piston pumps is controlled by the pump control valves (5 and 6) mounted on the main hydraulic pumps.
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This illustration shows the pump control valve for the drive pump. Except for the power shift solenoid, the components for the idler pump are identical. The power shift PRV solenoid (1) provides a common power shift pressure for both pumps. The power shift PRV solenoid is controlled by the Machine ECM. The pump output pressure sensors (2) signals the Machine ECM of each pump's output pressure. The Machine ECM uses the pump output pressure, actual engine speed, and desired engine speed to determine the power shift pressure. The pressure sensors also signal the Machine ECM to cancel the AEC settings if the pump pressure increases above approximately 7370 kPa (1100 psi) and the engine rpm is still at an AEC setting. The horsepower adjusting screws (3) adjust the hydraulic horsepower output of each pump. The maximum angle screw (4) limits the maximum flow of each pump. The pressure tap (5) above the power shift PRV solenoid can be used to check the PRV signal pressure. The pressure tap (6) just above the pressure sensor can be used to check the drive pump supply pressure. Another the pressure tap (not shown) can be used to check the idler pump supply pressure. Cat ET can also be used to check these two pressures.
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This illustration shows the pumps in STANDBY condition. The pump control valves will upstroke, destroke, or maintain the displacement of the pump depending on the conditions the pump control valve senses. The pump control valve controls oil pressure (stroking pressure) to the right side of the actuator, which controls the angle of the pump swashplate. Each pump has a pump control valve which senses the three following control signals: - a pump specific Negative Flow Control (NFC) signal from the main control valve group - a common power shift signal pressure generated by the power shift PRV - a common cross sensing signal pressure from the output of the two main pumps NFC: NFC pressure is the most significant controlling signal in a negative flow controlled hydraulic system. Each pump control valve receives a specific NFC signal that is based upon the hydraulic demand for that specific pump.
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Flow from the drive pump supplies the right half of the main control valve group, and has a corresponding NFC signal for the drive pump. Flow from the idler pump supplies the left half of the main control valve group, and has a corresponding NFC signal for the idler pump. The open-center valves in the main control valve group allow pump output to flow through unrestricted. An orifice in the NFC valve creates a restriction to the pump output which increase the NFC pressure. The NFC pressure then signals the corresponding pump control valve. Each pump will remain at STANDBY as long as a full NFC signal pressure is present. When a hydraulic control valve is shifted from the NEUTRAL position, the NFC signal pressure to the corresponding pump is reduced, which causes the pump to UPSTROKE. Any change in the movement of a valve in the main control valve group will effect the NFC signal because the valves send a variable NFC signal to the pump depending on the needed pump output. Output of each pump is unaffected by a change in the NFC signal to the other pump. NFC pressure has the following effect on the main hydraulic pumps: - As NFC pressure decreases, pump output increases, - As NFC pressure increases, pump output decreases. NFC signal pressure overrides all other control of the main hydraulic pumps. Cross Sensing: Cross sensing pressure is essentially an average pressure from the output of the drive pump and the idler pump.
The output of each pump flows respectively to the left and right halves of the main control valve group. The output of each pump also flows to the cross sensing orifices. The pressure on the pump control valve side of the cross sensing orifices is an average of the output pressure of the two pumps, and is referred to as cross sensing pressure. The cross sensing pressure compensates for the horsepower demand of each pump individually and for the two pumps together. With cross sensing assistance, the pumps constantly regulate to effectively use all of the available engine horsepower at any given time. This is referred to as constant horsepower control. Cross sensing pressure has the following effect on the main hydraulic pumps: - As cross sensing pressure decreases, pump output increases, - As cross sensing pressure increases, pump output decreases. Given a fixed NFC signal, cross sensing signal pressure regulates the output of the main hydraulic pumps. NOTE:
Hydraulic horsepower is a function of pump output flow and pressure. As pump flow or pressure increases, the horsepower demand increases. As pump flow or
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The above illustration shows a cross sectional view of one of the main hydraulic pump control valves in STANDBY. The main pumps will be in STANDBY condition when the engine is running and all control valves are in the NEUTRAL position. Under these conditions the NFC pressure signal to the pump control valves is high. The pump can not upstroke until NFC signal pressure is reduced. The high NFC signal pressure causes the NFC control piston to move left against the force of the NFC spring on the right. When the NFC control piston moves left it contacts the shoulder on the pilot piston, which causes the pilot piston to move the horsepower control spool against spring force. The passage between horsepower control spool and the sleeve is now open to tank, causing the right end of the actuator to be open to the tank. The actuator moves to the right, moving the swashplate to a minimum angle, which causes pump output flow to be minimum. NOTE:
With the S.B.S. pumps, system pressure destrokes the pumps, while the signal pressure varies to upstroke the pumps.
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The pumps must have a reduction in NFC pressure to upstroke from STANDBY. The illustration shows the pump control valves upstroking the pump due to a decrease in NFC signal pressure. As shown, there is no NFC signal pressure, indicating that at least one control valve has been fully shifted. When the joysticks or travel levers are moved from the NEUTRAL position, NFC signal pressure decreases proportionally to the amount the joystick or travel levers are moved. When the NFC signal pressure decreases, the spring on the control piston forces the control piston to the right. The horsepower control springs on the left overcome the cross sensing signal pressure and the power shift signal pressure to move the horsepower control spool to the right. With the horsepower control spool shifted to the right, the passages between the sleeve and the horsepower control spool are closed off to tank and pump output pressure is allowed to flow to the right side of the actuator. Because the right side of the actuator is larger than the left side, the greater force generated by the pressure on the right side causes the actuator to move left and upstroke the pump. The pump can also be upstroked by a decrease in either power shift or cross sensing pressure, but only after a reduction in NFC pressure has caused the pump to leave minimum angle.
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As the pump upstrokes, the movement of the actuator causes the control linkage to move the sleeve around the horsepower control spool. The sleeve moves to the right as the actuator moves to the left. Because of the geometry of the control linkage, a large movement of the actuator moves the sleeve a small amount (see Section D-D). The small movement of the sleeve causes the passages between the sleeve and the horsepower control spool to open partially to tank and partially to the pump output. The pressure signal that is sent to the right side of the actuator is now metered, which causes the actuator to reach a balance point where the pump does not upstroke or destroke. With the actuator at a fixed position the swashplate angle of the pump is fixed. Constant flow is now achieved. Due to varying loading and operating conditions, this fixed output is rarely maintained for very long. When operating conditions change, the pump will UPSTROKE or DESTROKE.
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The three things which can cause the pumps to DESTROKE are: - an increase in NFC pressure - an increase in cross sensing pressure - in increase in power shift pressure This illustration shows the system under a heavy hydraulic load. As the supply pressure increases due to the heavy load, the cross sensing signal pressure rises as an average of the left and right pump delivery pressures. The cross sensing signal acts on the difference of the two areas on the pilot piston. As the cross sensing signal increases, the pilot piston moves to the left, which pushes the horsepower control spool left against the force of the horsepower control springs on the left. As the spool moves left, the large end of the actuator is opened to tank by a passage between the horsepower control spool and the sleeve. The pressure decreases on the right end of the actuator and the actuator moves to the right, which causes the pump to DESTROKE.
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An increase in power shift signal pressure has a similar effect as an increase in cross sensing signal pressure. If the hydraulic pump lugs the engine below full load speed, the Machine ECM increases the current to the power shift solenoid. The increased signal causes a higher power shift signal to be sent to the pump control valves. The power shift pressure acts on the right side of the pilot piston. The force generated from the power shift pressure assists cross sensing pressure to destroke the pump. As the pump destrokes the engine speed will increase due to the reduction in load. An increase in NFC signal pressure will cause the pump to destroke. If all control valves were returned to NEUTRAL, the NFC signal causes the pump to fully destroke and return to STANDBY.
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330D Main Hydraulic Pump Group
The 330D uses a new Kawasaki designed main hydraulic pump group (1) rated at 2 x 280 L/Min (2 x 74 gpm). The pump group is different from the pump group used on the 330C, however it continues using an NFC control system. This pump group is similar to the pump group used on the 345C. The right (drive) pump (2) is driven by the engine via a flexible coupling. The left (idler) pump (3) is driven directly off the right pump. Each pump rotating group has its own pump control valve. The pump control valves are used to adjust the output flow of the pumps. Each pump rotating group also has its own pressure tap and pressure sensor. A power shift PRV (4) is mounted on the top, center of the pump group case. The power shift PRV uses pilot system oil and sends it to the main hydraulic pump control valves as a control signal pressure. The power shift pressure is measured at pressure tap (5). Additional pump components shown in this photo are: the right pump control valve (6), the left pump swashplate minimum angle adjustment (7) and the left pump control valve (8). The pilot pump (9) is driven off the idler pump and the demand fan pump (10) is driven off of the drive pump.
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This is a view of the right pump pump control valve. The pump control valve is located above and behind the power shift solenoid and proportional reducing valve. This view shows: - right (drive) pump negative flow control adjustment (1) - right (drive) pump horsepower control adjustment (2) The left pump control valve has similar adjustment screws.
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Each pump receives four different signals to control the output flow of the pumps: - Power shift pressure - System pressure from that pump - Cross-sensing pressure (from the other pump) - Negative flow control pressure Power Shift Pressure: The power shift PRV receives a control signal from the ECM. The ECM sends an electrical signal to the power shift PRV to regulate power shift pressure in relation to the engine speed.
The power shift signal to the pump control valves enables the machine to maintain the target engine speed for maximum productivity.
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If the Machine ECM senses that the engine is below the target speed due to a high hydraulic load from the main pumps, the Machine ECM will increase the power shift pressure. The target speed is the full load for the no load engine speed. (The new no load speed is taken 2.5 seconds after the implement/swing and the travel pressure switches open when the joysticks and the travel control pilot controls are returned to NEUTRAL). As power shift pressure increases, the pump control valves destroke the main pumps accordingly. This reduces the load on the engine, and consequently enables the engine to maintain the target engine speed. If the engine speed is above the target speed, the Machine ECM will decrease power shift pressure, causing the pumps to upstroke and produce more flow. Cross-sensing Control Pressure: Each pump control valve gets a cross-sensing control pressure from the other pump system pressure. Negative Flow Control (NFC): NFC is the primary controlling signal for the main pump output. The NFC signal to the main pump control valve is generated in the main control valve group. The NFC signal is delivered to the left and right pump control valves from the left and right halves of the main control valve group, respectively.
When the joysticks or travel levers are in the NEUTRAL position, the oil flows from the main pumps through the open center bypass passages of the control valves. The oil flows to the valves and returns to the tank by way of the NFC control orifices. The restriction of the NFC orifices causes a pressure signal to be sent to the right and left pump control valves, respectively, as an NFC signal. When the main pump control valves receive a high NFC signal from the main control valves, the pumps remain at a standby output flow at or near minimum pump displacement. When a joystick or travel lever is moved from a NEUTRAL position, the open-center passage of the corresponding implement/travel function is closed in proportion to spool movement. This reduces the NFC signal to the main pump control valve and the pump output flow is increased proportionally. When the control valve is fully shifted, then NFC pressure is reduced to slow return check valve pressure. The use of an NFC hydraulic system maximizes efficiency of the machine by only producing flow from the pumps when the flow is needed. NOTE:
A high NFC signal will always overcomes the horsepower control and decrease pump flow to minimum.
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This illustration shows the pumps in STANDBY condition. Each pump control valve senses the Negative Flow Control (NFC) signal, the power shift pressure, the cross sensing pressure, and the system pressure for that pump. When one of more circuits are activated, the pump control valves will upstroke or destroke the pumps to maintain the pump flow depending on the four signal pressures to the pump control valves. The pump control valve controls oil pressure to the left side of the actuator. This controls the angle of the pump swashplate. The 330D hydraulic pumps are always trying to upstroke to increase flow. The pump control valves vary the oil pressure used to destroke the hydraulic pumps.
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The idler (left) pump supplies oil to the following valves: - left travel control valve - swing control valve - stick I control valve - boom II control valve - idler (left) pump negative flow control valve - auxiliary valve (if equipped) The drive (right) pump supplies oil to the following valves: - right travel control valve - standard attachment control valve - bucket control valve - boom I control valve - stick II control valve - drive (right) pump negative flow control valve.
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This illustration shows the three separate control sections of the pump control group. Individual parts are also shown. The three control sections are connected with a series of pins and linkages. The separate control sections work together to regulate pump flow according to demand and hydraulic horsepower requirements. The horsepower control section directs system pressure to and from the minimum angle end of the large actuator piston. The large actuator piston moves the swashplate for increased or decreased pump flow. The lower end of the feedback lever is connected to the actuator piston. The feedback lever works as a follow-up linkage to move the horsepower control spool when the large actuator piston moves. The negative flow control (NFC) section works in conjunction with the horsepower control section to destroke the swashplate when all hydraulic controls are in NEUTRAL or during implement or travel MODULATION. The torque control section works in conjunction with the horsepower control section to regulate pump flow while the hydraulic circuits are actuated . Full pump system pressure is directed to the maximum angle small actuator piston to upstroke the pump. A regulated pressure signal is directed to the minimum angle large actuator piston to destroke the pump.
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This illustration shows an end sectional view of the pump controls. The NFC spool is connected to the lower end of the NFC lever with a pin. The upper end of the NFC lever pivots on a fixed pin in the housing. The torque control rod is connected to the lower end of the torque control lever with a pin. The upper end of the torque control lever pivots on a fixed pin in the housing. The upper end of the feedback lever is connected to the horsepower control spool with a pin. The lower end of the feedback lever is connected to the actuator piston. The feedback lever pin fits tightly into the feedback lever. The feedback lever pin extends into large holes in the torque control lever and the NFC lever. The large holes permit individual control from the torque control lever and the NFC lever. Movement of the actuator piston causes the feedback lever to pivot on the feedback lever pin and move the horsepower control spool.
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This illustration shows the NFC portion of the pump controls. When all hydraulic control valves are in NEUTRAL, a high NFC pressure is directed to the left end of the NFC spool. The NFC pressure pushes the NFC spool to the right against the spring force. The NFC adjusting screw changes the effect of the NFC pressure on the NFC spool. Turning the screw in (clockwise) causes the NFC pressure to increase higher before the NFC spool moves. This condition causes the pump to upstroke sooner (less modulation) when the hydraulic control valve is ACTIVATED. Turning the screw out (counterclockwise) causes the NFC spool to move at a lower NFC pressure. This condition causes the pump to upstroke later (more modulation) when the hydraulic control valve is ACTIVATED. In the STANDBY condition, the horsepower control spool directs a signal pressure, which is part of system pressure, to the minimum angle end of the actuator piston. The increase in pressure moves the actuator piston to the right against the minimum angle stop screw. The pump flow will remain constant until the NFC pressure from the control valve decreases.
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This illustration shows the pump controls at the beginning of an upstroke that was caused by a decrease in NFC pressure. When a hydraulic control valve in the main control valve is shifted, the NFC pressure is decreased. Due to reduced NFC pressure, spring force moves the NFC piston to the left. The NFC piston moves the lower end of the NFC lever to the left with the pin on the upper end of the NFC lever as the pivot point. As the lower end of the NFC lever moves to the left, the large hole through the lever also moves to the left. As the large hole moves to the left, spring force pulls the horsepower control spool and the upper end of the feedback lever to the left because the feedback lever pin is allowed to move to the left. The minimum angle actuator piston is opened to case drain through the right orifice in the horsepower control sleeve and the right end of the horsepower control spool. System pressure pushes the maximum angle actuator piston to the left to upstroke the pump.
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As the actuator piston moves, the lower end of the feedback lever moves to the left. The feedback lever rotates clockwise with the feedback lever pin as the pivot point. The upper end of the feedback lever pulls the horsepower control spool to the right until the right land on the horsepower control spool reaches a balance point between the orifices through the horsepower control sleeve. Flow to and from the minimum angle piston is metered by the horsepower control spool and the horsepower control sleeve. The swashplate angle remains constant until the NFC pressure is again changed. The amount of reduction in NFC signal pressure determines the amount of pump upstroke. If NFC pressure is reduced to minimum, the pump will upstroke until the actuator piston contacts the maximum angle stop screw. A decrease in power shift pressure will cause an increase in flow from the pump in the same manner as described for a decrease in system pressure, since both power shift pressure and system pressures act on the torque control piston.
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Pump Flow Decrease - Due to Increased Pump Load
This illustration shows the torque control piston and horsepower control spool sections of the pump control valve with the pump in the upstroked position at the beginning of DESTROKE. For the purpose of this presentation, assume that power shift pressure remains constant. - Power shift pressure from the PRV enters the pump controls and pushes on the plug at the left end of the torque control piston. - System pressure from this pump enters the pump controls and goes to the right shoulder area on the torque control piston. - The cross-sensing signal pressure from the other pump goes to the left shoulder area on the torque control piston. - The combination of power shift pressure and the two system pressures push the torque control piston to the right against the force of the horsepower control adjustment springs. - The horsepower control spool directs the signal pressure to the minimum angle end of the actuator piston to destroke the hydraulic pump.
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The large horsepower adjustment screw regulates the pressure or point that the pump starts to destroke (large spring adjustment). The small adjustment screw regulates the rate that the pump destrokes (small spring adjustment). When the system pressures and power shift pressure push the torque control piston to the right: - The torque control rod moves to the right to compress the horsepower control springs. - The torque control rod moves the lower end of the torque control lever to the right with the fixed pin on the upper end of the torque control lever as the pivot point. - The torque control lever pulls the feedback lever pin and the upper end of the feed back lever to the right. - The feedback lever pulls the horsepower control spool to the right against the spring force. - System pressure is directed around the horsepower control spool through the center orifice of the horsepower control sleeve and to the minimum angle end of the actuator piston. - The increase in pressure in the minimum angle piston moves the actuator piston to destroke the pump.
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This illustration shows the pump controls at the end of DESTROKE. When the actuator piston moves toward minimum angle, the lower end of the feedback lever moves to the right turning the lever counterclockwise with the feedback lever pin as the pivot point. The lever movement shifts the horsepower control spool to the left so system pressure is metered through the two orifices to and from the minimum angle end of the actuator piston. Pump flow is held constant until one of the signal pressures changes. An increase in power shift pressure will cause a decrease in flow from the pump in the same manner as described for an increase in system pressure since both the power shift pressure and system pressure act on the torque control piston.
