word transmission

Hydrostatic Transmission Systems Operation

HYDROSTATIC TRANSMISSIONIntroduction

The hydrostatic transmission is a hydraulic transmission with variable speed. Mechanical power líom the engine is changed to hydraulic power by a variable displacement pump. This power is sent through hydraulic fines to the motor where it is changed back to mechanical power to drive a load. To meet the needs of

INTRODUCTlON TO HYDROSTATIC TRANSMISSIONS

C10058X1

This diagram shows the most basic type of hydrostatic drive system. The required components are: a variable displacement piston pump, two hydraulic lines (one for forward and one for reverse) and a fixed displacement piston motor. These components make up a single hydrostatic “drive loop.” A complete drive system requires two drive loops, one for each track. The 973 Track Loader has variable two-speed drive motors, not fixed displacement drive motors.

The drive loop changes the mechanical power to hydraulic power and then changes the hydraulic power back into mechanical power to drive a load. The mechanical power, provided by the engine, is changed to hydraulic power (flow and pressure) at the variable displacement piston pump. The piston pump delivers varied rates of flow to the piston motor, through one of the hydraulic fines. The rate and direction of flow is determined by a swashplate inside the piston pump. The motor drives the track in either forward or reverse. The direction offlow from the purnp deterrnines the direction of ti‘ack movernenl. The pressure in the drive line is determined by the load on the drive motor. If the load on the motor increases, the pressure in the drivc line increases. The drive line proYiding the

a vehicle, the pump is designed to give a flow that can be varied and also sent through either of the two hydraulic lines to the motor. Thus, the load (tracks) can be driven at differerit speeds and in eitller forward or rex erse direction.

flow to the motor is referred to as the “high pressu re side” of the drive loop. The drix'e l ine prov iding the path for the return oil from the motor to the pump is

referred to as the “low pressure side” of the drive loop.

Leakage in the pump and motor is necessary to provide lubrication for the internal components. For this reason, the hydrostatic drive system has a charge purnp to replenish the oil lost due to normal leakage. The system also has a charge valve and a charge relief valve. The charge valve connects to both sides of the drive loop. A shuttle spool, inst de the charge valve, directs the flow from the charge pump to the low pressure side of the drive circuit. The low pressure side, remember, servcs to supply the variable displacement pump. The shuttlc spool also connects the low pressure sidc to the charge relief valve. The charge relief va1ve limite the pressure in the l ow pressure side by limiting thc maximum pressure of the charge oil to approximately 1380 kPa (200 psi). Thc charge valve also contains the main relief valve for the high pressurc side of the drive loop. Thcrefore, the charge valvc is common- fy referred to as the charge and main relicf valvc. The main relief valve limits thc maximurn prssurc in the high pressure side to approximately 38 000 kPa (5500 psi).


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This diagram also shows an oil cooler positioned Most of the oil from the low pressure side is sent directly below the charge relief valve. During normal through the charge relief val ve to the transmission oil operation, the low pressure side of the drive loop does cooler. The cooler reduces the temperature of the oil not require all the flow provided by the charge pump. before it returns to tank.

Hydrosialic Transmission Systems Operation

To control the operation of a hydrostatic drive system, the operator must be able to control the rate and the direction of flow from the variable displace— ment pump. Remernber, the rate and direction offlow from the pump to the motor determines the speed and direction of the track. Both the flow rate and the direction of flow is determined by the angle of the swashplate inside the variable displacement pump. When swashplate angle is increased, the rate of flow to the drive motor is increased. Similarly, a decrease in swashplate angle results in a decrease in the flow rate.

The hydrostatic transmission has a servovalve to control the positioning of the swashplate. Servovalve movemeiit is mechanically controlled by an underspeed valve and a steering pedal (not shown). When a steering pedal is depressed, a mechanical linkage arrangement moves, causing the servovalve to move. As a result, the angle of thC SWaShplate changes.

The underspeed valve connects to the transmission control fever, in the operator’s compartmenl, through

a mechanical linkage. Wher the operator changes the position of the transmission control lever, the servovalve will again move, causing the angle of the swashplate to change.

The underspeed valve will also change the position— ing of the servovalve automatically during operation to increase or decrease the angle ofthe swashplate. The underspeed valve senses increases and decreases in engine speed caused by the total load on the vehicle. “Total load” is defined as the load felt by the engine from both the track drive system and the implement hydraulic system. When the total loild causes the engine speed to go below a specified rpm, the underspeed valve will react and mechanically mox e the servovalve. As a result, the angle of the swashplate will decrease, reducing the load from ihe drive system. The decrease in total load causes the engine spccd to increase. When the speed of the engine increases beyond a specified rpm, the underspeed valve will react to return the scrvovalve and swashplate to thcir original positions.

Hydrosialic Transmission Systems Operation


This diagram shows one drive circuit at the top and a second drive circuit at the bottom. Each drive loop controls the o¡ieration of one track. The underspeed valve, charge pum¡i, charge relief valve and oil cooler are common to both circuits. The drive loops are connected through these common components but oper— ate independently and can be controlled separately by the steering peclals. The two drive loops are combined to provide the steering capaliilities necessary to operate a machine.

Each drive loop has its own servovalve and charge valve. Each charge valve contains a main relief valve to limit thc maximum drive pressure in that particular circuit. Next is a discussion of the dTive system operation during two basic functions; machine moving in a straight line and steering.

When the transmission control fever is moved from the PARK position toward either FORWARD or RE- VERSE, the mechanical linkage from the undcrspced valve simultaneously moves both servovalvcs thc same amount. The amount of servovalve movement is controlled by the positioning ofthe transmission fever. The mechanical linkage arrangement causes the servovalves to nieve the swashplates to identical angles. This ensures that the output (flow) from each pump is the same. If the angles of the swashplates are NOT the sarne, the outputs from both pumps will not be equal.

As a result, one track will turn at a faster rate and the machine will not travel in a straight line. The coiiect adjustment of the control linkage arrangenlent is ci iti- cal to swashplate positioning.

Each servovalve connects to a steering pedal through a linkage arrangement. When the machine is moving in either direction, the operator can depress a steering pedal and cause a servovalve to reduce or even reverse the swashplate angle of one pump. This in turn causes the rotation of one track to either slow, stop or reverse direction. The amount of steering pedal movement determines the rate and direction of track rotation. When a pedal is depressed a small amount, the rotation ofone track will decrease, resulting in a gradu- al turn. Depressing the pedal a little farther will move the swashplate back to a zero angle. This stops the rotation of one track and causes a pivot turn. When a steering pedal is depressed even farther, the rotation of the track will reverse. This is referred to as a spot turn. When a steering pedal is released, the servovalve and swashplate will return to their original positions.

With a general understanding of“how a hydrostatic drive system operates,” we are now ready for a more detailed explanation of the hydrostatic drive system used in the current family of Caterpillar Ti‘ack Loaders.


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COMPONENT LOCATIONS

CROSS SECTION OF TRANSMISSION

1. Left hand axial piston 5. Engine driven gear. pump. 6. Drive gear for right hand

2. Left hand servo cylinder. pump.3. Drive gear for left hand 7. Manifold for

charge pump. pump oil supply.4. Swashplates. 8. Right hand axial piston

pump.

9. Right hand servocylinder.

10.Case.F. FORWARD.P. PARK.R. REVERSE.

The Hydrostatic Transmission is a single modular and (8). On the engine end of the case are three gears unit that includes ×11 the working components except (3), (5) and (6), that transfer power from the engine to

the oil cooler, the track drive motors and connecting the pumps. On the other end ofthe case is the head and hoses, the track brakes and the external control two charge and main relief valve groups. The head

and linkage. the charge valves make a manifold system that sends oil from pumps (1) and (8) to and through the high

The hydrostatic transmission has a case ( 10) thatcontains the two drive system axial piston pumps ( l )

pressure [38 000 kPa (5500 psi)] hoses to the motors that drive the tracks.


11. Top cover.12. Filter.13. Locatíon for filling

transmission.

HYDROSTATIC TRANSMISSION(943 Illustrated)

14. Head.15. Case.16. Charge valve.17. Cooler bypass valve.

18. Bottom cover.19. Charge pressure relief

valve.20. Drain valve.

B 3355 X 1

On the top ofthe hydrostatic transmission is a cover A splined shaft sends power from engine drive gear‘ over the valves and linkage that control the displace- (5) through the center of the case to charge pump (32) ment (output) of the axial piston pumps. A similar which is installed on head (l 4). Oil is taken from the cover is used on the bottom of the case to provide bottom pan by the pump and sent through filter (12) to sufficient oil capacity for the system. the system control valvcs.

NOTE: The 953 has a larger bottom cover (18) (oil pan) and an oil filler tubc that is externally mounted.

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HYDROSTATIC TRANSMISSION(943 lllustrated)

21. Inlet to right hand sync 25. Input shaft for 28. Input shaft for speed andadjustment screw. underspeed override direction control.

22. Inlet to right hand neutral valve. 29. Underspeed cut-inadjust screw. 26. Inlet for underspeed adjustment valve.

23. Inlet to left hand sync valve cut-in tool. 30. Sync cutoff valve.adjustment screw. 27. input shafts for steering 31. Main control valve.

24. Inlet to left hand neutral control. 32. Charge pump.adjust screw. 52. SAIVO SlJ[J)3l/ Téliéf VRIVO.


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HYDROSTATIC TRANSMISSION

12. Filter. 13. Location for filling transmission. 21. Inlet to right hand sync adjustment screw. 22. Inlet to right hand neutral adjust screw. 23. Inlet to left hand sync adjustment screw. 24. Inlet to left hand neutral adjust screw. 26. Inlet for underspeed valve cut-in tool. 38. Lever for steer left.54. Lever for underspeed overríde valve.


7. Manifold for charge pump oil supply. 12. Filter.31. Main control valve. 32. Charge pump. 33. Pressure tap (quick disconnect) for underspeed valve throat pressure. 34. Pressure tap (quick disconnect) for underspeed valve upstream pressure. 35. Pressure tap (quick disconnect) for brake pressure. 38. Lever for steer left. 3g. FoR- WARD-PARK-REVERSE fever. 40. Lever for steer right.45. Relief valve for main pressure to right hand drive mo-tor. 52. Servo supply relief vaive.


16. Charge valve (right hand). 29. Underspeed cut-in adjustment valve. 30. Sync cutoff valve. 32. Charge pump.36. Pressure tap (quick disconnect) for charge pump pres-sure. 37. Lever for control of speed-brake valve. 38. Le- ver for steering left. 39. FORWARD-PARK-REVERSE lever. 40. Lever for steer right. 41. Pressure tap (quick disconnect) for venturi upstream prrssure. 42. Pressure tap (quick disconnect) for servo supply pressure. 43. Pres- sure tap (quick disconnect) for venturi throat pressure.47. Sync valve. 48. Balance line. 49. Pressure tap (quick disconnect) for right hand drive pressure. 50. Relief valve for main pressure to left hand drive motor.


16. Charge valve (right hand). 17. Cooler bypass valve.19. Charge pressure relief valve. 37. Lever for control of speed-brake valve. 38. Lever for steer left. 40. Lever tor steer right. 45. Relief valve for main pressure to right hand drive motor. 47. Sync valve. 51. Pressure tap (quick disconnect) for charge pressure. 52. Outlet for main pressure to right hand drive motor (reverse). 53. Outlet for main syGtem pressure to right hand drive motor (forward).



12. Filter. 26. Inlet for underspeed valve cut-in tool.36. Pressure tap (quick disconnect) for charge pump pressure. 39. FORWARD-PARK-REVERSE fever. 48. Balance line. 50. Relief valve for main prssure to left hand drive motor. 54. Lever for underspeed override valve. 55. Out- let for main system pressure to left hand drive motor (forward). 56. Outlet for main system pressure to left hand drive motor (reverse).


Main Control Valve

MAIN CONTROL VALVE GROUP TRANSMISSION

1. Brake line. 5. speed-brahe valve stern. 9. Orifice.2. Servo relief valve. 6. Start vent spool. 10. Passage for underspeed3. Charge pressure check 7. Venturi. venturi throat pressure.

vaive. 8. Passage for venturi 11. Underspeed cut-in4. Operate brake spool. upstream pressure. adjustment valve.

12.Quick response valve.This valve is installed on the front of the transmis- change in engine rpm. The two pressures from the

sion case. In addition to the components shown, it also venturi are used in the automatic load control system. includes the relief valve for the filter. Upstream pressure from the venturi goes through pas-

sage (8) and quick response valve (l 2) to the inlet onOil flow from the charge pump gocs into the left end

of venturi (7). As the flow goes through the venturi it makes two pressures, the venturi upstream pressure and the venturi throat pressure. The differcnce between these two pressures (upstream prcssure minus throat pressure) is used to indicate engine speed. Since the charge pump is driven directly from thc engine, a change in output flow will be seen whcnever there is a

the bottom of the underspeed valve. Throat pressure from the venturi goes through a small orifice (9) and passage ( 10) to the inlet on the top of the underspeed valve. The automatic load control system works to automatically reduce the drive system part of the engine load whenever ihe total load causes the engine


rpm to go below approxi iiiately 2325 for the 943/95 3 and 2 125 for the 963/973. The engine rpm at which the automatic load conti’ol system i‘eacts can be adjustcd with untlcrspeed cut-in adjustrnent valvc (11).

Oil flow from the right end of the venturi (7) goes through a passage to servo relief valve (2). At this point the flow is divided, with part of it going through the sere o relief valve and the rest is sent to the servovalves. The flow to the servovalves will be used as signal oil to the servo cylinders and will control the swashplates and thus the output of the axial piston pumps. The pressure ofthis l4ow is kept at approximately 2415 kPa (355 psi) by the action ot servo relief valve (2).

The oil that is dumped through the servo relief valve is divided again with part of it going to charge pressure check valve (3) and the rest going to the charge and main relief valves. This oil is now called charge pressure. As will be seen later, the flow to the charge and main relief valves will be used to replace system losses and the pressure will be held at approximately 1275 kPa (200 psi) by the charge pressure relief valve.

The oil that goes to the charge pressure check valve goes through it and into the area of operate brake spool (4). It goes across the operate brake spool and start vent spool (6) and then to the ¡iilot valve. If the pilot val ve is in the ment (RR AKE €l N) Position, the oil will go through it [passage (15) to passage ( 16)] and return to the chamber at the right end ofstart vent spool (6). If the pressure of the oil is more tha 730 kPa (106 psi), start vent spool (6) will move to the left against the force of the spring. Movement of the spool closes an outlet to tank and opens a passage up past the operate brake spool to the operate brake reset passage (14) in the pilot valve. With the pilot valve in BRAKE ON (vent), the oil will go to tank through outlet ( 13).

PILOT AND OVERSPEED VALVES

13.Outlet to tank. 14. Operate vent valve reset passage.15. Inlet passage from start vent system. 16. Outlet to end of start vent system.

When the FORWARD-PARK-REVERSE lever is

behind operate brake spool (4). This will cause operate brake spool (4) to move and open a passage that will send charge pressure through brake line (1). The pressure in the line from the brake line is used to release the brakes. When the pilot valve was moved to the BRAKES OFF position, the pilot oil flow to and from the start vent spool was cut off. The start vent spool stays in the shifted position but will return to the right if the charge pressure goes below 730 kPa ( 106 psi).

Automatic Load Control

The automatic load control system uses the two signal pressures from the venturi to control the under— speed valve. The underspeed valve changes the two signal pressures into mechanical signals to the servovalves. The servovalves in turn control the angle of the swashplates on the axial piston pumps. Thus, the un— derspeed valve controls the speed of the machine.

EXPLODED VIEW OF UNDERSPEED YALVE GROUP

1. Inlet for underspeed venturi throat pressure. 2. Retainer (two). 3. Guide assembly. 4. Lever assembly. 5. Lever assembly. 6. Pin. 7. Lever assembly. 8. Speed stops (two-one forward-one reverse). 9. Track. 10. Pin.11. Spring. 12. Retainer. 13. Track. 14. Spring cartridges (two). 15. Input shaft. 16. Retainer. 17. Spool.18. Roller. 19. Direction-speed link. 20. Bolt. 21. Bolt.22. Retainer. 23. Spring. 24. Inlet for underspeed up-

moved to FORWARD or REVERSE position, the pi- stream signal pressure. 25. Bracketlot valve stern will move with it. This action causes thepilot pressure to stop at outlet (13). Pressure will increase in operate brake valve reset passage ( 14) and

Hydrostatic Transmission Systems Operation18


c

The two signal prssiires from the venturi go to the top and bottom of the underspeed valve. The throat pressure goes to inlet ( 1) and the upstream prssure goes to inlet (24). Thus, the movement ol spool (1 7) is determined by the difference in pressure between these two signal oils.

When the engine governor is set on high idle rpm, the flow of oil through the venturi will be maximum. This means that the difference bctween the two pressures will be maximum. While the throat pressure re- mains fairly constant at about 1 720 kPa (250 psi), the upstream pressurc will vary with an increase or de- crcasc in flow from the pump. As the flow decreases. the differential will also decrease.

When engine is running at high idle, upstream pressure will be approximately 2950 kPa (430 psi) and throat pressure will be approximately 17211 kPa (250 psi). This is a large enough difference in the two pressures so that the upstream pressure, against the force of spring (1 1) and the throat pressure, nieves the underspeed valve up to its maximum activate‹l position. This puts the underspeed valve in a position where it will allow the machine to move in one direction or the other.

UNDERSPEED VALVE GROUP

1. Inlet for underspeed venturi throat pressure. 2. Retain- er. 3. Guide assembly. 4. Lever assembly. 5. Lever assembly. 6. Pin. 7. Lever assembly. 8. Speed stops (two). 11. Spring. 14. Spring cartridge. 15. Input shaft.16. Retainer. 17. Spool. 18. Roller. 19. Direction-speed link. 20. Bolt. 21. Bolt. 22. Retainer. 23. Spring. 24. In- let for underspeed upstream signal pressure.

Bolt (20) goes through rollers ( 1 8), retainer (12), spool (17) and retainer ( 16). Guide assembly (3) has a slot that lets the bolt and rollers ( 18) move up whenever spool (17) moves up. The roller is a mov able pivot point about which fever asscmbly (4) rotates. Since the dircction-speed link is connected to the pin in the bottom of fever assembly (4), the lever must have side- to-side movement to activatc the link. When the roller

is down, the center of the roller is almost in line with the center ofthe link. This, if input shaft ( 15) is turiied, there would be little or no movement l›y direction— speed link (19).

LOCATION OF UNDERSPEED VALVE GROUP

2. Retainer. 3. Guide assembly. 4. Lever assembly.5. Lever assembly. 7. Lever assembly. 8. Speed stops (two). 11. Spring. 14. Spring cartridge (two). 19. Direc- tion-speed link. 2d. Oil line for underspeed venturi throat pressure. 27. Oil line to underspeed-override valve. 28. Underspeed-override valve. 29. FORWARO -PARK-RE- VERSE fever. 30. Input shaft for underspeed-override valve. 31. Oil l.r.c for upstream pressure.

To get the side-to-side movement needed for direc— tion-speed link ( 19), roller ( 18) will have to move up in lever (4). This will happen whenever the underspeed upstream pressure at the bottom of the underspeed valve becomes high enough to overcome the force of spring (11) and the underspeed throat pressure at the top of the underspeed valve. With the roller rnoved up in fever assembly (4), maximum FORWARD move— ment ofinput shaft (1 5) will cause lever assembly (5) to rotate untilthe spring cartridge makes contact with pin (10). Since the pilot valve linkage is connected to the top of lever assembly (5), the pilot valve will have moved to BRAKES OFF, a condition that must hap— pen before the machine can be made to move. Lever (5), through the contact with lever assembly (7), will cause pin (6) to move lever assembl) (4) until pin (6) makes contact with speed stop (8) for forward travel. Since direction—speed link ( 19) is connected to the bottom of fever assembly (4), the movement of the lever has caused the direction-speed link to movc to the right and activate the servovalves that control thc angle of the swashplates on the axial piston pumps. When the input shaft is rotated to the maximum RE— VERSE position, the same sequence will happen cx- cept that the direction-speed link will move in the opposite direction.

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MAX FORWARD

SPEED STOPREVERSE

SPEED STOP

SECTION A•A

UNDERSPEED VALVE GROUP[Zero Speed (BRAKE OFF)

Operator Input-High Idle Engine Speed]

B6930X1

UNDERSPEED VALVE GROUP(Maximum Forward Speed Operator Input

and Hi Idle Engine Speed)2. Retainer. 3. Guide assembly. 4. Lever assembly.

6. Pin. 7. Lever assembly. 8. Speed stops (two).11. Spring. 18. Roller. 19. Direction-speed link.

The spool in the underspeed valve and roller (18) will change their position and move downward when the total load on the engine causes it to lug. The total load one the engine is caused by the drive system and the implement system. When the total load causes the

3. Guide assembly. 4. Lever assembly. 6. Pin. 8. Speed stops (two). 11. Spring. 18. Roller. 19. Dlrection-speed link.

If the engine is startd with the FORWARD—PARK— REVERSE lever in either FORWARD or REVERSE, the machine will not move until the lever is moved to PARK.

When the FORWARD-PARK-REVERSE lever isenginc speed to go below cut-in rpm (2325 25 rpm moved, lever (5) will move. Since the linkage for thefor the 943/95 3 and 2125 ± 25 for the 963/973) thevalve spool and roller will move down. As the roller moves down, the dircction-speed link moves thc servovalves toward PARK position. This action causes a decrease in the angle of the swashplates and a decrease in output flow from the axial piston pumps. At this position, the engine horsepower is enough to develop relief pressure in the implement system, track crowd- ing, drawbar pull and speed for acceptable bucket loading.

pilot valve is connected to the top of lever (5), movement by the lever will cause the spool in the pilot valve to move. Pilot oil in passage (3 l ) goes into thc main control valve and comes out in passage (32). Passage(32) takes the oil to the pilot valve. With the pilot valve closed, the oil carinot get through to passage (33). Pas- sage (33) from the pilot valve goes back to a chamber at the end of start vent spool (34). Start vcnt spool (34) will not move, so the drive system stays in a x ented condition.


NVERTE

MAX FORWARD SPEED STOP MAX REVERSE SPEED STOP

SECTION A- A


UNDERSPEED VALVE GROUP(M8XlfftlJfñ R6V6reé Speed Operator Input

and Hi Idle Engine Speed)UNDERSPEED VALVE GROUP

(MaxlmLt£ñ FOfWBrd 5p8Bd Operator Input and MaximumLug Engine Speed)

3. Guide assembly. 4. Lever assembly. 6. Pin. 8. Speed 3. Guide assembly. 6. Pin. 8. Speed stops (two). 11.stops (two). 18. Roller. 19. Direction-speed link.

TRANSMISSION MAIN CONTROL VALVE GROUP31. Oil passage. 32. Oil passage. 33. Oii passage.34. Start vent spool.

Spring. 18. Rollor. 19. Direction-spe»d link.

PILOT AND OVERSPEED VALVES

32. Oil passage. 33. Oil passage.


Charge Valve and Main Pressure Relief Valve

CHARGE AND MAIN RELIEF VALVE(Shuttle Valve in Forward Drive Position)

1. Inlet for charge flow. 2. Main pressure relief valve.3. Outlet to sync valve. 4. Outlet to tank. 5. Bah. 6. Pilot piston. 7. Inlet for high pressure supply line. 8. Shuttle spool. 9. Outlet for low pressure supply line. 10. Outlet to cooler. 11. Inlet for high pressure supply line. 12. Check valve. 13. Outlet for low pressure supply line. 14. Outlet to cooler. 15. Inlet for charge pressure.

Two of these valve groups are used and they are identical except for the sync valve on the right hand

group and the sync shut-off valve on the left handgroup. Each group has a shuttle spool (8) and (l 8), a vent check VdlVC ( 12) and a relief valve (2) for rnaxi- mum drive pressure. The shuttle spools (8) and ( 18) are meved by the pressure in the supply lines between the valves and the drive motors. Thc supply line (forward or reverse) with the highcst pressure will cause the spool to move in the direction nccessary to let charge flow go into the supply line with the lowest pressure (return side from the motors and inlet side io the pumps). In this way, thc losses, which are designed into the system for lubrication and cooling, are re- placed by the flow from the charge pump.

The vent check valve (12) will stay closed as long as there is charge pressure behind pilot piston (6). Loss of charge pressure will let ball (5) become unseated and let vent check valve (12) open. When the vent check valve opens, it makes a passage that connects the high pressure sid eofthe drix’e loop to the low pressure side. This stops the flow of high pressure oil to the track motors and the vehicle stops. The loss of charge pressure also causes the brakes to be applied. This loss of charge pressure can be caused by the operator or it can happen automatically as the result of a failure.

CHARGE AND MAIN RELIEF VALVE(Shuttle Valve in Forward Drive Position)

2. Main pressure relief valve. 5. Ball. 6. Pilot piston.12. Check valve. 15. Inlet for charge pressure. 16. Inlet for charge oil supply. 17. Outlet for charge oil supply to charge valve for right track motor. 18. Shuttle spool. 19. Outlet to cooler. 20. Inlet for high pressure supply line. 21. Inlet for low pressure supply line. 22. Outlet to cooler. 23. Inlet for high pressure supply line. 24. Outlet for low pressure supply line.

The ti‘ack syiiclii‘oniziiig system makes it possible for a small flow of oil to go between the two charge and relief valves. This insures that the pressure and flow to the track motors will be equal un der 1200 psi. This means that the tractor will mox e straight ahead when motion occurs.

This arrangement could cause a problem whenever the vehicle started to turn. This is because the pressure to the inside track is always tess than to the outside track during a turn. To prevent this from happening, the sync valve closes the flow path whenever the pres— sure from either of the motors becomes higher than 8280 kPa (1200 psi).

The shutoff valve (26) makes it possible to manually cut the circuit off during periods of adjustrnent or troubleshooting.


NVERTE


TRACK SYNCHRONIZING SYSTEM

25. Left side charge and relief valve. 26. Sync cutoffvalve. 27. Sync valve. 28. Right side charge and reliefvalve.

Charge PPé5SUPé CheCk Valve

scrvo cylindcrs will fill. This causes the pressure in the charge circuit to decrease momentarily. The momen- tary decrease in pressure will cause the charge pressure check valve to closc. Thc check valve has orifice (3) to slow thc rcvcrsc flow of oil and maintain the pressurc in the control valvc.

OVERSPEED VALVE

PILOT-OVERSPEED VALVE

1. Adjustment screw. 2. Locknut. 3. Passage for underspeed throat pressure. 4. Spool. 5. Passage for underspeed upstream pressure. 6. Passage for venturi upstream pressure. 7. Piston. 8. Pilot valve. 9. Spring.10. Passage.

LOCATION OF PILOT OVERSPEED VALVE

1. Adjustment screw. 2. Locknut. 11. Oil line to passage(5). 12. Oil line to passage (3). 13. Overspeed valve.

C13 21 7X1

CHARGE PRESSURE CHECK VALVE

1. Spring. 2. Check valve. 3. Orifice.

Under certain conditions, when the machine is moving, it is possible for the speed of the machine to cause the track motors to act like pumps. This could cause either the right or left drive loop to vent, which could make the machine turn abruptly.

To prevent this, the overspeed valve has been installed, Its purpose is to cause the ground speed to go no higher than the position ofthe FORWARD-PARK- REVERSE lever.

The oil to the main control valve and pilot valve flows through the charge pressure check valve. The charge pressure check valve prevents sudden pressure drops in the main control valve during large swashplate movement. When the servovalves are moved, the

Tht? Clil pTessure in 1iRt2 ( 1 2) is the same as the pressure on the top of the underspeed valve. The oil pressure in line (1 l ) is the same as the pressure at the bottom of the underspeed valve. See AUTOMATIC LOAD CONTROL..


G O NVERTE


As explained iii the section on the Automatic Load Control, two signal pressures are caused by the flow of oil from the charge pump through the venturi. The difference in pressure between the two signal pressures controls the action of the underspeed valve.

When the engine is running at high idle, the upstream pressure will be approximately 2950 kPa (430 psi) and the throat pressure will be approximately 1720 kPa (250 psi). This is a large enough difference to make the underspeed valve raise and put it in a position where an input signal from the transmission control lever will cause the machine to move.

Sirice the difference in pressure between the upstream and throat sections ofthe venturi are created by

the oil flow through it, any increase in oil flow will cause the differcncc in pressure to become greater.This will happen whenever the spced (rpm) of the engine increases.

As the engine speed (rpm) increases the flow from the charge pump increases and the flow of oil through the venturi increases. This causes an increase in prssure in the upstream part of the venturi. This increase in pressure is felt in line (l l) and passages (5) and (6). As the pressure increases it becomes high enough to push spool(4) against the force ofspring (9) and underspeed throat pressure in line (12) and passage (3). The spool will move when the upstream pressure through passage (10) becomes high enough to move piston (7). When the spool moves, it opens a passage between passages (3) and (5) and lets some of the underspeed upstream oil mix with the underspeed throat oil. The spring force of the underspeed valve will push the valve down toward the neutral position. This will cause the servovalves to move toward neutral and the swashplates on the piston pumps will move toward a position of zero input. This will cause the machine speed to be maintained at the position of the FORWARD-PARK-REVERSE lever.

Track Motors (943, 953 & 963)

The two track motors are the same. They are fixed displacement, link-type piston motors. Each motor gets its oil supply from one of the variable displacement pumps (the two pump-to-motor circuits are sepa- rate at all times). The displacement of each motor is the same as the maximum displacement pump. A change in direction of oil flow through a motor will change the direction but will not change the amount of output torque available from the shaft of the motor.

Oil flow through a motor can be in either direction. A change in the direction of oil flow will change the direction of rotation of plate assembly (4) and barrel assembly (8). The components in the motor that turn are: shaft (23), barret assembly (8), spring (11), spring

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(25) and the parts used in the assembly ofboth springs, piston assemblies (7) and (22), joint assembly ( 17) and plate assembly (4). The rest of the parts of the pump are fasteried to either the head assembly (13) or bodies(6) and ( 18) and do not turn. Oil flow from the pump goes into one of the inlets in the head, through a port plate (24) and into the cylinders in baFrel assembly (8). When each piston reaches the position of piston assembly (22), oil pressure in the cylinder pushes the piston assembly out of the cylinder. Because of the angle between the barrel assembly (8) and plate assembly (4), they will turn as the piston is forced out of the cylinder.

When rotation of the barret and plate turns piston assembly (22) to the position of piston assembly (7), the piston will be fully retracted. As the barret contin- ues to move, the piston assembly will be forced back into the cylinder. The oil in the front of the piston will be forced through the head assembly to a return line to the variable displacement pump.


NVERTE

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