Hydraulic Manual

- GENERAL INFORMATION - SYSTEM OPERATION - SCHEMATICS

HYDRAULIC SYSTEMS

SINGLE PUMP - DUAL WHEEL MOTOR and DUAL PUMP - DUAL WHEEL MOTOR

Revision 8/2000

GENERAL

SINGLE PUMP - DUAL WHEEL MOTOR HYDRAULIC SYSTEM In a single pump with two remote motor systems like the ones used on the STHM and Magnum III, highpressure hoses carry the high-pressure drive system oil between the pump and parallel drive motors. The major difference between the systems on the STHM and Magnum III is that the Magnum III incorporates a crossover relief valve to protect the system from high-pressure spikes. Oil flow through the single pump, dual wheel motor system begins in the oil reservoir 1 . This is where oil should be added if the system becomes low on fluid. The oil reservoir serves several functions. One function is an oil reserve for the system. Another function is cooling. As hot oil enters the tank, the heat from the oil is transferred to the walls of the tank and then into the surrounding air. This is why often times you see hydraulic tanks that are long and thin. A long and thin tank has more surface area and can cool the oil better than a more square design of tank. The oil reservoir also promotes the removal of small air bubbles from the system. From the oil reservoir, the oil passes through the filter 2 . After the oil has

SYSTEM OPERATION

been filtered, it travels to the charge pump 3 . The charge pump acts as a smaller hydraulic pump that keeps the main pump supplied with oil. The charge pump also takes oil from the closed loop and replaces it with cooled (by the reservoir), filtered (by the filter) oil. Now the oil has reached the main pump 4 . The oil leaves the pump from a single outlet, and is split to allow flow to each wheel motor 5 . Because both wheel motors are in the same closed loop system, the pressure is always equal at each motor inlet. In normal, straight-ahead operation, both motors have roughly the same torque load. The flow splits evenly between the right and left motor, resulting in both motors operating at the same speed. Once the oil has passed through the wheel motors, it returns to the main pump where the charge pump replaces it with cooled, filtered oil and then the main pump sends it back to the wheel motors. The STHM and Magnum III also have an oil cooler 6 (or heat exchanger) located in the oil return line running from the pump to the oil reservoir. When the machine goes into a turn, the outside wheel in the turn must travel further than the inside wheel. As this happens, the torque demand on the inner wheel increases at the same time the outer wheel requires increased oil flow to allow it to turn faster. Because the pressure is equal at both motors, the oil will follow the path of least resistance. With increased load and resistance to oil flow to the inner wheel, more oil flows to the outer wheel where resistance is lower, allowing it to turn faster, to complete the turn. The pressure balances wheel speed and load to inherently provide differential action between the two drive wheels. The Magnum III incorporates a dual cross over relief valve 7 to protect the hydraulic system from pressure spikes. The relief valve is required on the Magnum III because of its increased weight versus the STHM. The valve senses high pressure spikes and will protect the hydraulic components by returning excess fluid to the low pressure side of the system. It is located after the pump, where the fluid is split to the two wheel motors. With the single pump and dual wheel motor design, if the drive is weak or slow at both wheels, common elements such as linkage, oil condition, oil reservoir, oil filter, the pump, and lines running to the pump should be examined for faults. If one drive wheel is slower or weaker than the other, the slower wheel motor should be examined as it may have an internal problem causing fluid leakage and loss of performance. On the Magnum III, a defective cross over relief valve could also cause loss of power to both drive wheels. Note: The arrows displaying the direction of oil flow in the closed loop system (oil flow between the pump and wheel motors) represent the machine moving forward. The direction of the oil flow in the closed loop would be reversed if the pumps were stroked into reverse.

1

SINGLE PUMP - DUAL WHEEL MOTOR HYDRAULIC SYSTEM 1

2 6

LEGEND 1

STHM

2 3 4 5 6

3

reservoir oil filter charge pump main pump wheel motor oil cooler (heat exchanger) high pressure line low pressure line direction of oil flow

4

5

5

1

1

2

LEGEND

MAGNUM III

1 2 3 4 5 6 7

6 3

reservoir oil filter charge pump main pump wheel motor oil cooler (heat exchanger) dual cross-over relief valve high pressure line low pressure line direction of oil flow

4

7 5

5

1

2

GENERAL

DUAL PUMP - DUAL WHEEL MOTOR HYDRAULIC SYSTEM In two pump with two remote motor systems like the ones used on the SWZ, SWZU, STC, STT, STTBSG, and SCR, each pump and motor system operates as an isolated unit or a “closed loop”, only sharing the system reservoir and filter. In other words, the left hand pump drives only the left hand wheel motor and the right hand pump drives only the right hand wheel motor. Oil is shared between the two closed loops (left and right side) however, oil ALWAYS passes through the tank (for cooling purposes) and then through the 10 micron oil filter (filtration purposes) before reaching the other side. In the event of a pump or motor failure in one closed loop (left or right side), any contamination will get trapped in the filter and NOT be able to cause damage the other closed loop system. The only exceptions to this rule are contamination to the oil with water or excessive amounts of air. The hydraulic filter can not remove water or air from the oil. If the oil is contaminated with water, the system should be flushed and the oil should be renewed. If the oil is contaminated with air, the source of the air leak should be identified and repaired before refilling the system with fresh oil. For these systems, high-pressure hydraulic hoses connect the main pump and wheel motor. As demonstrated in the Hydro-Gear video program (reference the Scag Technical Video Series Fundamentals Tape - Hydraulic Section), a flowmeter can easily be installed in the system to evaluate pump performance and to isolate main pump performance from the wheel motor.

SYSTEM OPERATION

Oil flow through the dual pump, dual wheel motor system begins in the oil reservoir 1 . This is where oil should be added if the system becomes low on fluid. The oil reservoir serves several functions. One function is an oil reserve for the system. Another function is cooling. As hot oil enters the tank, the heat from the oil is transferred to the walls of the tank and then into the surrounding air. This is why often times you see hydraulic tanks that are long and thin. A long and thin tank has more surface area and can cool the oil better than a more square design of tank. The oil reservoir also promotes the removal of small air bubbles from the system. From the oil reservoir, the oil passes through the filter

2

. After the oil has been filtered, it’s flow is split

into two lines, one going to each charge pump 3 . The charge pump can be identified on the main pump as the “diamond” shaped extrusion on the cast, black portion of the main pump (reference the Scag Technical Video Series - Fundamentals Tape - Hydraulic Section). The charge pump acts as a smaller hydraulic pump that keeps the main pump supplied with oil. The charge pump also takes oil from the closed loop and replaces it with cooled (by the reservoir), filtered (by the filter) oil. Now the oil has reached the main pump 4 . There it is pressurized and sent to the wheel motor 5 . This high pressure oil is what drives the wheel motor and makes it turn. Once the oil has passed through the wheel motor, it returns to the main pump where the charge pump replaces it with cooled, filtered oil and then the main pump sends it back to the wheel motor. Certain Scag mowers (Sabre Tooth Tiger and STT72A-27KA) have oil coolers 6 (or heat exchangers) mounted in the return lines leading from the pump to the reservoir. Note: The arrows displaying the direction of oil flow in the closed loop system (oil flow between the pumps and wheel motors) represent the machine moving forward. The direction of the oil flow in the closed loop would be reversed if the pumps were stroked into reverse.

3

DUAL PUMP - DUAL WHEEL MOTOR HYDRAULIC SYSTEM LEGEND 1 2 3 4 5 6

reservoir oil filter charge pump main pump wheel motor oil cooler (heat exchanger) high pressure line low pressure line direction of oil flow

1

2

6 3

6 3

4

4

5

5

4